Industrial diesel engines are big, because they are expected to have a service life of 15'000 hours and must meet emission standards (ie low RPM). I have uncovered some interesting facts:
Rudolf Diesel intended to achieve 240 Bar in his engine, with fuel injected to achieve isothermic expansion. This would get around most emission problems, allowing much higher RPM. Modern diesels are up to 170 Bar and so, with improved injection technology, can spin at higher RPMs. Some way to go though...
The limiting factor on pressure is the small end bush. This must package into the piston, and is splash lubricated. Ceramic coatings are allowing higher bearing stresses, with less cooling requirements. Larger pistons, however, allow more heat to be rejected at TDC, resulting in less benefit from higher pressures.
Cylinder liners can be coated in ceramic material, to reduce heat rejection. Higher pressures become more practical. This also allows lower friction losses, improving efficiency. Liners also result in a block that weighs no more than a gasoline engine.
The practical upshot is that diesel engines are the fastest evolving engine technology out there. A 2-stroke turbo diesel could be thought of as a 2-stage gas turbine, with very high pressure ratios. I expect that not only will all (small) helicopters go diesel, but some may even go diesel/electric hybrid! This allows optimum rotor design for many different mission profiles, without the complexity (say) of multiple engines...
Mart
Last edited by Graviman; 28th Apr 2014 at 17:25.
Reason: Cause I got bored of designing non aviation equipment.