We’ve learnt that petrol engines ignite the fuel-air mixture in the cylinder with the help of a spark, unlike diesels, which squeeze the mixture of air and diesel oil until it gives up and ignites all on its own, much like tempers get lost in a train during crush hour if you squeeze enough people into a single compartment. This week we’ll examine what role spark plugs play in an engine.
Starting with a glow
Have you ever heard of ‘glow plugs’? You’ll find these in diesel engines. They don’t ignite fuel, but are necessary for diesels, especially in cold weather conditions. When a car’s been standing in sub-zero temperatures all night, it won’t generate enough heat to start up immediately in the morning. A glow plug is positioned inside the cylinder and what it essentially does is heat up the cylinder, especially its walls, so that the engine can start operating without hiccups because it’s too cold. They’re called ‘glow’ plugs because when they’re functioning, they heat up so much as to become incandescent, like a red-hot iron rod.
Spark plugs, like the name suggests, generate a spark to light the fire in a cylinder. Bosch has the honour of being awarded the first spark plug patent. Today, you can find spark plugs from companies like Bosch, NGK and Champion in the market. Spark plugs comprise of two electrodes, which are the tips that the spark jumps across.
The voltage required to make it across the great divide is about 20,000 volts, but today’s cars prefer about 40,000 volts. Modern performance cars can turn the dial up to 60,000 volts as well. You don’t want to be anywhere near these sparks when they occur — they’re of such high voltage because air isn’t a good conductor of electricity (and thank goodness for that, else we’d all be walking around with our hair on end, or in rubber suits.) Therefore, it takes a lot of effort to make it past the gap to the other electrode.
What affects the spark
The material of the electrodes does, to begin with. They’re usually made of copper, but high-performance spark plugs use precious metals like iridium or platinum. The reason claimed for the upgrade is a cleaner spark, but that’s only half the story. The ends of the electrodes are sharpened to points, so that the spark will be clean and require the least amount of energy. If there’s a wide area available for the spark to jump, it will try to cover the entire area with itself and will require more energy. As the spark plug fires, a little bit of the electrodes gets eroded, increasing the area of the tip. This means more energy is required for the next spark, thus reducing performance. Metals like platinum and iridium don’t erode as easily as copper, so have a longer life.
The erosion of the plug means that the distance between the tips of the electrodes increases. If the gap is too large, the spark won’t make it across. If it’s too small, the spark won’t make the fuel burn completely. There are fixed intervals at which the spark plugs have to be ‘gapped’ so that one gets maximum performance from them. This gap is usually a range, to allow for erosion due to the spark. If you set it at the maximum, you’ll get maximum performance right from the word go but may get a misfire after a few thousand kilometres because the gap might be too large for the spark.
We aren’t finished talking about spark plugs yet, but we have run out of space this week. We’ll discuss them next week as well.