High Compression
Discussion
Mr2Mike said:
bearman68 said:
Not directly, though the engine temperature gauge is reading OK, and this must be taken from the EC sensor.
Why must it? Plenty of cars either have two separate sensors, or four wire sensors that include two thermistors in the same housing.
I'd like to come up with a long fangled explanation that says it's not my fault, but......
I drained a 206 diesel that had been filled with petrol, and put it in a marked can...... and then used it to fill the ford.
Funnily enough, just today after it's been running fine, it started to misfire, and lo and behold one of the new plugs I fitted last week has gone belly up. Fitted the old one, and all good again.
I can't believe I was so stupid to fill the damn thing with contaminated fuel.
I drained a 206 diesel that had been filled with petrol, and put it in a marked can...... and then used it to fill the ford.
Funnily enough, just today after it's been running fine, it started to misfire, and lo and behold one of the new plugs I fitted last week has gone belly up. Fitted the old one, and all good again.
I can't believe I was so stupid to fill the damn thing with contaminated fuel.
I notice that the table and the equations have now been changed in Wikipedia, with no information on how the new table values are derived.
Auntieroll said:
The compression ratio of a PISTON engine is calculated thus:- The swept volume of the cylinder +the combustion chamber volume divided by the combustion chamber volume. V+v/v.
The definition of static compression ratio is not in doubt. The thread is about the relationship between static compression ratio and the measured pressure ratio when cranking.Mr2Mike said:
The definition of static compression ratio is not in doubt. The thread is about the relationship between static compression ratio and the measured pressure ratio when cranking.
I agree, but the way that the static compression ratio is calculated has ,so far not been mentioned in the thread.The Wikipaedia chart pasted in one of the above postings is for gas turbine engines, indeed a quick glance at the paragraph above the pasted equations from the Wiki entry specifically states this very fact.
I would think that any truly accurate correlation between the static C/R and the measured pressure when cranking is rather unlikely, the number of variables to be measured i.e. temps, atmospheric pressure, gas density etc; these equations are for "ideal gases" and laboratory conditions are required to make precise measurement possible.
A quick compression test, followed by a cylinder leakdown test should give a good idea of the condition of the top end though, however, the idea that a simple compression tester (calibrated accurately?)can determine the C/R of an engine is frankly laughable.
Auntieroll said:
I would think that any truly accurate correlation between the static C/R and the measured pressure when cranking is rather unlikely,
Agreed, as already been mentioned by a couple of other people. However, if you read Dave Walkers section in PPC he's had a few cars in to map that suffered from bad detonation with any reasonable amount of ignition advance and he used a compression tester to discover the engine builder had set the CR way too high (pegged the gauge on his tester).RealSquirrels said:
Boyles law states that the product of pressure and volume is constant in a sealed system when you change the volume, the pressure increases or decreases accordingly.
i.e. p1 x v1 = p2 x v2
Thus if you have a vessel with an internal pressure of 1 bar (roughly atmospheric) and decrease its volume by a factor of 14, the resulting pressure is 14 bar.
e.g. 1 bar x 14 cc = 14 bar x 1 cc
Obviously the real cylinder has a volume greater than 14cc but you see my point.
So yes, you can measure compression ratios by pressure changes.
But you are forgetting, as others have pointed out, that an engine cylinder is not effectively sealed until the valves close, which varies on different engines and can vary with cam timing. In general the inlet valve is still open when the piston is starting to make its way back up the cylinder on the compression stroke.i.e. p1 x v1 = p2 x v2
Thus if you have a vessel with an internal pressure of 1 bar (roughly atmospheric) and decrease its volume by a factor of 14, the resulting pressure is 14 bar.
e.g. 1 bar x 14 cc = 14 bar x 1 cc
Obviously the real cylinder has a volume greater than 14cc but you see my point.
So yes, you can measure compression ratios by pressure changes.
EDIT, looks like you deleted your post after I quoted you, sorry...
Edited by Mikey G on Saturday 30th November 10:42
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