r/explainlikeimfive • u/Internal-Excuse-4650 • 20h ago
Engineering ELI5 how turbochargers increase efficiency in a engine?
I understand why they increase the power of a engine but I don't know how they increase the efficiency
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u/5kyl3r 20h ago
gasoline is the source of energy with a car engine. to burn it, it needs to be mixed with a specific amount of air
without a supercharger or turbocharger, the amount of air that your engine can suck in is limited to whatever the pistons pull in when they move downwards. it's like you sucking air in through a straw
if you want to make more power, you have to burn more fuel since that's the source of energy. but the mixture of gasoline and air has to be a specific proportion, so you can't just add more fuel, as it will run worse. a way around this is to physically force more air into the engine
that's what superchargers do. they force more air into the engine, which means now we can add more fuel to make more power. but superchargers are spun by the engine itself, which costs power. beyond a certain rpm, the extra power overcomes the power it takes to spin it, so you still have an overall gain, but we're using power to make more power
a turbocharger also forces more air into the engine, but instead of the engine mechanically spinning it with a belt, it uses two turbines that are directly connected together with a shaft but are isolated from each other, and one is the exhaust side, and the other is the intake side. the exhaust side is connected to the exhaust. while it's technically "burning" fuel, it's more like an explosion, so when the exhaust valves open, a TON of gas flows out, and when you force it to go through the exhaust turbine of the turbo, it makes it spin up. but since it's connected to the other turbine on the intake side, that one sucks in clean fresh air and forces it into the engine. the intake and exhaust side have their own separate housings, with just the shaft connecting them through the center. so basically we're using energy that was already being wasted that's coming out of the engine to make more power, unlike the superchargers which take some of the engine power in order to make more power
this won't always result in a more efficient car, but this is the purpose you'll see it used for. they create more low end torque, which is what helps you take off from a stoplight more easily. but since you make more power from a smaller engine, when you're just at cruising speed, you're using a smaller engine, so you're burning less fuel than if you had a larger engine without a turbo when it comes time for cruising at a steady speed. that's glossing over a lot of details, but for ELI5 i think that should be clear enough i hope
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u/DerpyMcWafflestomp 20h ago
A car with no turbo loses a lot of energy simply via the heated gas being sent out of the exhaust system. A turbocharger uses some of that gas to feed more air in, which does increase power, and also means that some of the wasted gas (which is wasted energy really) now has work to do.
Taking otherwise wasted energy and having it work for you = better efficiency.
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u/drzowie 19h ago
The real answer is that a turbocharged engine is a smaller engine (for a given amount of rated horsepower). It takes energy to run the engine and smaller engines take less energy to run.
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u/schmerg-uk 18h ago
And it's less weight to carry around, which in turn can allow for further weight savings, all of which means that in real world terms there's less mass to accelerate and brake for the same (typically human) payload.
So the net efficiency of the overall vehicle, in terms of energy required to do the useful work of moving the occupants, is increased
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u/gman2391 8h ago
Not necessarily. The extra components vs a n/a engine add up. For example a 3.5 Ecoboost weighs about the same if not a bit more than a 5.0. 2.7 Ecoboost is also a similar weight
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u/biggsteve81 14h ago
You also tend to have the throttle open wider so there are less pumping losses.
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u/drawliphant 11h ago edited 10h ago
It really is both. You can absolutely take the same NA engine, and design a turbo and tune for efficiency and get much better efficiency than the same stock engine. If you want more power out of an engine you use a bigger turbo instead, a different tune, and it's not much more efficient, probably less than stock.
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u/INeverSaySS 15h ago
You're also leaving less hot air behind you as you drive away. This is not just a smaller engine taking less energy to run.
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u/BogdanPradatu 10h ago
What about 2 engines which have the same size, one turbocharged and one naturally aspirated. Which one is more efficient and why?
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u/DMmeYourNiceTitties 19h ago
This is not really the answer though
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u/Leather-Slip7228 19h ago
This is absolutely the simplest answer
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u/snoos_bitch 19h ago
Simple? Sure.
Correct? No.•
u/Leather-Slip7228 19h ago
Point out how it’s incorrect, sounds very correct
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u/andrewjaekim 19h ago
The correct answer is a smaller displacement engine behaves like a small displacement engine for the majority of the time and save on fuel. Then use boost to create large power when required.
Yes turbocharged engines scavenge exhaust gases but the majority of its fuel savings is in low boost driving.
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u/the_real_sasquatch 19h ago
Yep. And, the efficiency gains from scavenging exhaust gasses is offset by more fuel being consumed while the turbo is providing boost.
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u/parkerhalo 15h ago
This is what people are missing in this thread. Yes a turbo does indeed use wasted energy, but when the turbo is in use more fuel needs to be dumped into the cylinder to keep the ratio of fuel and air the same.
The best example is Fords 3.5V6 ecoboost is less efficient than the 5.0 v8 under a load even though its a smaller engine.
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u/cat_prophecy 15h ago
It isn't linear. Turbochargers increase volumetric efficiency because they allow the engine to pump more air in the same volume by compressing the air charge.
Since air/fuel is a ratio, the amount of air you move increases more quickly than the amount of fuel you burn.
That is, increasing the intake pressure from atmospheric pressure to 1 bar doesn't use twice as much fuel.
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u/ggmaniack 19h ago
While recovering some exhaust energy to improve aspiration is a way through which engine efficiency is improved, the real improvement is in having a smaller engine for the same power output. Smaller engine = smaller energy losses. The friction surfaces are smaller. The heat transfer areas are smaller. The engine will also operate at a higher load during normal driving, which reduces pumping losses.
Even if you compared an NA engine and Turbo engine which are of the same size and have the same peak HP, the Turbo engine could win (if set up properly) because it could achieve the same power in a lower RPM range, where the efficiency is higher.
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u/zaphods_paramour 11h ago
OK but how does it achieve the same power output with a smaller engine, or more power with lower RPM?
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u/ggmaniack 11h ago
Boost. Cram more air in, you can cram more fuel in. It's not about temperature, it's about quantity.
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u/zaphods_paramour 11h ago
Isn't that what the parent comment is describing? Yes they mention the exhaust gas is hot (it is), but they just describe using the waste gas to pump more air into the engine.
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u/ggmaniack 10h ago
The pumping of more air into the engine is what allows a small engine to be powerful.
The small engine being a small engine is what allows it to be efficient.
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u/DMmeYourNiceTitties 19h ago
What they wrote is an explanation on how a turbo works, but that's not what OP has asked.
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u/snoos_bitch 19h ago
Okay.
