Turbocharging to extend HCCI Operating Range in a Multi Cylinder Engine- Benefits and Limitations

In the last decade there has been interest in Homogenous Charge Compression Ignition- HCCI as a way to increase the efficiency of the gasoline engine. The attractive properties are increased fuel efficiency due to reduced throttling losses, increased expansion ratio and higher thermodynamic efficiency. The main drawback of HCCI is the absence of direct combustion timing control. Therefore all the right conditions for auto ignition have to be set before combustion starts. The operating range in HCCI mode is restricted to relatively low load due to rapid combustion which results in high pressure rise rates and thus high combustion induced noise. In the last couple of years different car manufactures have shown concept engines operating in HCCI mode. General Motors has showcased a naturally aspirated HCCI engine operating in the low load regime. To evaluate if the load range in HCCI can be extended with increased inlet pressure our objective was to turbocharge an engine in HCCI mode. Through turbocharging the in-cylinder dilution is increased and the relative pressure rise rate can be decreased leading to a reduction of the combustion noise for a given load. The turbocharged test engine is an in-line four cylinder gasoline engine with a total displacement of 2.2 l. The engine is direct injected with spray-guided design. To achieve HCCI combustion the engine is operated with negative valve overlap by low lift and short duration valve timings where variable valve timing is used for combustion control. The effect of intake temperature, boost levels, combustion timing, intake valve timing, residual fraction and injection timing are demonstrated. From the results it can be seen how different settings affect intake pressure and hence combustion noise. The results show that the load range in turbocharged HCCI has been increased substantially compared to a naturally aspirated HCCI engine. The evaluation of different turbochargers and cam profiles has contributed to increased efficiency and load range capability. It can be operated from 1 bar indicated mean effective pressure to more than 6 bar, between 1000 and 3000 rpm. A comparison between a modern SI engine and this turbocharged HCCI engine shows that the HCCI engine can have up to 35 % higher fuel efficiency.