A turbulent jet ignition engine enables operation with lean mixtures, decreasing nitrogen oxide (NO_X) emissions up to 92%, while the engine efficiency can be increased compared to conventional spark-ignition engines. The geometry of the pre-chamber and engine operating parameters play the most important role in the performance of turbulent jet ignition engines and, therefore, must be optimized. The initial experimental and 3D CFD results of a single-cylinder engine fueled by gasoline were used for the calibration of a 0D/1D simulation model. The 0D/1D simulation model was upgraded to capture the effects of multiple flame propagations, and the evolution of the turbulence level was described by the new K-k-ε turbulence model, which considers the strong turbulent jets occurring in the main chamber. The optimization of the pre-chamber volume, the orifice diameter, the injected fuel mass in the pre-chamber and the spark timing was made over 9 different operating points covering the variation in engine speed and load with the objective of minimizing the fuel consumption while avoiding knock. Two optimization methods using 0D/1D simulations were presented: an individual optimization method for each operating point and a simultaneous optimization method over 9 operating points. It was found that the optimal pre-chamber volume at each operating point was around 5% of the clearance volume, while the favorable orifice diameters depended on engine load, with optimal values around 2.5 mm and 1.2 mm at stoichiometric mixtures and lean mixtures, respectively. Simultaneous optimization of the pre-chamber geometry for all considered operating points resulted in a pre-chamber volume equal to 5.14% of the clearance volume and an orifice diameter of 1.1 mm.

This paper presented an experimental and numerical study of pre-chamber volume, number of orifices and orifice diameter influence on engine performance and emissions. All the measurements were performed on a single cylinder test engine at fixed engine speed of 1600 rpm, while engine load was varied by a change of the excess air ratio in the main chamber from a stochiometric mixture to a lean limit. The total of nine pre-chamber variants comprised three different pre-chamber volumes, two orifice number combinations (six and four orifices) and nine different orifice diameters. It was observed that the pre-chamber volume affects the indicated efficiency in a trend which is mostly independent of excess air ratio, with the efficiency gain between the best and worst results ranging from 1 to 4.4%. While keeping the same pre-chamber volume and the total cross-sectional area of the orifices, the larger number of orifices show better performance on two out of three investigated pre-chamber volumes, with the efficiency gains more pronounced at higher excess air ratios. Finally, on a fixed pre-chamber volume and number of orifices, the variation of orifice diameter leads to a trend in efficiency gains which favor larger orifice diameter. The comparison of the obtained efficiencies between all pre-chamber variants identified two pre-chambers, differing in each of the varied geometrical parameters, that show the best performance depending on excess air ratio range. On the other hand, a single variant which showed the worst performance on each excess ratio was identified. An additional investigation was performed by the application of the cycle-simulation model to quantify the share of emissions which are formed in the pre-chamber. The presented results showed that when PC volume is lowered, PC emission shares of NO_X and CO grow larger. The influence of orifice number and size has a minor effect on the pre-chamber emissions shares. The maximum PC emission shares of 54.8% and 80.6% are achieved at lean limit (λ = 2.2) for NO_X and CO, respectively. THC emission share, on the other hand, is not affected in a significant manner by either the pre-chamber geometry or operating conditions.

