Engine Air, EGR and Cooling
Intake air helps reduce emissions
Downsizing of engines has been embraced by almost every automotive OEM as the KEY approach to reducing fuel consumption. Basically this involves reducing an engine’s displacement while at the same time still using a comparable mass of air for each intake stroke. As a result, downsizing in most cases involves the combined use of boosting (turbocharging or supercharging) together with direct fuel injection. In some cases such as at Audi and Toyota advanced fuel injection concepts have involved a combination of port and direct fuel injection approaches.
The exhibit below shows the many drivers that are resulting in changes to powertrain design. These key drivers stem from changing expectations of consumers, developments in government regulations, changing performance requirements sought by the automotive OEMs (primarily towards reduced fuel consumption) and changes to the supply base.
The moves to downsized engines have resulted in significant improvements to boosting systems, both superchargers and turbochargers. These systems may also be used in combination on an engine with the most notable recent example being by Volvo for one of its gasoline four-cylinder engines. Compared with turbocharging, supercharging provides an almost lag-free response. The use of a supercharger also reduces the thermal mass of the exhaust system since no exhaust turbine is used. This helps the exhaust system to reach its light-off temperature more rapidly. Currently there is intense interest in the use of eBoosters in combination with a traditional turbocharger so as to overcome turbo lag.
A number of OEMs have introduced TwinTurbo systems for improved performance from their premier engines. General Motors has introduced a twin-turbocharged V-6 engine for its 2014 Cadillac CTS and VTS. The twin turbo system was designed so that it could be packaged for the 90° mounting orientation of the transverse front-drive V-6 engine. Furthermore, the top of the engine also includes the air intake box together with the charge air cooler and throttle body. This tight packaging provides short airflow paths from the air filter box, through the charge air cooler, the throttle body and into the intake ports of the cylinders. The turbochargers used on the Cadillac CTS are supplied by Mitsubishi Heavy Industries.
As the automotive OEMs strive towards developing engines with improved fuel consumption and improved performance (power, torque, “fun to drive”), there have evolved a number of opportunities for engine air, EGR and cooling systems to help in enabling these goals. These are illustrated in the following exhibit.
The move to downsized engines has had a major impact on the performance needs of charge air coolers (CAC). In fact, it is the very cooling of the charge air that increases the charge air density so that the needs of downsizing are met. The increase in charge air density compared with that for a naturally aspirated engine needs to make up for the loss in engine displacement as a result of downsizing. With downsized engines, CACs are required to transfer more thermal energy and in turn, increase the density of the charge air. Furthermore, cooling the charge air reduces the propensity of the engine to knock.
The impact that the CAC can have on increasing the air density is dependent on the temperature of the air reached at the output of the compressor. In general, temperatures at the compressor have been limited to 190 to 210°C so as to avoid the need for expensive materials. Boost pressures are frequently greater than 2 bar. More recently, some outlet temperatures have been increasing as a result of an increase in the boost pressure. This increase in boost pressure and temperature has required development efforts in improved materials.
As the industry moves to more stringent emission and fuel consumption levels, there is a need for improved control of air intake temperatures. Indirect CACs cool the charge air with coolant and the coolant itself is then cooled by ambient air. Indirect CACs are increasingly being selected by the OEMs as a methodology to enhance control of charge air temperatures entering the combustion chamber. The higher heat capacity of coolant compared with air improves the heat transfer characteristics for energy removal from the charge air and as a result, the charge air is more effectively cooled and its temperature more effectively controlled. Some in the industry refer to indirect CACs as iCACs.
The move towards the use of indirect CACs has opened the door to the possibility of integrating the CAC within the intake manifold. Examples of such systems can now be found at BMW, Cadillac, Renault, Smart, and Volkswagen.
Integration has been a major enabler of the many success of engine air, EGR and cooling systems in helping an engine meet its performance goals. Examples of such integration is shown in the following exhibit.