Whats the compression of a running engine?
Whats the compression of a running engine?
(OP)
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.
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.





RE: Whats the compression of a running engine?
Regards
Pat
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RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
Cheers
Greg Locock
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RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
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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RE: Whats the compression of a running engine?
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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RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
If that 400 HP engine is from passenger car the bmep (P) is likely much higher.
HP=PLAN/33000
RE: Whats the compression of a running engine?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Whats the compression of a running engine?
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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for site rules
RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Whats the compression of a running engine?
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.
RE: Whats the compression of a running engine?
A typical number for peak cylinder pressure will be closer to 1000 psi (for an engine at full power):
RE: Whats the compression of a running engine?
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
RE: Whats the compression of a running engine?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Whats the compression of a running engine?
RE: Whats the compression of a running engine?
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RE: Whats the compression of a running engine?
All the above when operating as designed, not outside the intended envelope. And I'm talking about mainstream commercial production engines, not modified engines for racing or other shenanigans.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Whats the compression of a running engine?
This engine is rated at 345 Bhp.
Where this engine runs on the highway is highly dependent on the vehicle it is mounted in, it's weight, it's Cd, it's frontal area etc.
If we assume that it's running at 4 bar BMEP ( peak BMEP of this engine is about 11-12 bar BMEP)my data shows it will be running between 22 and 26 bar peak cylinder pressure.
Another example- a medium duty diesel truck engine- between 7 and 8 litre capacity.
At 1500 rpm 50% load this engine shows a PCP of 65 bar and at 1800 rpm PCP of 88 bar.
This engine makes peak power of about 220 Bhp (at 2100 rpm) and peak torque of about 550 lb ft.
The engines PCP is a modest (by todays standards) 150 bar.
www.auto-scape.com