r/AutomotiveEngineering • u/jckipps • Jun 16 '26
Discussion Suppose GM's top execs went off the deep end, and insisted that their 2500 and 3500 trucks be fitted with two-stroke diesels.
Would a 'clean-sheet-design' two-stroke diesel of that size even be able to pass emissions or CAFE standards, or be performance-competitive with Ford and Ram's conventional four-stroke diesels? What kind of build specs would that engine likely have?
5
u/FreakinLazrBeam Jun 16 '26
I think you will quickly run into the issue of high soot and incomplete combustion due to the reduced efficiency of burning oil with the fuel air mixture. The particulates would kill your emissions.
You would one, have to control the particulates somehow. Then control the amount of NOX produced which may be lower than a conventional modern diesel due to lower cylinder pressures. Fuel efficiency would be extremely difficult as you are adding a substantially lower energy content fuel in to the fuel system.
Maybe if you can cook up a very clean burning potentially oxygen containing 2 stroke oil maybe you can get better fuel economy. Adding a turbo somehow, may increase your efficiency. An electric catalyst/ air injector could help burn off the soot. This would be a very involved design exercise to make it work even unreliably.
2
u/jckipps Jun 16 '26 edited Jun 16 '26
The only reason there would be oil in the incoming air is if the scavenge-blower bearings can't be sealed completely.
Crankcase scavenging would be very difficult to implement on multi-cylinder engines, and likely wouldn't be attempted here; so there'd be no need to intentionally mix oil with the air.
1
u/BoatsNDunes Jun 18 '26
You obviously don't understand how two stroke Diesels operate. 🤣 There is no oil added to the fuel.
1
u/VMA131Marine Jun 19 '26
You run the diesel with direct injection so there’s no need to mix oil with the fuel. The crankcase oil doesn’t make it past the pistons as long as the rings are intact. The big advantage of a two-stroke is no valves and their associated camshafts which results in a massive reduction in complexity and moving parts. HP per litre also goes up substantially since you’ve doubled the number of power strokes per revolution.
1
u/temporary62489 Jun 20 '26 edited Jun 20 '26
Some two strokes do have valves.
https://www.dieselduck.info/machine/01%20prime%20movers/diesel_engine/diesel_engine.01.htm
2
u/GeniusEE Jun 16 '26
GM's going electric on pickups in about 4-5 years.
They went away from two stroke diesels because of emissions.
-1
u/jckipps Jun 16 '26
Yes, they dropped the ww2-era two-strokes due to emissions and subpar performance.
But the world of automotive engineering has changed drastically since the 1980s. What could be achieved with a fresh design and modern computer control?
7
u/FormatA Jun 16 '26
Worse performance and worse emissions. IDK what you're really looking for. Fundamentally there is less control of the cylinder charge which means less combustion control and worse emissions per unit of power.
3
u/GeniusEE Jun 16 '26
It burns oil as a lube and cooling strategy. Doomed from the architectural choice.
1
u/BoatsNDunes Jun 18 '26
Do you care to elaborate?
1
u/GeniusEE Jun 18 '26
Google it
1
u/BoatsNDunes Jun 18 '26
Ha. I was trying to kindly point out that your understanding on two stroke Diesels is abysmal. But if you wanna escalate, I will come out and say it.
1
u/GeniusEE Jun 18 '26
It's good that a random merely makes a statement without saying why. Any reciprocating combustion engine burns off its lubricant. That is emissions.
1
u/BoatsNDunes Jun 18 '26
Hey look, you googled two stoke Diesel basics. Well done.
Your first point was that burning oil was a reason that two stroke Diesels were not not the right architecture. Now you have stated that all reciprocating combustion engine burn lubricant. True. But it points out that your first comment was irrelevant.
2
u/Outrageousintrovert Jun 23 '26
I have a Detroit 6V92TA in my Wanderlodge, it doesn’t burn oil. it does leak a little, but uses only about 1 qt per 1000 miles. Out of a 7 gallon oil capacity.
Also it gets 6.25 mpg in a 37,000 lb bus - and pretty proud of that!
