Optimal Use of Boosting Configurations and Valve Strategies for High Load HCCI - A Modeling Study

This study investigates a novel approach towards boosted HCCI operation, which makes use of all engine system components in order to maximize overall efficiency. Four-cylinder boosted HCCI engines have been modeled employing valve strategies and turbomachines that enable high load operation with significant efficiency benefits. A commercially available engine simulation software, coupled to the University of Michigan HCCI combustion and heat transfer correlations, was used to model the HCCI engines with three different boosting configurations: turbocharging, variable geometry turbocharging and combined supercharging with turbocharging. The valve strategy features switching from low-lift Negative Valve Overlap (NVO) to high-lift Positive Valve Overlap (PVO) at medium loads. The new operating approach indicates that heating of the charge from external compression is more efficient than heating by residual gas retention strategies. Hot intake charge allows for valve events and combustion phasing that enable high turbine performance and alleviate the backpressure problems often associated with boosted HCCI. Reduced pumping work and high intake pressure allow for further improvements in the upper load limit and efficiency, while avoiding NO_x formation and ringing.

Boosting enables high load HCCI but combustion needs to occur in such a way as to allow for increased exhaust energy directed to the turbine. The addition of supercharging allows for higher intake pressures and temperatures, while the associated mechanical work penalty can be minimized by optimizing the supercharger gearing. The successful use of PVO and currently available turbochargers supports the realization of a dual mode SI/HCCI engine. This is also supported by the presence of high intake temperatures which allow compression ratio values closer to those of mainstream direct injection SI engines.