Even the most efficient internal combustion engine only converts about one-third of the energy derived from fossil fuels into the mechanical kinetic energy needed to power a motor vehicle.
Over the past years, BMW EfficientDynamics has made improvements in engine efficiency with technologies such as direct injection, variable valve timing, turbos, brake energy regeneration and Auto Start Stop. However, about 60 percent of the energy is still lost, half of it being exhaust heat, with the remaining half as heat absorbed by the cooling system.
Finding ways to recover the lost heat energy is one of the major goals being pursued by BMW engineers to utilize the dissipated heat energy. Among the most promising innovations are the turbosteamer, thermoelectric generator, engine encapsulation and a waste heat exchanger for oil heating.
The Turbosteamer and Thermoelectric Generator (TEG) projects are focused on generating electric current from waste heat to improve engine efficiency, but each project follows a different approach. There is great potential for fuel savings if the electrical energy required by all an automobile’s systems can be produced using waste heat rather than relying solely on the vehicle's alternator.
Modeled after a power station, the Turbosteamer Project specialists at BMW are working on a heat recovery system based on the principle of a steam process. The process of recovering energy from waste heat is already practiced on a large scale in modern gas and steam power stations that combine 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. The BMW turbosteamer is based on this two-stage stationary power generation method – but reduced in scale.
Researchers proved the feasibility of the technology in December 2005 with the first-generation turbosteamer. They designed a dual-cycle system with the primary high-temperature circuit employing a heat exchanger to recover energy from the exhaust gases.
This was connected with a secondary circuit that collected heat from the engine cooling system and combined it with the high-temperature heat from the primary circuit to create lower temperature heat.
When this design was laboratory tested on four-cylinder gasoline engines, the dual system boosted the performance by 15 percent. In order to further develop the system for production, attention was given to reducing the size and making the system simpler. Thus researchers focused on designing a component having only one high-temperature circuit.
For the latest generation of turbosteamer, engineers developed an innovative expansion turbine based on the principle of the impulse turbine, which offered advantages in terms of cost, weight and size. When completed, this system will weigh only 10-15kg and will be capable of supplying all of the electrical energy required by an automobile while cruising along the freeway or on country roads. Under these conditions, the developers estimated the average driver will be able to reduce fuel consumption by up to 10 percent on long journeys.
All of the system components have been configured into a module that can be integrated in vehicles. This has been done successfully by installing a mock-up system in a BMW 5 Series.
Considerable progress has also been made in the Thermoelectric Generator (TEG) Project that is also focused on production of an energy-saving component. One unit is designed for the exhaust system, while the other is intended for exhaust gas recirculation. The development phase focused on integrating units in the exhaust system, which has led to considerable component improvements, especially in terms of weight and size.
The thermoelectric generator converts heat into electricity and BMW’s engineers o refined a technology used by NASA to power space probes for more than four decades.
The principle behind this technology is known as the Seebeck Effect, namely that an electrical voltage can be generated between two thermoelectric semiconducters if they have different temperatures. Since the percentage degree of efficiency of TEGs was rather low, this technology was previously unsuitable for automotive applications. However, recent progress in material research has led to discoveries that have improved the performance of TEG modules.
The first step was to integrate a thermoelectric generator in the exhaust system to generate electrical current. The first such system was shown to the public in 2008 and delivered a maximum of 200 watts, which was relatively low in terms of power efficiency.
The use of new materials and improvements in the weight and size led to rapid developments, so the latest generation are capable of generating 600 watts. And it’s predicted that it will not be long before 1000 watts is reached.
The current prototype – a BMW X6 – was built as part of a development project funded by the US Department of Energy.
In 2009, the BMW Group decided to integrate the TEG in the radiator of the exhaust gas recirculation system rather than in the exhaust system. Testing showed that 250 watts can be generated while CO2 emissions and fuel consumption are reduced by 2 percent at the same time.
What's more, this energy recovery system offers some added benefits, such as supplying additional engine or passenger compartment heating during cold starts.
Thermoelectric generator is also the ideal counterpart for BMW’s Brake Energy Regeneration. While the brakes generate energy during deceleration and stopping, the TEG functions during acceleration. Researchers forecast that TEGs will lead to fuel consumption savings of up to 5 percent under everyday conditions in the future.
In the future, insulation and encapsulation of the engine compartment will ensure that the temperature of the drive train is stabilized by residual heat even before starting the car, thus shortening the cold-start phase. An exhaust heat exchanger will also keep gearbox oil warm to reduce friction and fuel consumption. And a TEG or turbosteamer will supply the vehicle's electrical systems with ample power.
Depending on the vehicle environment and driving habits, heat management can deliver measurable benefits for specific driving scenarios. For both short and long-distance driving various features can reduce fuel consumption. Insulation of the engine compartment, gearbox oil heating with exhaust heat exchangers installed with gasoline engines, or the heating function of the exhaust heat exchanger for diesel engines, are well-suited to vehicles t driven over short distances. During longer journeys, the thermoelectric generator or turbosteamer add to that. And by utilizing synergy effects, heat management will play a major role in reducing CO2 emissions in the future.