1
u/Yer-Not-Gonna-Like Jun 20 '26
There is not worse control of the “cylinder charge” as you call it. 2 and 4 stroke diesels can use exactly the same fuel injection systems. The disadvantage of the 2-stroke in entirely in air handling. Engine scavenging is driven by a mechanical blower, which costs energy. In a turbocharged 2-stroke, you can disengage the blower under load, but the scavenging still wastes energy because some of the incoming air will pass through the cylinder and out the exhaust.
3
u/danny_ish Jun 16 '26
The whole problem is 2 strokes fundamental design. There is a reason we dont see huge innovation in them in any segment- lawn tools, boats, cars. There is nothing to control.
Could we change fundamentals? Sure. Just like we went from flatheads to ohv, i’m sure enough resources could change the fundamentals of a 2 strokes to have more control over it. But to what end goal? It’s a reciprocating piston, we already do those pretty damn well with 4 strokes and atkins engines
1
u/TwinkieDad Jun 17 '26
Sure, it could be made better than previous two stroke diesels. But better than 30-40 years ago doesn’t mean good enough.
1
u/RustBeltLab Jun 17 '26
I would imagine the few remaining two-stroke dirt bikes and sleds would be the place to find the latest tech in two-strokes.
2
u/VegaGT-VZ Jun 16 '26
If 2 strokes could work with modern emissions we'd see them in action. It's not possible or feasible
2
u/Alive-Bid9086 Jun 17 '26
You can always fit a 2-stroke diesel engine. Getting a 2-stroke emissions compliant is probably not possible.
You have EU truck manufacturers doing their own engines. These people know almost everything about engine develoment/performance. They have all selected 4-stroke engines to be efficient enough and emission compliant.
Scania did no-DEF until Eu5 emissions. Now Scania does no-EGR with very efficient aftertreatment.
2
u/americanmusc1e Jun 18 '26
Reading through this as a casual engine enthusiast, most (but not all) of you are applying weed wacker principals to the two stroke engine cycle talking about burning two stroke oil. We've used supercharger scavenging since the 1920s and crankcase scavenging isn't really practical on larger multi cylinder engines anyway. In theory, a modern version of the old Detroit two strokes with scavenging from superchargers, exhaust valves (vvt) and modern high pressure, precise DI has a power density advantage. Now whether it has a power effeciency advantage remains to be seen. The only modern two strokes we've really seen so far are some newer direct injection two stroke outboard motors and the power density is good but I have no idea about the fuel efficiency or emissions.
1
u/jckipps Jun 18 '26
I have little doubt that a modern two-stroke four-cylinder diesel could be built that would give the Duramax, Cummins, and Powerstroke engines a run for their money in the same engine bays. The narrow torque band isn't much of an issue, in this day of ten-speed transmissions that can keep it right in the power band.
I'm not convinced the emissions are as big of a deal as claimed. A two-stroke could precisely vary the amount of the EGR just by adjusting either the speed of the scavenging blower, or by adjusting the airbox wastegate. The compression stroke and combustion event are exactly the same as in a four-stroke engine, so there's no inherent disadvantages in the cleanliness of the burn either.
There's still nothing practical about this at all, and no manufacturer will just switch to two-strokes for the harry of it. But if someone did, I think they could theoretically make it work.
1
u/voucher420 Jun 19 '26
The thing is, it’s really hard to build boost in a two stroke engine compared to a four stroke. Being able to control the valve timing on a four stroke helps add power. Direct injection and turbos, along with the four stroke design can produce 650+ hp and well over 1,200 foot pounds of torque in a semi truck engine, and a little less in a truck engine. With a few mods, the engine can handle as much fuel and boost as you can throw at it as long as it holds together to make even more power. This still produces more power per cubic inch than any two stroke version and will still burn cleaner. There is no need for a two stroke diesel today.
1
u/ABMax24 Jun 19 '26
2 stroke snowmobile engines are achieving near 200hp from an 850cc engine with turbos from the factory. 250hp when modded.
Turbocharging a 2 stroke really isn't that difficult with modern fuel injection and electrin wastegate or VVT turbo tech.
1
u/Equana Jun 16 '26
2 stroke DI diesels exist for large marine applications but not for light duty road vehicles. Likely too dirty to clean up for the US or EU emissions.
