Dec 01, 2016 05:29 PM
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, I wondered “how does an engine really work”. I wasn’t just thinking of explosion in cylinders pushing pistons up and down. I was thinking of the little detail.
We want the engine to turn the back wheel. Or, more practically, to turn the gearbox since we want the bike to move both a very slow speeds and very fast speeds. The range of engine speeds available is roughly 1000 to 10, 000 rpm which is a factor of ten. We want to ride a motorbike at anything from 1 to 100mph which is a factor of 100. So, we need a gearbox to provide a suitable range of gears. The engine drives the gearbox which drives the wheel Combustion isn’t always complete. The fuel/air mixture might now have completely burned by the time it gets punted out of the exhaust port.The fuel/air ratio. Different proportions of fuel and air will affect combustion. Let’s look more at petrol first
The carburettor is the bit of the motorbike which mixes the fuel with air. The fuel comes the tank, through a fuel tap which lets you turn the fuel on or off, then into the carburettor. Normally, we leave it up to gravity to get the fuel from the tank into the carburettor – we don’t need to pump it. This wouldn’t work in an aeroplane which has to fly upside down, but bikes spend most of their life right-way-up so it works fine. The air is sucked in from the outside world, and passes through an air cleaner before reaching the carburettor. There’s a few different types of air cleaner, but their purpose is the same. They remove grit, dust and other undesirable gunk from the air. You don’t want little bits of grit flying into your engine where they’d cause havoc.
The carburettor has its own little reserve of fuel called the float chamber. This work in the same way as a toilet cistern. When the fuel level drops, a value is opened which lets more fuel in. When the fuel level is high enough, the valve closes. You can get an idea of how much fuel is stored in the float chamber by turning the fuel tap off while the bike is still running. It’ll keep running for a minute or so.
There’s a problem with this description though. There’s not yet any way for the rider of the bike to control the fuel/air mixture. That’s going to be a problem, because the rider isn’t going to be happy with an engine which only runs at one speed. We can introduce speed control by arranging for a metal needle to sit above the jet. We can slow the engine down by moving the needle down, so that it partially blocks the hole in the jet. This throttles the fuel supply, and so it’s known as the throttle control. The right-hand grip on a motorbike simply controls the height of the needle. When you’re at full throttle, the needle is totally out of the jet. When you’re going very slowly, the needle will be blocking the jet more.
