The influence of cylinder-to-cylinder variation in EGR distribution on the NOx–PM trade-off (while varying EGR rate) is studied on an automotive high-speed direct injection Diesel engine. Experiments have been conducted on an engine test bench with and without air-EGR mixer and demonstrate that variations in cylinder-to-cylinder EGR distribution results in a deteriorated NOx–PM trade-off (increased NOx emissions level at a given PM emissions level, or increased PM emissions level at a given NOx emissions level) compared to the well mixed configuration with equal EGR rate for all the cylinders. A qualitative study as well an original experiment is conducted to explain this emissions increase induced by unequal distribution of EGR. When recirculating hot exhaust gases, the emissions increase is due to cylinder-to-cylinder variations in intake gas composition and temperature.
Future emissions regulations like EURO 6 in Europe force Diesel engine manufacturers to find ever more complex ways to reduce exhaust gas pollutant emissions, in particular NOx and particulate matter (PM) emissions. Exhaust gas recirculation (EGR) into the engine intake is an established technology to reduce NOx emissions [1] and [2]. The decrease of NOx emissions with EGR is the result of complex and sometimes opposite phenomena occurring during combustion [3], [4], [5], [6], [7], [8], [9] and [10].
At the same time, the decrease in combustion temperatures and oxygen concentration while increasing EGR rate reduces both soot production in the spray core and soot oxidation in the diffusion flame around the jet [11]. Thus the final impact of EGR on PM emissions is complex and is the result of contradictory phenomena. In the conventional Diesel high-temperature combustion (HTC), the increase of EGR rate (at constant boost pressure) is accompanied by an increase of PM emissions, resulting in a trade-off between NOx and PM emissions while varying EGR rate [4], [12], [13], [14] and [15].
Moreover, practical EGR systems often lead to EGR unequal distribution from cylinder to cylinder, air and EGR being imperfectly mixed. This phenomenon has been studied by various researchers [16], [17], [18], [19], [20], [21], [22], [23] and [24]. By measuring CO2 instantaneous concentration at each inlet port during the intake stroke owing to mid-infrared laser absorption spectroscopy, Green [16] has demonstrated that even when operating at a steady condition, the engine’s EGR system can produce large temporal variations in the EGR concentration within the flow of fresh charge during the intake stroke, that are different for each cylinder. Furthermore, CFD analyses [17], [18], [20] and [21] have demonstrated that a standard engine’s EGR system results in a highly stratified concentration field within the inlet manifold. Many experimental and numerical studies [17], [19], [23] and [24] have proposed improved inlet manifolds or air–EGR connections in order to improve cylinder-to-cylinder EGR distribution.
If some studies have shown that cylinder-to-cylinder variations in EGR can lead to higher NOx and PM emissions compared to a configuration where the EGR is equally distributed among all cylinders [17], the influence of on the NOx–PM trade-off (while varying EGR rate) has not been experimentally studied in details or explained. Thus, the aim of this study is to quantify and explain the influence of this phenomenon on the NOx–PM trade-off (while varying EGR rate at constant boost pressure) of an automotive HSDI Diesel engine.
In this study, the influence of cylinder-to-cylinder variations in EGR distribution on the resulting NOx–PM trade-off (while varying EGR rate) has been experimentally investigated on an automotive high-speed direct injection Diesel engine. Main conclusions are as follows:
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Unequal EGR distribution results in increased NOx and PM emissions compared to engine running with well mixed air and EGR gases.
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The increase in emissions is due to cylinder-to-cylinder variations in both gas composition and intake temperature.
From the above experiments, it is concluded that the suppression of unequal cylinder-to-cylinder EGR distribution results in a large reduction of NOx and PM emissions, especially when running with high EGR rates. An optimised air–EGR connection will be one of the ways to achieve future emissions standards.
