Engine in-cylinder pressure validation

[Fadzli]

Hi all,
I’m currently investigating the engine in-cylinder pressure by simulation and experimental method. All inputs parameters in the simulation were exactly follow the engine technical specification (engine geometry, compression ratio, fuel injection parameters, initial pressure and temperature etc.) When both data were plotted, they are significantly different (please refer to the attached picture). Based on my literature study, most of the pressure vs crank angle graph are almost similar to my simulation result. So any idea why this is happen? Or at least what else parameters need to consider in order to validate these two results?

 

[JBA]

Are you doing your test by hand cranking a cold engine?

 

[Jac5522]

Your experimental plot does not seem to take valve event timing and charge momentum into consideration. Cylinder pressure will not start to rise significantly until the intake valve closes and cylinder pressure does drop off quicker than piston speed on the power stroke which is why exhaust valve timing opens the valve before bottom dead center.

 

[JBA]

Are you modeling a 2 stroke or 4 stroke engine?

What I am seeing is the opposite of the above valuation with the test indicating a sooner initiation of pressure rise than your simulation indicates.

Assuming your crank rotation to piston displacement calculation is correct, this would indicate an additional source of pressure rise during the compression stroke. Are you accounting for adiabatic compression heating in your simulation calculations? This is not an isothermal compression process.

Also, to repeat my original question, what is the method and engine conditions under which you are getting your compression test data. The basis of this question is that on a hot engine the compression pressure rise will be enhanced by heat transfer to the enclosed gas from surrounding preheated engine components.

 

[Fadzli]

Thanks JBA and Jac5522,
This is a modelling for 2 stroke engine and I already take into consideration of heat transfer inside cylinder. But my initial discussion with the engine contractor, we found that most probably the crank rotation to piston displacement calculation was not correct which is due to encoder reading. The encoder was installed not directly connected with engine shaft, but only with belting which is assumed power lost will be there. I hope this assumption is true and I will try to fix this first.
But anyway, thanks again for the advice.

 

[Mech_Engineer]

As Jac and JBA indicated, your simulation prediction doesn't seem to take into account intake/exhaust events or fuel mixture ignition. How are you taking these confounding factors into account?

Your captured data looks similar to an expected result from a 2-stroke diesel engine, see below image:

http://www.marinediesels.info/Theory/actual_diesel_cycle.htm

 

[Fadzli]

Thanks Mech_Engineer,
My simulation was not predict for the whole engine cycle but only from intake valve closing (IVC) to exhaust valve opening (EVO) which mean only in close-cycle. But the software is capable to simulate detailed chemical reaction process (in CHEMKIN format), fuel spray modeling, combustion modeling and also turbulence modeling. So I believe that by taking all those consideration, the results should be accurate enough. Instead of simulation result, I think experimental result was more doubtful. Correct me if I'm wrong.
Thanks

 

[Mech_Engineer]

What leads you to think the experimental result is wrong?

 

[Fadzli]

I'm not saying it's totally wrong, but based on my literature, all the references paper/journal reported that for crank angle between -150 degree to -50 degree, the in-cylinder pressure would be not more than 1 MPa. And also, by referring to my previous answer, most probably there is problem with the encoder which record all the crank angle data in my experiment. So maybe I need to verify this first.

 

[cjl]

I'm more inclined to believe your experimental data is accurate, since the simulation result seems surprisingly featureless, especially the lack of a pressure spike due to combustion. The experiment seems to agree fairly well in general shape with the plot from Mech_engineer, on the other hand. What kind of 2 stroke is it - is it a small 2 stroke? Large 2 stroke diesel? Turbocharged? Tuned exhaust?

 

[Fadzli]

It is a small 2 stroke with 85mm x 87mm (bore x stroke) without turbocharged or tuned exhaust

 

[Mech_Engineer]

Do you have any examples in literature which show a similar simulated pressure curve to your result?

 

[Fadzli]

Here are 2 examples of articles with quite similar engine condition for comparison. Both showing low in-cylinder pressure (less than 1 MPa) in the earlier or later combustion process.

 

[Mech_Engineer]

You should confirm that the encoder was reading your angular position correctly, but overall the "shape" of the curve seems reasonable. I agree that the pressure rise so early in the cylinder's travel seems suspicious, but if the encoder checks out maybe something else is happening...

 

[Fadzli]

Yes, agreed. Now working to check the encoder. Thanks