Steaming into the future

A steam engine powered by heat from a car exhaust could replace conventional batteries in hybrid vehicles, according to its UK developers.

A team from the University of Sussex has recently received funding from the EPSRC to develop the steam accumulator, which it claims could rival battery energy storage and offer significantly more power output – especially for heavier hybrid vehicles.

The Sussex team, along with Spirax Sarco, a specialist steam component supplier, and advanced vehicle concepts company Menard Engineering, plan to power the steam engine with heat from the main engine exhaust. Prof Richard Stobart, of the automotive engineering department at Sussex, said that up to one third of the input energy in a normal engine is lost to exhaust.

‘We’re using a by-product of the internal combustion engine and taking what would be otherwise wasted,’ he said. The system re-routes the hot exhaust from the main engine through a pressurised steam accumulator, where the pressure and temperature rises until steam forms, and the energy is stored.

An electronically-controlled valve then feeds the steam through three stages of expansion in a reciprocating, piston cylinder engine. The steam is then re-circulated back into a liquid reserve feeding the accumulator.

This closed system could allow the team to investigate the use of other fluids with better thermodynamic properties, said Stobart.

‘Steam power represents a whole different path to take as an alternative to electric hybrids,’ he said. ‘This technology has been neglected since the last steam locomotive was built.’

The team believes its steam engine could ‘easily achieve and go beyond’ the power output of the battery on the hybrid Toyota Prius, which operates around 900w/kg. It could yield stronger acceleration for example. ‘You’re always going to be limited by the rate you can take out of the battery. But if you want to take a lot of steam out of the accumulator you can,’ said Stobart.

The energy storage could also be comparable, he said. ‘In our initial calculations with a 20 litre water reservoir – around half the size of a fuel tank – we could extract five kWh of steady energy, with peak powers much higher than that. That’s quite a lot of energy – about half the level a heavy truck would need to run air conditioning all night.’

The accumulator cools the exhaust, so it could also be coupled with exhaust gas recirculation technology (EGR), which requires lower exhaust temperatures to enhance combustion.

The researchers admit that there are a number of questions to be resolved before a working prototype can be built. Stobart said the biggest challenge for the team during the three year project will be how to manage the heat flow in and out of the device. ‘There’s no science around the electronic control of heat flow. For example how precisely a valve second-by-second controls the steam.’

He also admitted that starting up the steam engine from cold would pose problems, but an externally powered electric heater could warm the accumulator when the engine is not operating in the same way as a battery charges.