Idle curiosity: How fuel-efficient are turbines compared to IC engines, horsepower for horsepower?
Now THAT is a good question DE.
I am not an engine expert but as I understand it, the question is fairly complex. Gas turbines are universally poor - fuel economy wise - at sea level but they do very well at high altitudes where:
- aerodynamic drag is lower (less resistance to the aircraft's motion through the air, so less power is required);
- there is less oxygen - so less fuel is needed to keep the fire lit.
I am not sure I remember the exact values, but as I recall it, the P&W J75 turbojet engine on a U-2 reconnaissance aircraft (Francis Gary Powers) could produce about 20,000 lbs of thrust at sea level and do maybe 500 mph. They never flew it that fast down low mind you, they used that thrust to achieve an extremely rapid climb to very high altitudes at which the cameras and other sensors could do their job. So, at 70-80,000 ft. the engine was producing only about 4-500 (!!) lbs of thrust - and the plane could do....about 5-600 mph. The numbers may not be quite right - but it was something like that.
Now, the weight of fuel needed to produce 20,000 lbs of thrust was huge, but to make only 4-500 lbs of thrust it was miniscule - so the airplane had an extremely long range as long as it flew waaaayyyy up there. That is also why airliners nearly always fly about 30,000 ft. If they tried to fly at sea level, their range would be cut to a small fraction of what they can achieve at altitude.
The same rules obviously apply to piston engines - which use superchargers (either mechanically driven or turbos) to increase the density of the intake charge - but they produce so much less power than gas turbines in the first place that it isn't much of a contest.
Here is some more interesting math for you:
- horsepower is defined as doing work at the rate of 33,000 ft-lb/minute or 550 ft-lb/second;
- so - if you do the math, 1 lb of jet engine thrust is equivalent to one horsepower at about 375 mph (i.e. 375 mph is 550 feet per second);
- so, a 747 with four engines each producing 50,000 lbs of thrust at 375 mph is producing around 200,000 hp;
- in contrast, a Boeing B17 with four Wright Cyclone R1820s each producing about 1200 hp, has about 4800 hp on-tap (i.e. about 2% - really) of the power of a 747.
That is why a 747 can take-off and climb faster than ANY WW-2 fighter while weighing nearly 800,000 lbs. and a B17 struggled to get off the ground with a full load at about 65,000 lbs.
There is obviously A LOT more to this than meets the eye, but those are the basis as I understand them.
BTW -That tractor may be a small APU or perhaps a small Lycoming helicopter engine or even a P&WC PT-6 (the smallest versions of which could still churn out around 5-600 HP).
Pete