Engines are a waste of energy

by Simon Hargreaves

Given the amount of work petrol does, it's amazing your motor uses so little to go so far so fast

It takes just 100cc of unleaded mixed with 800 litres of air – or two and a bit Tequila shots in four large garden water butts – for your engine to accelerate you from a standing start to 150mph under half of a mile away in around ten seconds. And it's even more impressive when you consider how little of that fuel energy is actually used to turn a wheel.

The waste starts when petrol is ignited in a combustion chamber. Chemical energy is converted into thermal energy. If the initial charge has 100% of its energy available for work, around 65% of it is 'wasted' as heat, half passing into the exhaust and half passing into cylinder walls, oil and engine components.

Some of this lost energy could be recovered. Exhaust gas energy is half kinetic and half thermal. The movement of exhaust gas could be used by a turbo or supercharger to pressurise intakes (improving efficiency by cramming in more air per cc). Meanwhile BMW and Honda, among others, are developing thermal recovery systems using the same principle driving almost all the world's power stations: heat converts water to steam, which drives a turbine and supplements crank torque. BMW, charmingly, call their system Turbosteamer. It's improved fuel consumption in test cars by up to 15%. But don't expect to see steam turbines on bikes anytime soon (although superchargers are almost certainly on the way).

Back inside the engine, more energy is lost as heat into the surrounding engine itself. Heat-rejecting composite ceramics in cylinder walls, piston rings, skirts and valves could improve the low thermal efficiency of piston engines, because of their strength at high temperatures. Experiments in the 1990s showed ceramic-coated engine internals could increase power by up to 18% and reduce fuel consumption by 10%. Although composite coatings like Nikasil have been used on cylinder walls for years, the technology has limited value in conventional bike engines because increasing combustion temperatures leads to other problems, like detonation.

The next big 'waste' in the energy path is mechanical. With only 35% left of the energy available at the start, friction, throttling and transmission losses steal more. Friction is generated by pistons moving in bores, rods on the crank, the crank spinning, driving counterbalancers and valvetrain, and spinning a clutch and gearbox. There's also 'dead weight' in a four-stroke engine – each cylinder contributes a power stroke once every 720° of crank rotation; the rest of the time it's a passenger, more in a multicylinder engine. Throttling or pumping losses are impediments to airflow: the intake, throttle plates, valves, and imbalances in crankcase pressure. Over the years improvements in piston skirt design, piston ring profile, and inserting cylinder wall ports to balance under-piston pressure have all helped reduce friction and pumping losses, but even so, something like 20% of the remaining 35% fuel energy is sapped.

Meaning of the original 100cc of fuel, around 28cc ends up overcoming inertia and the tyres' rolling resistance to turn the rear wheel and propelling you to 150mph.
Blimey, you'll need a drink after all that effort. Shot of Tequila, anyone?