How A Motorcycle Engine Works

Motorcycle Engines 101

How do the gasoline engines that power motorcycles work? There are two parts to the answer – the basic principle involved, and a practical way to apply that principle.

The physical part is that when a confined gas is heated, its pressure increases. The hotter a gas becomes, the faster its molecules zoom around, colliding constantly with each other and with the walls of the container. The pressure of the gas is the result of the gas molecules, colliding with the walls. That gas pressure can be used to drive the piston of a gasoline engine. One easy way to heat air is to mix into it a fuel and then to ignite the mixture.

The mechanical part is how we go about using this idea of letting fuel-heated gas expand to make a power-producing machine that can turn the rear wheel of a motorcycle.

GSX-R Cut-Away Engine Illustration
Cut-away engine drawingPhoto courtesy of Suzuki Motor Corporation

We begin with a cylinder, closed at its top end, in which a close-fitting piston can slide up and down. A rod, pivoting inside the piston, extends out the open bottom end of the cylinder to attach to a crank on a shaft that rotates in bearings, located below the cylinder. If we rotate this “crank-shaft”, the rod attached to the crank will drive the piston up and down in its cylinder.

This gives us a movable piston that gas pressure can drive down the cylinder, and that motion, transmitted through the rod that connects piston to crank, can give the crank a push, thereby doing useful work.

In the closed top of the cylinder, called the cylinder head, are three essential parts. Two of them are valves – an intake valve to let fresh fuel-air mixture enter the cylinder from a mixing device, and an exhaust valve to let expanded combustion gas leave the cylinder. The third essential part is a spark plug, screwed into a threaded hole in the head so that an electric spark can jump when needed across the plug’s electrodes, located in the cylinder.

A valve operating mechanism is provided to open and close the intake and exhaust valves when needed, and another timing mechanism causes a spark to jump across the spark plug electrodes at the right moment and at no other time.

We begin the operating cycle with the piston at its position of closest approach to the top, or head of the cylinder. We turn the crank. The intake valve opens and the piston moves down, away from the head, creating a partial vacuum in the cylinder. A mixture of air and fuel rushes in through the open intake valve. When the piston reaches the bottom of its travel, the intake valve closes. Now the piston reverses direction, rising toward the head, compressing the fuel-air mixture. When the piston has nearly reached its top-most position, a spark is sent across the spark plug’s electrodes. The intense heat of this spark ignites the compressed fuel-air mixture, burning it and releasing heat. This heat causes the pressure of the burned gases to rise by about 7 times, to a high pressure. This pressure now drives the piston down again, but with great force, giving the crank-shaft a strong spin. Attached to the crankshaft is a flywheel, whose purpose is to keep the engine rotating smoothly even though power is delivered by the piston only intermittently.

As the piston, driven by expanding hot, high-pressure combustion gas nears its bottom position, the exhaust valve opens, allowing exhaust gas to rush out of the cylinder. The piston rises, helping the exhaust process by pushing the burned gases. The exhaust valve closes as the piston arrives at it top position, and the cycle repeats

If you have kept track, this sequence of events, or “engine cycle”, takes four end-to-end strokes of the piston to complete. It is therefore called the “four-stroke cycle”. Engines operating on this cycle are called “four-strokes”.

The four piston strokes of the cycle have names;

  1. Intake
  2. Compression
  3. Power
  4. Exhaust

Because each valve opens and closes only once in this cycle, which requires two rotations of the crankshaft, the mechanism driving the valves is made to turn at only ½ of crankshaft speed.