Making some of that waste heat energy available through intelligent heat management offers significant potential for reducing fuel consumption and CO₂ emissions, BMW said. While a small share of the thermal energy available is already used today (for example when warming up the engine or through exhaust gas turbocharging), further, specific improvements will serve to reduce fuel consumption and, accordingly, CO₂ emissions once again by several percentage points.

BMW Group development engineers are working on a number of promising projects for using waste heat, and presented three projects as examples: no more cold starts; heating with waste heat; and a thermoelectric generator.

Encapsulating the engine for heat retention. Click to enlarge.

No more cold starts. Starting the engine cold is a critical phase in fuel consumption, with the engine consuming the largest amount of fuel at this point due to greater internal friction and the high viscosity of the engine oil as long as it is still cold. Compared with an engine already warm, fuel consumption may therefore increase under such circumstances by up to 10%.

We want cars to warm up as quickly as possible, since higher temperatures mean less friction, less friction means less fuel consumption and, therefore, less CO₂.

—Dr Andreas Eder, Head of Heat Management Pre-Development Projects

BMW engineers are working on largely avoiding cold start conditions through intelligent heat management and significantly shortening the warm-up period. The solutions considered include technologies for improving heat insulation on the engine in order to retain heat built up in the engine and engine compartment as long as possible.

A great deal of thermal energy is generated while driving and is available after coming to a stop—energy stored in components such as the engine and transmission. The engineers are therefore seeking to prevent the engine from cooling down quickly and to retain as much residual heat as possible so that the engine is not completely cold when started the next time. Particularly under realistic customer driving conditions, this can contribute to a significant reduction of fuel consumption.

To keep temperatures within the engine compartment at a high level for as long as possible and to avoid having to warm up the engine from the regular ambient temperature, the engine is fully encapsulated. In addition to the air flaps behind the BMW kidney grille already introduced in 2007 in the context of BMW EfficientDynamics, an engine on a prototype that is already developed is completely surrounded by fully clad walls and panels, the engineers using proven materials from the underfloor of the car for insulation purposes.

Components in the engine compartment which previously had to be cooled at a great effort are now protected better from engine heat by the encapsulation. With encapsulation, an engine running at a temperature of 80 °C or 176 °F cools down much more slowly after being switched off and still has a temperature of approximately 40 °C or 104 °F after 12 hours. Studies show that customers park their car for more than 16 hours in a row only in 12% of cases.

With each degree of temperature having a significant influence on fuel consumption, this improvement alone provided by encapsulation reduces fuel consumption by up to 0.2% for each extra degree of temperature (in °C).

This method of maintaining temperatures is equally suited for all kinds of vehicles and in all climate zones, although it is somewhat more effective at low temperatures. Highly efficient dampening of heat on the drivetrain also has some positive side effects.

First, measures previously required to dampen noise in the engine compartment are no longer required, since now the source of noise is insulated directly. This saves weight and improves the acoustic behaviour of the car. Second, the customer benefits from such innovative insulation of the engine compartment not only through lower fuel consumption, but also through additional comfort, since, apart from acoustic improvements, the insulation also helps to warm up the interior faster in cold weather, as the coolant is also kept warm.

Heating with waste heat. Use of the thermal energy contained in the car’s exhaust gas also offers major saving and comfort potentials. Using such heat to warm up the interior helps to reduce fuel consumption by a diesel engine, for example, by up to 10%, depending on the outside temperature and the driver’s driving profile.

With gasoline engines an exhaust gas heat exchanger would be very effective in warming up the drivetrain more quickly to the right temperature, avoiding friction in, for example, the gearbox. Such a heat exchanger conveys heat or thermal energy from one flow to another, in this case the heat of the exhaust gas going to the oil in the automatic transmission, with additional heat being pumped in consistently from the start.

Modern direct injection diesel engines are now so efficient that the energy going into the coolant circuit and, therefore, to the heater is no longer always sufficient to meet the customer’s requirements. It has therefore become quite normal to fit cars with an additional electric heater providing such extra heat on up to 1,000 W of electrical energy.

To deliver such energy for additional heating, the engine has to develop up to 2,000 W&mash;since about twice the amount of mechanical energy is required to provide one watt of electrical energy. In all, therefore, such additional heating involves an increase in fuel consumption of up to one liter.

To avoid this extra fuel consumption the hot exhaust emissions may be used by means of a heat exchanger positioned as close as possible to the catalytic converter and diesel particulates filter, thus providing an additional source of heat for the interior. Heat otherwise lost on the exhaust system therefore serves to warm up the interior as an additional source of driving comfort. Properly designed and engineered, such a system may provide the same heating output as an electrical heater, avoiding the need for electrical heating modules consuming additional fuel.

