Supercharging a single-cylinder diesel engine has proved to be a viable methodology to reduce engine-out emissions and increase full-load torque and power. The increased air availability of the supercharger (SC) system helps to inject more fuel quantity that can improve the engine's full-load brake mean effective pressure (BMEP) without elevating soot emissions. However, the increased inlet temperature of the boosted air and the availability of excess oxygen can pose significant challenges to contain oxides of nitrogen (NOx) emissions. Hence, it is important to investigate the potential NOx reduction options in supercharged diesel engines. In the present work, the potential of low-pressure exhaust gas recirculation (LP EGR) was evaluated in a single-cylinder supercharged diesel engine for its benefits in NOx emission reduction and impact on other criteria emissions and brake specific fuel consumption (BSFC).
A mass-production single-cylinder diesel engine was used for the present work after reducing its compression ratio (CR) from 18:1 to 14:1 that was needed to contain the peak firing pressure (PFP) with the SC setup. The experimental investigations revealed that the migration from the stock naturally aspirated (NA) intake system to the supercharged system resulted in a considerable increase in NOx emissions across the loads. Hence, LP EGR was introduced in the engine by an additional circuit that connected the diesel particulate filter (DPF) to the inlet port of the supercharger. An electric valve was used to control the mass flow rate of the EGR. The results concluded that the introduction of LP EGR and increased EGR rates up to 20% could reduce the NOx emissions by 52% at a reference operating point of 2000 rpm, 40 N-m. Moreover, the benefits could be obtained without any penalty on BSFC. On the contrary, in the SC system without LP EGR, for the same level of NOx reduction, the main injection timing had to be retarded significantly, resulting in a 5 % penalty on BSFC. Thus, the LP EGR system was highly beneficial in supercharged diesel engines to reduce engine-out NOx emissions without elevating the BSFC results.