Can anyone tell me what the difference is between Compression ratio and cranking pressure?
COMPRESSION RATIO:
Is the ratio between the volume of ALL the space inside ONE cylinder when the piston is at BOTTOM dead centre, compared to when the piston is at TOP dead centre.
In a TD5, 2,500 cc, 5 cylinders, each cylinder is 500cc swept volume.
If the compression ratio was 19 to 1, then the 500cc PLUS THE VOLUME OF THE COMBUSTION CHAMBER has to be squashed into a ninteteenth of the original total space.
So, the combustion chamber would be about 28 cc, thus the total is 528cc, squashing into 28cc, which is near enough one ninteenth the original total. It is a HELL OF A SQUEEZE! Half a litre of air squashed into half an egg-cup!
A compression ratio of 19:1 would theoretically provide a compression pressure of about 280 psi (pounds per square inch) but .....
CRANKING PRESSURE is the ACTUAL pressure achieved in the cylinder at starter motor speed, usually about 100 rpm.
Because of valve timing and overlap, the pressure would be expected to be LOWER than the theoretical, BUT because the air is being compressed it HEATS UP, causing the air to try to expand, but there is nowhere for it to expand to, so the pressure goes up instead.
In practice, as long as the engine isn't VERY cold, diesel engine cranking pressures will be much the same as the compression ratio times 14.7 psi.
CharlesY
