How does Motorcycle Engine Work

A while ago, 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 details. This file is a tidied up version of my random notes and questions.

If you want more detail, then grab a copy of the book Vehicle and Engine Technology from Amazon. It tells you how engines, carburettors, chassis, tyres etc work. It's where I learned all this stuff from, and it's probably the only book on the subject I'll ever need.
How does an engine work

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. How do we get rotational motion from the engine?

We do this by a piston which provides periodic linear motion, connected by a con-rod to a crankshaft which turns the linear motion into rotation. The piston experiences a downward force on the ignition stage, which increases its monentum. The crankshaft acts as a flywheel which moves the piston through the other three stages (exhaust, intake and compression).

What affects the force on the piston when we ignite the fuel/air mixture?

* Combustion is not instantaneous. A flame spreads from the spark plug across the cylinder. This takes a significant amount of time. Remember that an engine running at 10,000 rpm completes one revolution in 0.006 secs.
* 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 amount of fuel/air mixture ignited. We can get more energy in each explosion by having more fuel and oxygen molecules in the cylinder to react together. This is why turbos and supercharges make an engine more powerful - they squeeze more fuel and air molecules into the cylinder. Nitros add an oxygen-rich chemical into the cylinder, which means we can burn more fuel.
* The fuel/air ratio. Different proportions of fuel and air will affect combustion. Let's look more at petrol first ...