Energy Conversion Efficiency of Various Engines

[redpoint5]

I have an overall interest in numbers and especially numbers dealing with efficiency. After much reading on these forums, it appears the average car engine is about 20% efficient. This means that 80% of the energy generated by burning fuel is lost as heat that provides no work.

I stumbled upon a Wiki chart and observed that the gas turbine engine is listed as 40% efficient. Can it be that commercial planes produce more power per BTU than our cars? If so, why aren't we all riding these?


Does anyone know the typical efficiency of a rocket engine? How about the solid-fuel rocket booster on the space shuttle? This never struck me as particularly efficient, but perhaps it is?


Energy conversion efficiency - Wikipedia, the free encyclopedia

[baldlobo]

they are more efficient because it's straight through
induction(single stage centrifugal compressor on early models; multiple stage axial compressor on newer models)
combustion(flame canisters)
exhaust(single stage centrifugal turbine on early models; 2 max axial turbines)

1-2 rotating parts

vs.

multiple rotating/moving parts that use energy in an otto cycle engine


we don't use them because they are fuel inefficient; think keeping a flame thrower going for 100km(they use pounds per hour for flight)

on the plus side that engine you have posted with the motorcycle behind it is an alison 250 turbo shaft (15 stage axial compressor with a centrifugal at the end and 2 turbines, one runs the compressor the other runs the gearbox (900 shaft hp;something like 1000hp goes to just running the engine)

[serialk11r]

That's peak efficiency. Most current production car engines are ~36% efficient or so at peak, maybe a little higher.

Gasoline engines and diesel engines have very consistent performance across a wide range of rpm compared to turbines, which are not efficient at all short of max power.

40% is about the best you can do with a single cycle, combined cycle it is obviously higher. Gasoline engines of the typical design can probably hit 45% if optimized fully, I think. Extremely high compression ratio, simulated Atkinson, titanium rods/valves, race engine balance and friction reduction, should get you there or close (F1 engines are ~37% running at rpms so high the piston is about to outrun the flame front). A good exhaust heat regeneration system can probably pick up 5% more.

If you compare to what percent Carnot cycle efficiency at the same temperature ratio, current gas engines are like 50% of that, which to be honest isn't that bad. Much room for improvement sure, but it's not that bad.

[redpoint5]

Originally Posted by serialk11r

Gasoline engines and diesel engines have very consistent performance across a wide range of rpm compared to turbines, which are not efficient at all short of max power.

Well I guess when you put it that way, the answer seems quite obvious.

What are some other engine types and peak efficiencies? Very curious to see the efficiency of rocket motors.

Edit: As an interesting aside, the muscle "motor" is only 20% efficient. Funny, food conversion is the one energy source most people wish they weren't so efficient at processing.

[Piwoslaw]

Turbines spin faster than an ICE and aren't as elastic rpm-wise (correct me if I'm wrong).

Solid fuel rockets can't really be controlled on the fly - once they're lit, they burn at full blast until they are out of fuel. I believe that one of the Darwin Award laureates put a solid fuel take-off assist booster in the bed of his pickup. The skid marks from his brakes were supposedly a few miles long.

[mort]

Originally Posted by redpoint5

What are some other engine types and peak efficiencies? Very curious to see the efficiency of rocket motors...

Liquid fuel burning rocket motors are essentially pumps which supply fuel and oxidizer into a high pressure chamber. The pressure is released through a de Laval nozzle. So like any engine there are 3 components to efficiency:
1. The thermodynamic efficiency, which is lower than the Carnot efficiency for the operating temperatures of the engine.
2. The mechanical efficiency of the conversion from heat and pressure to motion.
3. The peripheral losses, such as coolant, lubricant, and fuel pumps, valve mechanisms, etc.

For a gasoline ICE the Carnot efficiency can be as high as 50%, the mechanical efficiency of a piston for extracting power is around 99%, but ancillary losses (pumps and friction) are generally about 10% to 15% so over all peak efficiency is about 35% to 40% Throttled efficiency, say 10% of maximum output, is much lower 10% to 15%

For a liquid fueled rocket, the Carnot efficiency can be very high, 80% for a rocket burning hydrogen and oxygen in the cold vacuum of earth orbit. The nozzle, which turns temperature into thrust, is designed for one specific external pressure, exhaust speed and vehicle speed - efficiency is lower under any other conditions. At the design point nozzles are about 98% efficient. Fuel and oxidizer pumps use between 10% and 15%

Solid fuel rocket motors have practically no ancillary losses. But Carnot efficiency is generally lower than liquid fuel rockets because the combustion temperature is limited to keep the nozzle from disintegrating.
Peak efficiency for rocket motors is 65% to 70%
-mort

[user removed]

If you let your rocket engine or electric motor idle without doing any useful work, then they are both 0% efficient.

The Nissan Leaf, when tested by Consumer Reports only provided 85% of the energy used to charge the battery, and that does not consider the percentage of battery capacity that can not be utilized without damaging the battery permanently (about 30%).

The most efficient IC engine is right at 55% and it can run on basically crude oil, virtually unrefined. That engine would not work in your car, with a 3 foot bore and an 9 foot stroke running at 92 RPM.

Turbines used in airplanes are very different from turbines used in power generation. One has to deal with air entering the engine at 100 below 0 and 650 MPH. The other has to not melt in its restricted fixed application. Efficiency increases in power generation turbines can be very high when co-generation is utilized where the exhaust heat is used to generate additional power in a power plant, like making steam for another turbine, instead of thrust in an airplane.

Fuel mileage in cars as well as AVERAGE efficiency could be doubled with a system that was capable of highly efficient short term energy storage that allowed the liquid fuel consuming engine to only run at it's highest efficiency.

regards
Mech

[serialk11r]

You can't directly compare things like jet engines and rockets to car engines, because there is the component of propulsive efficiency; At 0 mph, they're all 0% efficient overall because they're not moving anything. Rockets and jets are great at putting out pressurized gas, but it's very hard to convert that to mechanical power.

[oil pan 4]

Originally Posted by Old Mechanic

when co-generation is utilized where the exhaust heat is used to generate additional power

Sounds a little like a turbodiesel.

I work on small power generating gas turbines. At light load they are horribly inefficient.
At or near full load they are very impressive for their size.

Piston engines manage to keep a good portion of their efficiency even at light load.

[user removed]

Fairly sure co-generation in commercial power plants uses the turbines exhaust heat to boil water for a steam trubine. Could be other steps in a fashion similar to the old triple and quadruple expansion steam engines or prior centuries. These used the exahust from the first piston to drive a larger lower pressure second, third, and even 4th pistons, further extracting energy out of the steam pressure until it had dropped to practically nothing, more than hot water under 212 degrees, which could then be used for other purposes, like hot showers.

http://www.bing.com/images/search?q=...EBAE49&first=1

regards
Mech