Efficency of an IC engine as an air pump?

[Milanoguy]

What is the thermal efficiency of an IC engine, strictly as an air pump? Turbochargers typically have a quoted efficiency of 90%, modern roots superchargers(Eaton) have a max of about 60%, a twin screw supercharger(Lysholm, Autorotor) is a little better, at about 70%.

Charles Taylor’s “The Internal Combustion Engine”, states that piston compressors can give excellent efficiency at pressure ratios of 1 to 2. From this I gather that as the compression ratio goes up the thermal efficiency goes down.

What is the efficiency of an IC engine at say the 8 to 1 compression rating typically found in forced induction engines?

Do other factors beside compression ratio affect the thermal efficiency of an IC engine as an air pump?

 

[tbuelna]

The recip piston compressor is probably the most efficient type of compressor at lower flow rates (<5000 cfm) and higher pressure ratios (>2:1), thus its widespread use for air compressors. This is due to its high internal single-stage compression capability and its low leakage. At higher flow rates, they become too bulky and other type of compressors such as positive displacement rotary (screw types) or dynamic devices (centrifugal or axial) prevail.

Recip piston compressors can easily exceed 85% efficiency throughout their operating range. No other type of compressor can match that. As mentioned though, they are bulky and relatively expensive.

 

[JimCasey]

TMoose. I agree.

I am puzzled by the attitude in this thread that reed valves are somehow desirable. Reed valves are just like having a throttle plate that never gets over 1/4 open. It has been pointed out that it takes power to open a valve. With roller tappets you get most of that back. AND with the valve open, it's OPEN. It takes power to suck intake through a reed valve, too, and to push the discharge gas through the outlet reed valve.

It is possible to have a geometric compression ratio of 15:1, but if the Volumetric efficiency is only 33%, then the net compression ratio is really only 5:1.

 

[NickE]

JimCasey- Then why do most if not all recip compressors use Reed Valves?

(especially those used for compression of refrigerant in highly efficient airconditioning and heat pump applications)

 

[GregLocock]

Let's see. Typically a normal car sized engine takes sevral hp to drive the camshaft. The cylinder head valving arrangements, including FEAD, are (WAG) 1/3 of the cost of the engine. Reed valves let air in when it wants to get in, and out when it wants to get out, as such you don't have to worry about back pressure etc.

efficiency may not be a concern.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dcasto]

The equations for HP per unit of gas moved are listed in several text. The function (from my old thermal class) is ratio ^ 1/cp-cv or something like that. Having gotten over all the functions, slide rules, charts, (see also GPSA data book for all this) I now use the ariel compressor program from

http://www.arielcorp.com.

I run my thermal similation program with an 80% effiency, then run the ariel program to get exact power requirements and then adjust the thermal effiency in the silulator till I get the same energy. After several thound runs I can guess the efirncy fairly well now.

 

[dcasto]

I apologise if my responce is to much jargon.

The Horsepower per million standard cubic feet per day of air is :
BHPM=.99*[{50.8*((ratio^.288)-1)*(ratio-.715)}/(.288*(ratio-.793))]
For 14 psia in and 42 psia out or ratio=3, the answer is 66.6 BHPM
For a 100% efficient compressor, the BHPM would be 57.5 so for air at 3 ratios the efficiency is 57.5/66.6 = 86.3% subtract some mechanical losses of about 5% and you get a net 81.3%.