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.

Research in the Medium-Duty Diesel Combustion Laboratory is conducted using a single-cylinder, medium-duty diesel research engine based on Ford’s 6.7 liter PowerStroke® engine. Shown below is the all-metal variant of this engine. We are in the process of developing an optical section in the exhaust runner and a full optical engine variant to enable high-speed imaging; laser-based optical techniques; and detailed study of the thermal, fluid dynamical, and chemical processes occurring inside the combustion chamber.

Sandia’s Medium-Duty Diesel Research Engine (shown above) will be utilized to support the development and calibration of the next generations of medium-duty diesel combustion systems.

Currently, medium-duty diesel combustion research is focused in two project areas—Science-driven piston bowl designs and Catalyst heating operation—discussed below.

Science-driven piston bowl design

Intelligent design of diesel combustion systems is crucial to promote rapid, efficient combustion with very low pollutant emissions. Combustion system design must account for the shape of the bowl in the piston crown and the injector nozzle parameters, which include the number and geometry of nozzle holes, the injector opening angle, and the location of the nozzle holes relative to the cylinder head.

Image shows an example of a stepped-lip diesel piston.

These parameters influence the interactions between the fuel sprays and the piston bowl walls, establishing the turbulent flow patterns that help mix fuel and air. Improved fuel-air mixing processes lead to faster, cleaner, more efficient combustion. Because the world’s fleet of medium- and heavy-duty diesel-powered vehicles is very large, even modest combustion-system design improvements can result in significant reductions in overall CO2 emissions. Scientific study inside state-of-the-art diesel combustion chambers produces fundamental insights necessary to improve piston bowl shapes.

The project focuses on a specific type of commonly used diesel-piston design with stepped-lip or chamfered-lip bowls (shown above). High-speed imaging and thermodynamic experiments in an optical diesel engine have provided evidence that spray-wall interactions can create strong, long-lived vortices under some conditions. The appearance of these vortices correlates with faster, more efficient combustion and reductions in soot emissions (as illustrated, below). However, the vortices become weaker as the fuel injection timing is advanced toward top-dead center (TDC), which is necessary to achieve peak efficiency.

The strength and longevity of squish-region vortices corresponds to faster combustion with reduced soot emissions.

Researchers have modeled this complex turbulent flow behavior using numerical simulations. In-depth analysis of the results has provided insight into the physics responsible for the observed vortex behavior, leading to the following hypothesis: If a change in combustion system design creates stronger, longer-lived vortices at near-TDC injection timings, then peak efficiency will be improved while soot emissions are reduced. A modified stepped-lip piston design has been conceived to test this hypothesis, with the resulting design called a dimpled stepped-lip (DSL) piston.

An example of a discrete-axisymmetric, DSL piston design.

Researchers predict that DSL pistons promote vortex strength and longevity for injection timings near TDC. Using simulation tools, we are developing a DSL piston design for Sandia’s medium-duty diesel research engine and performing experiments with a metal version of the DSL piston to quantify its benefits on efficiency and emissions over a wide range of operating conditions.

Catalyst heating operation

Meeting current and future exhaust emissions regulations requires the use of after-treatment systems in diesel engine exhaust. These systems use catalysts to help convert harmful criteria pollutants into CO2 and water, but the catalysts are only effective once they have reached their so-called “light-off” temperature. A cold-started engine must be operated to heat up the exhaust catalysts to their light-off temperatures as quickly and efficiently as possible, while minimizing pollutant emissions that will leave the exhaust system untreated. When operated at low loads, the engine exhaust temperatures must remain high enough to keep the catalysts above their light-off temperature. Delivering clean, compliant, efficient diesel engines to market requires effective catalyst heating and thermal management strategies. At the CRF, we are improving our understanding of how in-cylinder processes during catalyst heating operation work to create tradeoffs between exhaust heat, pollutant emissions, and efficiency. Further investigations help CRF researchers understand how changes in fuel properties influence these tradeoffs.

The Heavy-Duty Engine Combustion and Fuel Effects Laboratory centers around a modern 1.7-liter, single-cylinder diesel engine that has been modified to provide extensive optical access to the combustion chamber. Researchers apply advanced imaging, laser-based, and conventional diagnostics to gain a fundamental understanding of the detailed in-cylinder mixing and combustion processes that govern engine efficiency, emissions, and performance. The results facilitate stakeholder collaboration to achieve practical, clean, and sustainable engines and fuels for the future.

The current research focus is on ducted fuel injection (DFI) with sustainable fuels. DFI is a new and conceptually simple mechanical technology to enhance the preparation of fuel/charge-gas mixtures within the combustion chambers of engines, with the goal of dramatically attenuating soot formation. Based loosely on the concept of the Bunsen burner, DFI involves injecting fuel along the axis of a small cylindrical duct within the combustion chamber to achieve more-complete local premixing at or near the end of the duct where ignition occurs. Although DFI is in a relatively early stage of development, it has been shown to profoundly curtail soot formation in optical-engine experiments (see figure).

