Whats the compression of a running engine?

[PorkChop808]

Hi everyone,

So i am wondering what types of compression's a engine is making while its operating under normal condition. i dont mean a compression test i want to know what its making while its operating.

 

[patprimmer]

That depends a lot on throttle position and cam timing if it's some form of vtec which many now are.

Regards
Pat
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[PorkChop808]

im just looking for a general number

 

[GregLocock]

You need to find a cylinder pressure diagram for the engine you are interested in and look at the pressure just before ignition. It will be confusing because the air heats as it is compressed for a couple of reasons at least, so you you should see more than (CR*VE-1)*atmospheric gauge pressure.

Cheers

Greg Locock


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[PorkChop808]

for a engine that puts out around 400 hp

 

[PorkChop808]

i want to know the pressure after ignition, the force that is being applied in a cylinder.

 

[pontiacjack]

For a "general number" calculate the so-called DCR (dynamic compression ratio). In addition to the usual parameters required to calculate static compression ratio, the crank angle at intake-valve-closing is needed. Do the geometry/math yourself or use one of the many online DCR "calculators" on the Internet (I tend to use wallaceracing.com for such things). The main drawback of looking at DCR is that it ignores the varying VE (volumetric efficiency) of a running engine.

 

[GregLocock]

PorkChop808: "i want to know the pressure after ignition, the force that is being applied in a cylinder."

OK, so you want the peak cylinder pressure. A typical number for a gasoline engine is an instant peak of 400 psi, if it doesn't detonate, and say double that if it is in det. That number is highly dependent on the signal filtering that is used, for a given engine, and very dependent on the engine in question.

A google search for images on cylinder pressure diagram will probably help.





Cheers

Greg Locock


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[patprimmer]

Basically it is limited by engine strength. To much pressure blows things up. To little makes anaemic engines.

Cam timing, intake air temperature, octane rating of your fuel, static compression, and ignition timing all have a significant impact. If you ask for to much, you generally melt a piston or break a ring land, gudgeon or rod.

I am sorry, but there is no straight answer to the very general question as you asked it. Why do you ask? Is it homework? If it is homework, so far you have gotten the benefit of the doubt.

Regards
Pat
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[swall]

An interesting low buck experiment might be to take an engine air compressor (aka "chuffer pump") and hook it to a pressure guage. Make sure you ground the spark plug after removing to avoid coil damage with the engine running and no way to dump the spark.

 

[Tmoose]

Here's some info from a research engine from the late 1940s with a puny BMEP in the 80s.
If that 400 HP engine is from passenger car the bmep (P) is likely much higher.
HP=PLAN/33000

 

[GregLocock]

Hang on, bmep is an average over the cycle, not a measured cylinder pressure.

Cheers

Greg Locock


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[patprimmer]

From what I understand this guy is actually asking for PEAK cylinder pressure which should normally occur at about 12 deg ATDC on a properly tuned engine without any detonation occurring.

Regards
Pat
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[pontiacjack]

Oops! When 'Porkchop' posted at 1:19 AM I was busily typing my "DCR" post... sorry...

 

[patprimmer]

Don't worry Jack, we have all been there.

Regards
Pat
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[BigClive]

GL - I would have thought that the peak combustion pressure after ignition was more like 1000psi (or maybe a bit more) rather than 400psi.

SWall - You can actually just screw a normal compression gauge into a plug hole and measure the compression with the engine idling. I have discovered that a big six cylinder engine idles at about 45psi with the small throttle plate opening. One thing to note is that the rubber hose to the gauge overheats very quickly from the compression temperature.
I have never tried this but I suppose you could make up a gauge with a metal pipe replacing the rubber hose and actually road test the engine. You would need to achieve WOT at the maximum torque RPM (maybe by driving with one foot on the brake in a lower gear?) or on a chassis dyno.

 

[JackAction]

OK, so you want the peak cylinder pressure. A typical number for a gasoline engine is an instant peak of 400 psi, if it doesn't detonate, and say double that if it is in det. That number is highly dependent on the signal filtering that is used, for a given engine, and very dependent on the engine in question.

A typical number for peak cylinder pressure will be closer to 1000 psi (for an engine at full power):

http://performancetrends.com/Definitions/Cylinder-Pressure.htm[/URL]]Peak cylinder pressures near TDC (where spark occurs) will be in the range of 300 psi for engine's at light load, to a 1000 psi for production engines at full power to 1500 psi or greater for race engines.

 

[Tmoose]

Hi Greg,

You replied "Hang on, bmep is an average over the cycle, not a measured cylinder pressure. "

My reference to 80-ish psi bmep was related to the chart image I attached. It shows peak cylinder of between 400 and 540 while producing bmep 82-84 psi depending on ignition timing for a 1200 rpm 1947 vintage research engine. I mentioned the bmep (and included the formula used to extract it) only because I think a more modern engine would have a higher BMEP, but do not know what impact that would have on peak pressure. Certainly the research engine did not gain much bmep (2 psi) relative to the whopping peak pressure increase. I gotta think there was probably a spark advance between 13 and 26 degrees where the bmep would be as high, or higher, without the peak pressure penalty.

Some old generic Shell oil info suggested the spark plug temperature increased a lot, and un-necessarily rapidly when ignition timing was even a few degrees more than "best".

regards,

Dan T

 

[GregLocock]

I got the 400 psi from an actual cylinder pressure plot, admittedly off a very old engine. I'm quite willing to believe higher numbers, but there is a signal processing issue if the pressure rise rate is rapid (what you might call inaudible detonation).

Cheers

Greg Locock


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[dicer]

Depending on engine, about into the 4,000 psi range with nitromethane, and likely 1,400 psi gasoline.