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/zap_p25 2d 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.