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In this paper, a multimode combustion system was developed in a gasoline direct injection engine. A two-stage fuel-injection strategy, including flexible injection timings and flexible fuel quantity, is adopted as a main means to form desired mixture in the cylinder. The combustion system can realize five combustion modes. The homogeneous charge spark ignition (HCSI) mode was used at high load to achieve high-power output density; stratified charge spark ignition (SCSI) was adopted at intermediate load to get optimum fuel economy; stratified charge compression ignition (SCCI) was introduced at transient operation between SI and CI mode. Homogeneous charge compression ignition (HCCI) was utilized at part load to obtain ultralow emissions. Reformed charge compression ignition (RCCI) was imposed at low load to extend the HCCI operation range. In SI mode, the stratified concentration is formed by introducing a second fuel injection in the compression stroke. This kind of stratified mixture has a faster heat release than the homogeneous mixture and is primarily optimized to reduce the fuel consumption. In CI mode, the cam phase configurations are switched from positive valve overlap to negative valve overlap (NVO). The test results reveal that the CI combustion is featured with a high gradient pressure after ignition and has advantages in high thermal efficiency and low NOx emissions over SI combustion at part load.

Homogeneous Charge Compression Ignition (HCCI) has challenges of ignition control. In this paper, HCCI ignition timing and combustion rate were controlled by two-stage direct injection (TSDI) strategies on a four-stroke gasoline HCCI engine. TSDI strategy was proposed to solve the two major problems of HCCI application-ignition control and load extension. Both simulation and experiments were carried out on a gasoline HCCI engine with negative valve overlap (NVO). An engine model with detailed chemical kinetics was established to study the gas exchange process and the direct injection strategy in the gasoline HCCI engine with TSDI and NVO. Simulation results were compared with experiments and good agreement was achieved. The simulated and experimental results provided a detailed insight into the processes governing ignition in the HCCI engine. Using TSDI, the fuel concentration, temperature as well as chemical species can be controlled. The effects of different injection parameters, such as split injection ratio and start-of-injection (SOI) timing, were studied. The experimental results indicate that, two-stage direct injection is a practical technology to control the ignition timing and combustion rate effectively in four-stroke gasoline HCCI engines. Both the high load and low load limits of HCCI operation were extended.

Partially premixed compression ignition (PPCI) and multiple premixed compression ignition (MPCI) mode of straight-run naphtha have been investigated under different injection strategies. The MPCI mode is realized by the multiple premixed combustion processes in a sequence of “spray-combustion-spray-combustion” around the compression top dead center. The spray and combustion events are preferred to be completely separated, without any overlap in the temporal sequence in order to ensure the multiple-stage premixed compression ignition. The PPCI mode is well known as the “spray-spray-combustion” sequence, with the start of combustion separated from the end of injection. Straight-run naphtha with a research octane number (RON) of 58.8 is tested in a single cylinder compression ignition engine whose compression ratio is 16.7 and displacement is 0.5 l. Double and triple injection strategies are investigated as the last injection timing sweeping at 1.0 MPa IMEP and 1800 rpm conditions. The MPCI mode is achieved using the double injection strategy, but its soot emission is higher than the PPCI mode under triple injection strategy. This is mainly because of the lower RON of the straight-run naphtha and the ignition delay is too short to form an ideally premixed combustion process after the second injection of straight-run naphtha. Diesel fuel is also tested under the same operating conditions, except for employing a single injection strategy. The naphtha PPCI and MPCI mode both have lower fuel consumption and soot emission than diesel fuel single injection mode, but the THC emissions are both higher than that of diesel fuel.

Knocking is the main obstacle of increasing the compression ratio in order to improve the thermal efficiency of gasoline engines. This paper proposes a concept of a stratified stoichiometric mixture (SSM) with two-zone homogeneity (TZH) for suppressing knocking and validated the concept by means of a numerical simulation and an experimental study in a DISI engine. The results show that the SSM can effectively suppress knocking and the knocking intensity decreases when the zone around the spark plug is richer and the end-gas zone is leaner. The less rich zone (fuel/air equivalent ratio of ϕ ≤ 1.2) of the SSM can speed up the initial burning velocity in order to avoid a thermal efficiency decrease, while the over rich zone (ϕ > 1.2) would decrease the combustion velocity when knocking was suppressed. The SSM leads to higher CO and lower HC and NOx emissions, which can be effectively after-treated using a three-way catalyst. The SSM can also reduce the decrease of power output compared to the method of retarding spark timing for suppressing knocking and has better fuel economy and fewer emissions than the method of enriching the mixture. The TZH can effectively alleviate combustion deterioration and soot formation due to the stratified mixture combustion. As a result, the SSM with TZH suppresses knocking, thus simultaneously lowering fuel consumption and emissions.