Now that we know how long one degree turn of the crankshaft takes, how far the spark flame must travel to completely ignite all of the air/fuel mix, and the speed of the flame propagated, let's start putting pieces of the puzzle together...


The key to making power is timing when the combustion event creates the most downward force upon the piston top. The expansion of combusted air and fuel pushes down upon the top of the piston for the power stroke. This downward force turns the crankshaft and propels your car forward.

All of the air-fuel mix inside the combustion chamber must be ignited for the biggest explosion to occur.


When is the best time to have the combustion event completely finished?:

Below is a graph looking at what happens to cylinder pressure as the crankshaft rotates.

On the X-axis is crankshaft degrees in which zero degrees is Top Dead Center (TDC) or the top most point of the piston's travel as it moves up and down the cylinder.

On the Y-axis is cylinder pressure as the piston is squeezing the air-fuel mix through to the time the combustion event is finished.

The graph shows 5 consecutive combustion events in the same cylinder at the same engine settings. The spark is ignited in this Mercedes diesel engine at 30 degrees BTDC on the far left (-30 on the X-axis).



Notice that there is variation in peak cylinder pressures between each of the 5 combustion events , even within the same engine and igntion timing, fuel injector timing, and cam timing. This can be caused by variations in how the air:fuel mix fills the cylinder (mixture quality) and the mixing in the cylinder during each intake stroke.

The dotted line at the very bottom represents the cylinder pressure if there was no ignition and lighting up the air/fuel mix. It shows you the level of the cylinder pressure from the piston squeezing air/fuel mix alone without combustion. This is what you measure when you perform a compression test. It is also called the "cranking pressure". Clearly, the combustion event spikes the cylinder pressure much higher than the cranking pressure.
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