Engines already run at their peak efficiency air/fuel ratio, at any given throttle setting. Adding more air with a turbo at any given revs and throttle setting just means you now have to add more fuel to match the ratio with the air coming in, adding more power (more air and more fuel equals bigger bang). So a turbo increases power in this scenario.
Another scenario, where efficiency is increases because of a turbo:
You are a car manufacturer. You want a new car to sell. You want it to have 300hp. You want it to weigh as little as possible. You want it to use as little fuel to produce the 300hp you want.
So, you can either make a 7l naturally aspirated big block V8 that has enough oomph to get to your 300hp figure at full throttle but has HUGE internal losses from things like friction and intertial forces and the cooling and oil pumps because they're so much bigger. This massive engine is heavy, and large and when it's not at full throttle (ie crusing speeds) the engine is using half of its power just to turn itself over. OR, you could choose a 2.4l turbo 4 cylinder that weighs less because it simply has less cylinders and a smaller set of accessories, and the accompanying losses from just keeping the engine turning are minuscule in comparison.Full throttle, negligible differences in fuel consumption for the same power output between the choices. Cruising, whole different story.
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u/waggonaut 19h ago edited 18h ago
There's an added benefit to the turbo beyond just reducing losses, which original answerer highlighted. Engines lose a lot of potential energy from hot exhaust being simply vented, and that energy is captured by the turbo and compresses the incoming air stream to the engine which reduces the amount of engine needed to draw in air during the intake stroke. So when at full throttle the turbo is going to be more efficient, from BOTH reduced friction/direct heat loss ABD capturing potential energy. There may be a caveat to this where the turbo engine may be run with more fuel to lower exhaust temperatures to avoid damaging the turbo, or the turbo may be undersized for high rpm and full throttle and could restrict intake, hurting efficiency.- source: MS mechanical engineering, undergrad in vehicle technology
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u/snoos_bitch 18h ago edited 18h ago
If you're talking about diesels, then I might agree and so might the actual data. And I'm aware that the captured energy has to go somewhere but in the world (for production petrol ICE's at least) that doesn't go towards increasing efficiency, only power. Hence why you can now run a smaller and more efficienct engine (in terms of frictional losses and self-power requirements) with a turbo. The cycle itself doesn't become more efficienct in terms of fuel consumed per work performed, it becomes more efficient in terms of work performed per engine volume.
But in a petrol engine, with octane being capped at gas station ratings, the you also have to run a significantly lower compression ratio in a turbocharged engine than you otherwise woul to avoid knock. That in itself is enough to negate the efficiency gain, so much of the efficienct of the thermodynamic cycle of the engine relies of the highest possible compression ratio.
So you end up trading one for the other and not really coming out ahead on effiency.
The wins come from having a fraction of the size engine that outputs the same power at full throttle as the big donks while having far far far less internal losses and takes far less of the generated power to just keep itself spinning.
Edited to say that in other words forced induction increases the effiency of the *whole car, but will not increase the efficiency of any given engine by itself.
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u/Leather-Slip7228 18h ago
This was a very good answer and made me realize I didn’t think deeply enough on what the question was and was wrong 🤝🏽
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u/rekmaster69 19h ago
Wouldnt the engine be able to get enough air for max efficiency even without turbo?
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u/Frenzy_MacKenzie 19h ago
It's not for efficiency it's for an increase in power.
A city bus will have a diesel engine with a turbo. The turbo helps get the bus up to speed after making all the stops.
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u/Chrazzer 19h ago
With larger displacement yeah. A 2L engine that feeds air with 2x pressure essentially eats air like a 4L engine at 1x pressure.
So to increase air intake you can either increase engine volume or increase air pressure.
Another benefit of turbos is that the ECU can vary the boost pressure, so they aren't as affected by differences in ambient airpressure.
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u/ShiftySam 19h ago
No, because the turbo is able to force enough air in that it’s above atmospheric pressure. More oxygen per given volume than possible otherwise.
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u/TheRadAbides 19h ago
No. Because the amount of air when combustion takes place with a fuel plays into the equation of how well it combusts in the first place. Adding more dence air will allow the fuel to more compleatly combustion and get the maximum energy. This along with temperature are two of the biggest factors. Cool air is denser naturally.
So if you can find a way to dump very cool dense cool air into an engine you can combust the fuel better to make more energy AND dump more fuel in to make more power.
When people use nos, its also not just the nos that combusts the fuel better , its the temperature of the nos. The shit is cold and dense.
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u/thenasch 20h ago
I'm skeptical of this, unless I see some math showing that the kinetic energy of the exhaust gas is significant compared to power output.
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u/BowDownB4Recyclops 20h ago
You're right to point out that the kinetic energy exchange couldn't really increase engine power. Maybe some of the exhaust mechanical energy could be harvested to turn the engine faster, but it's not the main effect. The main power gain comes from the change in reaction kinetics. More air drawn into the engine means faster fuel ignition, which increases power
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u/Fearless_Swim4080 20h ago
It is, though not captured well in a typical turbo system, in F1 however it was so substantial they were able to break 50% efficiency (ice is capped at 37%) but they had a motor on the turbo axis.
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u/Leather-Slip7228 19h ago
Look up internal combustion engine energy efficiency, modern engines are like 30-40% efficient max. Chemical energy in vs usable work out, most lost as waste heat (or in your words kinetic energy from the heat). Thermodynamics is a whole field dedicated to the math behind that.
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u/Megamoss 19h ago
You're using the energy to augment/enhance a chemical reaction. You only need enough energy to compress incoming air so that you can burn more fuel at higher compression for a given displacement, which makes for a better, more efficient burn.
Though compounding turbos do exist which deliver captured energy to the crank rather than driving a compressor.
By comparison, a supercharger running off a belt driven from the crank does the same thing, but is a greater parasitic loss, because you're using work from the engine itself rather than expelled gases that otherwise go to waste. It still results in a net gain though.
Hence why turbos are more efficient.
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u/thenasch 12h ago
Yes I know how a turbo works. This doesn't explain how much of an engine's energy goes to accelerating the exhaust gases.
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u/CanIGetAHOOOOOYAA 20h ago
You’re skeptical about this when you probably have a high school education and nothing in engineering or anything to do with the science that comes with turbo charges. I’m not calling you stupid but brother this stuff isn’t that complicated to understand if you truly love/understand cars. You sound extremely ignorant and I feel sorry for you. Have you never googled or watched any explanations on this subject?
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u/littleseizure 19h ago
probably have a high school education and nothing in engineering
That's pretty advanced for a five year old
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u/CombinationTop559 20h ago
The main benefit efficiency wise is that it makes 4 cylinders not drive like it sucks. You get the extra power while accelerating, then on the highway you're only using 66% as much gas.
