scrinch
Silver Member
Consider a 180 hp aircraft engine and a 180 hp auto engine. Both of these engines are honest 180 hp engines...both are rated at 180 peak hp at max rpm. The auto engine is engineered for operating all day at 50% of max rpm, occasionally at 75% rpm, and almost never at max rpm. If it is used this way it will last 100,000 miles or about 2000 hours. The aircraft engine is engineered to operate all day at 90% of max rpm (and at 80% power) and occasionally at 100% rpm and power. If it is used this way it will also last about 2000 hours. But if the auto engine is used to put out aircraft engine-like power all day long, it will probably last only 200 hours. So while both engines are honestly rated at 180 max hp, you can get a lot more work out of the aircraft engine if you need to operate at high power settings. (Alternatively, the auto engine has tighter tolerances and can be considerably more fuel efficient.)
I think this is analogous to the situation with small gas and diesel engines. We can operate our diesel engines at high power settings all day long, whereas a gas engine would wear itself (and our ears) out if we operated it near max power for too long. So the hp available for doing work all day long are greater for the diesel because you can operate it at that higher power setting. I think with the proper gearing you could get your bush hog moving and spinning with the gas engine (ask the Power Trac guys what they can do with a 22 hp Kohler), but you just wouldn't be able to keep it running very long if you are doing heavy cutting. Accordingly, a gas engine with a higher hp rating is required to do the same job as a lower hp diesel because it will only be continuously operating at 50-60% load. Looking at it from a manufacturer's perspective, the equipment that is typically built around a 22 hp gas engine is a lot less capable than the equipment built around a 22 hp diesel engine...because the day-in, day-out hp available will usually be considerably lower.
Given the aircraft example, gas engines obviously don't have to be rated for lower power loads, but I think the perception is that they generally are.
I think this is analogous to the situation with small gas and diesel engines. We can operate our diesel engines at high power settings all day long, whereas a gas engine would wear itself (and our ears) out if we operated it near max power for too long. So the hp available for doing work all day long are greater for the diesel because you can operate it at that higher power setting. I think with the proper gearing you could get your bush hog moving and spinning with the gas engine (ask the Power Trac guys what they can do with a 22 hp Kohler), but you just wouldn't be able to keep it running very long if you are doing heavy cutting. Accordingly, a gas engine with a higher hp rating is required to do the same job as a lower hp diesel because it will only be continuously operating at 50-60% load. Looking at it from a manufacturer's perspective, the equipment that is typically built around a 22 hp gas engine is a lot less capable than the equipment built around a 22 hp diesel engine...because the day-in, day-out hp available will usually be considerably lower.
Given the aircraft example, gas engines obviously don't have to be rated for lower power loads, but I think the perception is that they generally are.
