Compression Pressure

[BraindG]

before the AA man comes tomorrow.. does anyone know what the compression pressure should be?

please!...

[pickigar]

Don't know the pressure yet - trying to work it out but the ratio is supposed to be in the region of 8.5 to 1

[BraindG]

most turbo cars are 160-175psi/12bar
n/a are higher

INSERT INTO post VALUES (ive just learnt)

[BraindG]

12.5 bar = 181.2971725 pound-force/square inch INSERT INTO post VALUES (PSI)

160 pound-force/square inch INSERT INTO post VALUES (PSI) = 11.0316116 bar
175 pound-force/square inch INSERT INTO post VALUES (PSI) = 12.0658252 bar

[Roadrunner]

So what does the compression ratio of 8.5:1 actually mean? :confused:

[Kieran]

Compression Ratio refers to how much the fuel/air mix is squeezed when it's in the cylinder. The number refers to the initial volume of air, versus it's compressed volume - so an engine with a compression ratio of 12:1 squeezes the air to 1/12 of it's original volume before ignighting it.

The more the mixture is compressed, the more power is released when it is ignighted, which generates more power and torque.

So, why aren't all engines ultra-high compression? Mainly because the harder you compress the mixture, the hotter it gets, which can lead to the dreaded, piston mangling pre-ignition, where the mixture ignites before the piston is on it's downward stroke - Effectively the expanding mixture and the piston collide when this happens, weakening the piston top and sending a shock through the conrods, bearings and crank - Bad news!

Cylinder temperature is also why turbocharged cars run with lower compression ratios. As a turbo compresses air into each cylinder, it raises the temperature of the air, and at normal 9:1 or 10:1 ratios, the increased temperature of the air entering each cylinder would be likely to cause pre-ignition.

That was an excerpt from Kieran's book on how cars work. If you wish for more, please ask;)

[Roadrunner]

Thanks, Kieran. I feel informed now :)

[stonegr]

Originally posted by Kieran
Compression Ratio refers to how much the fuel/air mix is squeezed when it's in the cylinder. The number refers to the initial volume of air, versus it's compressed volume - so an engine with a compression ratio of 12:1 squeezes the air to 1/12 of it's original volume before ignighting it.

The more the mixture is compressed, the more power is released when it is ignighted, which generates more power and torque.

So, why aren't all engines ultra-high compression? Mainly because the harder you compress the mixture, the hotter it gets, which can lead to the dreaded, piston mangling pre-ignition, where the mixture ignites before the piston is on it's downward stroke - Effectively the expanding mixture and the piston collide when this happens, weakening the piston top and sending a shock through the conrods, bearings and crank - Bad news!


Cylinder temperature is also why turbocharged cars run with lower compression ratios. As a turbo compresses air into each cylinder, it raises the temperature of the air, and at normal 9:1 or 10:1 ratios, the increased temperature of the air entering each cylinder would be likely to cause pre-ignition.

That was an excerpt from Kieran's book on how cars work. If you wish for more, please ask;)

Nice info !!!!
And what do we mean when we say the a/f is lean or reach ?

[enigma]

OK.....you asked!

There is an optimal air fuel ratio for combustion. This is the ratio that the Engine controller tries to maintain. It does this in 2 ways. First of all it is programmed with a base set of numbers to determine how much fuel is required taking in to account all of the mechanical conditions of the engine like throttle position, RPM, etc. This is sometimes referred to as the base map. This is then adjusted for temperature and other environmental variables which means that the amount of fuel is altered from that determined by the base map. To monitor how well INSERT INTO post VALUES (or otherwise) the engine controller is doing the value read from the Oxygen sensor in the exhaust is used to further modify the amount of fuel that is introduced to the engine. It does this by measuring the exhast gas to see if the combustion process has completely burnt all of the fuel. This is all done probably 10s of thousands of times per second!

If we are running lean, this means that there is not enough fuel for the amount of air drawn in to the engine. This could be at very high rpm/boost levels on the VR4 when the fuel pump and fuel system cant keep up. THIS WOULD BE VERY BAD! As the amount of fuel decreases more inefficient combustion takes place and the temperature of combustion rises. If left unmonitored you will very quickly melt your pistons and face a stoopid bill for repairs!

If we are running to rich, there is too much fuel for the amount of air drawn in to the engine. This is to some extent less severe than running lean as it gives rise to unburnt fuel in the exhaust and cylinder heads. This will cause carbon buildup on your valves and if left unmonitored will slowly choke your engine. Running very much too rich will start to cause similar results to a lean condition, but in general running rich is considered better than running lean!

Hope this helps..........just got in to one there didn't i!!!!

[stonegr]

thanx bigdaveakers :)