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.

Aggressive DOE Vehicle Technologies Program (VTP) gasoline engine fuel-economy and pollutant- emission targets can be met through some combination of reduced heat transfer, lower throttling losses, shorter combustion durations, lower combustion temperatures, improved mixture preparation, and higher compression ratios. Charge dilution by air or exhaust gas recirculation (EGR) is the most common method to achieve many of these benefits for low-power conditions, but increased dilution is offset by poorer combustion stability. A primary limitation of higher dilution levels is a slow, early burning rate when using conventional inductor coil ignition systems. Consequently, advanced ignition technologies are needed to produce larger and more energetic ignition volumes, while still performing well under elevated cylinder densities and charge motions. Ignition systems may also be an important source of radicals and heat, which are then used to tailor gasoline reactivity for advanced compression ignition (ACI) strategies. However, we currently lack foundational understanding of igniter mechanisms for new technologies, which inhibits the development of production-ready systems.

Gasoline Combustion Fundamentals Laboratory researchers seek to improve understanding of ignition physical and chemical processes and to use this information to develop more reliable engine simulation sub-models. Relevant ignition processes include plasma formation from deposited electrical energy, plasma-to-flame transition, and flame kernel development. Experiments are performed in a single-cylinder research engine and ignition test vessels, all containing suitable optical access. In situ optical diagnostics and ex situ gas sampling measurements elucidate important ignition process details.

To reduce CO2 emissions, we must supplement traditional gasoline with renewable fuels and improve the fuel efficiency of automotive engines. Research conducted in the Alternative Fuels Direct-Injection Spark-Ignition (DISI) Engine Lab strengthens the scientific underpinnings necessary for the automotive industry to meet increasingly stringent efficiency and emission standards. The single-cylinder research engine is based on the cylinder head and combustion system for an advanced spray-guided stratified-charge engine. It has a bore of 86 mm and a stroke of 95 mm, for a swept volume of 0.55 liter, which corresponds to a 2.2-liter swept volume in a typical 4-cylinder configuration. The engine can be operated in an all-metal configuration for extended performance mapping. It also offers extensive optical access to the combustion chamber for probing the in-cylinder processes.

DISI engine in optical configuration for high-speed dual-camera imaging of fuel sprays.

Drop-down cylinder for easy access to the piston

Because this engine can operate overall fuel-lean, it may substantially improve thermal efficiency relative to the traditional port-injected stoichiometric gasoline engine. With lean operation, the intake flow requires less throttling to control engine load, reducing the fuel economy penalty associated with the use of throttling for load control. However, operating the engine in an overall lean but stratified mode requires precise and robust control of fuel/air mixing and charge preparation to ensure that an ignitable and flammable mixture exists around the spark-plug gap at the time of ignition. Such control is particularly challenging for flex-fuel engines because the fuel properties vary substantially between traditional gasoline and alternative fuel blends. For example, when using E85, an 85%/15% volume mix of ethanol and gasoline, roughly 50% more fuel mass must be supplied to obtain the same engine torque due to E85’s high oxygen content (i.e., its low energy content). To fully understand the implications of the change of fuel type on the in-cylinder processes, researchers perform a combination of laser-based flow-field and fuel-concentration measurements.

Ray tracing of laser-sheet that enters through piston-bowl window.

Another approach for improving the vehicle fuel economy is based on the use of engine “downsizing.” By equipping the vehicle with a smaller-displacement engine, the engine must work with higher cylinder pressures, which reduces the need to throttle the intake air flow and the fuel efficiency losses associated with throttling. However, to ensure acceptable vehicle performance at high loads, the specific power output must be increased, typically by turbocharging. To operate with high peak in-cylinder pressures, the fuel must have sufficient resistance to autoignition to avoid damaging engine knock. Fortunately, ethanol has very low autoignition reactivity (i.e., low knocking propensity) even under highly boosted conditions. E85 can therefore enable aggressive downsizing to improve the fuel economy. Given the industry trend of increasingly aggressive turbocharging, researchers are evaluating the autoignition reactivity of other proposed alternative fuels. These aspects are being investigated using knock-detection in the all-metal configuration of the DISI engine, and by supporting measurements of autoignition reactivity performed in the nearby HCCI Fundamentals Lab at Sandia.

CRF researchers focused on the automotive-scale engine area, are investigating Low-Temperature Combustion (LTC) strategies appropriate for diesel fuel, advanced diesel combustion strategies, Direct-Injection Spark-Ignition (DISI) combustion, Homogenous-Charge Compression-Ignition (HCCI) and Stratified-Charge, Compression-Ignition (SCCI) combustion strategies, and the fundamentals of fuel sprays for these applications. Researchers are particularly interested in the fundamental mechanisms controlling HCCI.