The Reminder is making its archives back to 2003 available on our website. Please note that, due to technical limitations, archive articles are presented without the usual formatting.
Burnin' Grass Using the example of the burning patch of grass, we can say the match (spark plug) starts the fire (combustion), the wind (squish) blows to accelerate and feed the flame until the grass (air/fuel) mixture is consumed. Good things can happen during this process, but some very bad things can occur also. If all goes well, the burn rate will provide peak cylinder pressure at or about 15 to 20 degrees past top dead center, the piston will be pushed down with maximum available energy and the engine will run as planned. Change a few of those parameters and you're reaching for the wallet. If the spark is lit too soon, peak pressure is reached too soon, causing excessive loads on the pistons and rods robbing horsepower. Problems of detonation, pre-ignition and excessive piston and cylinder wall temperature can grow and eventually lead to failure of critical engine components. On the other side, if the spark is lit too late, peak pressure is past and energy is wasted because the exhaust valve or port is going to open when there is still energy available. Now it is diverted into the exhaust system doing no real good at all. All this energy and fire needs to be controlled. Spark timing becomes real critical in making things happen on time. Spark timing is usually set to start combustion sometime before the piston reaches top dead center, and is usually adjustable for a good reason. Fuel burns at a given rate and as rpms increase, the linear speed of the piston increases, reducing the amount of time for the combustion process to occur. Therefore, if we want to control when peak pressure occurs, we need to continue to advance the timing as rpms increase to get peak pressure at or just after top dead center.
