SAGE PublicationsBenajes Calvo, JV.; Novella Rosa, R.; De Lima Moradell, DA.; Tribotté, P. (2015). Analysis of combustion concepts in a newly designed two-stroke high-speed direct injection compression ignition engine. AbstractTwo main research paths are being followed to develop compression ignition (CI) engines, the extreme optimization of the conventional diesel combustion (CDC) concept and the development of alternative combustion concepts. The optimization of the CDC concept focuses on exploring the potential of the flexibility provided by the new engine subsystems to control the combustion development in an attempt to improve pollutant emissions and efficiency levels. In the frame of the development of alternative combustion concepts, the Partially Premixed Combustion (PPC) using high volatility and low reactivity fuels, such as gasoline-like fuels with different octane numbers, has been extensively evaluated in 4-stroke engines, confirming its benefits in terms of emissions and efficiency at medium to high loads, but also its critical problems at low loads including difficulties to assure stable ignition and engine efficiency deterioration. The already confirmed high flexibility of the innovative 2-stroke poppet valves high speed direct injection (HSDI) CI engine under development in terms of air management settings to control the cylinder conditions and affect final combustion environment encouraged the authors to perform a detailed optimization of the CDC concept in an attempt to find the real limits of this engine configuration. Additionally, its compatibility with the PPC concept using a high octane fuel (Research Octane Number 95) with a triple injection strategy for reducing pollutant emissions at medium-to-low load conditions has been evaluated considering also the impact on engine efficiency.Results at low speed and medium load confirm how the engine configuration provides potential for attaining stateof-the-art emission levels operating with the CDC concept, and how emissions and efficiency can be optimized by adjusting the air management settings without facing any additional trade-off aside from that usual between NO X and soot. The feasibility of combining this engine configuration with the PPC concept using gasoline as fuel for controlling pollutant emissions has been also corroborated and, with a fine tuned triple injection strategy, engine efficiency even improves compared to that obtained operating with a well optimized CDC concept.

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ElsevierBenajes Calvo, JV.; Novella Rosa, R.; De Lima Moradell, DA.; Tribotte, P.; Quechon, N.; Obernesser, P.; Dugue, V. (2013). Analysis of the combustion process, pollutant emissions and efficiency of an innovative 2-stroke HSDI engine designed for automotive applications. Applied Thermal Engineering. 58(1-2): 181-193. doi:10.1016/j.applthermaleng.2013 AbstractOn the last years engine researchers has been focused on improving engine efficiency in order to decrease CO 2 emissions and fuel consumption, while fulfilling the increasingly stringent pollutant emissions regulations. In this framework, engine downsizing arises as a promising solution, and 2-stroke cycle operation offers the possibility of reducing the number of cylinders without incurring in NVH penalties. An experimental investigation has been performed to evaluate the performance of a newly-designed poppet valves 2-stroke engine, in terms of finding the proper incylinder conditions to fulfill the emission limits in terms of NO X and soot, keeping competitive fuel consumption levels. Moreover, present research work aims to improve the existing knowledge about the gas exchange processes in a 2-stroke engine with poppet valves architecture, and its impact over the combustion conditions, final exhaust emissions levels and engine efficiency. The experimental results confirm how this engine architecture presents high flexibility in terms of air management control to substantially affect the in-cylinder conditions. The in-cylinder oxygen concentration and density, which are the product of a given trapping ratio and delivered mass flow, were linked to pollutant emissions and performance by their impact on instantaneous adiabatic flame temperature and spray mixing conditions. After the optimization process, it was possible to minimize simultaneously NO X , soot and indicated fuel consumption, without observing a critical trade-off between the pollutant emissions and the fuel consumption.

Partially Premixed Combustion (PPC) of fuels in the gasoline octane range has proven to combine low NOX and soot emissions with high indicated efficiencies, while still retaining control over combustion phasing with the injection event. Previous research performed in four-stroke engines, has shown how the operating region where gasoline PPC concept can be successfully implemented is largely linked to the octane number of the fuel, making difficult to cover the entire load range with a fixed fuel.In this framework, 2-stroke engines arise as a promising solution to extend the load range of gasoline PPC concept, since it intrinsically provides equivalent torque response with only half the IMEP required in a 4-stroke cycle. Moreover, 2-stroke architecture provides high flexibility on the air management parameters to substantially control the cylinder conditions and affect the combustion environment, allowing proper combustion control even in low load conditions. An experimental investigation has been performed to evaluate the potential of the PPC concept for pollutant control, using a commercial gasoline with Research Octane Number of 95 in a newly-designed 2-stroke poppet valves automotive diesel engine. The experimental results confirm how it is possible to achieve stable gasoline PPC combustion at a low speed medium load point (1200 rpm, 5 bar IMEP); with good combustion stability (σIMEP below 3%), high combustion efficiency (over 98%), and low NOX and zero soot levels; thanks to the wide control of the cylinder gas temperature provided by the air management settings. Nevertheless, in agreement to the results reported in the literature, the indicated cycle efficiency attained at this low load operating condition is lower than the obtained in conventional diesel combustion conditions. Therefore, a dedicated optimization process of the engine hardware and engine settings is required to fully exploit the benefits of gasoline PPC concept in the investigated 2-stroke engine architecture.

Partially Premixed Combustion (PPC) of fuels in the gasoline octane range has proven its potential to achieve simultaneous reduction in soot and NOX emissions, combined with high indicated efficiencies; while still retaining proper control over combustion phasing with the injection event, contrary to fully premixed strategies. However, gasoline fuels with high octane number as the commonly available for the public provide a challenge to ensure reliable ignition especially in the low load range, while fuel blends with lower octane numbers present problems for extending the ignition delay in the high load range and avoid the onset of knocking-like combustion. Thus, choosing an appropriate fuel and injection strategy is critical to solve these issues, assuring successful PPC operation in the full engine map.In this framework, the objective of the present investigation consists of evaluating the use of multiple injection strategies for achieving stable PPC operation, attaining low NOX and soot emissions together with high efficiencies. This research was carried out in a singlecylinder DOHC 2-stroke HSDI CI engine using 95 Research Octane Number (RON) gasoline fuel. Three different operating conditions in terms of indicated mean effective pressure (IMEP) and speed were investigated: 3.1 bar IMEP and 1250 rpm, 5.5 bar IMEP and 1500 rpm and 10.4 bar IMEP and 1500 rpm. Parametric variations of injection timings, at different rail pressures and different fuel split between injections were experimentally performed to analyze the effect of the injection strategy over the combustion process, exhaust emissions and efficiency levels.Experimental results confirm how using an appropriate injection strategy helps to achieve stable PPC operation in the selected operating conditions; with competitive combustion stability, lower NOX and soot levels, and moderate CO and HC emissions with combustion efficiency over 96%, compared to Conventional Diesel Combustion (CDC).Finally, a detailed analysis of the local cylinder conditions was performed by means of 3D-CFD simulations in order to provide guidelines for further optimization of the gasoline PPC concept, when using multiple injection strategies in the 2-stroke engine under development.

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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.

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