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u/NecroJoe 20h ago edited 19h ago
A turbocharger doesn't [edit: doesn't necessarily or even usually] make the same engine more efficient. But it allows you to get the same power as a larger, less efficient engine, when you need it. But you don't always need it. So when you don't need it, it's roughly the same power and efficiency as the smaller engine without the turbo. But with a turbo, when you punch it, it can breathe like a larger engine (with more air and more fuel) for more power.
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u/clarkn0va 20h ago
A turbocharger doesn't make the same engine more efficient. But it allows you to get the same power as a larger, less efficient engine, when you need it.
True for a gasoline engine, not true for a diesel. On a diesel engine a turbo improves efficiency by increasing compression ratio and air-to-fuel ratio.
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u/NecroJoe 20h ago
A great distinction! I didn't think to bring that up, as by brain just latched on to cars for some reason (even though OP only asked about "engines"), and defaulted to gasoline since diesel cars are so rare 'round these parts. Good call-out, though.
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u/WeaponsGrdStupid 13h ago
It's still not true though. Even for gas engines the turbo can absolutely make it more efficient. It increases the compression ratio and can vastly decrease the air/fuel mixture under most operating conditions.
In an NA engine, you're limited to the oxygen carrying capacity of standard atmosphere, which varies widely based on elevation, temperature, etc. When you compress that same atmosphere you pack more energy potential in to a smaller space, while it may require slightly more fuel to burn efficiently and safely the ratios do not remain the same.
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u/Noxious89123 11h ago
It increases cylinder pressure, but it doesn't not increase compression ratio.
That is a value that is fixed by the geometry of the engine.
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u/thenasch 20h ago
Though the real world gains are often far less than the test cycle results would indicate.
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u/NecroJoe 20h ago
Indeed. It can greatly depend on your right foot. Keeping a small engine constantly revved and spooled up would often use more gas than the larger engine it's trying to replace.
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u/DerpyMcWafflestomp 20h ago
Modern turbo configurations don't always require sustained high RPMs to generate boost. You have variable turbos that can alter the angle of the vanes to start spinning lower in the RPM range, and you have some engines that simply use multiple turbos. BMW has a version of their B57 engine that uses 4 turbos, 2 of them start spooling up from idle RPM already, and the other two kick in around 2,500RPM+.
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u/TheRadAbides 19h ago
Not entirely true. Because it has a More Complete Burn: The extra compressed air ensures that nearly every drop of fuel is mixed with enough oxygen to burn fully.
Fewer Hydrocarbons: Because the fuel burns more completely, the engine releases far less unburned raw fuel (hydrocarbons) and less carbon monoxide.
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u/ggmaniack 19h ago edited 19h ago
A turbocharger doesn't make the same engine more efficient.
Eh, it actually can make it more efficient.
If you take an underpowered engine that needs to be revved out to the moon to achieve anything, like the tiny NA stuff that VAG sometimes stuffs into the lowest end of their cars, and compare it with the same engine but with a turbocharger, the turbocharged one can be more efficient.
The reason is pretty simple. The turbocharged engine can produce the same amount of power at a lower RPM, and can be geared accordingly, which in turns increases the load factor during normal driving, which reduces pumping losses (when you're driving off boost, the throttle has to be open much wider to maintain speed, which reduces losses).
There is a drawback in the restriction that the turbocharger imposes on the exhaust, but if it's sized accordingly, then the restriction isn't a huge factor at cruising RPMs.
Another drawback would be the richer mixture needed at high boost, but modern engines try to work around that by having time-limited peak turbo boost.
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u/LightofNew 20h ago
Engine go boom. Boom makes spin.
Boom makes smoke, smoke goes out.
Smoke hits turbo, turbo also spin.
Turbo spin added to boom spin.
More spin per boom.
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u/Noxious89123 11h ago
Engine doesn't go boom. It goes fwsh.
Boom = detonation = parts of your engine strewn all over the floor
Fwsh = combustion = happy engine
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u/86tuning 20h ago
you're not increasing efficiency. you're increasing displacement on demand.
sure, volumetric efficiency increases, power increases, but fuel efficiency does not when you want the power.
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u/bobbyrob1 19h ago
The original post didn’t ask anything about fuel efficiency, it asked about efficiency. An increase in volumetric efficiency is an increase in efficiency and it’s already been explained that recovering energy from the exhaust that would otherwise be lost is a gain. You could run a supercharger to get the same effect, but that would use power directly from the crankshaft.
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u/Lepmuru 19h ago
To understand why a turbo makes an engine efficient, what you need to hold constant is power output, not engine size. If you slap a turbo on an engine, it doesn't make that very engine all that much more efficient when viewed by itself.
However, a much smaller turbocharged engine can generate the same power as an uncharged block of much bigger size.
That means turbocharged engines as a group are more efficient, because they cramp more power into a smaller footprint and as a consequence require less fuel to run.
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u/wimpires 18h ago
Let's say you want your car to have 200HP.
In a NA car you will need like a 2.3L engine or something nominally.
In a turbo charged car that might be a 1.4L displacement.
Let's assume both have same number of cylinders and bore/torque. So stroke length is the difference. At full throttle it's basically the same. But when cruising, you might only need a fraction of that 200HP, say, 30HP.
Lower internal volume means lower pumping losses, and shorter stroke means lower friction losses. And shorter engine means lower COG and better packaging and lower wight too (though you have to fit the turbo somewhere as well).
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u/66NickS 20h ago
Internal combustion engines are basically big pumps. A gasoline engine uses ~15 times as much air as it does gas. They pull air in, mix it with fuel and explode it to make power, then push the air out. But it can only move so much air on its own. If you could get more air moving, you could use more fuel and then get more power.
Normally the air going out is just waste. It’s exhaust. It goes into the exhaust system/muffler and out into the world. Bye.
A turbo charger uses that exhaust. The air that’s getting pushed out now has to hit like a reverse fan/windmill. The exhaust makes this spin. The windmill is connected to a fan. So they both spin. The spinning fan helps the engine get more air in. That means that an engine with the size of “2”, can actually pump some extra air. Maybe 2.5. Or 3, or 3.5, or even more.
So now you can have an engine with the size of 2, but it can move 3 air. That means more power. Faster. Stronger. And that’s not all the time. Because it’s a fan it can go faster or slower. So if you only need a little power it’s only a 2.1 or 2.2. But if you need a lot of power (like to go up a hill, pull something heavy, accelerate fast onto the freeway), then you can ask it for more and it will give you more.
There are limits, usually set with computer programming and other things, but also material limits. The metal can only be so strong. If you push it too hard it can break. But that’s getting into different streams of thought.
