Spark plugs are actually one of the weaker parts of the ignition system. It needs to work under severe conditions, mounted on top of the combustion chambers. In modern car engines, the temperature may reach 2500 degrees Celsius and pressure at about 7Mpa. Spark plugs need to withstand the high temperature and pressure. Other than that, plugs also have to endure corrosive atmosphere, mechanical vibrations and high voltage. Modern spark plugs could last up to 20,000km before being replaced. When replacing spark plugs, there are a number of requirements that have to be met.
- Excellent resistance to current leakage
- Continued closure of gaps under all conditions
- Good gas tightness. Leaked hot gas could disrupt the normal state of the plugs. At a very high temperature, the electrode and insulators of plugs could literally disintegrate.
- Corrosion resistance: Corrosion could increase the length of the gap, making plugs to work improperly.
- Proper reaches into the combustion chamber. In this case, reach is defined as the length of the threaded portion of the plugs that enter the engine cylinder. If we use long-reach plugs for short-reach hole, there will be a reduction in the space of the combustion chamber. In this case, more heat can be produced, the compression ratio will increase and the edges of the threaded portion will reach a very high temperature, causing pre-ignition. Also, if we use short-reach plugs into long-reach holes, the compression ratio will be reduced due to the increased space in the combustion chamber. It means that we will have carbon deposits, causing different kinds of problems.
- Ideal electrode temperature: The temperature of the electrodes should be ideal. Corrosion of the electrodes and risks of preignition may increase when the temperature goes beyond 850 degrees Celsius. If the temperature is lower than 400 degrees Celsius, we could find the formation of oil and carbon deposits. This will cause the leakage of current, degrading the engine performance. The optimum range of temperature should be between 500 to 800 degrees Celsius.
We also need to consider the proper construction of the spark plugs. There are multiple parts of spark plugs, including insulation, ground electrode and center electrode. There are also gasket washers, sealing ring and body shell. It is important that spark plugs have solid construction. Check gas tight seal, which is needed to prevent the leakage of hot gas. There are different types of seals, such as metal powder that’s fused into glass, dry powder and solid ring.