James A. Eng

A Mean-Value Model for Control of Homogeneous Charge Compression Ignition (HCCI) Engines

A Mean Value Model (MVM) for a Homogeneous Charge Compression Ignition (HCCI) engine is presented. Using a phenomenological zero-dimensional approach with five continuous and three discrete states we first model the effects of the Exhaust Gas Recirculation (EGR) valve, the exhaust Rebreathing Lift (RBL), and the fueling rate on the state of charge in the cylinder at intake valve closing. An Arrhenius integral is then used to model the start of combustion, θ soc . A series of simple algebraic relations that captures the combustion duration and heat release is finally used to model the state of charge after the HCCI combustion and the Location of Peak Pressure (LPP). The model is parametrized and validated using steady-state test data from an experimental gasoline engine at the General Motors Corporation. The simple model captures the temperature, pressure, air-to-fuel ratio, and inert gas fraction of the exhausted mass flow. This characterization is important for the overall HCCI dynamics because the thermodynamic state (pressure, temperature) and concentration (oxygen and inert gas) of the exhausted mass flow affect the next combustion event. The high dilution level in HCCI engines increases the significance of this internal feedback that generally exists to a smaller extent in conventional spark-ignition and compression-ignition internal combustion engines.

The Effect of Spark Retard on Engine-out Hydrocarbon Emissions

Using spark retard during a cold-start is a very effective means of achieving fast catalyst light-off. In addition to obtaining faster catalyst light-off, retarding the spark also results in lower engine-out HC emissions. The objective of this research was to understand the reasons for the decrease in HC emissions with spark retard. In order to make the results as unambiguous as possible, the experiments were performed on a dynamometer at constant speed and load conditions using pre-vaporized, premixed gasoline. A zero-dimensional ring-pack crevice flow model was used to determine the mass flows into and out of the piston crevice during the engine cycle. The analysis showed that with spark retard a large fraction of the unburned fuel from the ring-pack re-entered the cylinder before the end of flame propagation, and was consumed by the flame when it extinguished on the cylinder wall. The level of post-flame HC consumption was estimated as the difference between the measured engine-out HC emissions and the unburned fuel re-entering the cylinder after the end of flame propagation, which was taken to be the crankangle of 90% mass fraction burned. Even with the most severe levels of spark retard the majority of the HC consumption, up to 70%, takes place by the flame within the cylinder before exhaust valve opening.

The effects of intake charge preheating in a gasoline-fueled hcci engine

Experiments were performed on a homogeneously fueled compression ignition gasoline-type engine with a high degree of intake charge preheating. It was observed that fuels that contained lower end and/or non-branched hydrocarbons (gasoline and an 87 octane primary reference fuel (PRF) blend) exhibited sensitivity to thermal conditions in the surge tanks upstream of the intake valves. The window of intake charge temperatures, measured near the intake valve, that provided acceptable combustion was shifted to lower values when the upstream surge tank gas temperatures were elevated. The same behavior, however, was not observed while using isooctane as a fuel. Gas chromatograph mass spectrometer analysis of the intake charge revealed that oxygenated species were present with PRF 87, and the abundance of the oxygenated species appeared to increase with increasing surge tank gas temperatures. No significant oxygenated species were detected when running with isooctane. The presence of the oxygenated species for PRF 87 fueling indicated that reactions were occurring in the intake surge tanks which resulted in needing lower intake charge temperatures to achieve autoignition.