To make things even more complicated, lets throw a torque converter into the mix. This is a device that converts a high input speed into torque, usually with the ability to work even if the output is held at zero speed.
The right torque converter would allow some high-revving gas engine to be used in a low-torque application and would tend to flatten out the torque curve down near zero RPM. Great for pulling on things.
The problem with torque converters is efficeincy. Most are viscous-fluid-based. They turn a lot of the input power into heat.
Feathering a clutch is also a torque conversion operation. Do it too long and the clutch burns up.
A real-world hydrostat system also functions somewhat like a torque converter as well. The pressure-relief and leakage/bypass of the pump allow for output torque at low output speeds. Even if the output is held at zero RPM's, the input engine can still run at RPM's up in its torque band instead of stalling.
Again, the problem here is efficiency. All that power has to go somewhere. Since it is not going to the output shaft (its not turning, remember), it goes out as heat.
On the other side of the coin are gear-drives. Gear drives can be built at very high ratios and still be fairly efficient. As long as the output shaft is not stopped dead, the input engine torque can be leveraged up quite a bit. In most situations, the slight slippage of the wheels and elasticity in the drive-train provides that little bit of torque conversion to prevent stalling at start-up.
Finally, I will call your attention to top-fuel tractor-pulling machines. They are not low RPM diesel devices at all. They are top-fuel, very-high revving machines. Also, the wheels are not geared down all that much. They work by using the slippage of the wheels on the ground as a torque conversion mechanism. The cool thing about doing it this way is that half the heat is disappated for free - literally left behind in the dust. The other half of the heat of torque conversion is in the tires, but since they spin pretty fast, there is a lot of air cooling going on. Since the event is so short, the tires don't completely melt off.
Meanwhile, the high-revving, top-fuel engine is spinning like mad, way past any kind of long-term sustainable red-line, making 1000's of horsepower and 10's of thousands of RPM'.s
So, I think that to to make the 300 HP gas-powered pickup out-pull the 90HP diesel tractor in a tug-of-war style pull-off, you could just put some big meaty (monster truck) tires on it and spin them like mad. (assuming equal ballasted weights as well).
- Rick
