Study of an EGR System for Downsizing Turbocharged Gasoline Engine to Improve Fuel Economy 2014-01-1199
This paper presents a study of a cooled exhaust gas recirculation (EGR) system applied to a turbocharged gasoline engine for improving fuel economy. The use of a higher compression ratio and further engine downsizing have been examined in recent years as ways of improving the fuel efficiency of turbocharged gasoline engines. It is particularly important to improve fuel economy under high load conditions, especially in the turbocharged region. The key points for improving fuel economy in this region are to suppress knocking, reduce the exhaust temperature and increase the specific heat ratio.
There are several varieties of cooled EGR systems such as low-pressure loop EGR (LP-EGR), high-pressure loop EGR (HP-EGR) and other systems. The LP-EGR system was chosen for the following reasons. It is possible to supply sufficient EGR under a comparatively highly turbocharged condition at low engine speed. It is important for knocking suppression to remove nitrogen oxides (NOx) from the EGR gas, which means using EGR gas from the catalyst downstream.
On the other hand, the lower differential pressure and longer EGR route that characterize the LP-EGR system make it necessary to apply more sophisticated EGR rate control. EGR flow is generated by the differential pressure at the EGR valve, which varies according to the air flow rate, so the EGR rate can be maintained with a constant EGR valve opening. Although this principle is usable only in a steady-state condition, we have developed a new compensation control that can maintain the EGR rate even under transient conditions by estimating the pressure delay at the point in the exhaust pipe where the EGR gas is extracted.
Test results showed that fuel economy was improved by as much as 5% with LP-EGR under a turbocharged condition, and the exhaust temperature was also reduced.