Pistons from your Motorcycle Engine

The piston is at the very heart of a motorcycle engine. It is subject to the heat and pressure of combustion and can be accelerated from 0 to 100km/h in 0.001 seconds – and back to zero in the same time. What's more, it must do this hour after hour, kilometer after kilometer, transmitting the gas force into the connecting rod and hence to the crankshaft where it turned into useful work. It is a wonder that pistons are as reliable as they are.

That is today. In earlier times pistons were less reliable, even though the forces imposed on them were also less. Providing an engine is running correctly the heat input into the piston is roughly proportional to the amount of fuel burned and the amount of fuel burned is roughly proportional to the power produced. Therefore a 500cc engine producing 30 horsepower is using approximately half the fuel of one producing 60 horsepower and for the same piston area will receive approximately half the heat input.

At the same time, the cylinder pressure generated by combustion in the past was usually lower than today due to generally lower compression ratios and poorer combustion efficiency. In a typical road going motorcycle engine this pressure is likely to be in the range of 50and 80 bar (725 to 1160psi). An older engine is likely to be towards the lower end of the range while a modern engine may be towards the upper end wen being driven hard.

What about the engine speed of modern engines, as surely the piston runs much faster? Well, consider the piston speed of a modern engine compared with a '50s classic motorcycle. To compare engines, this speed is often calculated as the Mean Piston Speed at rated power (MPS).
A typical value of mean piston speed for a modern motorcycle engine is between 16 and 23m/s, or to put it another way 58 to 100km/h. It is interesting to compare some classic and modern motorcycle mean piston speeds: Honda CBR600F @12500rpm 17.7m/s, Velocette Thruxton @6200rpm 17.7m/s, Ducati 999 @9500rpm 20.1m/s, BSA 500cc Gold Star @7000rpm 20.5m/s If we compare this four motorcycles not much things have changed.

This is the mean speed, and the maximum speed will probably be between 1.61 and 1.65 times higher, depending on crank and rod geometry. So we can see the piston may have to go from rest to 113km/h and back to rest all in one stroke. So, consider not the speed but the acceleration of the piston.

The piston in an motorcycle engine running at 6200rpm goes from zero to maximum speed in approximately 0.0012 seconds. So twice the engine speed means twice the piston acceleration. Isaac Newton told us that 'Force equals Mass times Acceleration.' Therefore twice the piston acceleration means twice the force exerted on it if it is the same weight. This acceleration can be more than 2500g. Something to think about when you open the throttle of your motorcycle!

So the modern piston has twice the heat input, one-and-a-half times the pressure, and twice the acceleration of an older motorcycle piston but is still more reliable. How come? Sixty years of development by designers, materials engineers and analysts (as well as some trial and error) explains most of it.

Early pistons were what are now described as 'baked bean cans', meaning they were simple cylinders closed at one end. Today's pistons are far from that, incorporating complex skirt geometry and features that control the shape and running clearances at operating conditions. To get from the baked bean can to today's piston has taken many stages, not least of which is the advance in the use of computers to handle complex calculations by a process called Finite Element Analysis (FEA). This has enabled the designer to better understand the distortions and shape changes of the piston and cylinder throughout the working range of the engine and give birth to the modern motorcycle.
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