BMW is carrying out exhaustive R&D work on recovering engine waste heat. It believes there is great potential for considerable fuel savings if electric energy required by all vehicle onboard systems could be produced via the use of waste heat.
The company estimates that despite a range of energy-saving technologies incorporated in its EfficientDynamics portfolio and now in series production, some 60% of the energy generated by the internal-combustion engine is lost—half via exhaust heat and the rest absorbed by the vehicle’s engine cooling system.
To try to lower this figure, BMW uses not only automotive engineering know-how but also knowledge gleaned from other industries, including power stations. “Large gas and steam power stations combine the principles of a gas turbine and a steam circuit to achieve a significantly higher level of efficiency,” explained Dr. Hermann Sebastian Rottengruber, Team Leader for Thermal Energy Converters at BMW Group Research and Technology.
The company’s research project, Turbosteamer and Thermoelectric Generator, uses the principles of a steam process to recover heat. It is based on a Rankine Cycle stationary power generation technique, which has been greatly scaled down to make it applicable for vehicle integration. (The thermodynamic Rankine Cycle, which forms the basis of steam engine technology, converts heat into work. It is used in a majority of power stations and is a key element in the generation of electricity. William Rankine was a 19th century Scottish physicist and engineer.)
Rottengruber said initial research work involves a high-temperature circuit. A heat exchanger recovers heat from the engine exhaust; the heat is used to warm a fluid under high pressure. “The heated fluid then turns into steam to power an expansion turbine that generates electric energy from the recovered heat,” he explained.
The reclaimed heat can provide up to 10% improvement in fuel consumption for long-distance journeys in a vehicle powered by a turbocharged four-cylinder engine, he added.
In its original form, the system was too large for series production application. BMW worked to make packaging easier by using smaller components.
The thermoelectric generator (TEG) also allows the HVAC to produce added heat more quickly in low ambient temperatures. Use of a TEG complements the EfficientDynamics brake energy regeneration system, as it operates at its most efficient level during the acceleration phase. Rottengruber and his team believe that by using synergy effects, heat management will play a major role in reducing CO2 emissions in the future.
BMW is expecting the production version of the TEG to reduce fuel consumption by up to 5%.
Regarding added weight and cost, BMW is saying nothing at present, nor is the company confirming when it might enter series production, as the project is described as being still in its predevelopment phase.