Ignition System Timing

     The ignition system on your car has to work in perfect concert with the rest of theengine. The goal is to ignite the fuel at exactly the right time so that the expanding gasescan do the maximum amount of work. If the ignition system fires at the wrong time,power will fall and gas consumption and emissions can increase.

    When the fuel/air mixture in the cylinder burns，the temperature rises and the fuel isconverted to exhaust gas. This transformation causes the pressure in the cylinder toincrease dramatically and forces the piston down.

    In order to get the most torque and power from the engine, the goal is to maximize the pressure in the cylinder during the power stroke.  Maximizing pressure will alsoproduce the best engine efficiency, which translates directly into better mileage. Thetiming of the spark is critical to success.

    There is a small delay from the time of the spark to the time when the fuel/airmixture is all burning and the pressure in the cylinder reaches its maximum. If the sparkoccurs right when the piston reaches the top of the compression stroke, the piston willhave already moved down part of the way into its power stroke before the gases in thecylinder have reached their highest pressures.

    To make the best use of the fuel，the spark should occur before the piston reaches thetop of the compression stroke，so by the time the piston starts down into its power strokethe pressures are high enough to start producing useful work.

Work＝Force X Distance

    In a cylinder：

Force＝Pressure X Area of the piston

Distance“Stroke length

    So when we're talking about a cylinder, work＝pressure X piston area X stroke length. And because the length of the stroke and the area of the piston are fixed，the onlyway to maximize work is by increasing pressure.

    The timing of the spark is important, and the timing can either be advanced orretarded depending on conditions.

    The time that the fuel takes to burn is roughly constant. But the speed of the pistonsincreases as the engine speed increases. This means that the faster the engine goes，theearlier the spark has to occur. This is called spark advance：The faster the engine speed is,the more advance is required.

    Other goals，like minimizing emissions，take priority when maximum power is notrequired. For instance，by retarding the spark timing (moving the spark closer to the topof the compression strokei"），maximum cylinder pressures and temperatures can bereduced. Lowering temperatures helps reduce the formation of nitrogen oxides（NO}），which are a regulated pollutant. Retarding the timing may also eliminate knocking；somecars that have knock sensors will do this automatically.