In Cylinder Pressure Testing With a Pressure Transducer and a Oscilliscope

Question

Recently I've been looking into buying a higher PSI pressure transducer to do in cylinder pressure testing. I've encountered vehicles which would of benefited from this approach and wish I had thought to try this in the past.

I've been watching videos on the subject and have a general idea of how in cylinder pressure diagnostics works.

Some examples I've seen so far

• Known good waveforms.
• Plugged exhaust waveforms.
• Waveforms that show issues with valve sealing.

Though I'd be willing to look at the same waveforms in different engines/conditions.

I'm interested in learning about how cam timing effects the waveform. Say a chain is stretching and/or a cam gear is one tooth off.

And just as a broader question, what can an in cylinder pressure test tell me and how can I interpret the waveform?

Answer 1

Analysing in cylinder pressure waveform is extremely useful in developing combustion strategies in OEM R&D environments and also for diagnosing lots of issues.

I have taken picture and its explanation from a Snap on product catalogue which I thought was well explained.

A—Ready Position

— Piston has stopped at TDC

— Cylinder contents are compressed

— Both valves are closed

— The spark pulse usually happens 7 to 10 degrees before the peak pressure.

B—Power Position

— Both valves remain closed

— Piston has rapidly resumed movement

— Piston reaches top speed at the halfway point

— Piston quickly decreases in speed and stop at the end of the stroke

C—Vacuum 1 Position

— Piston has stopped at BDC

— Both valves remain closed

— Cylinder pressure has dissipated, and the rapid downstroke of the piston has drawn a vacuum in the cylinder

— Exhaust valve is ready to open

D—Exhaust Position

— Exhaust valve has opened

— Piston has rapidly resumed movement

— Piston expels cylinder contents through the open exhaust valve

— Piston reaches top speed at the halfway point

— Piston quickly decreases in speed and stop at the end of the stroke

E—Zero Position

— Piston has stopped at TDC

— Exhaust valve has closed

— Intake valve is ready to open

F—Intake Position

— Intake valve has opened

— Piston has rapidly resumed movement

— Down-stroke of the piston draws air into the cylinder through the open intake valve

— Piston reaches top speed at the halfway point

— Piston quickly decreases in speed and stop at the end of the stroke

G—Vacuum 2 Position

— Intake valve has closed

— Piston down-stroke has drawn a slight, non-detectable vacuum in the cylinder. (A closed throttle registers a detectable vacuum.)

— Piston has stopped at BDC

H—Compression Position

— Both valves are closed

— Piston has rapidly resumed movement

— Cylinder contents are compressed on the piston upstroke

— Piston reaches top speed at the halfway point

— Piston quickly decreases in speed and stop at the end of the stroke

Now onto the diagnostics. Measuring in cylinder pressure will show up lots of problems if analysed correctly.

1. If the piston rings are damaged ( loss of compression will result in lower than normal peak pressure reading).
2. If there is restriction in Intake or exhaust and it will show up in C (exhaust) and D (intake) area in the waveform.
3. If the head gasket is damaged, the peak pressure will be lower than normal reading.
4. Valve timing failure will show up in the readings. If there is an open valve, the compression pressure (A) will be lower and there will not be any vaccuum created in the intake stroke. Camshaft timing problem will have a different compression and vaccuum pressure reading.

In cylinder pressure sensors are useful in finding knocking or detonation accurately.

Detonation is unwanted combustion which happens when hot air pockets with fuel are auto ignited and the yellow pressure curve with jagged ends at the top explains it clearly.But capturing this requires some high speed data acquisition hardware modules which are very expensive, which is why only well funded companies like OEM or F-1 teams have access to such sophisticated piece of equipment.

Cheers, Vish

Answer 2

I use pressure transducers to test for cam sync problems. And other problems. Cam sync problems are seen on the waveform in a section called the exhaust ramp. The middle of the ramp moves back and forth as the valve timing changes. It is easy to see if you know where to look. Training in what to look for is required. It is a bit like interpreting ignition waveforms. One supplier of these tools provides a real time interpretive program that can help, but will not negate the need for proper training.

In cylinder pressure readings can be used to see: Compression, cranking and running , VVT activation, cam timing, valve opening failures, exhaust restrictions and others.

Answer 3

Most accurate way to diagnose plugged exhaust (not severe) or chain stretch is to compare to known good waveforms on same engine and year. Like any other diagnostic tool of this type it takes time and experience to do it without comparing to good waveforms.

Another good tool in conjunction with compression tool you describe is looking at oscilloscope secondary ignition waveforms, which is harder to connect on cars without plug wires.