Victor Harman shares his wisdom when it comes to eking the most out out of your vehicle
Friction is a prime enemy of all mechanical devices. When a car, its engine, and its component parts are in motion, friction constantly dissipates the energy of their motion into heat which is dispersed into the atmosphere, where none of it is recoverable other than by plant life. Designers can help reduce this wastage, as we know. Low-friction bearings, low-viscosity oils and lightweight alloys can reduce engine friction, body design can minimise aerodynamic drag and tyre technology can reduce a car’s rolling resistance. Stop/start technology and braking energy recovery are the most recent energy-saving technology. As drivers, we can also reduce friction and energy wastage by how we drive and the speed that we travel at, and by good maintenance. At present, that is the best that we can do, and we hope that you have taken some of our economy driving advice on board. But friction is not how most of the available energy in our diesel is lost, and there are prospects that engineers and technologists can take giant steps to improve fuel efficiency, possibly even within the next decade. Electric cars will undoubtedly establish their place, largely as a means of urban transport, but the diesel engine is very far from dead, and we’ll speculate on how it might survive.
With any diesel engine, less than half of the fuel’s energy actually ends up moving the car. The rest is lost in the waste heat of combustion, something fundamental to the diesel combustion cycle and, with current engine technology, something largely irrecoverable. But a promising possibility now appearing is the conversion of heat directly into electricity, using new types of semiconductors. These have yet to be developed sufficiently to be of use in vehicles, but the scientists are making steady progress towards that target. They are creating and developing efficient new solid-state devices, called ‘thermal diodes’, that operate between 200 and 450 degrees Celsius – the sort of temperatures ideal for harnessing the waste heat of engine exhaust gases. This thermal-to-electric energy conversion works, in crude laymen’s terms, by using the heat to make electrons to flow within the semiconductor, and thus is an essentially pollution-free process. The secret will be to scale the principle up, in a compact enough form, to generate a few kilowatts rather than watts.
Such electrical energy can obviously be usefully stored in a battery to supplement conventional engine power via an electric motor, as in today’s hybrids. It might also be used to power all the electrical equipment in a vehicle, the demands of which are now significant, or maybe to drive a supercharger, raising engine efficiency above that of today’s turbocharged engines. Just as we might, a mere decade ago, never have imagined the technology of braking energy recovery appearing on everyday cars, this level of advanced energy recovery is now looking technologically possible, and man usually – eventually – manages to master such tempting opportunities. Given success, our children may be driving 100mpg cars, with smaller engines, cooling and exhaust systems that waste very little heat, and exhaust gas temperatures that are barely more than ambient. Such energy recovery systems would work very well with diesel engine EGR (exhaust gas recirculation) systems, the cooler recycled combustion gases facilitating virtual elimination of NOx emissions. But there’s a decent chance that the familiar diesel engine might be on its way out by then though, perhaps replaced by the ultra-efficient Stirling Cycle diesel engine, which is well-suited to this sort of energy recovery, and one of which is already used in Swedish submarines – but then that’s another story!