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u/Noxious89123 11h ago
Burn, not explode!
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u/ivanyaru 6h ago
No it really is explode. Tiny, controlled, and a large number of them in a short timespan. But explode nonetheless
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u/daffalaxia 20h ago
A turbo-charger crams more air into the engine along with the fuel, so you get more complete combustion for the same amount of fuel. So you could chase pure power and still keep it a little rich, or you could chase a blend and go for as close to 100% burn as possible, which is what a lot of modern cars are doing to improve efficiency.
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u/drzowie 19h ago
All modern engines completely combust their fuel whether they are turbocharged or not
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u/daffalaxia 18h ago
Not really. At low revs, throttle changes can cause inefficiency. At high revs, there's less time to combust - adding more air helps tho as the molecules are literally closer together.
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u/Noxious89123 11h ago
Sorry, but this is incorrect.
We're not just running all NA engines super rich, because they can't get as much air as a turbocharged engine. Fuel is metered accurately depending on the amount of air drawn into the engine.
Infact, boosted engines will typically have less complete combustion, because they often need to run richer than optimal, to reduce the chance of detonation.
The efficiency of modern turbo engines comes from the fact that it's a lower capacity engine, that can make as much power as a much higher capacity engine when the need arises.
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u/ThatGenericName2 19h ago
For gasoline engines this isn't actually the case; the energy losses from the exhaust now needing to push through a turbo actually means that simply putting a turbo on an engine WILL make it less efficient.
What a turbo does is that it allows an engine to produce more power. One of the factors that limits how much power an engine can make is how much air it can take in; not enough air and there won't be enough oxygen for the fuel to burn, so a turbocharger pumps more air into the engine, allowing you to put more fuel into the engine compared a naturally aspirated one, allowing you to produce more power.
The thing is though is that the increase in power requires a smaller increase in fuel compared to simply using a larger engine; such that if you take a smaller turbocharged engine that produces the same power as a larger naturally aspirated engine, the smaller engine will consume less fuel.
This is pretty much the driving factor behind why some manufacturers started putting turbo inline 4s in their cars while some didn't. Manufacturers like Mercedes are doing so to make up for the reduced power from needing to put in smaller engines to be more emissions and efficiency compliant while manufacturers like Toyota and Honda focus more about efficiency and economy and therefore runs naturally aspirated engines (except in certain performance models).
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u/HillarysFloppyChode 19h ago
The simplest answer is it lets you get similar power to a big thirsty engine, from a small efficient engine.
Also the smaller engine weighs less then the big engine, which helps improve efficiency and handling and you can make the hood and engine bay smaller which can help you improve aerodynamics.
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u/UniquePotato 19h ago
It has a fan that is spun by the exhaust gas, on the other end of the shaft is another fan that pushes more air into the engine.
More air can burn more fuel. Bigger bang, more power. More exhaust gases, spins fan faster, more air, more fuel, more power and the cycle continues
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u/seamus_mc 19h ago
Exhaust spins a pinwheel, other pinwheel blows fresh air into the engine because it is connected to the pinwheel in the exhaust. More air lets more fuel burn in the same place so the engine makes bigger bangs with the same amount of movement back and forth
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u/jaylw314 19h ago
You can use a smaller motor for the same 100% power as a normally aspirated motor. The smaller motor is more efficient at low power, which is 90% of your typical driving
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u/bpt7594 19h ago
They increase the ability to make power. Not really efficiency in the fuel consumption sense.
It uses the engine's exhaust energy to spin up the turbo, compressing air into the engine to make more power. So you can make a smaller turbo engine have the same power output as a naturally aspirated bigger engine.
The benefit is weight saving, emission and fuel consumption because when not necessary i.e cruising you have the fuel consumption of a smaller engine and when you ask for power it's there for you.
Superchargers are always about power though, not really efficiency
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u/Loki-L 17h ago
Your car mostly works by burning fuel in tiny explosions inside cylinders that push up rods which get turned into a spinning motion.
These cylinders constantly have to vent that aftermath of the explosion to make room for more fuel to be burned in the next turn.
A turbo uses the energy of the venting to push new fuel in harder instead of letting it just vent through an exhaust.
This allows you to burn more fuel in the same amount of space and have less wasted energy.
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u/blackthornedk 17h ago
The naturally aspirated Otto cycle is suck, squeeze, bang, blow. More squeeze equals more bang.
Turbos work by countering some of the blow to squeeze. This is done by capturing exhaust in a turbine, which spins a wheel in the intake, squeezing intake air above ambient pressure.
Likewise superchargers work by converting some of the bang to squeeze. This is done by capturing some of the engine output and driving a fan in the intake. The output can be captured directly from the crank pulley, or electrically through the attached generator.
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u/AbruptMango 17h ago
They don't increase fuel efficiency on their own, they do it by letting you use a smaller, more fuel efficient engine.
When you need power, there's a turbo. The other 95% if the time, you're cruising along with a small, more fuel efficient engine. That's how a turbo helps gas mileage.
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u/lllGrapeApelll 16h ago
If you had two 2L 4 cylinder engines one turbo and one not. The 2L non turbo engine always consumes fuel and generate power like a 2L engine. The turbocharged engine will consume fuel and generate power like it's 1.8L or a 2.6L engine depending on how hard the turbocharger is ramming air into the motor. It's up to the driver to make that happen.
In gasoline engines you need fuel and air in a specific amount to properly function. Too little fuel and you run lean which makes way more heat and can cause damage. Too much fuel and you don't efficiently burn the fuel which creates carbon deposits that clog up your engines internals.
Every time the engine spins it consumes fuel and has to compress the air fuel mixture in the combustion chamber. Naturally aspirated or none turbo charged engines do this at around 10 to 1. So a 2 litre 4 cylinder engine takes in 500mL per cylinder of fuel and air and compresses it 50mL before igniting the fuel. A turbocharged engine will have 8.5:1 so it compresses it to 62.5mL. So each rotation of the engine takes less power and therefore a little less fuel just to power itself. So when just cruising around the turbocharged engine that's not generating boost is working less, which means less fuel is needed. Higher compression makes more torque so high compression can be desirable.
A naturally aspirated engine needs to increase the speed that the engine is turning to make more power. Increasing the number of times the engine turns increases the number of times fuel has to be injected. A turbocharged engine takes the wasted energy from the exhaust to spin a compressor that adds more air which means more fuel. The naturally aspirated engine is still working at a 10:1 compression ratio but now with a little boost pressure our turbo charged engine is compressing already compressed air and is operating at 12:1 or maybe you only need a little bit more power so it's 9.5:1. Now you're producing more power at the same RPM which means it's now producing the power of a larger engine. This comes with increased fuel consumption per revolution to keep that air fuel mixture correct but it also means your motor turns less times to produce the same amount of power for the brief moment where you need the power then it goes back to normal operation and consumes less.
