Running Compression Question.

[George7941]

My question arises from this video by Pico Technology
http://www.youtube.com/watch?v=Kwy72aaAFvo

Towards the end of this video the engine throttle is opened wide for a brief time to get maximum cylinder compression pressure and the maximum pressure obtained is just under 25 bar. 1 bar is approximately 14.5 psi, so 25 bar = 362psi.

Relative compression at WOT is supposed to be roughly 80% cranking compression. 80% of 200psi is 160psi.

Why is there this huge discrepancy between 160psi and 362psi?

 

[supton]

I scanned thro the vid. Sounded like the engine was running. Not a static compression test! But rather peak cylinder pressure, after ignition.

 

[MolaKule]

Was the engine under full, half load, or no load, and does it have a turbo?


Engine Compression Modeling

 

[George7941]

Originally Posted By: supton
I scanned thro the vid. Sounded like the engine was running. Not a static compression test! But rather peak cylinder pressure, after ignition.


The injector for the cylinder under test was disconnected, plug wire for that cylinder was pulled off the plug and grounded (to avoid damage to ignition system). Spark plug was removed and a Pico pressure transducer was installed in the spark plug hole to record instantaneous pressure. All mentioned in the video. You are right in that the engine was running but it was running on the remaining cylinders.

 

[George7941]

Originally Posted By: MolaKule
Was the engine under full, half load, or no load, and does it have a turbo?


Engine Compression Modeling



The throttle was opened fully for an instant. So, no load except inertia.

I thought about a turbo but even turbocharged engines don't end up with 25:1 compression ratio which is what 25 bar translates to.

The ENGINE COMPRESSION MODELLING study linked to is studying pressure in a cylinder where fuel is burning.

 

[Rand]

compression wave.

 

[punisher]

Always remember the cranking compression is dependant upon more than just the "static" compression of BDC cylinder capacity/TDC chamber capacity.

Things such as cam timing, and the inertia of the incoming air and exhaust scavenge, make the higher RPM compression higher than cranking compression just because the VE is increased.

That being said, even a NA F1 engine tops out at around 140% VE, so where that 300+ PSI is coming from, I don't know.

 

[Shannow]

Isentropic compression PV^gamma = constant...gamma is ratio of specific heats

therefore P1V1^gamma = P2V2^gamma

P2 = P1 (V1/V2)^gamma

CR = (V1+V2)/V2...play some number games.

P2 = P1 (CR-1)^gamma

gamma's around 1.3, set P1 at a bar, make C.R. 11

P2 = 20 bar...simple example, change the numbers to suit.

Cranking isn't isentropic, as it's too slow, and allows heat leakage (e.g. diesels). Cranking also has cam timing effects, in that the inlet closes after BDC, and that can be a significant waster of volumetric efficiency at cranking speed.

For more fun, plunk the answers from the isentropic compression calcs into the gas laws

P1*V1/T1=P2*V2/T2

and see how hot it is before ignition...

 

[George7941]

Originally Posted By: Shannow

For more fun, plunk the answers from the isentropic compression calcs into the gas laws

P1*V1/T1=P2*V2/T2

and see how hot it is before ignition...


Ok, here goes.

P1 = 1bar
P2 = 20bar
V1 = 1
V2 = 1/11
T1 = 20 C

1*1/20 = 20*(1/11)/T2
T2 = 20*20/11 = 400/11 = 36 C, not hot at all.

I suspect T is in Kelvin (as is common in thermodynamics), so let us try again
T1 = 293
1*1/293 = 20*(1/11)/T2
T2 = 20*293/11 = 533 K = 260 C

Now, that is hot!

To put in non-mathematical terms, the compressed air heats up during the process of compression and this heat leads to the high pressures in the Pico video.

 

[Shannow]

George, good work...

It does get hot

Speaking of 500F

 

[dustyroads]

Originally Posted By: Shannow
George, good work...

It does get hot

Speaking of 500F





That is cool...compression ignition goes back a couple hundred years to Malaysia??

 

[Shannow]

Yep...it also happens inside hydraulic rams, and sometimes in bearings when an air bubble gets squeezed rapidly.

If you can imagine the same piston being pressed slowly down so that as the temperature rises, it is dissipated, the peak pressure and temperature are nothing compared to a rapid "bang"

 

[dustyroads]

I can picture that..(the slow motion heat dissipation) but never thought about it in hydraulics or bearings. Very interesting.