Compression vs. Compression Ratio

[Silvertip]

Re: Compression vs. Compression Ratio

Yup - brain phart!!!

 

[bruceb58]

Re: Compression vs. Compression Ratio

I don't think anyone was talking about diesel engines.

I stated in my post:

Doesn't matter if its a diesel engine or gasoline engine.

Read this:
http://en.wikipedia.org/wiki/Adiabatic_process

And then go read the example called Example of adiabatic compression and it will show you the reason.

am I going to have to start dreaming about adiabatic compression cycles again???

Yes! Brings back memories for me too when I was a Chem major!

 

[John_S]

Re: Compression vs. Compression Ratio

I have seen many posts stating that their compression is ~210 psi and I think they think this relates to a high compression ratio........however I do not.

In most cases >200psi has indicated a non-stock engine build. Many times it has been a flat-top piston used with 64cc heads. While you can't accuratly derive the comp ratio from psi, it is an indicator.

There are at least two active threads going on this. One has >200psi and has indicated the above being true. The other (Rapido 4.3L), has not been determined what was the build, yet. Another suspect on that build is cam timing. If the cam timing is retarded, or an oddball cam, it would have a higher dynamic compression than stock. ie intake valve closes sooner.

 

[gbeltran]

Re: Compression vs. Compression Ratio

Atmospheric pressure, nominally 1-BAR, is about 14.7-PSI. (at sea level) So if you divide your cylinder pressure readings by 14.7, you will get the compression ratio.(Roughly)

Cylinder pressure (psi) / 14.7 = Ratio

210/14.7 = 14.29:1

I also see in all those posts of people that are stating they have a compression reading that high (4 stroke Mercruiser/Volvo Penta) are being told to get a new gauge, replace the heads or get dished pistons...

As stated above this doesn't work. It doesn't because the cranking compression is dependent on the intake closing point as well as the static compression ratio. An example, a 383 I currently run has a 155-165 psi reading on a gauge. It has 9.8:1 static compression ratio. If I put a bigger cam in the psi readings will go down. If I put in a smaller cam they will go up.

 

[Aloysius]

Re: Compression vs. Compression Ratio

My point is..compression pressure vs. compression ratio is NOT a straight-line function. In a diesel the pressure/temperature goes so high it will automatically combust.

The factors which affect cranking PRESSURE are the compression ratio, cam specs including valve overlap, cylinder leakage (leakdown), engine and air temperature, and to a minor degree, cylinder head material..aluminum conducts heat faster than iron.

 

[mkast]

Re: Compression vs. Compression Ratio

http://www.youtube.com/watch?v=p-vPrLPk_jw

 

[gbeltran]

Re: Compression vs. Compression Ratio

My point is..compression pressure vs. compression ratio is NOT a straight-line function. In a diesel the pressure/temperature goes so high it will automatically combust.

The factors which affect cranking PRESSURE are the compression ratio, cam specs including valve overlap, cylinder leakage (leakdown), engine and air temperature, and to a minor degree, cylinder head material..aluminum conducts heat faster than iron.

I dont know about the cylinder head material affecting cranking pressure. It will allow you to build a motor with a higher static/dynamic compression ratio because it will resist detonation, but by itself has nothing to do with cranking compression. Valve overlap not so much either, it's more dependent on the intake valve closing point.

 

[Aloysius]

Re: Compression vs. Compression Ratio

Overlap, and/or lobe center, allows part of the compression to exit the exhaust.

Cylinder head..aluminum conducts the heat away from the combustion chamber quicker. The more heat retained in the head, the more power FROM the engine.

Engines are only about 30% efficient at converting heat to energy. Retaining heat and producing work is the trick.

The ideal engine would be one that runs at the same temperature as the temperature of combustion..over 1000 degrees fahrenheit. NO loss of thermal energy to the engine itself..only out the exhaust.

 

[Moggy23]

Re: Compression vs. Compression Ratio

Determinating a compression ratio from a compression test can be tricky as mentioned earlier in this thread due to cam shafts and timing.

A high mileage Jag V12 eng. Cam per bank with 11.5 cr came up with compressions of one bank being 210 psi and the other 175 psi.
Cam timing on one bank 210 psi was 4 deg. retarded
and the other 175 psi was 8 deg. retarded.
Retimed cams to zero and the result was 235 psi on both banks.

The later the inlet valve closes, less psi and low down torque.
The earlier the inlet valve closes, more psi and low down torque.
This has it limits too.


Generaly stock motors with around 150 psi is usally around 9-1

 

[John_S]

Re: Compression vs. Compression Ratio

Overlap, and/or lobe center, allows part of the compression to exit the exhaust.

The exhaust valve will not be open for any part of the compression stroke, or if it is, that is your problem

 

[gbeltran]

Re: Compression vs. Compression Ratio

Overlap, and/or lobe center, allows part of the compression to exit the exhaust.

Cylinder head..aluminum conducts the heat away from the combustion chamber quicker. The more heat retained in the head, the more power FROM the engine.

Engines are only about 30% efficient at converting heat to energy. Retaining heat and producing work is the trick.

The ideal engine would be one that runs at the same temperature as the temperature of combustion..over 1000 degrees fahrenheit. NO loss of thermal energy to the engine itself..only out the exhaust.

Again, what does cylinder head material have to do with cranking compression? Just trying to understand the though process. And overlap, as stated in the above post, will not affect cranking compression. It's the intake closing point that will. Overlap, at high rpm, can increase the efficiency of cylinder filling, but it will by no means influence a compression test.

If I take an engine with iron heads, take a compression reading, and replace them with a set of aluminum heads that have the same size combustion chamber and similar style of valve heads, and do another compression check it will be the same.

 

[bruceb58]

Re: Compression vs. Compression Ratio

If I take an engine with iron heads, take a compression reading, and replace them with a set of aluminum heads that have the same size combustion chamber and similar style of valve heads, and do another compression check it will be the same.

+1 ^^^^ Agree. At least for a compression test, the heat isn't drawn away fast enough to affect that singular test.