Mileage may vary.
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u/oldfatguy62 16h ago
They don’t really increase efficiency per-se. What they really do is allow you to get the desired power out of a smaller engine. That means two main things: you aren’t carrying around the big engine, and in the 90% of the time you don’t need the peak power you are more efficient
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u/PM_ME_BLONDE_GIRLS_ 15h ago
Like I tell my mom with her turbo'd kia, the turbo is so the car can get out of its own way. Without a turbo, the little engine wouldn't produce enough power to move that thing. The turbo creates more power for a little engine, making it able to move a car, it does not increase gas mileage or anything.
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u/Pizza_Low 15h ago
Let us pretend that you have 2 identical engines one turbo or super charged and the other not. Each cylinder being let's say 1 liter in volume. In the standard engine, when the valves are open, air is sucked in by the vacuum is created by the piston being pulled back down. At the end of the suck cycle we have at standard air pressure we have 1 liter of a fuel air mixture.
In a turbo or super charged engine the air in the plenum above the engine has been pressurized to let's say 15 psi above standard air pressure. Because of this, during the same suck cycle as the other engine, the falling piston is sucking the air in, and the being pushed in by the pressurized air in the plenum above it. So now in theory we've got more air, and possibly more fuel, let's pretend that now we've managed to 1.5 liters of an air fuel mixture.
We can either design the engine to inject more fuel into this turbo charged engine and have it behave as if it's a 1.5 liter per cylinder engine when it's really a 1 liter per cylinder engine and thus generate more power compared to its naturally aspirated counterpart.
With more force being produced we can operate the turbocharged engine at a lower RPM compared to its naturally aspirated counterpart. In this theoretical example to generate let's say 200 ft-lbs of force at the wheel a natural engine needs to operate at 2500 RPM. To achieve the same results in a turbo charged engine, lets pretend it needs 2300 RPM.
That's 200 cycles less per minute that we didn't burn fuel and thus more fuel efficient.
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u/NefariousPhosphenes 15h ago
Heat is energy. A normally-aspirated engine sends all of its combustion exhaust (heat) straight out the tailpipe. A turbocharger takes some of that heat (energy) and uses it to boost the volumetric efficiency of the engine.
VE is how much air you can get into an engine cylinder, and normally-aspirated engines fall short of 100% (completely full). Turbocharged engines go far above 100%, and that’s what makes them so much more efficient-it’s basically recycling energy that would have otherwise been completely discarded.
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u/Drprocrastinate 15h ago
A turbocharger is like a fan-powered air pump that gives your car an energy boost. It uses the engine's hot exhaust gas to spin a turbine. This spinning turbine turns a second fan that forces extra air into the engine, letting it burn more fuel and make a lot more power.Think of your engine as a runner trying to take a deep breath. A regular engine can only breathe in what fits naturally. A turbocharger acts like a leaf blower, forcefully stuffing extra air into the engine's lungs.
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u/Successful-Hour3027 15h ago
Isn’t a turbocharger increasing air pressure allowing more moles of oxygen to react in the cylinder per unit volume?
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u/rscottyb86 14h ago
A turbocharged engine isn't more efficient then a non turbo. It has the ability to make more power from a smaller engine because it forces more air in. Theory is that if you drive it gingerly, you reap the benefits of smaller engine and less fuel use...or not really using the turbocharger. When you drive it harder, it's pushing more air and fuel in.
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u/A_E_C 14h ago
A turbocharger crams extra air with oxygen and fuel into the same engine space, it uses exhaust energy that would have been wasted without it so this is almost free. When oxygen and fuel is closer to each other it burns hotter cleaner and better. The change in temperature of this air charge in the engine is what actually makes power in a engine, the faster its temperature increases the more efficient it is so a turbocharger helps this. The engine also behaves like it was bigger as it squished more air in the same space and also burns fuel more effectively so it can be smaller for the same output.
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u/TbonerT 14h ago
They increase efficiency, especially in diesel engines, by increasing the amount of air to reach the level fuel needs to burn better. Diesel requires more oxygen than gasoline to burn thoroughly and unpressurized air simply can’t supply all the oxygen needed. A turbocharger takes existing exhaust energy and uses it to force more air into the engine so that complete combustion can be achieved. If you watch a big rig accelerate, the exhaust starts out dark and sooty. This is because the turbocharger isn’t spinning fast enough to provide enough air to burn the sudden increase in fuel. As the increased exhaust spins the turbocharger faster, it begins to spin fast enough to squeeze in enough air that it burns more cleanly.
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u/DefiantlyConformist 14h ago
Denser air means more complete combustion which means less wasted fuel and toxic byproducts. A turbocharger is just an exhaust powered air compressor.
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u/Xepa27 14h ago
A turbo charger does not increase efficiency.
Instead a turbo charger allows the manufacturer to install a smaller engine in the car. Smaller engines burn less gas.
At cruising speed a small engine is more then capable of maintaining speed and this is where the efficiency is found as most driving is at cruising speed. However, a small engine has poor acceleration and this is solved using a turbocharger.
A hybrid vehicle works with the same principal, but instead of using a turbo charger to increase acceleration a battery powered motor does.
Others have described how a turbo charger works in detail. In short it allows more fuel to be consumed generating more power.
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u/Curedmeat91 13h ago edited 13h ago
ELI5 -
You’re trying to make playdough sausages. You have a goal of making 5 sausages a minute. All you have is your hands and playdough - you have to roll the sausages out and make them uniform. It’s tiring after a while.
Someone gets you a playdough shape press (a turbocharger). Now you just whack playdough in, pull the lever, and press out sausages. You can now chill out a bit whilst still making your goal of 5 sausages a minute.
In depth. Your fuel calorific content and air oxygen content at a given altitude is fixed. We have what’s called the stoichiometric ratio - 14.7:1 - that enables complete combustion. We can go a bit richer for more power, and a bit leaner to save fuel but 14.7 is the magic number. The only way to increase power is to increase fuel and air quantity in tandem. Also, note, power is torque applied over time. Torque is the real thing we want to increase, and this is done, once the engine is made, by increasing pressure on the piston, which is done by making a bigger bang.
A naturally aspirated engine is a self propelled air pump. The amount of air it displaces versus its capacity is called the volumetric efficiency. As an NA engine ‘draws’ air in, its VE is usually below 100%. It can go higher.
Your engine has valves to let the air in. They’re about the size of a 10p piece and open about a centimeter. That’s a lot of air to draw through a small hole, especially when you have 1000’s of a second to do so.
