SPARK IGNITION – COMPONENTS & FUNCTION

Engine is the device to convert heat energy of fuel into useful mechanical work. To use full potential of fuel to generate heat, full combustion of fuel is required which further needs proper ignition of the fuel. Poorly ignited fuel leads to loss of un-burnt fuel. So, engine ignition system is of high significance for getting maximum engine performance and efficiency. Engine ignition system has either of the following types.

· Spark ignition system

· Compression ignition system

Spark ignition system

In petrol engines where air fuel mixture is compressed in the cylinder and an external/additional element i.e spark plug is used to ignite the fuel and generate power stroke. When the air fuel mixture (charge) is compressed, a high voltage spark jumps across the gap in spark plug to ignite the charge. This jumping of spark should happen at a particular and predetermined time. If the spark occurs early while starting the engine i.e when piston is yet to reach TDC, it would tend to result into a reverse kick. However, when engine is running at full load, the spark is advanced to make fuel to have sufficient time to burn and produce maximum heat energy which would further be transferred by the piston to generate maximum mechanical power. Following are the components of electric system being used in spark ignition system.

Battery

It is required to produce electric current to the ignition circuit which comprises of distributor, ignition switch, ignition coil, condenser, breaker point, spark plug and a generator. The composition of battery is same as used in tractors for compression ignition and discussed in the previous lecture no. 29.

Distributor

It is a rotating device used to open and close the electric circuit between the battery and ignition coil which further supply current to the primary winding of the coil. Magnetic field is produced with this surging of current in the primary coil and then the distributor opens the circuit to collapse the magnetic field and generate the high voltage current in the secondary winding of the coil. The distributor rotor then guides this high voltage to the spark plug.

Ignition Coil

It is used as voltage transfer device which transfers the low voltage ( 6 – 12 volts) current drawn from the battery to high voltage ( around 20,000 volts) current which is necessary to jump the gap between spark plug. The ignition coil is made up of laminated core which has several hundreds turn of heavy wire as primary winding and the secondary winding consists of thousands of turns of fine wire. Primary and secondary windings are insulated from each other and are also sealed fully.

Condenser

To have sudden collapse of magnetic field (which is mandatory to generate high voltage current in the secondary coil, a condenser is provided which is further connected with the breaker point. This condenser also helps in avoiding spark being produced at the breaker points which may damage these points. As these contact points are being used continuously, the gaps (0.3 to 0.5 mm) between these contact breaker points are to be checked regularly and dressed up.

Generator

The battery need to be charged regularly for which generator is provided in the electric system of an engine. Generator is provided with cutout which makes the generator to get disconnected from the battery when the engine is running at very low speed. Generator is driven by a belt which is to be maintained at proper tension and bearing used should also be well lubricated.

Spark plug

This is used to generate the high voltage spark which is transferred from the tip of the spark plug to the engine combustion chamber. It consists of outer shell having threads and an electrode. It is available indifferent sizes as per the requirement of engine. As the point of spark plug is always exposed to the intense heat in the cylinder/combustion chamber, it may get damaged and carbon deposits are seen many times. These deposits are to be cleaned regularly and the gap should be kept between 0.5 to 0.85 mm. The spark plugs are to be tightened properly.

Magneto Ignition

In magneto ignition system there no battery being used but a magento is there to  generate its own small current which is further steps up to high voltage enough to jump over the spark plug. The spark should be produced at a right and predetermined time.  In magneto, a magnet is rotated so rapidly in a coil having turns of wire which generates the current just like it is produced in the battery ignition system. Similarly, a high voltage is also produced in the secondary winding of the magneto coil. A distributor is being used to carry this current to the spark plug. Magneto points are to maintained properly as per the service schedule prescribed by the manufacturer.