Got all the way through typing this and my connection timed out. Maybe the internet spirits are trying to tell me something.
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If I take a 30HP gas engine similar to my Kohler and run it at it's peak torque RPM and I take a 30HP diesel engine and run it at it's peak torque RPM, which one is going to produce more torque at the driveshaft? </font>
OK - looking at the crankshaft or crank pulley, with no gears or pumps:
The diesel will measure more, because you asked about peak torque, not peak horsepower. In fact, the diesel will probably make more torque at the crankshaft at every RPM in its - the diesel's - range. Without looking up real torque or horsepower curves, the following may not be far off: Assume both engines have peak torque at 2500, which is possible. The diesel may have 60 ft lb or torque for 28.56 hp, but the gasoline only 40 ft lb for 19 hp. at the 2500 torque peak of each. The diesel may then have max horsepower, 30 at 2750 rpm, but with only 57.29 ft lb of torque. The diesel won't turn much more than the 2750, but if it does, both torque and power will fall off.
The gasoline engine at its power peak may be 4500 rpm, where the 30 hp means it is only putting out 35 ft. lb of torque, down from the 40 at 2500, but developing 30 hp instead of 19.
What ultimately makes pulling force is the torque at the wheels, of course. If the gears reduce the diesel's 2750 rpm to 42 rpm (about 3 mph with a 24" wheel) at the wheels (65:1 ratio), and the gears on the gas carriage reduce the rpm from 4500 to 42 rpm (107:1 ratio) , they will have identical 3751 ft lb of torque, and thus 3751 lb of pulling force if the tires don't slide. This assumes, of course, the impossible 100% efficiency of the gears. [Please anyone chime in if I've screwed up the math. I'm working only with the HP=Torque X RPM / 5252 formula, 1 foot wheel radius and direct rpm relationships.]
Maximum torque at the wheels is delivered by maximum horsepower at the crankshaft, and gears (or hydraulic torque conversion). In order to get maximum torque at the wheels for drag racers, they configure the engines to turn as high as they can get them to go without exploding or running out of air, and gear them to turn the wheels from a stop. Not uncommonly, they turn big V8s at 9000 rpm, where horsepower is massive but torque fairly low. The torque at the rear axles, however, may wring them off.
It isn't just a diesel v. gasoline comparison, either, since some newer diesels are lighter and turning faster, and some workhorse gasoline engines actually have torque and power with peaks at low rpms. You could probably find a much larger and heavier gasoline engine than the Kohler, with large displacement, short cam duration, long intake and exhaust, etc. with torque and power curves like the Deutz diesel, and the same 30 hp maximum output, at 1500 or 2000 rpm. It would be even bigger and heavier than the diesel, however. One of the advantages of the Kohler, Robin and car engines, etc. is that you can get power out of a small light engine by turning it up.
And, the maximum power or maximum torque figures do not tell you much about the real working capability of the machine. If it has a steep and high power peak, it will deliver high power only at or near the best-horsepower rpm, and at any other range it will be deficient. The great advantage of the hydrostatic transmission is its continuous variability, so that you can pin the engine at its horsepower peak and vary the wheel or implement speed over a wide range with the hydraulics. Although the hydrostatic systems are a lot less efficient than hard gears, they allow you to hold the engine near high power so average power to the work is greater.
And boy can you find a lot of threads on TBN strenuously debating that point. /forums/images/graemlins/smile.gif