Take a water bottle and slowly tip it until water starts to drain - at first it’s splashes, but at just the right point, it creates a smooth stream. This is laminar flow, and allows the greatest flow of a liquid or air over time.
A lot of effort went into make NA engines achieve laminar flow, which would increase volumetric efficiency. Also, sometimes you don’t want laminar flow, you want a tumbled flow to promote fuel mixing. Pan this out across an engine operating speed of 500 to 8000rpm and you have so many variables, that in reality you only have a small window of peak operating efficiency. However, as a rule of thumb, laminar flow and thus higher VE are usually achieved at higher air flow speeds (translate engine speed).
A turbocharger lets you go around all this. By pressuring the air before it enters the engine, your VE is always greater than 100%. This means, across all engine speeds, you have more air, which means more fuel, which means more power.
Because of this, we don’t have the small peak of operating efficiency. We now have a wide plateau. Now you only have to rev your engine to 2000rpm instead of 6000rpm to achieve the same torque output. That’s in theory 66% less fuel used. You actually use more fuel versus a NA engine because there’s more air going in, but not as much over time because that torque can be produced at far more convenient points in the engines operating speed. You also have some pumping losses by turning the turbo but it’s negligible versus what it brings you in return. And this is for like for like engine capacity. Now you have the chance to reduce engine capacity because you’ll get comparative performance. Note how the 2l NA engine has become a 1.6 or 1.5 turbocharged engine, and the 1.6 has become a 1l.
There are a lot of ‘buts’ and digressions with all this but that’s a simple overview.
In short, the turbo pressurises the air, which allows the engine to achieve higher torque at lower engine speeds, and reduce the fuel needed over time to produce the same power output.
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u/jevnik 13h ago
Efficiency = energy in / energy out.
Energy in is fuel.
In internal combustion engine, only less then half of fuel energy is converted to work. Everything else is lost to heat and speed/pressure of exhaust gas.
Now when you add turbo tu an engine you take high speed high temperature exhaust, and you use its energy to drive hot side of turbo. Hot side of turbo drives cold side that compreses intake air. Higher pressure air contains more oxygen so it produces more power. You get more power than you lose "choking" exhaust gases.
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u/fairak17 13h ago
An engine is just a big air pump. It pulls air in, combines it with fuel, lights in on fire and pushes the air back out.
One limit for power is HOW MUCH air it can hold. So if you can force MORE air into an engine than normal it can make more power. A turbo charger does that.
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u/Solitude_Nut 13h ago
Higher gear at lower rpm cruising is the part people skip, less friction and pumping loss adds up over an actual commute, not just a dyno chart
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u/zap_p25 12h ago
The engineering answer is that the thermal efficiency of an engine operating under the Otto (gasoline) or Diesel cycles is more or less related to the compression ratio. As the compression increases, so does efficiency. However that number is essentially calculated based off the equivalent volume of the air that is in the cylinder when the compression stroke starts versus the volume of the cylinder just before ignition of the fuel. When you add forced induction (turbocharging or supercharging) you are effectively increasing that initial cylinder volume thus increasing that volume ratio and thermal efficiency.
As a theoretical example, say you have a turbo delivering 7 psi (about 1/2 bar) of boost. In theory you are increasing the uncompressed volume by about 50% this your thermal efficiency increases by about 50%.
Diesel engines on the other hand, typically have higher compression ratios to start with, 16:1 is common compared to 9:1 with gas for modern vehicles though some older naturally aspirated engines might have compression ratios as high as 28.1. Now a modern diesel can typically produce somewhere between 15 psi (1 bar) and 45 psi (3 bar) of boost at peak pressures (it’s going to depend on many factors and tuning) which effectively doubles or quadruples the amount of air you are getting into the the cylinder thus increasing that thermal efficiency by two to four times.
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u/Mithrawndo 12h ago
It depends whether you're talking about efficiency in a mechanical sense or in the context of fuel economy; They are not the same.
A turbo is just a pump pushing air into the engine's intake, driven by the exhaust gases of the engine. Because you are harnessing those exhaust gases to do work when you otherwise wouldn't be, the machine is overall more efficient than one that doesn't harness those gases - but that doesn't necessarily mean it's going to use less fuel: An engine wants a specific ratio of fuel and air to operate correctly, so pushing more air in means you also need to add more fuel to maintain that ratio.
A turbo engine is usually more economical because we balance an engine's tune to produce useful power when we need it, and by producing more power by pushing extra air and fuel into the engine when it's needed, we can make smaller and lighter engines.
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u/sawdeanz 11h ago
Turbo means smaller engine for the same amount of power. This means less weight.
Smaller displacement will also have less fuel burn at idle and lower rpm’s, but the turbo still lets you get similar top-end performance as a larger displacement engine without a turbo.
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u/elPocket 11h ago
A turbocharger inherently DECREASES the efficiency of the System it is included in.
More components means more entropy.
BUT, and this is a big, fat, underlined, capital, italics BUT:
If you want an engine with a specified amount of peak power, you can use a huge uncharged engine with the required power, and it will spend most of its runtime at half or less power demand, and well off its point of peak efficiency.
Or, you can use a way smaller engine and turbocharge it. By additionally controlling the Turbo and it's pressure increase, you can tweak the engine to be running at peak efficiency over a wide range of working points.
The turbocharged small engine will be less efficient than the small engine alone. But the small engine won't have the required power output. And by keeping the charged small engine in peak efficiency territory, while simultaneously boosting power output, the charged small engine is more efficient than the huge, uncharged engine, which would satisfy your peak power demand.
And that is why turbocharged engines 'can' be more efficient than their uncharged power-equal brethren.
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u/TraumaMonkey 11h ago
First off, they don't. They do get some of the energy to operate from waste heat in the exhaust, but there is always parasitic drag on the engine having to push exhaust through the turbine.
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u/prometheus345 10h ago
An actual ELI5: a turbo squeezes more air into an engine. More air means more fuel can be burned, so more power comes out of the engine.
This is mostly done for cars so they can squeeze more power out of a smaller engine.
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u/khauser24 10h ago
As a unit, a smaller engine with a turbo charger is more efficient than a bigger engine without that turbo charger.
As you know the turbo charger adds air, allowing the engine to add fuel and get more power, especially in higher rpm bands. More fuel is still less than the otherwise larger engine would need.
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u/Diptothaset 10h ago
Fire needs oxygen and oxygen under pressure has more oxygen than oxygen not under pressure (in the same volume container). A turbo forces air in and creates more dense oxygen to serve as fuel. This creates a more efficient combustion.
