A novel concept of active pre-chamber engine with a single injector – The passive main chamber approach

Abstract

The pre-chamber spark ignited combustion is recognized as one of the most promising concepts utilizing a lean burn combustion approach towards the increase of efficiency and reduction of NOx emissions in spark ignited engines. Numerous investigations indicate the possibility to reduce the engine-out NOx emissions under the regulation limits by using active pre-chamber at very high dilution levels. Regardless of the dilution approach, one of the main challenges is to achieve the proper mixture formation and scavenging of the pre-chamber from residual gases to ensure stable engine operation without misfires. Recent investigations imply that the auxiliary air injection in the pre-chamber might be necessary to ensure stable operation at such highly diluted mixtures, especially if an EGR dilution approach is utilized, further increasing the complexity of such a concept. In this work, a new approach is investigated which can be described as a passive main chamber approach and relies on a single fuel injector supplying both the pre-chamber and the main chamber. Contrary to the passive pre-chamber systems, the injector in this approach is mounted in the pre-chamber, while the main chamber is supplied indirectly through the pre-chamber nozzles. To verify the feasibility and the potential benefits of such an approach, a numerical framework consisting of 3D-CFD and 1D/0D models is applied. The performed study included different pre-chamber variants, previously investigated on an experimental gasoline-fuelled engine, to verify the applicability of such a concept regardless of the pre-chamber geometry. The performed study showed that the proposed concept is a feasible option and, despite the inability to directly control the excess air ratio in the pre-chamber, results with the desired values of λPC ≈ 0.8 at the MBT optimized spark timings. At very lean mixtures (λglobal > 2.0), the average residual gas concentration in the pre-chamber and around the spark plug at the ignition timing is reduced from 10 % and 13 % respectively to 2.5 %, along with a 1.4 percent point increase of indicated efficiency when compared to normal operation. When compared to the reference gasoline fuelled cases, a highest efficiency of 37.7 % is achieved at moderately lean mixture (λglobal ≈ 1.6) and MBT optimized spark timing with NOx levels of 1.41 g/kWh, which represents a 0.7 percent point increase in indicated efficiency but 75 % increase of NOx emissions. At NOx optimized spark timing however (NOx < 0.4 g/kWh), an identical indicated efficiency of 34.7 % is obtained. Overall, the results indicate that with the proposed concept, it is possible to achieve the same scavenging effect as with auxiliary air injection while eliminating the need for both the main chamber injection system and the pre-chamber auxiliary air supply system and without sacrificing the overall performance.