Doctor Diesel

Diesel energy efficiency

If you consider the energy content of the diesel fuel we use, and compare this with the energy used in moving a car from point A to point B, there’s a lot of energy wastage in between.

The laws of physics dictate how much mechanical energy you can harvest from the bang when fuel ignites in an engine’s cylinders, and more of that energy is harvested in a diesel engine than a petrol one, due to the higher compression ratios achieved with diesel fuel.

Turbo-charging raises the efficiency further, using exhaust gas energy to drive a turbine linked directly to a compressor that turbo-charges the air drawn into the engine, achieving higher cylinder pressures than possible by purely natural induction.

But even at engine speeds that are most efficient around their peak torque speed, only around 35 per cent of the fuel’s energy ends up pushing the pistons down in their cylinders, turning the crankshaft, and driving the car.

The rest of it is lost in hot exhaust gases and passed to the engine’s cooling system, from where it passes to the atmosphere. And of that crankshaft energy only a proportion is eventually available to overcome the car’s resistance to motion, after further losses between the crankshaft and the wheels, overcoming friction in the transmission, and driving ancillary equipment.

What energy then remains to move the car has to overcome three forces of resistance to motion: the “rolling resistance” of tyres on the road surface, aerodynamic resistance and, the energy needed to overcome hills and road undulations.

Some of that energy is recovered on downhill stretches, but there are always significant net energy losses when roads rise and fall.

The efforts of automotive design engineers in recent years have been mostly devoted to reducing aerodynamic resistance, using wind tunnels and computerised body design, reducing rolling resistance and energy lost in changes of direction with low-energy tyres, and recovering some of the vast amount of other wasted energy.

We’re a long way from recovering very much energy from the exhaust gases and cooling systems, although more efficient engine cooling can raise engine efficiency and advanced cooling systems such as that in Ford’s Focus are examples of reducing waste cooling energy along with improving aerodynamics.

Most progress has been made by reducing other previously wasted energy. Whenever you apply the brakes then some of the car’s kinetic energy, or momentum, that might have taken you halfway up the next hill is lost for ever as heat produced by friction at the braking surfaces.

With intelligent alternators though, car electrics use mostly stored battery power for much of the time, with the alternator freewheeling, drawing minimal engine power, and only switching to a high charging rate whenever the brakes are applied, drawing off some of the energy that would have been otherwise totally wasted in braking.

It’s not a huge proportion, but it is a significant amount and, as time passes, such systems will become more clever and recover more and more of such energy.

We now also have stop-start systems, multi-ratio manual and automatic transmissions that enable engines to work at speeds where they are most efficient, and cylinder deactivation that shuts down the fuel supply to some cylinders in low power demand situations.

Freewheels – much favoured in past decades – are beginning to reappear, saving fuel by reducing engine braking on the overrun.

Engineers are achieving faster engine warm-up, significantly reducing vehicle weights with thinner high-strength alloy steels, and making more efficient air-conditioning systems and other ancillary service units.

It’s an endless story, but progress is continuous, and we’re certainly nowhere near yet achieving peak efficiency for the diesel engined car which will be around for a few more decades yet, and become more and more efficient with each and every new model.

Doctor Diesel



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