Not every technology is equally sensible for each type of engine, BMW notes. BMW Group engineers consider which objective can be reached best with which technology and in which context. The exhaust gas heat exchanger, for example, is used in different ways in the BMW Group’s various development projects. It has a greater potential for saving fuel with diesel engines by additional heating of the interior, while on the gasoline engine the exhaust gas heat exchanger serves to shorten the warm-up period required.

The benefits of intelligent thermal management depend not only on the outside temperature, but also on the size and power of the engine, the size of the car and its main use (city traffic or long distances). In the EU test cycle the potential offered is not really noticeable, since here the engine is required from the beginning to start at a temperature of 20–30 °C (68–86 °F). Under realistic everyday driving conditions, on the other hand, the customer will feel the greater fuel economy, BMW says.

Thermoelectric Generator (TEG) integrated in the exhaust gas recirculation. Click to enlarge.

Thermoelectric generator. Having earlier presented the principle of the thermoelectric generator (TEG) as well as a steam-powered auxiliary drive for waste heat to power (earlier post), BMW presented the next level of development in the TEG project. The original TEG development was a stand-alone underfloor solution developed since 2004 also as part of a project promoted by the US Department of Energy. At Innovations Day 2009, BMW Group engineers presented the thermoelectric generator as an integrated component in the exhaust gas recirculation cooler.

In this new stage of development, the TEG is able to deliver up to 250 W of energy under typical customer driving conditions, reducing CO₂ and fuel consumption by up to 2%.

Exhaust gas recirculation (EGR) serves to keep temperatures in the engine low during the combustion process, thus minimizing the generation of nitric oxides. To do this some of the exhaust gas already burnt is fed back and added to the fresh intake air, the mixture burning in the combustion chamber therefore being made up not only of fresh air and fuel, but rather fresh air, fuel, and residual gas. This residual gas takes up combustion heat and thus reduces the peak temperature in combustion as well as the generation of nitric oxides.

To cool the residual gas and recirculate it as required, EGR comes with a water cooler (operating through the engine coolant) and a control flap, thus offering ideal conditions for enhanced efficiency, since, apart from water cooling on the cold side and a flap controlling the flow of exhaust gas, the EGR-TEG only requires thermoelectrical material.

“With the number of components limited to a minimum, the exhaust gas recirculation cooler is a very elegant solution which may quickly reach production level.”” —Dr. Andreas Eder

This material is taken up by the hot exhaust gas directly downstream of the exhaust gas manifold (upfront of the exhaust gas turbine) where the temperatures available are a lot higher and electricity can be generated more efficiently in the TEG.

A particular challenge in the integration of the thermoelectric generator is that the thermoelectric materials used have an insulating effect. While this increases the space required for the EGR, it continues to cool the residual gas with virtually no loss of pressure, even generating electric power in the process.

Thermoelectric Generator integrated into the exhaust gas recirculation system. Click to enlarge.

Engineers at the BMW Group are continuing their efforts to further improve the TEG. Currently they are looking for the best technology for integrating the thermoelectrical material efficiently into the EGR cooler and, with the steel housing on the heat exchanger, to make the TEG solution as light as possible.

Engineers at the BMW Group are continuing their efforts to further improve the TEG. Currently they are looking for the best technology for integrating the thermoelectrical material efficiently into the EGR cooler and, with the steel housing on the heat exchanger, to make the TEG solution as light as possible.

A further potential lies in the improved distribution of heat through the thermoelectrical material. The challenge in this case is to develop an intelligent structure providing as much heat as possible without at the same time losing too much pressure within exhaust gas recirculation.

Since only some of the exhaust gas is available within the recirculation process, the integrated TEG solution does not yet use the full potential for generating energy through heat. It is, however, an important project in the implementation of future TEG concepts.

BMW anticipates fitting a larger thermoelectric generator in the exhaust system in future—either as an underfloor solution or directly integrated in the catalyst. However, this solution is a lot more elaborate and involves a far greater design and construction effort than the integration of the TEG unit in the exhaust gas recirculation system.

Integration in the main exhaust gas pipe is nevertheless an attractive option for the future, since here, due to the greater mass flow, more electricity could be generated. Turning the energy generation process around, with electricity going from a battery into a semi-conductor (the Peltier Effect), the electric power unit might also serve to heat the catalyst in order to reduce untreated emissions when starting the engine cold. Then, once the exhaust gas has reached the temperature required and the catalyst is operating at its regular temperature, the process is turned around in order to generate electric power. In all, this offers a potential improvement of fuel economy under the customer’s typical driving conditions of up to 5%, according to BMW.