Propulsion Efficiency

[twistedenginestarter]

I wondered if anyone has any idea of the relative efficiency of piston propellor, turboprop and turbofan? I guess jets have become so pervasive because they can fly above the weather and do more sectors per day but maybe in due course fuel efficiency will start to be a bigger factor.

[wondering]

It depends on your airspeed. With increasing speed, jet engines become more efficient than turboprop engines. Depending on who you want to believe, the crossover point is at about 500mph. No idea how the piston engine is doing relative to those two.

Assuming fuel air ratio<<1, the overall efficiency is:

no=2*nth*((u/ue)/(1+u/ue))

no=overall efficiency
nth=thermal efficiency
u=velocity
ue=exhaust velocity

Since the jet engine has a high exhaust velocity, it is not very efficient at low speeds. -> low u/ue ratio

[411A]

Regarding piston engined airliners, specifically the DC-6B, the Hamilton Standard propellors fitted to this aeroplane were approximately 89% effective.
How do I know?
A close relative was the engineering projet manager on the type.

[chornedsnorkack]

Look at it this way: both high-bypass turbofan and turboprop have a turbine which powers a fan.

In a turbofan, the fan is connected to turbine by a shaft, so they have to have the same rpm. Since the fan is bigger than turbine, and works at lower temperature, the turbine is too slow for best efficiency, and the fan is too fast. Both have to be afflicted by design compromises.

In a turboprop, there is a gearbox between turbine and propeller. You can run both at the optimal speed. If you want to fly slower, you can just design the gearbox, and have a bigger propeller at a lower rpm, moving more air at a lower speed.

[barit1]

Carrying the turboprop one step further:

Put an 80:1 reduction in the gearbox, and turn the output 90 degrees, and put really long airfoils on the prop hub - you can directly lift the aircraft plus payload at zero airspeed. :8

[hawk37]

"In a turbofan, the fan is connected to turbine by a shaft, so they have to have the same rpm. Since the fan is bigger than turbine, and works at lower temperature, the turbine is too slow for best efficiency, and the fan is too fast. Both have to be afflicted by design compromises."

What engines would this be true for? The Honeywell 731 series has a sun/planetary gear system that allows an rpm reduction. The designers would have ensured at cruise that the ratio was optimized for SFC, since that seems to be the driving force in engine design.

Chorned, what do you think?

[barit1]

The geared turbofan is the exception rather than the rule, and is thus far limited to engines under 8000# thrust. (ALF702, TFE731, etc.)

Theoretically it's a wonderful idea, if the mechanical "features" :} can be kept under control. I believe all the big engine shops have studied the idea and are awaiting the big breakthroughs in materials, lubricants, etc. that will make it practical and reliable.

Perhaps the E. Hartford folk can join forces with their old colleagues in Windsor Locks to make it happen. :)