If we drive an internal combustion engine, for sure our car will need to use spark plug. The number of spark plug depends on the number of cylinder, most car will have one sparkplug per each cylinder, let say you are driving a Proton Saga BLM which have 4 cylinder,youwill have 4 spark plugs in your engine. But some engine such as the Honda Jazz I-DSI and Alfa Romeo Twinspark series engine have two spark plugs in each cylinder making the number of sparkplugs double the number of cylinder.
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As most of us should know, internal combustion works by detonating a mixture of air and fuel. The combustion will produce power and force the piston to go downwards. This event will repeat in every cycle of the operation. But how does the air fuel mixture detonate? This is where he spark plugs comes in. The main function of spark plug is to detonate the air fuel mixture. And to do so, the spark plug is located right on top of the engine cylinder, submerged in the air fuel mixture. Due to this, we can generally know whether your car is running rich on fuel or running lean. We can take out the sarkplug and look at the colour of it.
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There is three general condition of air fuel mixture. First is Lean, where there is too little fuel in the mixture. Lean mixture will save you fuel but in a piston engine, the fuel will act as a coolant for the combustion. So if you have a lean mixture, your engine will run hot, this condition will slowly harm your engine and in the worst case, especially in high performance car, you may blow your engine apart. This is possible when the hot temperature start to melt your head gasket or some other sensitive parts. So for any car, it is much better to have a richer air fuel mixture rather than having a lean air fuel mixture.
Lean Spark Plug
Second is stoichiometry or in simple term just right. This is the best air fuel mixture. The condition will let you have the best performance in the same time gives you a very good fuel consumption. The Stoichiometry condition wil differ from each car especially between turbo or supercharge car and normally aspirated engine.
Normal Spark Plug
The third general condition is rich. Rich means that there is more fuel in the mixture than it is necessary. This condition will gives you the lowest engine temperature. But by having the rich condition, you will waste your precious fuel. Having a rich mixture will not give you any extra power, in fact, too much fuel will reduce your car performance and may bog down your car.
Rich Spark Plug
To differentiate between this three condition, we can tell by looking at the ceramic insulator at the sark plug. A WHITE insulation colour means that your rngine is running lean. A BROWN/MOCHAinsulation colour means you have a stoichiometry or just right mixture. And a BLACK coloured insulator will tell you that your engine i running rich.
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This article only cover on the air fuel mixture parts. There is still few other things that our spark plugs can tell us. Look at all the pictures for other indication.