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?
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...)
For NA engines it was found that the extra thermodynamic efficiency was eaten up by the mechanical efficiency and pumping losses.
