In the classical combustion engine theory, petrol consumption is hard to calculate. Of course it can be measured. In the laboratory a diagram can be drawn with the consumption in gram/KWhour for each revolution at maximum throttle. An engine speed is getting visible with optimum fuel consumption, mostly about 275 gr/KWh at full torque for a four stroke petrol engine. Other revolutions cover a wide area of about 310 gr/KWh to 350 gr/KWh for maximum power. When the throttle is partly opened, efficiency is lowered and separate lines have to be drawn. At idling speed, no power is delivered, but fuel consumption is going on. Helmut Hütten has described in his book Motorrad Technik (ISBN 3-613-01175-1) an tremendous amount of measurements about motorbikes (unfortunately no website), also including the consumption of bikes at idling speed. I could make the conclusion that zero load fuel consumption is equal with piston displacement. The number of cylinders doesn't matter.
A German motor magazine took a Kawasaki GPZ900 to an English laboratory and powered the crankshaft with an electric motor. They discovered that the piston losses were more or less equal to the crankshaft speed.
Both items combining, I decided to split the engine formula in an ideal part and an useless part (Must be done before by somebody, but I never read about it). Consumption of the perfect engine is 185 gram per KWh and the useless one consumes 1 litre petrol per 1000cc per 1000 revs and per hour. The perfect engine has to supply the rear wheel with sufficient power to compensate tyre losses and wind pressure. Most workshop manuals contain diagrams for speed and power. I use in my calculations about 23,5 KW for a normal sitting driver at 160 km/hour. Tyre losses are about 2,7 KW for a machine of 200 kg and a driver of 90kg.
We put all those calculations in Excel and beautiful tables are the result.
I've been enough at school, let's get out of here
Practice.
So, the smaller the enginedisplacement, the lower fuelconsumption. Considering the fact, life must also be a pleasure on the high way, a 50cc Honda was not on my list of favourits. Below 36KW driving a motorbike gives less fun, but more then 250cc is not good for pistonlosses. A man have to make a decision. The Kawasaki GPz305 was famous for his low fuelconsumption and not hard to become on the market. I got mine free from a friend, because the engine was finished, something what is rather common when the mileage reach the 30.000. After repair, the experiments could start. The front chain sprocket was changed from 15 teeth to a 17 teeth from a Suzuki TL1000. Theoretical top speed 190 km/h @ 11.000 rev. Fuelconsumption was measured as follows:
4.5 litre/100km at 81 mile/h with full camping equipment.
3.4 litre/100km on highway at 63 miles/h in sixth gear.
3.0 litre/100km on mountainous roads.
2.4 litre/100km behind a lorry.
Last value was measured on a distance of 200 km in both directions, with a carefully levelmeasurement of the fuel. Clothing was leather and the road was protected against wind by trees. The same measurement on a windy road gives a value of 2.6 litre.
On the Mont Ventoux.
In Scotland.
