The spark plug ignites the air/fuel mixture in the combustion chamber of a gasoline engine by means of an electrical spark. During operation spark plugs become soiled and worn. The resulting larger electrode gap means a higher ignition voltage is required. If the required ignition voltage is greater than the voltage that can be supplied by the ignition coil this will cause the engine to misfire. For this reason, spark plugs need to be replaced on a regular basis.
On Bosch spark plugs the electrode gap is set during the production process. It is not usually necessary to reset the electrode gap when replacing spark plugs. You can find the electrode gap specifications for the vehicle application you require in the Bosch spark plug catalog. The spark plug electrode gap is stated on the packaging.
If the spark plug is not appropriate for the application this can cause pre-ignition or auto-ignition. For this reason, it is important to choose the right spark plug using the catalog.
Pre-ignition (auto-ignition) is an uncontrolled ignition process, in which the temperatures in the combustion chamber rise to such an extent that severe damage is caused to the engine and the spark plug. In the full throttle range, auto-ignition can occur at the following sites:
If the spark plug is not appropriate for the application this can cause engine knocking. For this reason, it is important to choose the right spark plug using the catalog.
Engine knocking refers to uncontrolled combustion with a rapid increase in pressure. Uncontrolled combustion of this type is caused by self-igniting parts of the mixture that are ignited without a spark. Combustion takes place significantly faster than for normal combustion. Pressure fluctuations take place with high peak pressures and high frequencies above the normal pressure profile. When the combustion pressure hits the walls of the combustion chamber, a metallic knocking noise is heard. If the problem is not remedied promptly, serious damage to the engine can occur.
The heat range is a measure of the thermal rating of a spark plug. It indicates the maximum thermal capacity that arises on the spark plug in the equilibrium between thermal absorption and supplied heat.
If the power of the engine is modified, e.g. by fitting a turbocharger, a blower system or other systems, the recommended spark plug cannot be used. Ideally, a spark plug with a colder heat range should be used in order to compensate for the additional heat generated by the engine. Bosch recommends seeking the advice of a qualified engine builder when determining the correct heat range.
In general, the change interval for spark plugs is indicated in the vehicle manual. To ensure optimum engine performance Bosch recommends that spark plugs are checked once a year and replaced if necessary.
The demands on a spark plug are electrical, mechanical, chemical and thermal.
The spark plug must function under extreme conditions. Voltages up to 40,000 V are produced during ignition and temperatures of more than 1,000 °C arise in the combustion chamber. Effective insulation is an absolute requirement.
Spark plugs must be able to withstand variable pressure of around 100 bar in the combustion chamber. They must have a high level of reliable mechanical strength in any form of operation.
Resistance to chemicals in the combustion chamber and aggressive (corrosive) residues from the combustion process.
Thermal impact resistance (hot exhaust gases and cold intake air/fuel mixture). Effective heat dissipation by the insulator and the electrodes is a prerequisite for reliable operation.