Exhaust gas recirculation is a technique commonly used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. This technique has proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. The exhaust gas recirculation technique primarily involves the recirculation of exhaust gas by-products into the intake air supply of the internal combustion engine. This exhaust gas thus reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxide. Furthermore, the exhaust gases typically contain a portion of unburned hydrocarbon which is burned on its reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
Another technique useful in the control and reduction of undesirable emissions from internal combustion engines is the use of pressure-charged intake air. This permits the use of relatively smaller cubic displacement and lighter weight internal combustion engines in mobile equipment, reducing in turn the specific fuel consumption of the vehicle and overall mass of the vehicle necessary to perform a given function. In addition to the benefits of reduced size and mass, the typical pressure-charging device may be controlled to provide improved emissions characteristics. Pressure-charging machines suitable for such applications include the exhaust gas driven turbocharger which is comprised typically of an exhaust gas driven turbine linked to a turbine disposed in the intake air stream to provide compression of the intake air. The typical turbocharger is controlled by providing a gate which controls exhaust gas flow and gates exhaust gas to bypass the exhaust gas turbine and control the charging rate of the turbocharger so that the maximum pressure limits of the associated internal combustion engine are not exceeded.
Still another technique for controlling emissions used by many engine manufactures is the use of aftercooling the intake air thereby reducing the intake manifold temperature. Some of the related art techniques have also considered intercooling the EGR gases by routing the recirculated exhaust gases through an aftercooler. It is well known that lower intake manifold temperatures tends to reduce the formation of nitrous oxides found in the exhaust.
When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted directly from the exhaust manifold. Likewise, the recirculated exhaust gas is preferably re-introduced to the intake air stream downstream of the compressor and air-to-air aftercooler. For example, in many EGR applications the recirculated exhaust gas is reintroduced to the intake manifold.
Reintroducing the exhaust gas downstream of the compressor and air-to-air aftercooler is preferred due to the reliability and maintainability concerns that arise should the exhaust gas is passed through the compressor and aftercooler. However at some engine operating conditions, there is a pressure differential between the intake manifold and the exhaust manifold which essentially prevents many conventional EGR systems from being utilized. For example, at high speed, high load conditions in a turbocharged engine, the exhaust gas does not readily flow from the exhaust manifold to the intake manifold. What is needed, therefore, is a simple and inexpensive technique for recirculating exhaust gas from the exhaust manifold to the intake manifold at all engine operating conditions.
Another problem associated with many conventional EGR systems is that the turbocharger efficiency is sacrificed when exhaust gas is diverted from the exhaust manifold. Removing the exhaust gas to be recirculated from the exhaust manifold or elsewhere upstream of the exhaust gas driven turbine depletes the mass flow and heat energy passing through the turbine which, in turn, lowers the boost levels created by the compressor. Most diesel engine turbochargers are fixed geometry turbochargers, in that they are specifically designed to operate efficiently when matched to the engine exhaust flow output. The reduction in mass flow and pressure due to the EGR creates a mismatch between the exhaust flow to the turbocharger and the turbine specifications during EGR operation. The mismatch results in a turbocharger output that is reduced in percentage more than the percentage reduction in exhaust flow to the turbocharger thereby creating significant losses in airflow and boost pressure. The reduction in airflow and boost pressure decreases the air to fuel ratio down to a point where particulates as well as the brake specific fuel consumption (BSFC) increase. Disadvantageously, the reduction in airflow and boost pressure also results in a noticeable difference in engine performance to the operator depending on whether EGR is on or off.