Overview: BMW’s waste heat research

When we burn fuel in the very best modern internal combustion engine, about 60 per cent. of the energy is wasted. Half of the lost energy taks the form of exhaust gas heat. The rest is absorbed by the engine’s cooling system. Finding ways of recovering this lost heat energy is one of the many ways we can extend the life of the internal combustion engine well into the age of electricity.

The BMW Group is involved in several projects aimed at catching and using the heat energy that is currently lost. The projects are operating at various stages of maturity: in research, pre-production and series development. 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 focus on generating electric current from waste heat to improve overall engine efficiency, but each project follows its own approach and time-frame.

There is potential for considerable fuel savings if the electrical energy required by all of the systems in a vehicle can be produced using waste heat rather than relying solely on the vehicle’s generator.

Turbosteamer: modelled on a power station.

The Turbosteamer project is based on the principle of a steam process. The recovery of energy from waste heat is already practised on a large scale in modern power generation plants: 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. 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 obviously reduced in scale and designed to form a component that can be used in a modern car engine.

The first-generation turbosteamer used what BMW calls a ‘maximalist’ approach. The feasibility of the technology was demonstrated in December 2005 with the unveiling of the first-generation turbosteamer, which was based on a dual-cycle system rather like a power-station. The primary element was a high-temperature circuit that employed a heat-exchanger to recover energy from the engine exhaust gases. This was connected with a secondary circuit that collected heat from the engine cooling system and combined this heat with the high-temperature heat from the primary circuit.

When this design was laboratory-tested on the four-cylinder petrol engines produced by BMW at the time, the dual system boosted the performance of these engines by 15 per cent.

BMW Turbosteamer system showing exhaust gas temperatures: cool beyond generator box.

Today’s turbosteamer is smaller and simpler. In order to develop the system further for use in series production, attention was given to reducing the size of the components and making the system simpler in order to improve its dynamics and reduce manufacturing costs. The Company decided to design a system that used only one high-temperature circuit.

A heat-exchanger recovers heat from the engine exhaust, and this energy is used to heat a fluid which is under high pressure. The fluid then vapourises, powering an expansion turbine that generates electrical energy from the recovered heat. The expansion turbine is based on the principle of the impulse turbine, which offers advantages in terms of cost, weight and size when compared to earlier concepts.

BMW’s original goal five years ago was to develop a system ready for series production within about 10 years, and the Company considers that it has ‘made considerable progress’ towards achieving this.

When completed, this system will weigh around 10kg to 15kg, and will be capable of supplying all of the electrical energy required by the car while cruising along the motorway or on country roads. BMW estimates that a normal driver should be able to reduce fuel consumption by ‘up to 10 per cent.’ on long-distance journeys.

All of the system components developed on the test bench have been configured to form a module that can be integrated in vehicles. This has been done successfully by installing a mock-up system in a BMW 5 saloon.

Thermoelectric generator

BMW has also made progress in its Thermoelectric Generator (TEG) project. This is also intended ultimately to result in an energy-saving component suitable for series production.

The two alternative systems developed to date differ in their positioning in the vehicle. One unit is designed for the exhaust system, while the other is intended for the exhaust gas recirculation (EGR) system. The development phase, which focused on integrating units in the exhaust system, has so far led to (amongst other things) substantial reductions in the mass and size of components.

The thermoelectric generator converts heat directly into electricity. BMW used a technology that has been used by NASA to power space probes for more than four decades. The principle behind this technology is known as the Seebeck effect: an electrical voltage can be generated between two thermoelectric semiconducters if they have different temperatures. Since the efficiency of TEGs has traditionally been rather low, this technology was considered unsuited to automotive applications. However, in recent years, progress in the area of materials research has led to developments that have improved the performance of TEG modules.

The first step taken by engineers 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 200W — relatively low in terms of power efficiency. But the use of new materials and improvements in the weight and size of the TEGs means that the latest generation of TEGs installed in a car exhaust are capable of generating 600W of electrical power, and it is thought likely that the technology will soon be capable of generating 1kW. The current prototype — a BMW X6 — was built as part of a development project funded by the U.S. Department of Energy.

In 2009, the BMW Group unveiled an alternative development in this project. Rather than installing the TEG as a separate module in the exhaust system underneath the vehicle, engineers decided to integrate the TEG in the radiator of the EGR system. In this configuration, customer testing has shown that 250W can be generated while CO2 emissions and fuel consumption are reduced by 2 per cent. at the same time.

Additionally, this energy recovery system offers some interesting added benefits, such as supplying the engine or passenger compartment heating with additional warmth during cold starts. And the thermoelectric generator works well with a brake energy regeneration system, complementing it. While the brakes generate energy during deceleration and stopping, the TEG functions at its best when the engine is under load. Researchers forecast that TEGs will lead to fuel consumption savings of up to 5 per cent. under real everyday driving conditions in the future.

In the future, even before starting the car, insulation and encapsulation of the engine compartment will ensure that the temperature of the drive train is stabilised by residual heat, thus shortening the cold-start phase. An exhaust heat exchanger will also keep gearbox oil warm to reduce friction and fuel consumption as well. And a TEG or turbosteamer will supply the vehicle’s electrical systems with power.

Depending on the vehicle environment and driving conditions, heat management can deliver measurable benefits for specific driving situations. 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 petrol engines, or the heating function of the exhaust heat exchanger for diesel engines, are features that are well-suited for vehicles that are predominately driven over short distances. During longer journeys, the thermoelectric generator or turbosteamer add to that.

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