One might think that the steam engine is an outdated technology that had its heyday centuries ago, but in fact steam is once again a hot topic with vehicle manufacturers. Indeed, the next generation of hybrid cars and trucks may incorporate some form of steam power. Honda, for example, has just released details of a new prototype hybrid car that recharges its battery using a steam engine that exploits waste heat from the exhaust pipe.

Typical cars only convert about a quarter of the energy produced during combustion into work, with the rest being lost as heat. Honda has managed to increase this efficiency by 4% to nearly 29% by using some of this lost heat to generate electricity.

Honda’s heat-recovery system is based on the Rankine cycle, which is also used in most steam-driven power plants. First, heat from the car’s catalytic converter is used to boil water. The high-temperature steam (400-500 °C) produced then turns an electric generator, before a condenser finally cools the steam back into water.

According to Honda, under normal driving conditions, the steam system recovered three times as much electric power as the hybrid’s regenerative braking system. Unfortunately, however, the 4% improvement in overall vehicle efficiency that resulted is not high enough to warrant commercialization, Honda claims.

Honda is not the only manufacturer interested in incorporating waste heat recovery into vehicle design. BMW, for example, is working on a steam-based unit that generates additional mechanical power, rather than electricity. In lab tests, their so-called ‘turbo-steamer’ reduced fuel consumption by as much as 15%.

It may be some time, however, before waste heat recovery reaches the mass market, because typical car drivers would probably not make a big enough saving on fuel to justify the extra several thousand dollars that these systems would presumably add to the price of a vehicle.

But the situation is different for long-haul truckers, who often spend over $100,000 per year on fuel. Indeed, several diesel-engine manufacturers are testing ways of recycling lost heat, with interest being driven by fuel prices and emissions reduction.

The engine maker Cummins Inc is also working on a Rankine-cycle system that uses a low-boiling point organic fluid, which they say performs better than other thermodynamic models, such as the Stirling cycle or the gas turbine.

The Rankine cycle can convert up to 20% of the wasted heat into useful energy, but dealing with the 80% that is not used poses a big challenge. Cummins Inc. plans to have a full working prototype by mid-2009, and hope to make the system available to customers by 2013.