Browse Publications Technical Papers 2019-01-0967
2019-04-02

Experimental investigations on the influence of valve timing and multi-pulse injection on GCAI Combustion 2019-01-0967

Gasoline Compressed Auto-Ignition (GCAI) technology, which can be categorized under Homogeneous Charge Compression Ignition (HCCI), is a low-temperature combustion system with promising benefits such as ultra-low in-cylinder NOx emissions and reduced brake-specific fuel consumption, which are the critical parameters in any modern engine. Since this technology is based on uncontrolled auto-ignition of a premixed charge, it is very sensitive to any change in boundary conditions during engine operation. Adopting real time valve timing and fuel-injection strategies can enable improved control over GCAI combustion. This work discusses the outcome of collaborative experimental research by the partnering institutes in this direction. Experiments were performed in a single cylinder GCAI engine with variable valve timing and Gasoline Direct Injection (GDI) at constant indicated mean effective pressure (IMEP). In the first phase, intake and exhaust valve timing sweeps were investigated. It was found that the Intake Valve Closing (IVC) timing and Exhaust Valve Closing (EVC) timing have a dominant influence on combustion, performance and emission parameters. Retarding the IVC timing to a certain extent increased the mass of air trapped due to the inertia effect, which made the mixture lean. This in-turn decreased the indicated specific fuel consumption (ISFC) and cylinder-out NOx emissions without much influence on combustion phasing. Advancing the EVC timing trapped more amount of residual gas, which resulted in an increased in-cylinder temperature during the ignition delay period and hence advanced the combustion phasing. In the second phase of experiments, multiple injection strategies were investigated. Here the influences of combining an injection pulse during the negative valve overlap phase (pilot injection), an injection pulse during the compression phase (pre-injection) and an injection pulse after combustion TDC (post- injection) along with the main injection pulse occurring at the end of IVC were studied. It was found that the presence of pilot injection advanced the combustion phasing considerably due to the low-temperature reactions between fuel and air during the negative valve-overlap period. The pre-injection quantity significantly affected the charge temperature during compression and hence changed the combustion phasing. Its timing was not found to be as influencing. However, introduction of post-injection advanced the combustion phasing of the next cycle without significant influence on cylinder-out NOx emissions. The potential of the valve timing and multi-pulse fuel injection strategies to effectively control combustion instabilities on a cycle-by-cycle basis in real time will also be discussed in the paper. In addition the possibility of extending the load range through the above strategies will be explained.

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