In cylinder pressure waveform

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dannydebont
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In cylinder pressure waveform

Post by dannydebont »

I did an in cylinder pressure test on a Ford Figo TDci (diesel) 2008 F6J6 engine. The engine is out of the car with all ancillaries removed except for the starter motor and fuel pump. The intake box, E.G.R system are all removed. The injectors and glow plugs are still in. I modified an old glow plug to measure pressures.

When I crank the engine I get a nice waveform except for a bump at the bottom plateau (red circle). This is when the piston has reached BDC (180°), and starts to move towards the cylinder head reducing the volume of the cylinder, but since the exhaust valve is now open, the gas charge escapes into the exhaust. So the plateau represents the time during which the piston is pushing exhaust gas out and during this time the pressure in the cylinder should be nearly equal to the atmospheric pressure.

But as you can see on the attachment there is a bit of a bump (4.1psi). If the exhaust valve is open at this point what can cause this bump?

Another thing I picked up is that the plateau does not dip again before compression starts (purple circle). I do not know if it should dip on cranking only cause there is no vacuum created because of throttle closing.

Also, the Ford manual states that the difference between the cylinder pressures should not be more than 5 bar but says nothing about what the pressure should be. #1, #2 and #3 pressures (158psi) are almost exactly the same, however #4 is 192 psi. I cannot find a reliable source which tells me what the in cylinder pressures should be.

I have the same profile on all cylinders except for #3 which do not have the hump
Attachments
20240607114547.png

wiki38andy
OneWave
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Re: In cylinder pressure waveform

Post by wiki38andy »

Could you upload psdata files for each of all 4 cylinders?

Technician
TwoWaves
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Re: In cylinder pressure waveform

Post by Technician »

Also, the Ford manual states that the difference between the cylinder pressures should not be more than 5 bar

I'll take that as a misprint in the technical data. I've never known any manufacturer to allow up to 5 bar variation between cylinders, and only produce around 150 - 160 psi compression per cylinder, which also seem too low.

Those cranking waveforms don't produce sufficient data for anaylsis to be honest. If you suspect a cylinder problem, maybe so a leak down test of each cylinder?

picowight
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Re: In cylinder pressure waveform

Post by picowight »

Looking at the bump the cylinder has just started to move down on the intake stroke after reaching TDC so exhaust valve has just closed and intake has just started to open. As it's a diesel there is no intake vacuum of any note (unless it is using the EGR throttle at the time!).
Not a clue what that bump is but if you are losing compression that will shift the tower peaks potentially confusing things

Littlejohn
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Re: In cylinder pressure waveform

Post by Littlejohn »

Would be interesting to see a capture with the intake restricted. (Just partly block with your hand). This should then give you the intake events and slightly lower compression and see how this affects the hump. I would say the hump is lack of valve overlap/ exhaust valve closing just before TDC.

wiki38andy
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Re: In cylinder pressure waveform

Post by wiki38andy »

Very few diesel engines with overlap design and VVT system. They use classic EGR system with valve, cooler... to reduce NOx emissions. Petrol ones use overlap in this purpose. Valve timing for 1.6 DCI R9M Renault engine.
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R9MCYL.png

wiki38andy
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Re: In cylinder pressure waveform

Post by wiki38andy »

Nissan Navara D40/Pathfinder YD25 engine valve timing.
Attachments
YD25CYL.png

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leonardorj
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Re: In cylinder pressure waveform

Post by leonardorj »

dannydebont wrote:
Fri Jun 07, 2024 1:52 pm
I did an in cylinder pressure test on a Ford Figo TDci (diesel) 2008 F6J6 engine. The engine is out of the car with all ancillaries removed except for the starter motor and fuel pump. The intake box, E.G.R system are all removed. The injectors and glow plugs are still in. I modified an old glow plug to measure pressures.

When I crank the engine I get a nice waveform except for a bump at the bottom plateau (red circle). This is when the piston has reached BDC (180°), and starts to move towards the cylinder head reducing the volume of the cylinder, but since the exhaust valve is now open, the gas charge escapes into the exhaust. So the plateau represents the time during which the piston is pushing exhaust gas out and during this time the pressure in the cylinder should be nearly equal to the atmospheric pressure.

But as you can see on the attachment there is a bit of a bump (4.1psi). If the exhaust valve is open at this point what can cause this bump?

Another thing I picked up is that the plateau does not dip again before compression starts (purple circle). I do not know if it should dip on cranking only cause there is no vacuum created because of throttle closing.

Also, the Ford manual states that the difference between the cylinder pressures should not be more than 5 bar but says nothing about what the pressure should be. #1, #2 and #3 pressures (158psi) are almost exactly the same, however #4 is 192 psi. I cannot find a reliable source which tells me what the in cylinder pressures should be.

I have the same profile on all cylinders except for #3 which do not have the hump
Tenho um caso semelhante num audi gasolina

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