Effect of hydrogen injection strategies on mixture formation and combustion process in a hydrogen direct injection plus natural gas port injection rotary engine

This work aimed to numerically study the effect of hydrogen injection strategies on mixture formation and combustion process in a hydrogen direct injection plus natural gas port injection (HDI + NGPI) rotary engine. Two major factors, namely hydrogen injection timing (HIT) and hydrogen injection angle (HIA), were considered. Four HITs which were used in simulation, were from the early stages of intake stroke to the late stages of compression stoke. Comparing the fuel movement and combustion process among the four HITs with three HIAs (-60 degrees, 0 degrees and 60 degrees), it was found that for hydrogen movement, when the HIT was at early stages of intake stroke, this advanced HIT led to the fact that the hydrogen distribution areas at ignition timing all spread across the whole combustion chamber for all three HIA cases. When the HIT was at the late stages of the intake stroke and the whole compression stroke, with an increased hydrogen injection angle (HIA) (from -60 degrees, 0 degrees to 60 degrees), the hydrogen accumulation area moved from the front towards the rear of combustion chamber. In addition, with retarded HIT, the hydrogen aggregation degree increased continuously. For combustion process, to obtain a higher overall combustion rate, the hydrogen injection strategy should make as much hydrogen spread between the two spark plugs at ignition timing. In addition, the hydrogen concentration near the LSP and the TSP should be conducive to the flame kernel formation. This is mainly because of the fact that the above hydrogen distributions, not only lead to the rapid formation of the flame kernel near the LSP and the TSP, but also make use of the hydrogen at the early stage of combustion stroke. Under the computational condition, the overall combustion rate for case A: 140 - 60 which had a HIT of 140 degrees CA (BTDC) and a HIA of -60 degrees CA, was the fastest. Compared with case C: 300 + 60 which had a HIT of 300 degrees CA (BTDC) and a HIA of 60 degrees CA, the peak pressure of case A: 140 - 60 was raised by 31.1%. However, compared with other cases, the NO emissions of case A: 140 - 60 at 80 degrees CA (ATDC) was the maximum. Furthermore, for engineering applications, this study also provided a scheme of the change of the optimum HIA under different HITs for improving the performance of the HDI + NGPI rotary engine.