We learnt about the basics of spark plugs last week. Today, we’ll take a closer look at them, and at things that might not have occurred to you at first glance.
Spark plugs are used in spark-ignition engines. That is, in petrol engines. Diesel engines do not need them as they compress their fuel-air mixture until it explodes. This can happen sometimes in petrol engines too. It is then called ‘detonation’ or ‘knocking’. This can happen if you use the wrong spark plug for your engine. They can be divided into ‘hot’ plugs and ‘cold’ plugs, depending on their ‘heat range’. The heat range is simply the ability of the spark plug to extract heat from the cylinder and pass it on to the cooling system so that the engine is operating within its optimum operating temperature range.
Feeling the heat
The part of the plug that surrounds the electrodes (the bits of metal across which the spark makes the great leap) is made of ceramic, which is a bad conductor of heat. A hot plug will have more insulation, to keep the temperature of the plug high, and a cold plug will have relatively little insulation to keep the plug cool. Cold plugs help with high-performance engines, because they are always generating a lot of heat and need to conduct a lot of excess heat away from the engine.
Hot plugs are more useful in applications like small-capacity bikes, like our 100 cc commuters, because you’d expect the user to ride the bike for short periods of time, and hence need a spark plug that will heat up and reach optimum operating temperatures quickly. The ceramic insulator has to heat up enough, else deposits of carbon will build up on the plug, reducing its efficiency. If the ceramic insulation overheats, it’ll melt, and a whole host of problems will raise their ugly heads, leading to possibly expensive repairs.
The installation of a spark plug is also a bit of an art. They screw in like normal nuts do, but they also have to prevent any gas leaking from the cylinder. Since under compression, the gases in the cylinder can have incredible pressures, they have to be torqued enough. Torque them too much, though, and you run the risk of them snapping — and I bet every one of us has experienced the pain of trying to unscrew a nut or screw whose head has worn down so much as to offer no grip to the screwdriver or spanner.
Spare fuel or save plug
Another factor that helps combustion is the duration of the spark, or for how long the spark is fired. It makes logical sense to keep the spark going for as long as possible, so that the fuel burns completely, but the longer you make the spark’s duration, the faster the electrodes are going to wear out and the sooner you’ll have to replace or gap the plugs.
This might not be a problem with an engine that has its cylinders in a line or a vee, but if you own a Porsche (except for the Cayenne) or a Chevy Forester that have flat-four or flat-six engines, you’ll find that changing spark plugs can be a complex job!
The TVS Apache has a spark that lasts 1200 milliseconds, a lot longer than the usual 200, which helps its fuel economy. Another solution for a cleaner burn is to add electrodes to the plug, but that needs more voltage.
Spark plugs can tell you if you are running rich or lean. Ask your local mechanic to show you how.