Xjyuk

It is widely know that early turbojets guzzled fuel and created thrust simply by spewing accelerated matter out the back. Modern jets have come a long way in earning a better reputation with efficiency improvements.

If an aircraft mounted the exact same propeller on a supercharged piston engine and a turbine, and flew under identical conditions at the same pitch and rpm, how much more fuel would the turbine use (possibly including weight savings benefits for extra credit).

I am picturing a two engined aircraft with the two types of engines flying side by side at around 350 knots.

The most efficient IC engines are large Diesels. At the extreme end are ship engines with better than 50% thermal efficiency resulting in a specific fuel consumption of only 0.260 lbs/hp/hour or 158 g/kW-h. But even supercharged truck diesels achieve above 40% thermal efficiency at high load (this NHTSA study gives 42%).

Aerodiesels have achieved 220 g/kW-h already with the Jumo 204 and 205 of the early 1930s. Even the modern Thielert diesels (now sold by Continental) are hardly better, claiming 214 g/kW-h. Also the Napier Nomad, a super- and turbocharged aero diesel with utmost efficiency as its design goal just reached 219 g/kW-h.

Gasoline engines start at about 240 g/kW-h; this value is achieved by the Lycoming IO-390 with fuel injection. Without fuel injection, specific consumption rises to 260 - 280 g/kW-h which is typical for a Lycoming O-360 at 65% power. Note that the Jumo 213, one of the more efficient WW II-era piston engines, already achieved 260 g/kW-h even with 87 octane fuel and a compression ratio of only 6.93:1 at its most favourable operating point. Advanced Innovative Engineering, who have taken over the Norton-Wankel engine, claim 310-350 g/kW-h for their 650CS with 120  PS.

Comparing that with turboprops needs some conversion of thrust into power. This is only valid for a specific flight speed. If you do that at cruise speed, the large turboprops Progress D27 and Europrop TP400 claim a consumption of around 240 g/kW-h. Smaller turboprops rarely achieve below 300 g/kW-h.

To save you from the trouble of looking up and converting the data in the last link, here is a selected list:

• Allison 250 $;;;;;;;$: 370 g/kW-h. This is a typical small helicopter engine.


• Garrett TPE331$;;$ : 310 g/kW-h. This is used on small turboprops like the Do-228 or the Merlin III.


• PWC 126A $;;;;;;;$: 280 g/kW-h. Getting larger - BAe ATP.


• Rolls-Royce Tyne : 237 g/kW-h. This has long been the largest turboprop in the West and used on aircraft like the Canadair 400 / CL-44.

Very poorly actually. What saves turboprops and turboshafts is power to weight, smoothness and reliability. If you want just maximum MPG and don't have to go trans sonic, recip wins hands down.

Piston engine Specific Fuel Consumption is roughly .45 lbs/hp/hr for a normally aspirated carbureted engine (that figure comes from my own Lycoming engine's power chart - I can't find an on-line source), going down from there with fuel injection, and turbo/super charging. Diesels are in the low .3s.

The most efficient recips were the Wright Turbo Compound radials that had both supercharging and direct horsepower extraction from the exhaust (about 300 hp was recovered from the exhaust of the R3350 by the two power recovery turbines) that were down in the high .3s to .4.

Turboprops? Worse than two stroke gasoline engines. Somewhere around .6 lb/hp/hr or worse (the PT-6 is .67), maybe in the high 5s on some of the latest.

This is one reason why you don't see that many turbine conversions on airplanes like the DC-3. Aside from the cost of the conversion, it's just way more economical to run, fuel wise, on gas.

Very favourably. The early turbojets had low thrust efficiency: they could not convert their gas generator power into thrust in an efficient way.

If all gas generator power is converted to mechanical energy we're talking about turboshafts. The best way to compare the fuel efficiency of the engine only and not get engaged in a discussion about thrust conversion, is a listing of engine brake horsepower.

• As can be seen, a large gas turbine engine like the GE LM6000 (a converted aeroplane turbofan engine) is among the most frugal engines around, with a brake specific fuel consumption of 0.329 lbs/(hp * h) = 200 g/kWh = 42% efficiency.


• Diesels can get efficiencies of over 50%, mainly the large, low RPM 2-stroke diesels generating a huge amount of torque. The Wärtsilä-Sulzer RTA96-C runs at 22-120 RPM, which would be problematic for automobile and aircraft propulsion.


• The highest efficiency of all is the combined cycle with 62.2%, a gas turbine combined with a steam turbine to utilise the exhaust energy.

Gas turbines have additional advantages for aircraft propulsion: they don’t have any sliding contacting moving parts, making them extremely reliable, and have a very good power-to-weight ratio.

That is for large gas turbine engines, they don’t scale down well because of the boundary layer effects: a smaller engine has relatively a larger circumference. Also, they only operate efficiently at full power, piston engines have the advantage at lower rpm percentage.

So the smaller the engine, the more advantageous are the circumstances for the piston engine: they scale down much more favourably than the gas turbines. But to claim that gas turbines are inherently wasteful with fuel is not correct.