In the paper the operation of a turbocharged dual fuel engine at mid-high load is investigated on a single cylinder experimental engine complemented by a full 0D/1D simulation model that provides boundary conditions for the experiment and full engine system results. When duel fuel combustion mode is used on a turbocharged engine with the variable geometry turbocharger, the mid-high load operating points can be obtained with number of different combinations of intake pressure and excess air ratio. Besides the impact on combustion, the specific combination of intake pressure - excess air ratio has also impact on the exhaust back pressure caused by the turbocharger and consequently on the obtained brake efficiency. Additionally, the dual fuel combustion is influenced by natural gas mass fraction and start of injection of diesel fuel and the search for the optimal solution could be a challenging task. The method presented here enables the use of a single cylinder experimental engine in this search, while simultaneously taking into account the influences of the effects of a full engine system. The results showed that for the load range IMEP = 10-13 bar the optimal excess air ratio for obtaining the highest efficiency is between 1.5-1.65, and that in respect to natural gas (NG) mass fraction the favorable option is to use the highest NG mass fraction possible; in this case 95%. Also, it was noticed that although the average excess air ratio is high and diesel fuel mass low, the obtained NO_X emissions were above Euro VI limit for heavy duty engines and it was not possible to lower the NO_X emissions below that limit by further leaning of the mixture, because the impact of the NO_X formation in the spray was too strong.

The paper presents the optimization of injection parameters of directly injected fuel in the dual fuel engine operation. The optimization is performed numerically by using a cycle simulation model of the considered engine. In the cycle simulation model, combustion is simulated by a newly developed quasi-dimensional dual fuel combustion model. The model is based on the modified multi-zone combustion model and the quasi-dimensional combustion model. The modified multi-zone combustion model handles the part of the combustion process that is governed by the mixing-controlled combustion, while the modified quasi-dimensional combustion model handles the part that is governed by the flame propagation through the combustion chamber. The developed dual fuel combustion model features a phenomenological description of spray processes, i.e. the liquid spray break-up, fresh charge entrainment, droplet heat-up, and evaporation processes. In order to capture chemical effects on the ignition delay, special ignition delay tables are made. Additionally, to capture the effect of entrained methane on the chemical reaction rate, a special table that features the chemical reaction time scale is also created. The turbulence is calculated by the modified k-ε turbulence model that takes into account the effect of diesel pilot injection on the increase in the in-cylinder turbulence level. Before being used for the optimization, the engine model is validated through a comparison with experimental data. After that, the direct injection parameters are changed in search for the optimal solution. The objective is to obtain the maximum efficiency while maintaining the limits on emissions and on the mechanical load. The study is conducted at the mid load point at a constant engine speed.

This paper presents an experimental and numerical evaluation of the pre-chamber induced HCCI combustion concept (PC-HCCI) in terms of engine performance, emissions, and controllability. In this concept, a spark-initiated combustion in the pre-chamber is utilized to trigger the kinetically controlled combustion of an ultra-lean mixture in the main combustion chamber. The experimental measurements were performed on a single-cylinder engine with a custom-made active pre-chamber. A high compression ratio of 17.5 was used, which limits the maximum achievable engine load due to high knocking tendency but enables both standard PCSI combustion (flame propagation) at very high dilution levels and HCCI combustion at reasonable intake temperatures. The analysis of combustion characteristics and the resulting performance is performed at indicated mean effective pressures (IMEPs) of 3.5 and 3.0 bars, and three different intake temperatures of 80 °C, 90 °C, and 100 °C. The variation in engine load was achieved by adjusting the excess air ratio in the main chamber. On each combination of intake temperature and engine load, a spark sweep and an injected PC fuel mass sweep were performed to obtain the highest indicated efficiency while satisfying the restrictions in terms of combustion stability and knock intensity. It was shown that, unlike in a conventional HCCI engine, the combustion phasing can be directly and reliably controlled by adjusting either spark timing or the reactivity of the pre-chamber mixture, ensuring adequate combustion stability and eliminating potential misfires. A similar indicated efficiency as with conventional HCCI combustion was obtained, while the NOx emissions, although slightly elevated, are still insignificant. Compared to PCSI combustion at the same engine load, a 4-percentage-point increase in indicated efficiency and two times lower NOx emissions were achieved. Compared to the most efficient PCSI operating point, it was 1 percentage point lower, indicating that efficiency was achieved, but the specific NOx emissions are reduced by approximately 70%. Most importantly, very similar performance was obtained with significant variations in intake temperature, proving the reliability and adaptability of this combustion concept.