Homogeneous-charge compression-ignition (HCCI) engines achieve high, diesel-like efficiencies and very low emissions of NOx and particulate. In an HCCI engine, the piston compresses a dilute, premixed fuel/air charge that autoignites and burns volumetrically. The charge is made dilute by either being very lean or by mixing with recycled exhaust gases.
Researchers face several technical barriers to implementing HCCI in production engines. Potentially, variations of HCCI in which the charge mixture and/or temperature are partially stratified (stratified-charge compression-ignition or SCCI) could overcome many of these barriers. Because of HCCI’s high efficiency and low emissions, most automobile and diesel-engine manufacturers have established HCCI/SCCI development programs. Supporting this industrial effort, the HCCI/SCCI Laboratory provides the science-based understanding required to develop HCCI/SCCI engines.
The laboratory is equipped with two Cummins B-series production engines, mounted at either end of a double-ended dynamometer, that have been converted for single-cylinder HCCI/SCCI operation. One engine (the so-called “all-metal engine”) is used to establish operating points, test various fuel types, develop combustion-control strategies, and investigate emissions. The second engine has extensive optical access for the application of advanced laser diagnostics for investigations of in-cylinder processes. The design includes an extended piston with piston-crown window, three large windows near the top of the cylinder wall, and a drop-down cylinder for rapid cleaning of fouled windows.
The engines facilitate a wide range of operating conditions and various fuel-injection, fuel/air/residual mixing, and control strategies for researchers to investigate the potential of overcoming the technical barriers to HCCI. Producing results relevant to both automotive and heavy-duty applications, the 0.98 liters/cylinder engines are equipped with the following features:
- variable in-cylinder swirl: swirl ratios of 0.9 to 3.2, convertible to swirl ratios up to 7.6
- multiple fuel systems: fully premixed, port fuel injection, gasoline-type direct-injection, and diesel-type direct-injection
- complete intake charge conditioning: simulated or real exhaust gas recirculation, intake pressures to 6 atmospheres, and intake temperatures to 220 °C
- speeds up to 3600 rpm (metal engine) and 1800 rpm (optical engine)
- variable compression ratios, currently from 12:1 to 21:1, through interchangeable pistons
- custom HCCI piston design
- full emissions measurements: CO2, CO, O2, HC, NOx, and smoke
- mechanical valves, with a conversion to fully flexible variable valve actuation under development
Current investigations address several issues, including:
- stratification of the fuel/air mixture as a means of improving emissions and combustion efficiency during part-load operation
- effects of fuel properties on performance and emissions over a range of speeds and loads
- use of biofuels, including ethanol and potential second-generation biofuels, such as iso-pentanol [in cooperation with Sandia bioscience groups and the Joint BioEnergy Institute (JBEI)]
- effects of heat transfer and thermal stratification on HCCI/SCCI performance and potential for extending operation to higher loads
- intake-pressure boosting and other techniques to extend operation to higher loads and further improve engine efficiency
Because fuel characteristics are central to HCCI engine design, researchers are examining a variety of fuels including gasoline, diesel, biofuels, alternative petroleum-based fuels, and representative constituents of real distillate fuels.