Color picture showing conventional diesel combustion (left spray) and ducted fuel injection (DFI, right spray) as viewed through the piston window in the optical engine. The whitish crescent on the left side is produced by incandescence from hot soot. The crescent on the right side is blue because the DFI combustion on this side of the chamber produces little to no soot.

When used with today’s diesel fuel, DFI has been found to curtail hot, in-cylinder soot by ~10X relative to conventional diesel combustion. When used with sustainable, oxygenated fuels, DFI can provide an additional ~10X soot-reduction benefit, resulting in an overall soot reduction of ~100X, and providing a market incentive for cleaner, sustainable fuels. Furthermore, the observed soot reductions can be achieved regardless of the intake-mixture oxygen concentration, enabling exhaust-gas recirculation to be used for simultaneous, cost-effective reductions of nitrogen-oxide emissions by 10X or more.

Current activities focus on elucidating the effects of duct geometric parameters and fuel properties on DFI performance, with the goal of advancing the technology for deployment in production engines with low-net-carbon fuels.

The Heavy-Duty Diesel and Gaseous-Fueled Engine Laboratory uses laser-based and imaging diagnostics to understand the science base of in-cylinder processes used in the heavy-duty engine industry. Our research provides engineers with the necessary science base to design cleaner, more fuel-efficient engines. Fuel injection, combustion processes, and emissions formation are studied in a research engine that retains the basic geometry of a Cummins six-cylinder N-14 highway truck engine, but the single-cylinder version has been modified extensively for research. Added to a standard common-rail diesel fuel injector in the cylinder head is a gasoline direct-injection fuel injector mounted in the cylinder wall. This feature allows the formation of a well-mixed charge for combustion research and for certain optical diagnostics. Two injectors also allow engine operation with two different fuels (dual fuel), which can have emissions and efficiency advantages.

Cross-sectional schematic of the Heavy-Duty Optical Diesel and Gaseous Fuels engine configuration

To facilitate optical diagnostics, the research engine has several windows. First, instead of the standard 150 mm (6 in.) tall all-metal piston, the research engine has a 650 mm (26 in.) tall extended piston with a 100 mm (4 in.) diameter crown window to allow imaging access to the piston bowl. An additional 50 mm (2 in.) diameter window mounted in the cylinder head provides imaging access to the squish region above the bowl. Smaller 30 mm by 50 mm (1.2 in. by 2 in.) rectangular windows in the cylinder walls provide access for laser illumination. The engine’s drop-down cylinder liner allows manual access to the combustion chamber, so the windows can be quickly cleaned between engine runs.

Pulsed light from a high-power Nd:YAG laser is used for various planar measurements, including

  • in-cylinder liquid fuel penetration via Mie scattering
  • vapor-phase fuel concentration via Rayleigh scattering or fuel-tracer fluorescence
  • in-cylinder soot formation via laser-induced incandescence (LII) of soot
  • laser-induced fluorescence (LIF) of combustion-generated formaldehyde and polycyclic aromatic hydrocarbons

A second Nd:YAG laser is coupled to an optical parametric oscillator to obtain color-tunable light emission for spectroscopic diagnostics, such as fluorescence of OH and NO molecules, that are used to measure flame structure and pollutant formation. The laboratory is also equipped with a flat-flame, gaseous-fuel burner for optical-diagnostic development and optimization of laser wavelength tuning on molecular absorption lines.

While electrification is expected to play an increasing role in reducing the carbon footprint of medium- and heavy-duty vehicles in the coming decades, internal combustion engines—many of which will be diesel engines—will continue to serve in applications where technical limitations preclude complete electrification, in both hybrid and conventionally powered vehicles. Diesel engines are the dominant power source for heavy-duty applications such as trucking, construction, and railroad transportation because they are the most fuel-efficient and reliable engine today. Future emissions regulations will require that the criteria pollutant emissions from these vehicles are near zero. Required reductions in CO2 emissions from internal combustion engines must be achieved through improvements in efficiency and through a sustained increase in the use of renewable, GHG-neutral, or GHG-negative fuels. Medium/heavy-duty diesel combustion research at the CRF is focused both on technologies to reduce fuel consumption and pollutant emissions and on effects that renewable fuels have on diesel combustion processes and performance. Researchers are investigating several approaches—increased exhaust-gas recirculation, swirling in-cylinder air flows, multiple fuel injection events, very early or very late timing of fuel injection, and dual fueling with both gasoline and diesel fuels—to optimize efficiency and reduce engine-out emissions. These approaches could enable discoveries of advanced diesel combustion or low-temperature combustion strategies with potential for enabling both increased fuel economy and reduced NOx and PM emissions. However, fully realizing these combustion strategies’ ultimate potential requires a better understanding of in-cylinder processes.