Combined Low and High Pressure EGR for Higher Brake Efficiency with Partially Premixed Combustion
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The concept of Partially Premixed Combustion (PPC) in internal combustion engines has shown to yield high gross indicated efficiencies, but at the expense of gas exchange efficiencies. Most of the experimental research on partially premixed combustion has been conducted on compression ignition engines designed to operate on diesel fuel and relatively high exhaust temperatures. The partially premixed combustion concept on the other hand relies on dilution with high exhaust gas recirculation (EGR) rates to slow down the combustion which results in low exhaust temperatures, but also high mass flows over cylinder, valves, ports and manifolds. A careful design of the gas exchange system, EGR arrangement and heat exchangers is therefore of utter importance. Experiments were performed on a heavy-duty, compression ignition engine using a fuel consisting of 80 volume % 95 RON service station gasoline and 20 volume % n-heptane. A wide range of engine speeds and loads were run using a low pressure EGR system. The experiments served as a validation basis for a one-dimensional simulation model. Using the model, a comparison between low pressure EGR, high pressure EGR and combined low and high pressure EGR was performed. The results showed that the combined low and high pressure EGR configuration could reach an average brake efficiency of 41.6 % while the low pressure EGR and high pressure EGR reached 39.5 % and 39.9 % respectively. The combined configuration reached a higher efficiency because it decreased the mass flow range in which the turbine and compressor needed to work which resulted in a higher overall turbocharger efficiency. The effect of varying the EGR and charge air cooler gas outlet temperatures was also studied. It was concluded that a higher cooler temperature decreased both the brake efficiency and the maximum achievable load due to a higher in-cylinder heat transfer loss.