Hi fellow TBN'ers, I have been struggling with something each time I think about building something that requires a small external gas powered engine. I'm aware of the differences between torque and horse power, cubic inch to cubic centimeters doesn't confuse me too much, but when I try to sort through all of this to select a motor for a project it becomes a little muddled. In my earlier experiences I could pretty accurately guesstimate what I could expect from a HP rating on a motor. I had plenty of experience pushing a 3.5 HP Briggs around the yard as a kid and knew exactly when it was going to stall. lol It meant something to me when a 350 Chevy block was mentioned even though there could be a huge range of torques and horse powers generated from the same block depending on how it was set up. These examples were a learned point of reference, something that I gained over time and experience. Now I seem to be confused again and don't necessarily want to go through the hard knocks of gaining that reference point again. Is there a general rule of thumb I can use to at least ball park a small engine's capabilities based on the limited information many distributors provide?
I sort through Harbor Freight's selection of small engines and the most I can gather is the cubic centimeter and cost. That tells me very little about where it fits in the food chain of engines. Maybe I've missed something but I wouldn't be asking here if I hadn't already spent some time trying to figure this out on my own. I've looked at manufactured equipment spec's and that does help a bit. Maybe I'm looking for the Dick Tracey secret decoder ring for small engines? Is there one?
Thanks,
DTS
Horsepower is horse power. Most of the misunderstanding has to do with the way an engine is rated versus how it is marketed.
Marketing amost always touts Maximum Horsepower, i.e the peak of the horsepower versus rpm curve. Aircraft engines are probably the only commercial applications that utilize maximum power and even then it is time limited.
There is flywheel power, pto power and draw bar power on tractors, and these are defined by either SAE or ASAE standard methods. The flywheel power is that measured at the flywheel with the engine accesories operating. The pto power is measured at the pro shaft and is less then the flywheel power due to transmission losses through the gear box. Drawbar power is the drawbar force (~lbs) and the vehicle velocity(~ft/sec) divided by 550(~ft-ib/sec/hp) and is a function of the tractor weight including ballast, tires, and soil conditions, slippage, etc.(The Nebraska Tests are conducted on concrete.)
The engine usually has a "rated" engine speed where it develops it's designed output power at the flywheel (for example 95 hp@2400 rpm). This is not usually the peak of the horsepower vs. rpm curve. I'm not certain, but I believe this a continuous rated power.
Then there is max hp at 540 (or 1000 rpm) pto shaft speed. This tells you what the max power that can be transmitted, via the pto shaft, to a powered implement at the implement's designed input shaft speeds.
Finally there's max drawbar power. That's determined by test on a ballasted tractor pulling a weighted sled around a concrete track. I believe it's determined by "trial and error" varying ballast weight, tire pressure, at rated engine speed.
Most tractors will deliver more than rated power at the design speed since individual engines output will vary. The manufacture tries to deliver no engine with less HP then the rated power.
For small engine that run at near fixed rpm ( the governor actually "droops" the speed somewhat with load), I believe they are rated for continuous operation at design speed. They are probably marketed at "peak power", which is not realistic.
An electric motor generally runs at a certain no load design speed and when the rated load is supplied , the speed sags down to the operating speed and it delivers it's rated power.
So a small gas engine that develops 7.5 hosepower under continuous 7.5 hp load will produce the same horsepower as an electric motor rated at 7.5 hp electric motor driving a 7.5 hp load. The only difference is that the gas engine is liable to marketed as a 10 hp engine!
The best way to match an engine to a load is with the certified power curves and they come from the engine manufacturer. If an engine has the same compression ratio, the same displacement, the same valve configuration and the same design rpm, then for all practical purposes it will have very nearly the same power output. The major differences between small engines in the same power and torque class are probably due to manufacturing differences and materials.
That's probably more than you wanted to know.
