Correct ignition timing is important to ensure good engine performance and fuel economy. Incorrect timing causes engines to work less efficiently, reducing power and increasing fuel consumption and exhaust gas emissions.
To make the most efficient use of fuel and to generate the most power, it is important that spark plugs fire at the proper time during a piston’s compression stroke. If a spark plug fires too early, the piston is still on its way up, and the combustion process interferes with its progress. If a spark plug fires too late, the piston is already on the way down, generating less power and wasting fuel.
Because of the time it takes the fuel and air mixture to completely burn after ignition, the optimum time to fire the spark plug is prior to the piston reaching the top of its stroke. With proper timing, the maximum pressure inside the cylinder caused by the combustion process occurs just as the piston completes its stroke and is started back down. This results in the most efficient use of fuel, extracting the most power from the combustion process. If the goal is to minimize emissions instead of maximizing power, adjusting the timing so that the spark occurs closer to when the piston reaches the top of its stroke results in lower pressures and temperatures and a corresponding reduction in nitrogen oxide formation.