Spark-Augmented HCCI Shown to Increase Fuel Efficiency
March 5, 2004
Research funded by DOE's FreedomCAR and Vehicle Technologies Program has demonstrated a 12% increase in fuel efficiency for a multimode combustion process that uses a spark to augment and stabilize HCCI (homogeneous charge compression ignition). HCCI is a relatively new combustion technology that researchers believe could be used to increase diesel engine efficiency and reduce emissions of nitrogen oxides (NOx) and particulate matter (PM). Results were achieved in experiments performed at AVL North America, Inc.'s, Michigan test facility, under contract and onsite guidance from Oak Ridge National Laboratory.
AVL's single-cylinder research engine uses variable valve timing to control the amount of exhaust gas from the previous combustion event and thus, controls the temperature in the cylinder. In classic HCCI, temperature-dominated kinetics govern the start of combustion, and the precise timing is difficult to control, since the temperature in the combustion chamber is subject to many variables. Researchers suspected that a spark might offer an additional means of control by initiating a small flame to nudge the bulk of the fuel-air mixture into simultaneous compression ignition.
Results to date show that the spark augmentation modestly improves the range and stability of this engine's HCCI regime. The spark also allows a continuous transition from HCCI-like to spark-ignited flame in the high EGR mode, by varying the relationship of spark timing and early exhaust valve closing. Although spark augmentation produced a 95% reduction in NOx emissions over conventional propagating flame combustion, NOx levels were still too high to eliminate the need for exhaust aftertreatment.
This project is also examining the effects of fuel properties on HCCI. The AVL engine uses a gasoline-like fuel, which has confirmed HCCI's sensitivity to the fuel's ignition properties, compared to conventional spark-ignition, throttled operation. The researchers plan additional experiments with a more robust and energetic spark system.