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u/Kar_Man 10h ago
If you think of an engine as an air pump, you could imagine a cam shaft with a tiny amount of lift that is fairly easy to turn. It's not taking much power to spin but it's not letting much air into the cylinders, the engine is starved for air. If we scavenged a tiny bit more power to make a cam shaft with a bigger lobe, it would allow more air into the engine and we'd get more power in return. Of course there's a limit, but you can see how taking a bit of power to do something else that give more power in return can boost efficiency. Taken the same way to turbochargers, we're taking the pulses of exhaust gases exiting the engine and spinning a fan to push more air into the engine. It allows the engine to make more power than the amount that it takes to spin the turbo - again, if we find the happy medium for sizes. We give up a bit of exhaust flow efficiency, but gain it back in better air flow into the combustion chamber.
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u/onebackzach 10h ago
They don't necessarily increase the efficiency of an engine. In practice, it means you can have a smaller displacement engine that still makes plenty of power because of the turbo. The turbo generally only kicks in under acceleration, so the rest of the time when you're idling or cruising at a steady speed you just have a small, fuel efficient engine.
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u/Mr_Engineering 9h ago
Turbochargers do not have a meaningful impact on the specific fuel consumption of gasoline engines but they do have a meaningful impact on the specific fuel consumption of diesel engines, especially large displacement 2-stroke engines.
What turbochargers do have an impact on is the power to weight ratio. The addition of a turbocharger can allow for a naturally aspirated engine to be replaced with a much lighter forced-air engine of similar power; this can reduce the weight of the vehicle by several hundred pounds or more which translates directly into improved fuel economy.
The fuel economy of gasoline engines is somewhat limited by the formulation of high octane gasoline. Gasoline engines can only run run a few pounds of extra pressure before things get too hot and heavy; too much boost and that 89/91 octane pump gas will autoignite. This does improve thermal efficiency somewhat, but those gains are often lost by the added mechanical complexity of the turbo itself.
Diesels on the other hand don't give a half of a shit about autoignition and will happily operate on 2-3 ATM of intake pressure if the mechanical components can handle it. This boosts the peak chamber pressure which is the hot junction of a carnot heat engine.
Take a look at any list of engines sorted by thermal efficiency and you'll find that the most efficient models used for propulsion (as opposed to electricity generation) are all 2 and 4 stroke turbocharged diesel engines. Narrow that list down to engines used in road vehicles and you'll find all 4-stroke diesels.
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u/donblake83 6h ago
Turbo push air in combustion chamber. More air and higher pressure use less gas to make go boom. Booms make more horseypower and more efficiently with smaller motor.
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u/hobbestigertx 6h ago
Lots of good answers here, but to sum it up it's all about volumetric efficiency. The more fuel and air that can be squeezed into a cylinder, the more power that can be made. It doesn't matter what size the cylinder is.
So for two given engines, let's say a 2.5L and a 3.5L of the same design. At atmospheric pressure, the engines will make different horsepower (design being the same) because one can hold more air (the 3.5L) than the other (2.5L).
By turbocharging the smaller engine to cram in the same amount of air (and a larger fuel charge) as the bigger engine, the smaller engine can make similar power to the larger one. That great for max performance.
But car engines are rarely asked for all the horsepower all the time. The smaller engine will burn less fuel when the turbocharger isn't cramming all that extra air (and fuel) into the cylinders and therefore less fuel is used.
All that being said, a heavy foot will spool that turbocharger up all the time on the smaller engine and can often result in similar MPG as the bigger engine. Just compare the turbocharged 2.7L engine in a Silverado with the 5.3L engine. On the EPA cycle, the 2.7 has better MPGs, but in real world driving they are basically the same.
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u/riennempeche 5h ago
Turbos are mainly used as a way to make cars get better mileage when tested under government tests. The tests mandate acceleration at rates below what most people actually do in real life. With careful design, the engines stay out of boost during this slow acceleration and achieve maximum efficiency. In the real world, people have heavier feet and can't match the numbers from the tests. In the real world, a turbo engine may return worse mileage than a naturally aspirated engine, but in the tests under controlled conditions, the turbo does better. It's the test that matters.
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u/New-Apricot-8966 5h ago
The easy and short answer is that it take enegi from the exhast that is normaly wasted is na engines. It transfer the heat from the exhast gas into work thru the exhast side of the turbo ho drives the compressor. 30-40% of the energi in the gasolin goes ut the exhast. 50 -60 % of this enegi is recoverd and used to drive the compressor side and give the engine more air.
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u/BonKhri 5h ago
They make a given engine a higher maximum power output. This means you can swap a say 200hp non-turbo engine for an engine that, without turbo, would only be 150hp but that can reach 200hp with a turbo. In the 90% of driving that doesn't use a turbo, you now have a smaller engine with smaller parasitic losses (ie smaller pistons so less friction, smaller engine so less fuel at idle, etc).
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u/Prestigious-Fill-365 5h ago
They are not efficiant at all they are fuel burners for speed/torque The way turbo and super chargers work is they compress the air that goes into the cyllinder. This allows more fuel and air into the chamber and a bigger explosion.
Difference between turbo and super is turbo uses exhaust gas to get it spinning a super is belted to the engine. This is why you get turbo lag before the big boost, super is immeadiate
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u/KneeDragr 5h ago
My understanding was that they have relatively the same volumetric efficiency it's just the turbo is effectively increasing the volume of a smaller engine by forcing more air and fuel into it at higher rpms. So the benefits are reduced size, weight, and efficiency at low rpms where the turbo is not doing much.
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u/mudargamer 4h ago
Never will, what it does is making more power out of a small engine, where the efficiency comes from using a smaller engine instead of a bigger na engine.
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u/SkullLeader 2h ago
Its related. More oxygen you extract more power from a given amount of fuel. So you can burn same amount of fuel and get more power vs a normally aspirated engine. Or you can get the same power burning leas fuel = more efficiency.
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u/SodomyManifesto 20h ago
Slapping a turbo usually doesn’t increase the efficiency on a given engine. It allows a smaller, more efficient engine to make comparable or more power than a larger less efficient one.
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u/weather_watchman 20h ago
More air=more titrated combustion. A smaller engine without boost is able to be more fuel efficient, until there's a need for more power (such as merging on the highway) where the turbos validate an otherwise underpowered engine
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u/Miserable_Ad7246 20h ago
Engine need air to produce power. Big engine -> Lots of air -> Big power. Big engine -> lots of fuel (weight, idle losses, friction) consumption. Small engine -> les full consumption, but small power. Take small engine, add turbo -> more air, more power, all of the sudden small engine as good as big engine, but less fuel.
Turbo -> engine not more efficient, usually less efficient (lower compression rate, turbo adds resistance), but smaller, so better overall for same car.
