The spark is ignited in the figure shown at 30 degrees BTDC (-30 on the X-axis to the far left).

At about 10 degrees BTDC on the graph (-10 on the X-axis), the igniting of the air-fuel mix is finished and the air-fuel mix starts to increase cylinder pressure from the expanding force of the combustion event. Cylinder pressure rises, as the burning air-fuel mix expands and pushes down on the piston top.



The greatest force or highest cylinder pressure occurs at 20-25 degrees After TDC (ATDC) [+20-25 degrees on the x-axis]. This is the finish of the combustion event.

You want to "time" the start of the igniting process so that the combustion is finished (the highest peak cylinder pressure) when there is a good connecting-rod-to-crankshaft angle.


What do I mean by that?:


What is a "good" Connecting Rod to Crankshaft Angle for making more power? When should peak cylinder pressures happen?

Cartoon of a Piston ( [O] ) and Connecting Rod ( I ) On Side View:

1. ...........[O]............... 2. [O]
.................I..................... /

.........Rod @ TDC...........Rod @ 12-15 degrees ATDC


If the force of the explosion (or peak cylinder pressure) occurs at TDC, the connecting rod is straight up and down or "in line" vertically with the piston (1. in the above cartoon figure). A downward force or push by the explosion on top of the piston at this time does NOT help turn the crank. The force just travels down the rod vertically.

If the highest cylinder pressure occurs later ATDC (2. in the above cartoon figure) , the connecting rod to crankshaft angle is more ideal. The downward force or push of the explosion on the piston top helps turn the crankshaft due to the rod not being straight up and down or in line vertically with the piston. The rod is travelling clockwise and downward already at 12-15 degrees ATDC from inertia and an additional "push" from the peak cylinder pressure would accelerate it faster in this direction. As a result, the force turning the crank would be greater.

Most people say that the downward push should be happening no earlier than 12-15 degrees ATDC (+ 12-15 degrees on the X-axis) so that there is a mechancial advantage of turning the crank.


The time at which you START the ignition of the air-fuel mix influences when the combustion event finishes and where peak cylinder pressures occur.

You want the peak cylinder pressure or greatest downward push to occur at a time when the rod is at a good angle relative to the crankshaft.

This ensures that you get the most out of the downward force from the explosion to turn the crank.

Cont....