Hi Everyone,
Currently in the automotive industry, diesel engines are now performing with better output than petrol engines.
The major manufacturers are building engines with 50Kw/Litre which is equivalent to most petrol engines (not racing engines). The difference is that the torque output is equivalent to a petrol engine almost double. This is achieved through the use of electronic injection and "common rail" technology.
An example of this is Peugeot's diesel in it's 407 line, it's 2 litre 4 cylinder engine produces 100Kw and 320Nm Torque @ 2000 rpm (236lbs/ft). That's equivalent torque to most 6 cylinder 4 litre engines!
Torque in both fixed and rotary wing is very important, that's why we have been using large capacity petrol engines for so long.
Fuel burn at maximum power/torque is around 25% below petrol engines, and would be significantly more compared to larger older designed air cooled flat 4's and 6's.
Although these engines (diesel) are very suited to aviation, especially their torque, reliability and the safety of a lower flash point (less likely to catch fire), it will be sometime before we see them in our rotorcraft, mainly due to the high cost of r&d and the low unit sales figures of helicopters (very low return on investment).
However I am sure it will happen. The same as hydrogen powered cars are our future, but we will not see them on the roads in the near future due to the large revenue return governments around the world derive from the oil industry.