Michael Cattanach, of Waimairi School, asks :-

Can motorbikes be made to run on fuels other than petrol?

John Raine, a mechanical engineer at the University of Canterbury, responded.

Yes, provided certain conditions are met.

Motorbikes are powered by spark ignition engines. A mixture of air and fuel is drawn in and ignited by a spark-plug. The heated gases in the cylinders of the engine push pistons down the cylinders producing a turning effort on the crankshaft which is connected through the motorbike's gearbox and chain or shaft drive to the back wheel.

A number of fuels can be burnt with air in the motorbike engine but must be mixed in the right proportions to burn properly. Apart from petrol, you could use alcohols such as methanol or ethanol, compressed natural gas (CNG), liquified petroleum gas (LPG), and other less common gaseous fuels.

Before you could use a fuel other than petrol you would need to modify the parts that feed fuel and air to the engine. With alcohol fuels a larger volume of fuel must be metered into the engine with each lot of air, and more preheating provided, because alcohols contain less energy than petrol in each litre and take more heat to evaporate them. Gaseous fuels need a special arrangement of valves and (jets) small holes to reduce the gas pressure from that in the storage tank and feed the gas into the air which the engine is breathing in each time it starts a new cycle. The timing of the ignition spark for best performance must also be adjusted for each different fuel as they all burn at different rates.