I was only bringing up riding mower transmissions to point out that it only takes 2hp to move something of that size and 'rolling resistance' at 5mph. So the majority of your engine power is available to your PTO and there is not as much tied up in vehicle propulsion as you might expect. But yes, a heavier tracked vehicle will still have a much higher power requirement to go the same speed, and it's always good to design 'pessimistically' and over-deliver in the end, as long as it doesn't cost you too much time and money to do so.
Horsepower is Horsepower. If a small gas engine and a larger diesel say they make the same HP, then.. they do! In the 'small engine' world the gas engines are usually even spinning a similar rpm to the diesels while doing it, as well. But the difference you may be referring to can come from a couple of other things:
One is simply momentum, or inertia, in the rotating assembly of the engine. The other is how the engines are 'fueled'. As far as momentum, if 2 engines make the same power but one is 'storing' a lot more energy in its own parts because it weighs more, it will seem to react better to the same load being applied to it, than the other one. It is not
making more torque, but it can
apply more torque to the load because of its stored energy. It just has more momentum to 'soak up a hit'. This would definitely be the case between, for example, a ~500cc single cylinder briggs in a Craftsman DYT4000 riding mower i have, and the 600cc 3cyl diesel in my Kubota
B6100. BUT.. it's just weight. You can
add flywheel weight reasonably easily if you desire to. For example, I put a ~120cc push mower engine on one of those Craftsman DYT4000 riding mowers (have 2) to make room for a tiny crane on the front (yup..) and noticed that without the 'flywheel effect' of the 22" blade, the engine was extremely easy to stall with a load and did not like to run smoothly at anything lower than the governor rpm of 3000-something. I added a hydraulic fan clutch from a car (with no fan on it!) to the bottom of the crankshaft, and that spinning weight made the engine run a WHOLE lot smoother at low rpm, and you could apply a load to it a lot quicker without stalling it. So just adding flywheel weight is one option.
The other difference is not easy to change or improve, which is just a difference in how diesels and gas engines receive and use their fuel. A diesel engine sitting at a steady rpm with no load is basically using 100% of the fuel you're giving it. If you give it 4hp of fuel, it will rev up and up until it has 4hp of frictional losses, and then it will sit there at that rpm just 'breaking even'. If you apply a load and drop its rpm quickly, even before the injection pump has time to physically react, you're now receiving 4hp of fuel but only using 2 or 3 of it to overcome the engine's frictional losses, so the amount of power available to go out the end of the crankshaft goes up automatically! And then the injection pump guts shift a little and give
more fuel. That's an oversimplification, but by nature a diesel has some amount of 'automatic, instantaneous load compensation' that doesn't require ANY of its parts to move or shift, and that in combination with them usually having more rotating weight for the same power level makes them feel like they struggle less when you apply a load.
I'm not against using 2 engines if it's justified/cheap/easy enough! But you may also consider adding some flywheel weight or at least leaving yourself the room to do so in the design.
But belts themselves are perfectly sufficient for these <15hp applications. You may have noticed most of the small engine equipment using belts in that ~15ish HP range are using 5/8" belts rather than 1/2" belts you would typically see on old cars and stuff, and if they go much into the 20s hp they'll usually be running TWO belts, but yeah properly sized the belts are not an issue and give you a nice built in 'slip clutch' or 'fuse' in the design.
