Actuator Comparison for Closed Loop Control of HCCIC Combustion Timing 2009-01-1135

Homogeneous Charge Compression Ignition (HCCI) is an emerging combustion technology due to its increased efficiency and decreased NO_x emissions. One of the most challenging aspects of HCCI is the regulation of the combustion timing. Unlike conventional combustion modes there is no direct control over the start of combustion. Autoignition timing is a function of the temperature, pressure and composition of the mixture, so to adjust the combustion timing of HCCI changes have to be made to these. Both variable valve timing and variable fuel octane number are effective inputs to achieve cycle-to-cycle combustion control of HCCI combustion timing. The application of these control methods are investigated in this paper.

A one-cylinder Ricardo engine is fitted with a 4-valve spark ignition cylinder head equipped with camshaft phasers. These phasers independently adjust both the intake and exhaust camshaft phasing. By modifying the intake valve timing the effective compression ratio is changed, which affects the temperature-pressure condition of the mixture. Variable fuel octane is realized using two independent fuel injector systems, one equipped with iso-Octane and the other with n-Heptane. The CA50 (crank angle of 50% mass fraction burned) is regulated using feedback control and two separate actuators for combustion timing are implemented; intake camshaft phasing and variable fuel octane. These actuators are compared according to their range of operation and ability to reject system disturbances. The different combustion controllers are subjected to disturbances of both engine speed and engine load (changes in injected fuel energy). The results show the benefits and limitations of each actuator.