ElsevierBenajes Calvo, JV.; Martín Díaz, J.; Novella Rosa, R.; Thein, KJL. (2016). Understanding the performance of the multiple injection gasoline partially premixed combustion concept implemented in a 2-Stroke high speed direct injection compression ignition engine. Applied Energy. 161:465-475. doi:10.1016Energy. 161:465-475. doi:10. /j.apenergy.2015 Understanding the performance of the multiple injection Gasoline PPC concept implemented in a 2-stroke HSDI diesel engine with good combustion stability (CoV (IMEP) below 3%), high combustion efficiency (over 97%), and low NO x /soot levels.In this context, present research focuses on the two main specific drawbacks of this concept.Firstly, the high BSFC resulting from the work required by the mechanical supercharger to sustain the needs in air management since the turbocharging system is not sufficient at low speeds. Secondly the high level of noise generated by the combustion process is known as a matter regarding customers' expectations, especially at high loads. Therefore, a dedicated analysis has been carried out to fully exploit the benefits of the gasoline PPC concept combined with the innovative 2-stroke engine architecture with the aim of identify and break the most relevant trade-offs.
Partially Premixed Combustion (PPC) using gasoline-like fuels has proven its potential to control or even break the NO x and soot emissions trade-off, retaining the high efficiency levels characteristic of the conventional diesel combustion (CDC) concept. However, selecting an appropriate fuel and a suitable injection strategy is essential to assure a successful PPC operation in the full engine map. Additionally, extending the limit of PPC beyond 10 bar IMEP was not possible due to excessively high pressure gradients and onset of knocking-like combustion, so the CDC concept has to be adopted and the conventional trade-off between NO x and soot emissions was recovered. Present investigation focuses on evaluating the use of a multiple injection strategy for extending the load range of the PPC concept to medium/high load conditions, when using a commercial RON95 gasoline in a 2-stroke engine under development. Experimental results confirm how with a fine tuned triple injection strategy it is possible to reach extremely low NO x and soot levels keeping combustion efficiency over 96%, while indicated efficiency is improved compared to a well-optimized point obtained operating with the CDC concept. Finally, the research work is completed by including 3D-CFD modeling activities that are carried out to contribute to the understanding on how the mixture preparation and stratification prior to the start of combustion impacts its development and particularly the experimentally observed pollutant emissions trends.
New combustion concepts and engine designs are being currently investigated in order to comply with upcoming pollutant regulations and reduce fuel consumption. In this context, two-stroke architectures appear as a promising solution for the implementation of some combustion concepts. However, scavenging processes in a two-stroke engine are much more challenging than for a four-stroke engine, and the residual mass of burnt gases retained inside the cylinder needs to be properly determined in order to keep control over the in-cylinder composition, hence over the combustion conditions and pollutant emissions. In this study, a new methodology for the estimation of the internal residual gas fraction is introduced, which is based on the thermodynamic processes occurring in the engine investigated and makes use of basic engine instrumentation and measurement equipment usually available in a conventional test cell. Several versions of the estimator were developed so that different requirements could be met, such as those of real-time estimation on an engine test bench but with reduced precision or, on the contrary, highly precise but time-consuming computations for post-processing purposes and combustion diagnosis. The consistency of the internal residual gas estimator was then validated through its application to real engine tests at different operating points.
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