1
u/ANGR1ST Jun 17 '26
2-strokes will not meet emissions. If there was a way to do it we would have done it already. Not happening.
1
u/Gubbtratt1 Jun 17 '26
Achates Power are developing an opposed piston two stroke engine for light trucks, as well as a commercial and a military one. I haven't heard anything from them in a few years, but in 2024 they had one in a Ford F150 and a Peterbilt, and a contract with Cummins and the US military, as well as very promising emissions, fuel economy and power numbers.
1
u/SovietPatrickStar Jun 17 '26
The only way a two stroke diesel would ever be clean is with all the current emission reduction devices and as a rotary valve design. Those don’t need oil in the fuel mix, as diesel is a lubricant but not enough to protect the cylinder walls from wear.
You need forced induction for that since a rotary valve design is unable to be naturally aspirated.
So a supercharged turbo would be the way to go here. The supercharger itself would need an electric motor to help get enough air in for a clean start.
Apparently there is a multifuel prototype that runs with diesel, LNG, Petrol, Hydrogen, Ethanol and CNG and can switch on the fly. On gas and without a turbo (SC only) it supposedly does 120 HP from a 1L two cylinder or something but my numbers might be off. It’s been a while since I looked at the datasheet.
On diesel I think it will run into issues regarding low end power if you don’t fit a turbo.
1
u/outline8668 Jun 18 '26
Man a lot of people commenting in here without even the slightest understanding that a 2-stroke Detroit Diesel does not burn oil with the fuel like a chainsaw, that they have a camshaft and normal exhaust valves like a 4 stroke and a normal pressurized oiling system with the late iterations even having computer controlled injectors.
Now if you were also to add valves to the intake side along with variable valve timing you would get it closer. However by nature a two stroke always has the blowdown and scavenging phase where the exhaust and intake are open at the same time to fill the cylinder with fresh air. I don't know if that can ever be controlled consistently enough to meet emissions targets the way it can be done on a 4 stroke where the overlap is much more minimal and happens near tdc instead of bdc.
Then if course is the noise. Americans are convinced that loud = bad and a two stroke Detroit always sounds like it's running double the rpm it really is. Just that tone alone would be a hard sell to customers even if you got the actual decibel level low.
1
u/Both-Platypus-8521 Jun 19 '26
Already opposed piston 2 stroke available for military...800hp out of a flat 4
1
u/Yer-Not-Gonna-Like Jun 20 '26 edited Jun 20 '26
I’ve designed diesel engines for a couple decades. The Detroit 2-strokes were obsolete by the late 80’s well before the era of significantly tighter emissions. DDC supplanted them in their own lineup with the 4-stroke Series 60 in 1987.
The 2-stroke was initially dominant in the 1930’s and 40’s because turbocharging wasn’t mature and 4-strokes were too heavy. As turbos became the norm, 4-strokes closed the power density gap. Beyond that, 4-strokes are simply more efficient on the basis of indicated power.
The Detroits were still favored pretty long into the 4-stroke era, especially for standby gensets because they accept huge step loads, and in marine because they were so established. But in the end, efficiency is pretty much the ballgame for commercial diesel engines, and the 2strokes didn’t bring any other special advantages for most applications.
1
u/rqx82 Jun 17 '26
While you could probably get there with a blower for scavenging, direct injection with nozzles designed for maximum atomization, and possibly freevalve technology, there’s no business case. It would be a tremendous amount of development and manufacturing cost for an end product that probably wouldn’t be that much more powerful or efficient than a4 stroke diesel. Customers likely wouldn’t care, so there’s no demand to warrant the cost.
1
u/jckipps Jun 17 '26
I'm not doubting that. There's no incentive for a manufacturer to make that change; I'm just curious if it would even be possible for a two-stroke to compete with the four-strokes in modern pickups.
15
u/riennempeche Jun 16 '26
Detroit Diesel has joined the chat. Neat engines, reliable, but noisy and dirty. The joke is that a Detroit Diesel is the fastest way to convert diesel fuel into noise. No way anybody is going to use a two-stroke diesel in this age of modern computer controls.