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Modern diesel engines have low compression ratio (some only 14:1, old diesel has 23:1), if compression ratio is directly connected to thermal efficency, how this engines achieve good efficency?

Can turbo boost compensate low compression ratio, is higher boost same as you have engine with higher compression ratio, in sense of efficinecy?

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    $\begingroup$ Super-nerdy comment: it's not the compression ratio, it's the expansion ratio. Hence the claims on the original Atkinson cycle engine (it was really patent-dodging, but hey -- it worked for a while). But for all practical purposes: compression ratio. $\endgroup$
    – TimWescott
    Apr 4, 2023 at 1:12
  • $\begingroup$ Turbocharged engines typically have a lower compression ratio - but the boost pressure allows for a higher mean effective pressure - this is what affects the output force from the combustion process. $\endgroup$ Apr 4, 2023 at 1:53
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    $\begingroup$ @sebastian323 yes. I find it a bit misleading to call it "compression ratio" without further clarification when a supercharger is involved. One should really separate three notions there: the pure piston compression ratio, the pure supercharger compression ratio, and the combined compression ratio, which is what actually matters for thermal efficiency – and it's typically higher for supercharged engines. Only the piston compression ratio is lower compared to a NA engine. $\endgroup$ Apr 4, 2023 at 13:33

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Diesel engines with turbos have a lower compression ratio.

Also the compression ratio has been lowered generally to help reduce the nitrogen oxide emissions.

The better fuel delivery control and spray patterns have also contributed to controlling emissions. (unless you look at the tractor pulling stuff...)

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  • $\begingroup$ There is always a balance between particulate matter generation and production of NOx in the diesel combustion cycle. We are using after treatments to control PM. $\endgroup$
    – Takala
    Apr 5, 2023 at 16:41
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For NA engines it was found that the extra thermodynamic efficiency was eaten up by the mechanical efficiency and pumping losses.

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