Re: Is jet engine has better fuel efficiency than propeller engine

Charlie,

You ask a very interesting question, although, I wish you had been a little 
more specific.  Fuel efficiency can mean different things for different 
purposes.  Futhermore, there are many types of jet engines.  You can guess 
by now that I'm not going give a simple yes-or-no.

You've almost surely noticed that commercial airplanes use some form of jet 
engine and small, cheap, privately-owned aircraft mostly use reciprocating 
engines driving propellors.  There are clearly some useful observations to 
be made here.  However, since you brought up this question, I think I 
should speak a little about fuel-effiency for these kinds of engines.

Instead of simply paraphrasing my favorite reference (Introduction to 
Flight, by John D. Anderson, Jr., McGraw-Hill, 1989, 3rd edition), I'm 
going to quote it a little.

Section 6.12, p.302, "One of the most critical factors influencing range 
and endurance is the *specific fuel consumption*, a characteristic of the 
engine.  For a reciprocating engine, specific fuel consumption (SFC) is 
defined as the weight of the fuel consumed per unit power per unit time."  
This gives units of pounds per hour per brake-horsepower.

This is the fuel consumption rate divided by the brake horsepower of the 
engine ("the word *brake* stems from a method of laboratory testing which 
measures the power of an engine by loading it with a calibrated brake 
mechanism")

Section 6.13, p.310, "For a jet airplane, the specific fuel consumption is 
defined as the weight of fuel consumed per unit thrust per unit time.  Not 
that thrust is used here, in contradistinction to power, as in the previous 
case for a reciprocating-engine-propeller combination.  The fuel 
consumption of a jet engine physically depends on the thrust produced by 
the engine, wherease the fuel consumption of a reciprocating engine 
physically depends on the brake power produced." ... " In the literature, 
thrust-specific fuel consumption (TSFC) for jet engines is commonly given 
as  TSFC = pounds of fuel per hour per pond of thrust"

Unfortuantely, Dr. Anderson doesn't list typical values.  However, I did 
find some in Mechanics and Thermodynamics of Propulsion by Philip G. Hill 
and Carl R. Peterson, Addison-Wesley, 1970 (My copy is a little dated).  In 
Section 6-2, p. 148, we find that "typical values of TSFC for modern 
engines are:

For ramjets: 1.7 to 2.6 pounds per hour per pound of thrust at Mach 2,
for turbojets: 0.75 to 1.0 pounds per hour per pound of thrust (static),
for turbofans: 0.5 to 0.6 pounds per hour per pound of thrust (static) "

Hill and Peterson go on to add that, in terms of SFC (defined above for 
reciprocating engines), "the best turboprop engines are as efficient as the 
best piston engines.  In addition, the turboprop engine is considerably 
lighter and smaller (in frontal area) than a piston engine of equal power, 
at least in the high-power sizes."

A ramjet is a very pure jet engine; air comes in, mixes with fuel, burns, 
and is shot out the back.  Turbojets and turbofans use the jet engine to 
drive propellors and improve efficiency, hence the lower TSFC.  However, 
they are not combinations of jet and reciprocating engines, so that I don't 
suggest you infer from the trend that a pure propellor engine would have 
the lowest TSFC.

Unfortunately, these don't give us good ways in which to compare jet 
engines agains reciprocating engines.  Reciprocating and jet engines 
function is physically different ways, producing power or producing thrust, 
respectively.

I suggest that you find a copy of "Jane's All the Worlds Aircraft" in your 
library and check out characteristics of engines (yes, they list info on 
engines, too).  You might also look at the figures for range of aircraft 
with different engines.

Troy
http://surf.to/tdg/