In any internal combustion engine, the fuel and air mixture gets ignited and power is generated. But in the entire process there is one more thing that helps in the process and that is pressure. This pressure is created due to the compression in the combustion chamber by the piston. If you check out the specs of any bike, you will notice a column that reads compression and that reads anywhere from 8:1 up to 14:1. That number is actually the rate of compression where the area that 1 unit takes up, in the same area 8 units now acquire. This compression causes spike in the pressure that helps in the quick ignition.READ ALSO: What Is Compression Ratio And How It Affects The Performance & Economy Of Your Motorcycle?
But sometimes it so happens that this compression can get fatal or inefficient for working purposes and there is a need for the pressure to be released a little which is called decompression. You reduce the compression or the pressure inside the chamber for everything to work well. This is generally needed at certain times like during the initial ignition of the engine start up. During the first start there is already pressure buildup due to end of the engine running in the previous cycle of running.
So when the starter is applied, more air and fuel mixture enters inside the combustion chamber and causes extra compression than required. So if this compression exceeds the level that the bike is designed for, then there are chances of the engine to burst altogether and that is very much fatal. So to take care of that problem, one of the exit valve is designed in such a way as to open a little bit to allow some more breathing space and let the compression level fall a bit and the engine can start up properly.
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Another main use of decompressor is during kick starts, the low compression allows the rider to kick the engine easily; else if the compression was too much, then the reverse action to the kick start would be too high and could even injure the rider. So a small release of pressure to cause a decompression helps. Here those who have ridden old Royal Enfield (Bullet) can recall the Decompressor Lever, which is mandatory to pull while kick starting till the ammeter needle comes back to neutral. One more important use of decompression is when the engine is started without the need for decompression.
The turning of the engine can make the crankshaft sometimes turn in the opposite direction of normal movement and this event is more tend to happen with 2-stroke engines. This would not be a problem generally, but when put into gear, the bike would move in the opposite direction. It would still have been fine if the bike has a reverse gear, but then that is not the case here. So the decompression allows for the engine to release the pressure a little until the crankshaft has had a chance to move back in the direction it is supposed to move and then the pressure is restored again.
Now a days the way decompression systems are designed is simply by connecting one of the exhaust valves using a cable similar to a clutch cable, so when starting up if there is any issue encountered that need decompression, the cable gets yanked up a little and the exhaust valve opens up just a little to give a little breathing space allowing for the engine to work properly. And that is how decompression works.
So next time you have a little problem starting your bike even when everything is proper, get the decompression unit of bike checked out. Though it is not a problem that you will have because it is a very rare occurrence and will occur in a very rare case.
By: Pratik Patole