Point of Combustion in Piston Engine

There are many factors that will affect the cylinder pressure. The mechanical compression ratio, the camshaft specifications, the throttle position, the play in the timing chain, even the ignition timing will have an affect. I could not give you a formula, although I would be able to tell you how each of the above would affect the pressure.

Yea, that's what I said. GA.

Sounds like a patent application to me.

I understood what you said.

Thanks.

Thank you for your explanation but I have no way to interpret this complex information to find the PSI at the point of combustion. So paper and pencil wont get me anywhere and it made me think of a more hands on practical approach.

Drill-tap a hole next to the spark plug, install a pressure gage and take a reading at the low and high RPM and I don't care at what angle it occurs or worn out cam and gears.

compression gage in sparkplug hole. If want to know pressure at higher rpm than on starter. just fire up. no smark plug, no fire for combustion in that cylinder. rev up and watch gage. I would be interested in knowing how pressure at higher rpm compares with low on starter rpm. But not ready to put my gage on to find out yet.

Thanks Don.

having reread your original question I understand you want to know pressure with the combustion. So a seperate line is probably warrented. I don't know how much pressure would go up for increased rpm. I am sure pressure would go up dramatically with increased load on engine.

Thanks Don.

No, that won't work because you are NOT measuring the increased pressure from the explosion of the fuel in the cylinder...

I have an adaptor that screws into a spark plug hole, that lets me screw a spark plug into one side of the adaptor & a pressure transducer into the other side of the adaptor. That way I can use a lab scope to watch the pressure waveforms while the engine is running, and under all engine loads. I am not an engineer, I am an old auto mechanic that does a lot of troubleshooting of engine problems, and has the need to know. I do not know of any formula for the information that you are wanting, since there are way too many variables that enter into what the engine compression is. Even the brand of gasoline and the ambient temperature have a lot to do with the internal combustion chamber pressure.

I would be interested to know what ballpark pressures do you see in the cyllinder with the combustion at idle and under load. Sounds like you already have seen a represenative sample.

Thanks Don.

You seem to have the answer to my question. I didn't know such gadget is available.

Now I like to ask you a favor, would you make a test on any engine and post your finding also the quebec in of the cylinder and never mind of any variables.

I just found out the Ford hemmi has 2 spark plugs , but I don't know anybody with such an engine .

Thanks for your help.

I wonder if your gadget is still working. Still like to have a reply, if you could do it.

thanks for your trouble

It is common practice to measure combustion/firing pressure in diesel engines. Large diesels actually have a pitcock valve for engine blowdown prior to starting. A special pressure gauge, specific to the manufacturer, like Fairbanks-Morse, provides as part of the tool package for the engine the guage needed to do the combustion pressure test. Normal compression of the engine is taken by simply racking out the fuel injector/s for the cyclinder of interest, firing pressure is taken by racking it back in.

As far a formula . . . compression ratio gives you the non-firing pressure.

Gasoline engines . . . could be measured the same way . . . if you realize that the small cavity represented by the guage connection device may in fact represent a significant increase in the volume of compression. Also that any increase in the volume of compression does represent "loss of compression" to the engine and firing pressure will be signficantly lower.

Additionally, gasoline engine "combustion gas" activity and turbulence, is highly engineered by shape of the cylinder head design. It is entirely possible that changing the location of the spark plug, by raising the gap out of the "designed" location can have significant impact on the peak pressure developed during firing. As the pressure wave/ignition wave as it travels inside the compression chamber is greatly influenced by where ignition occurs.

You seem to know a lot about engines but haven't answered my question. Very simply stated. What is the psi at the point of combustion and I don't care about angle or exact time after igniting the fuel mixture .Also need to know , volume of piston , Psi at low and high RPM type of engine optional.

I had the same question years ago . Nobody came up with a practical answer I could understand. My thinking was : any piston has a given amount of compresst air before combustion. At this point a given amount of fuel is injected at idel and more at max RPM. Lets not go into vatations and say the piston is allways at the high point only the fuel injected varies from low to high RPM.