A side benefit however, is more torque. It gets that higher compression by having longer throws on the crankshaft, meaning more leverage.
Horsepower is more related to how fast you can turn the engine, not how strongly it twists the prop. Gassers turn the engines faster, diesels turn more slowly, but with more torque. Diesels generally max out at speeds where fixed props work more efficiently, but they have a narrower optimum speed range which is why trucks have so many speeds in their transmissions.
You need both horsepower and torque. You can turn that engine at 100,000 rpm, but if it can't twist against a load, it doesn't do you any good; that's where torque comes in. There have been some engineerspeak discussions here on the importance of torque vs. horsepower & little was said that makes sense to a non engineer. Anyway, the higher horsepower of the gas engine does not relate; it's an apples and oranges thing. Somewhere for a given aircraft and prop (and perhaps aircraft speed) combo there's an optimum mix of horsepower and torque. Which kind of engine provides the best mix is for those engineers to discuss; I hope they can do it in plain English.
Richard,
Here's the engineer's (simple?) description of how this works.
Diesels don't generate higher compression through longer strokes. They simply have less space between the piston and the head (smaller squish volume). The only reason diesels can be found with longer stroke/bore ratios in many cases is that the lower rpm charactoristics of the diesel engine mean that the big valves, found in large bore/short stroke engines, don't offer any flow advantage so there is no point in throwing a heavier piston around as the con-rod already has to deal with higher pressures.
Torque : torque is rotary force and is dependant entirely on cylinder pressure and volume. If you take two single cylinder 100cu.in engines, one with a small piston and long stroke and one with a big piston and short stroke, provided the cylinder pressure is the same, the torque is the same. The small piston (say 4sq.in) has the same psi (say 400psi) pushing on a small area giving 1600 lbs of force but on a long lever. The big piston (say 16sq.in) produces 6400lbs of force but on a short lever. Same torque.
Power: power is force over distance. In an engine, the force is torque and the distance is rpm.
If you have two 200hp engines, one giving power at 2500rpm and one at 5000rpm, they will do the same work. However, the second has half the torque of the first. If you put a 100% efficient PSRU with a 2:1 reduction on engine 2, you end up with two packages with identical performance. This is why you can get 200+ hp from an H-6 Subaru (3.0L) and spin the same prop as a IO-360 (5.9L). You simply spin the engine faster to get the power (but with less torque) and use a gearbox to get the torque back, equalling the bigger Lycoming crank output.
Diesel burns slower than gas but has more energy and the engines have much higher compression. The slower burn requires lower piston speeds (directly reducing rpm potential) but the higher cylinder pressures result in more torque. That is why a 2.0L turbo diesel can produce the same torque at 2700rpm as a 5.2L (O-320) normally aspirated gas engine.
To keep it simple, any engine producing the same power can also develop the same torque, you just need to gear it up/down to the same rpm. A 300hp/1500rpm engine geared up (1:2) to 3000rpm will give the same torque out of the box as a 300hp/6000rpm engine geared down (2:1) to 3000rpm.
In simple terms, for an aero-conversion, pick the power you want, find out what rpm the engine produces it at and get a psru that gears it down to the prop rpm. It will give the same performance as the same power Lycosaurus driving off the crank.
As a last note, the narrow power band of a diesel is less of an issue on an aircraft than in a car. This is especially true where a constant-speed prop would allow the pilot to select the point in the power band most suitable for his/her flight conditions and fly on MAP, leaving the prop to keep the engine at the best performance rpm.
