Thermal efficiency of the compression stroke of the Otto 4 cycle?

[Milanoguy]

What is the thermal efficiency of the compression stroke of the otto 4 cycle engine? For this question we will consider the motion of the piston from BDC to TDC, for simplicity we will assume no valve overlap and that both the intake and exhaust valves are closed when the piston starts coming up from BDC.

I gather at low static compression ratios (under 3 to 1) the thermal efficiency of a piston(positive displacement) compressor is very good - above 80%. I also understand that as the static compression ratio goes up, the thermal efficiency goes down. Why is this?

Note this question is only about thermal efficiency and only about the compression stroke. Please no posts about volumetric efficiency, or valvetrain issues.

 

[patprimmer]

Why do you ask.

The way you pose the question excludes all practical application.

The compression stroke heats the air and heat is lost to the cooling system.

Some air blows past the rings.

Regards

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[GregLocock]

Check out Heywood, he thoroughly discusses the efficiency of each part of the cycle. Or at least, he discusses it in eye-glazing detail, to me.



Cheers

Greg Locock

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

 

[globi5]

The efficiency during compression doesn't go down significantly with higher static compression ratios otherwise engines with higher compression ratios wouldn't reach higher overall efficiencies.

And reasons why it goes down:
The higher the static compression ratio the higher the pressure and the more air will blow past the rings (as Pat already mentioned). Also, when the compression ratio is increased the stroke is usually increased as well, which results in increased mechanical friction.

 

[patprimmer]

Also, more adiabatic heat is generated, so more is lost to the cooling system, BUT it is more than recovered by improvements on the expansion stroke.

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[SPOrotary]

Actually very little heat generated by compression is lost to the cooling system on the compression stroke. Graphs showing internal pressures tend to show this (as they show an increase of pressure from the dynamic compression as well as the thermal energy increase) Compression in todays engines happens very quickly and the temperatures reached aren't that much higher than the coolant temp.

Also almost no air gets past the rings on a well maintained well running engine, however pressure is lost to perform the work of sealing the cylinder. Does anybody still think the spring tension of the piston rings is the only thing holding back several BAR of pressure?

After all that we still have to consider that superchargers like the Twin-screw" system are more efficient at compressing air than piston type compressors. Piston type compressors tend to achieve about 70% efficiency. Turbochargers can also compress air at higher efficiency but only over a small operational range.

Current project car: '98 BMW 316i
Main targets: 950kg, 250rwhp, min. 1G lateral grip. Special: Twincharger system, hydraulic ebrake, New suspension.

 

[dcasto]

at a 9:1 compression ratio, air in a piston compressor will have over a 700 degree rise. When the fuel is added or as it is added, the net temperature will drop 100 degrees.

Screw compressor are always less efficient and centrifugal are even less yet given the same compression ratio. By adding more stages and intercooling between each stage of compression, the compression be come more efficient. Lets assume that we are looking to get the pressure inside the engine to 168 psi. This is about a 12 to 1 compression ratio. The piston compressor would be running at about a 60% efficiency to achieve this, a screw about 20% and a centrifugal would be 10% (a single wheel compressor can’t do much more than 3 ratios no matter what). If we were to instead have the compression broken up into 3 stages, then the ratio on each stage would be about 2.3 to 1 (2.3 x 2.3 x 2.3 = 12) and we cooled the air in between, the effiecy would be 83% for the piston, 76% for the screw and 72% for the centrifugal. The air would be so cool after the last compression stage that you could run 75 octane fuel without knocking. So in closing an engine with a 6 to one compression ration being twin or triple charged at 60 psia should be more efficient.

 

[SPOrotary]

dcasto:
Ummm at a 9:1 static compression ratio you would theoretically get an increase in temperature of roughly 280 degrees C but since a static compression ratio of 9:1 only gets a dynamic compression ratio of about 7:1 and the piston is roughly 65-70% efficient the temperature increase is roughly 250 degrees C.

I disagree with your assumption of screw and centrifugal efficiencies at 12:1 (even though its closer to 11:1). However you make the point well.

Current project car: '98 BMW 316i
Main targets: 950kg, 250rwhp, min. 1G lateral grip. Special: Twincharger system, hydraulic ebrake, New suspension.

 

[patprimmer]

Getting back to the OP

Ummm

If the engine has 100% VE which many high performance engines exceed, that means the mass of air in the cylinder when the intake valve closes is the same as it would be if full of atmospheric temperature and pressure air at BDC, so cylinder pressure and temperature are the same as static, not the same as dynamic unless the inlet valve is deliberately closed late to produce the Miller cycle. The miller cycle allows higher compression by reducing VE, then taking advantage of the higher expansion ratio.

Therefore on an efficient modern engine, the temperature generated by the compression stroke only, at engine speeds that match maximum VE, will be substantially above the coolant temperature, so a small but measurable heat loss will occur.



Regards

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