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Reading engine efficiency

Posted by MKruse 
Reading engine efficiency
January 30, 2009 09:05PM
Hi all,
How exactly engine efficiency be interpreted? When an engine has an efficiency of 30%, does that mean 30% of the heat is extracted for useful work? Also, looking at Barber-Nichols site. There is a chart that says efficiency of a turbine is 80% (sounds very efficient to me), does that mean 80% of heat is converted to work? I am confused here as I never heard of any engine can achieve 80% efficiency.
Re: Reading engine efficiency
January 30, 2009 09:41PM
Hi Max:

You can rate an engine by overall thermal efficiency, which is the percentage of heat energy converted to work. Or you can rate it by cycle efficiency. If a given cycle has a theoretical efficiency of 50% thermal efficiency, and your engine acheives 40% thermal efficiency, then you have a cycle efficiency of 40/50 or 80%.


Re: Reading engine efficiency
January 31, 2009 09:22AM
I see. Thanks Ken.
Re: Reading engine efficiency
January 31, 2009 09:23AM
Hi,,Same answer,,different words?? A boiler can be found to be 50% efficient,,,then the steam goes over to the engine,,[small loss in piping again 10%]and the engine is 5
0% efficient,,so starting w/100%,,first loss brings it to 50%,,mius pipe loss,,makes it 40%,,and then the engine loss at 50% , makes the system 20%,[ half of the 40],,,,So the numbers may reflect the system as a whole,,or as components,,,that need to be multiplied to get the overall picture,,,,Hope this helps,,Cheers Ben
Re: Reading engine efficiency
January 31, 2009 11:23AM

2 cents worth:

For my steam engine design I use the amount of work in BTU/lb produced by the engine minus the losses for the feedpump, or what work would be at the crankshaft; all divided by the enthalpy of the steam coming into the steam chest minus the enthalpy of the feed water going into the boiler.

I use an enthalpy of 180 Btu for the feed water going back into the boiler. Thus the engine efficiency does not include the losses of the boiler. So if say the engine is 30% efficient and the boiler is 85% efficient then the system is 25.5% efficient.

The steam rate of an engine is a fair approximation of engine efficiency as long as the engine exhaust is not too superheated. The efficiency of a steam engine is related to work out in BTUs divided by the heat in in BTUs. The work is figured from heat in minus heat rejected and the pounds/hr of steam used relates pretty closely to the heat rejected as long as the exhaust/condenser pressure range is similar among engines. For most auto engines this is around atmospheric pressure that the heat is rejected.

If the exhaust is superheated though, then this superheat must be removed before condensation can take place and more heat is being rejected than the steam rate would indicate.

Best, ---- Bill G.
Re: Reading engine efficiency
January 31, 2009 12:00PM
Thanks for your comments. It is difficult to calculation the efficiency when feeder water heater, and condenser air heater is involved. The way I try to calculate it is by the dyno read out divided by the energy (in fuel) input.
Re: Reading engine efficiency
January 31, 2009 12:16PM
Hi Max,

Yeah calculating the work out divided by fuel usage would then give the overall system efficiency.. In the end this is what it's about.

For design purposes and engine comparisons though the engine efficiency only is what is of interest. IC engines can't use this type of comparison so they are calculated by fuel efficiency. With steam engines it's a little closer to apples to apples just considering the engine by itself and, in a sense, the steam as the heat source.

Bill G.
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