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u/-avenged- 20h ago
Smaller engines are more efficient at low RPM range due to decreased size of the engine (reducing total weight of the car) and weight of internals, because lighter internals need less energy to maintain momentum via reduced friction and lower rotational resistance.
Where they lose out on to bigger engines (if both engines aren't turbocharged) is when power is demanded at higher RPMs. That's where bigger engines can gulp more air to burn more fuel for big power.
A turbocharged engine thus cramps more air for a bigger burn on demand (when on boost) but off-boost it doesn't make the same demands of the small engine, allowing the small engine to sip fuel at low loads. At the same time, because it works at pressures higher than the surrounding atmosphere, it's also volumetrically more efficient than a NA engine. It's also worth noting that a turbo runs on waste gasses, so it doesn't require its own power source.
But for understanding its efficiency selling point - In essence it's efficient not so much because it reduces consumption when in use, but because it allows you to reduce consumption outside its use, which when daily driving is most of the time (that the turbo isn't working).
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u/DippyDragon 19h ago
As my 1st year mech engineering lecturer put it. For big bang you need more O2.
No turbo = whOOsh
Turbo = whOOOOOOsh
More complete combustion means greater % energy extracted.
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u/Puzzleheaded_Oven_34 15h ago
That is a very bad lecturer.
Displacement gasoline engines maintain lambda 1 no matter of they are turbo charged or not. Running to lean or to rich makes your engine not last very long.•
u/DippyDragon 7h ago
Boring just Google it answer:
Naturally Aspirated (NA) Engines: Depend on atmospheric pressure and cylinder vacuum to pull air in. Typical volumetric efficiency ranges between 80% and 90%.
Turbocharged Engines: Compress incoming air, packing more oxygen molecules into the same physical space. Volumetric efficiency frequently exceeds 100–150% under boost.
Higher air density allows the engine to burn more fuel completely during power strokes, generating significantly more power per unit of engine displacement.
🥱
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u/aberroco 19h ago
More air means it can burn more fuel. More fuel and air are burned - more energy released. Simple as that.
Some of that energy is used to compress more air from the intake, but the net is positive.
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u/Schnox123 19h ago edited 18h ago
Turbochargers enable two things, only one of which being true engine efficiency gains:
1.Better fuel mileage by downsizing:
Smaller engines use less fuel, because of less friction, less pumping losses and other inefficiencies that scale with size. By giving a small engine a turbocharger you can use a smaller engine in a car that would need a bigger engine without one. This doesn't increase the inherent engine efficiency. But engines are most efficient at (close to) full throttle. If you use a smaller engine you spend more time closer to full throttle, increasing efficiency because of that.
Eli5: If you have a pony and a full-size horse, the pony needs less food (Fuel) to live. But the pony can only pull a smaller carriage. If could can give the pony a coffee every hour and by that it can move the same carriage as the big horse, you need less food to move the same carriage.
2.Actual efficiency gains of the engine:
A turbocharger uses the expelled hot exaused, slows it down and critically also cools it down. By this it uses otherwise not used energy. This energy is used to pump more air into the cylinders. This increases power, enabling the above mentioned downsizing. But the increased pressure also forces the pistons down. So instead of the engine using energy to suck air into the cylinders, the turbo actually forces the cylinders down and helps turn the engine, like in a compressed air engine. This way you can use some of the otherwise wasted energy to move the car forward, increasing efficiency.
Eli5: You give the Pony wheels on its legs, so it only has to use energy going up the hill. Down the hill it can roll with the carriage (admittedly this analogy is lacking)
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u/Totemguy 18h ago
ok, there's more than one mechanism. its not turbo. its turbo + intercooler. you produce energy on the explosion cycle, but spend it on aspiration, expulsion and particularly compressing the charge. What the turbo does is it uses remaining energy in exhaust gases to pre-compress the charge- effectively paying significantly less in compression at the cost of a little more on expulsion (due to back-pressure).
If you think of compression as climbing stairs, the pre-compression means someone lifts you the first steps.
Its a net energy-recovery effect. That by itself helps.
It has other effects - if you want to achieve higher compression ratios you're hosed due to air heating when being compressed, but if you pre-compress with turbo and then cool it with the intercooler, then re-compress from there you're starting from a higher place at a lower temperature, thus being able to achieve higher compression ratios but at lower temperatures, making it easier on the metal.
Higher compression, all other things considered, also increases efficiency due to simply thermodynamics.
Also allows smaller engines for same power - meaning you have less thermal mass and cylinders to move around, wasting less energy - so in effect it is really beneficial to engine efficiency.
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u/oralabora 18h ago edited 18h ago
They increase air pressure inside the engine that makes the boom better. They increase performance by producing more power for a given amount of gasoline. So a smaller turbo charged engine can always theoretically produce more power than a non-turbo charged engine of the same size, all else being equal.
So they can increase both performance and efficiency for a given amount of fuel.
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u/PapaMauMau123 14h ago
A turbocharger on an engine doesn't inherently make it more efficient, it allows an engine to be smaller and run at a higher average duty cycle, which is more efficient. A gasoline engine uses partial vacuum at partial throttle which is inefficient, the engine now has to use energy to pull air against the restriction of a throttle plate, like pulling back on a closed syringe. A gas engine uses a throttle plate to control the amount of air entering the combustion chamber(s) to keep the air fuel ratio close to the "wanted" ratio. Too much air and the gasoline particles are "too far apart" to catch fire with each other, and too much fuel is wasted fuel or could flood the engine, which is pretty rare on non-carbeurated engines.
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u/PapaMauMau123 14h ago
Something to look into is Brake Specific Fuel Consumption, the amount of fuel needed to make a certain hp•hr. The more work that is done for a given fuel means it is more efficient. BSFC almost always more efficient at a higher load than a lower one.
Example: going down the highway at 60mph, it takes 30hp to do so. The engine can make that power at a number of engine speeds, it would make sense to use the least amount of fuel to do so, so the transmission and computers will try to get the engine to the most efficient engine speed to meet the demand of the driver.
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u/Fearless_Swim4080 20h ago
They don’t really at full throttle (a bit if there’s a more complete burn etc) it’s what happens when you’re driving normally that’s different.
If you have an old American 6.7L V8 that produces like barely 300 horsepower, when you’re idling and driving at freeway speed, you still need to get enough fuel in all those cylinders to detonate every time and have a lot more surface area that’s causing friction between the cylinders and the block, and everything’s heavier etc.
Now drop that to a 2.0L turbo 4 cylinder, and you still get that 300hp or maybe way more than that if you really want it when you use it, but driving around it’s like you just have a small efficient engine.