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An exhaustive search for higher engine efficiency

31 August 2011

Even the most efficient internal combustion engine can only convert about one-third of the energy derived from fossil fuels into the mechanical kinetic energy needed to power a motor vehicle. In recent years, BMW, under its ‘EfficientDynamics’ brand, has achieved some notable improvements in engine efficiency through various technologies - direct fuel injection, variable valve timing, exhaust-driven turbochargers, brake energy regeneration and its own version of the Auto Start Stop function, being among them.

Despite these advances, about 60 percent of the generated energy is still lost; half of it from exhaust heat, with the remaining half as heat absorbed by the engine cooling system. Finding ways of recovering this lost heat energy is now one of BMW EfficientDynamics’ goals, and the carmaker is currently involved in several projects, each with different approaches to utilising dissipated heat energy - and at various levels – in research, pre-production and series development. Among the most promising innovations are the Turbosteamer and a thermoelectric generator.

Both the Turbosteamer and thermoelectric generator projects are focused on generating electricity from waste heat to improve overall engine efficiency. There is considerable potential for fuel savings if the electrical energy required by all of the systems in an automobile can be produced using waste heat rather than relying solely on the vehicle's belt-driven alternator.

The process of recovering energy from waste heat has been a factor of power generation plant operation for years, large power stations combining the principles of a gas turbine and a steam circuit to achieve a significantly higher level of efficiency. The gas turbine process is the first phase of the energy conversion and serves as the source of heat for the downstream steam cycle in the second phase.

BMW’s Turbosteamer is based on this two-stage stationary power generation method, though clearly reduced in scale and design to provide a component small enough to be used in modern automobile engines. Researchers proved the feasibility of this technology back in 2005 with the unveiling of a first-generation Turbosteamer dual-cycle system. The primary element was a high-temperature circuit that employed a heat exchanger to recover energy from the engine exhaust gases. This was connected to a secondary circuit that collected heat from the engine cooling system, combining this with the high-temperature heat from the primary circuit to create lower temperature heat.

When this design was laboratory tested on the four-cylinder petrol engines produced by BMW at the time, the dual system boosted their performance by 15 percent. The design effort that followed concentrated on reducing the size of the components and making the system simpler to achieve an optimum cost-benefit ratio. The key was to produce a component with just one high-temperature circuit. Jürgen Ringler from BMW Group Research and Technology takes up the story:

"A heat exchanger recovers heat from the engine exhaust, and this energy is used to heat a fluid which is under high pressure; this heated fluid then turns into steam, which powers an expansion turbine that generates electrical energy from the recovered heat. For the latest generation of the Turbosteamer, we developed an innovative expansion turbine based on the principle of the impulse turbine, which offered many advantages in terms of cost, weight and size when compared with earlier concepts, and these are factors that are very beneficial when it comes to series production.

"We have made great progress toward achieving our original goal, which was to develop a system ready for series production within about ten years. When completed, this system will weigh only ten to fifteen kilograms and will be capable of supplying all of the electrical energy required by an automobile while cruising along the motorway or on country roads. Under these conditions we are sure that the average driver will be able to reduce fuel consumption by up to ten percent on long-distance journeys."

BMW says that it has also made considerable progress on its thermoelectric generator (TEG) project – currently the subject of a series production development effort. Two alternative systems have surfaced so far, differing only in their positioning within the vehicle; one unit has been designed for the exhaust system, while the other is intended for the exhaust gas recirculation system.

The TEG converts heat directly into electricity, and is a technology that NASA has used to power space probes for more than four decades. It is based on the Seebeck effect, which most will recognise as the principle behind the thermocouple. Since the efficiency of TEGs has historically been rather low, however, this technology was considered unsuitable for automotive applications.

In refining the technology for its more earthly bound duty, BMW has taken advantage of recent progress in materials research to improve the performance of TEG modules. Initial work, involving the integration of a thermoelectric generator in the exhaust system, delivered a maximum of 200W but with new materials and improvements in the weight and size of the TEG, some 600W was achieved. BMW believes it will not be long before the goal of 1,000W is reached as research progresses. The current prototype – a BMW X6 – was built as part of a development project funded by the US Department of Energy.

Some two years ago, the carmaker unveiled an alternative development in which the TEG was integrated with the radiator of the exhaust gas recirculation system. In this configuration, the module produced 250W while simultaneously reducing CO2 emissions by 2 percent. What's more, this energy recovery system offered additional benefits, such as supplying the engine or passenger compartment heating with additional warmth during cold starts.

Researchers are currently forecasting that TEGs will lead to fuel consumption savings of up to 5 percent under real, everyday driving conditions.

Harnessing the potential of thorium
Gaining percentage point efficiency gains in conventional internal combustion engines may be a hot research topic among the big names of the automotive industry at the moment, but at least one enterprising individual is now seriously proposing the use of a certain type of fission reactor to power the cars of the future.

Charles Stevens, an inventor and entrepreneur, recently revealed that his Massachusetts based R&D firm, Laser Power Systems (LPS), is working on a turbine/electric generator system that is powered by - in his words - an ‘accelerator-driven thorium-based laser’.

Thorium, a mildly radioactive element, is as abundant as lead, and is present in large quantities in India where the long-term goal has been to develop an advanced heavy-water thorium cycle. Thorium is a much-touted stand-in for uranium in nuclear reactors because its fission is not self-sustaining, a type of reaction termed ‘sub-critical’.

The LPS idea has exercised the small but active thorium community, which holds that it is the answer to our clean energy needs because it could, for all intents and purposes, power a car forever. The new technology "would be totally emissions-free," Stevens says, "with no need for recharging."

For all the claims made on its behalf, the proposed LPS power plant isn’t a complete departure from traditional power generation. Lasing thorium creates a lot of heat, which is used to produce steam within a pressurised closed-loop system that subsequently drives a turbine coupled to an electric generator.

A 250kW unit weighing less than 230kg would be small and light enough to put under the bonnet of a car, Stevens claims. And because a gram of thorium has the equivalent potential energy content of 6,245 gallons of petrol, LPS calculates that using just 8g of thorium in the unit could power an average car for 5,000 hours, or about 300,000 miles of normal driving.

Stevens isn’t the only one who believes thorium could power cars. In 2009 Cadillac introduced a thorium-powered concept car at the Chicago Auto Show. Designed by Lorus Kulesus, the sleek World Thorium Fuel Concept didn’t quite go so far as to have a working thorium-fuelled nuclear-fission reactor that could generate the electricity to power it. But somebody at General Motors thought the idea to be sufficiently interesting to build a vehicle to show it off!

Les Hunt

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