Advice: how a motorcycle engine works

Different strokes for different strokes pretty much sums up the motorcycle divide.

Most of us throw a leg over a four-stroke machine; though those with a penchant for older bikes or serious off-roaders may still be riding a two-stroker.

The two-stroke engine is an ingenious approach to making an engine as simple as possible, and performs the same operation as a four-stroke in half of the crankcase revolutions.

It works by using the crankcase itself as part of the combustion chamber and using ports – effectively holes – in the cylinder to let air in and out rather than relying on more complex intake/exhaust valves.

As the piston starts to descend after combustion, it drops below the exhaust port while compressing the fuel/oil/air mix beneath it (oil is mixed directly with the fuel in two-stokes to help lubricate the crankcase bearings because – as part of the combustion chamber – they can’t be lubricated in a separate oil bath). This fresh charge is then forced through transfer ports back to the top of the cylinder, where it helps blow out the exhaust gases.

As the piston starts to rise again it covers up the inlet and exhaust ports and compresses the fuel/air mix. This creates a vacuum in the crankcase below the piston, sucking the next charge into the crankcase. As the piston reaches the top, the spark plug fires, combustion occurs and the process repeats itself.

One of the disadvantages of traditional two-stroke engines is they’re not as fuel efficient and emissions friendly as their four-stroke counterparts. KTM has led the way in developing direct injection two-stroke motorbikes.

They work by replacing the carburetor with a pair of downdraft injectors that spray atomised fuel in the transfer ports that then mixes with the flow of air and oil. The injectors are linked to electronic sensors to deliver a precise fuel charge based on throttle opening and engine load, just like a fuel-inject four-stroke.

A four-stroke engine relies on intake and exhaust valves and takes four strokes of the piston – or two revolutions of the crankcase – to achieve the same process. Think of it as the two-stroke engine doing two jobs on each up or down stroke, while the four-stroke engine achieves just one.

As the piston is forced down after combustion in a four-stroke engine, it generates the power to turn the crankcase. As it ascends again, the exhaust valve opens and the spent gases are ejected by the force of the piston. The piston descends and the intake valve opens to provide a new charge of fuel/oil. The intake valve closes as the piston comes back up, compressing the charge for the next spark.

While there are two basic engine types, there’s a huge variety of engine configurations. The naming typically relates to how the pistons are aligned – below are the most popular types.

A single is just that – one piston pumping out the power. Parallel twin means two cylinders sitting side by side. A V-twin has the same number of cylinders but each on is aligned as part of the V with the base being the crankcase.

A flat twin, or boxer engine, has the cylinders laid out horizontally and tends to be relatively vibration free.

Triple-cylinder engines are typically laid out side-by-side (transversely), though bigger versions can be longitudinal (front-to-back).

In-line four-cylinder engines are the most common approach for big capacity road bikes and are typically laid out transversely.

Honda’s VFR, the Aprilia RSV4 and the Ducati Panigale V4 are three of the most prominent V-four engines in use today. Popular as a racing layout, the difficulty in bolting bits on makes the V4 expensive as a production unit.