Search
Cooled Exhaust Gas Recirculation system applied to gasoline engines
The primary function of Cooled EGR Systems has been to reduce the formation of nitrogen oxides in diesel engines. Recently, Cooled EGR has been applied to gasoline engines to improve fuel economy by allowing higher compression ratios.
Valeo is a European market leader in exhaust gas recirculation; offering systems that further improve dynamic and thermal performance.
The success of diesel engines in Europe is largely due to their low fuel consumption and reduced CO2 emissions. The growth in diesel engine’s market share has been made possible by reducing pollutant emissions with optimized combustion and post-treatment devices. The carbon oxide (CO) and unburnt gas (HC) are reduced by a catalytic converter and the particles are processed in a self-regenerating filter (DPF: Diesel Particle Filter). The nitrogen oxides (NOx) are reduced by the Exhaust Gas Recirculation (EGR) system. The EGR system takes part of the exhaust gases at the cylinder head outlet and re-injects them into the air intake.
The main benefit is that NOx is reduced at the source by limiting the quantities formed in the combustion process, rather than by post-treating the gases. Consequently, the combustion process is cleaner. The gases are only recirculated under specific conditions, when the engine is idling or only lightly loaded. An electric valve fully or partially opens the recirculating circuit, depending on the operating phase of the engine. The valve is driven by a powerful electric motor that guarantees reliable operation, even under the severe constraints imposed by temperature, exhaust pressure, condensates and a highly corrosive environment. The maximum throughput is high and the control of the valve position allows the recirculation rate to be precisely adjusted. The system works in association with an electric throttle valve on the intake side that reduces the quantity of intake air. It is completed by a heat exchanger between the recirculated gases and the engine’s cooling circuit, which lowers the temperature, thereby increasing the reduction of NOx. Valeo is one of Europe’s leading EGR system suppliers.
With the expert knowledge of the Engine Management Systems and Engine Cooling branches, Valeo has the skills required for research and development of the exhaust gas recirculation function. EGR systems for gasoline engines are also available to reduce CO2 emissions.
The new electric EGR valve is made up of an electric motor, gearing, a cam system, a valve and a position sensor. With a high-power DC motor, the valve can be fully opened or closed in just 100 and 75 milliseconds respectively. One of the system’s most distinctive features is the progressive cam-driven control device that converts the rotary movement of the engine into controlled downward travel of the valve. The valve’s opening mechanism must be very reliable; in a highly corrosive environment, it must take into account the risk of the valve being jammed by soot deposits or oil combustion residues. The rotation/travel ratio of the cam increases the thrust force by significantly gearing down the movement. Once the valve is open, the second priority is the speed of travel of the valve. The gradient of the cam is then gradually increased. The cam also ensures that the valve can be closed tightly and quickly. The cams are linked to the valve by two bearings that eliminate friction and risk of seizure. Valeo has opted for an outward-opening valve for greater dependability, since the high exhaust gas pressure generated by a turbocharger applies a force that tends to close the valve. Pressure loss in the gas recycling circuit is low thanks to the clearance area, the aerodynamics of the valve head and the small diameter of the shaft. This property is especially important when idling and with very low loads. A Hall-effect sensor precisely measures the position of the valve. This is an important point, since a precise recirculation rate reduces the flow tolerances and enables the engine to comfortably meet the requirements of the Euro IV standard and, more importantly, the Euro V standard.
The capacity to cool the exhaust gases is another major factor contributing to the reduction of NOx, and Valeo has succeeded in improvingthe heat exchange capacity with engine coolant. The round corrugated tubes developed for Euro IV have been replaced by flat corrugated tubes to meet Euro V emission standards. This increases the heat exchange surface, resulting in thermal efficiency of 85%, and a reduced exhaust gas pressure drop. A U-shaped heat exchange circuit reduces the dimensions of the part. The gases can bypass the cooler via a pneumatically controlled flap. This function is essential for Euro V-compliance and is useful after engine cold start, when the temperature in the combustion chamber must increase quickly in order to minimize toxic emissions.
High quantities of nitrogen oxides are produced in combustion at high temperatures with excess air. Consequently, both of these parameters must be reduced. By recircuating part of the exhaust gases into the air intake, the mass of intake air is reduced, as are the quantities of oxygen and nitrogen available for combustion. The mass of intake air is also limited by partly closing the intake circuit. Lower quantities of oxygen also help to lower the combustion temperature. The heat exchanger, which is cooled by the engine’s coolant, lowers the temperature of the recycled exhaust gases, which in turn reduces the combustion temperature. The temperature can be further reduced by incorporating the UltimateCooling™ concept, which is capable of further lowering the temperature of the coolant whenever necessary.Exhaust gases are only recycled under certain conditions, when the engine is idling or only partly loaded, because in these circumstances, combustion occurs with excess air.
© Copyright Valeo Management Services 2008
