The Ignition System in a petrol vehicle ensures that each cylinder gets a spark through the Spark Plug at the right time when the piston is about to complete its compression stroke. This spark ignites the air-fuel mixture and hot expanding gases produced by this combustion of air-fuel mixture provide the power to push the piston into what is called the “Power Stroke”.
The ignition system controls when and how each spark plug will get the current to produce the spark. The critical components of a distribution system are a Ignition Coil, Distributor, High Voltage Spark Plug Leads and a Spark Plug.
The Ignition Coil builds up a high voltage which is discharged into the Ignition system through a rotor in the distributor when the Contact Breaker points open up. The opening and closing of the contact breaker points is governed by a camshaft that is driven off the Over Head Cam Shaft. When a cylinder is just about to complete it is compression stroke, the cam-shaft in the distributor opens the contact breaker points. At the same time the rotor in the distributor cap is opposite the electrode connected to the Spark Plug in the same cylinder via Spark Plug Leads. As soon as the contact breaker points open up the high voltage current from the Ignition coil jumps across the rotor tip and the electrode connected to the particular cylinder’s spark plug. This high voltage current then reaches the spark plug electrode where because of its high voltage it jumps across the gap between the Spark Plug electrode and ground terminal, thus creating a spark. This spark ignites the air-fuel mixture which is nearing its full compression within the cylinder by the upward moving piston. The gap between the Spark Plug center electrode and the L shaped ground terminal of the Spark plug determines how fat and powerful the spark would be when the high voltage current is supplied. The gap is specified by the manufacturer of the vehicle for each engine.
The time at which the Spark Plug fires or the high voltage current jumps between the electrode and the ground terminal is called Ignition Timing. It is measured as number of degrees before the piston reaches the top dead center (BTDC). The system is so designed that it fires a couple of degrees before the piston reaches its TDC (the top most point that the piston can travel to in the cylinder). This ensures that the while the spark starts igniting the air-fuel mixture the piston is still in it up-ward stroke and by the time the combustion is completed and the hot gases start expanding to push the piston downward the piston is already at its TDC. This ensures complete combustion and maximum efficiency. Retarding the timing will lead to incomplete combustion and part of the air-fuel mixture will remain un-burnt and will be expended in the exhaust. This will lead to higher pollution as well as lower fuel efficiency. Advancing the timing may lead to knocking or pinging. This is a metallic sound that emanates from the cylinder during acceleration. This happens because the advanced timing means that the spark is generated much before the piston reaches its TDC and therefore the combustion of the air-fuel mixture is completed before the piston completes its compression stroke. Since the combustion is completed before the piston reaches TDC the hot expanding gases start exerting force on the piston opposing its up-ward motion. This causes the knock or the ping. This phenomenon can harm the engine and piston if it continues for a sustained period of operation.
The mechanical distributor based ignition system requires regular maintenance for the wear tear of the cam based system which opens and closes the contact breaker points.
When this mechanical system is changed by the Electronic Distributor-less Ignition System all the moving mechanical parts of the Distributor type system are done away with. Instead the signal to provide high voltage current to the Spark plugs is provided by Crank Position Sensor and the Cam Position Sensor. Often enough the single Ignition coil is replaced in such a system by individual Coil on Plugs (COP)which give more time to the coil between two successive discharges of high voltage current ( could be in excess of 1000 volts). This is a very efficient ignition system that does not rely on mechanical parts and therefore is maintenance free as well as very accurate. Hence EFI and Electronic Distributor-less Ignition Systems make the engine more efficient, less polluting and more powerful and responsive.