Search Results

Homogeneous Charge Compression Ignition (HCCI) engines can have efficiencies as high as compression-ignition, direct-injection (CIDI) engines (an advanced version of the commonly known diesel engine), while producing ultra-low emissions of oxides of nitrogen (NOx) and particulate matter (PM). HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels. While HCCI has been demonstrated and known for quite some time, only the recent advent of electronic sensors and controls has made HCCI engines a potential practical reality. This paper provides a comprehensive overview of the current state-of-the-art in HCCI technology, estimates the potential benefits HCCI engines could bring to U.S. transportation vehicles, and lists the R&D barriers that need to be overcome before HCCI engines might be considered for commercial application.

The objective of this investigation is to verify and characterize the influence of fuel volatility on maximum liquid-phase fuel penetration for a variety of actual Diesel fuels under realistic Diesel engine operating conditions. To do so, liquid-phase fuel penetration was measured for a total of eight Diesel fuels using laser elastic-scatter imaging. The experiments were carried out in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. In addition to liquid-phase fuel penetration, ignition delay was assessed for each fuel based on pressure-derived apparent heat release rate and needle lift data. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum characteristic length. Beyond this characteristic length, the fuel is entirely vapor phase and not just smaller fuel droplets.

The effects of injection timing and diluent addition on the late-combustion soot burnout in a direct-injection (DI) diesel engine have been investigated using simultaneous planar imaging of the OH-radical and soot distributions. Measurements were made in an optically accessible DI diesel engine of the heavy-duty size class at a 1680 rpm, high-load operating condition. A dual-laser, dual-camera system was used to obtain the simultaneous “single-shot” images using planar laser-induced fluorescence (PLIF) and planar laser-induced incandescence (PLII) for the OH and soot, respectively. The two laser beams were combined into overlapping laser sheets before being directed into the combustion chamber, and the optical signal was separated into the two cameras by means of an edge filter.

A parametric study of the liquid-phase fuel penetration of evaporating Diesel fuel jets has been conducted in a direct-injection Diesel engine using laser elastic-scatter imaging. The experiments were conducted in an optically accessible Diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. The density and temperature at TDC were varied systematically by adjusting the intake temperature and pressure. At all operating conditions the measurements show that initially the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum length. Then, the liquid-fuel penetration length remains fairly constant although fuel injection continues. At a TDC density of 16.6 kg/m3 and a temperature of about 1000 K the maximum penetration length is approximately 23 mm. However, it varies significantly as TDC conditions are changed, with the liquid-length being less at higher temperatures and at higher densities.

The potential for catalytically enhanced ignition in low-heat rejection Diesel engines has been experimentally studied under engine simulated conditions in a high pressure chemical flow reactor. Results are presented for propane oxidation on platinum at 6 and 10 atmospheres, at temperatures from 800K to 1050K, and at equivalence ratios from 0.5 to 4.0. For turbulent transport rates which are typical of those in an engine, as much as 20% of the fuel was found to react on the catalyst before the onset of the gas-phase ignition reactions. Depending on the adiabaticity of the combustion chamber walls, this could lead to significant thermal enhancement of the gas-phase ignition process. Evidence of chemical enhancement was also observed, at 10 atm under very fuel rich conditions, in terms of a change in the concentration and distribution of the hydrocarbon intermediate species. Possible mechanisms for the observed chemical enhancement due to surface generated species are discussed.

During the vehicle braking, the Regenerative braking system (RBS) transforms the kinetic energy into electric power, storing it in the power sources. To secure the baking process, it is required to use hydraulic braking pressure to coordinately compensate the regenerative braking pressure. The traditional hydraulic pressure control algorithm which is used in regenerative braking system coordinated control has obvious laddering effect in braking. Unit control cycle pressure deviations seriously affect the comfort and the braking feeling on the vehicle.

We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio ϕ = 0.26 to very low loads (ϕ = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio.

Laser-induced incandescence (LII) has been explored as a diagnostic for qualitative two-dimensional imaging of the in-cylinder soot distribution in a diesel engine. Advantages of LII over elastic-scatter soot imaging techniques include no interfering signals from liquid fuel droplets, easy rejection of laser light scattered by in-cylinder surfaces, and the signal intensity being proportional to the soot volume fraction. LII images were obtained in a 2.3-liter, single cylinder, direct-injection diesel engine, modified for optical access. To minimize laser sheet and signal attenuation (which can affect almost any planar imaging technique applied to diesel engine combustion), a low-sooting fuel was used whose vaporization and combustion characteristics are typical of standard diesel fuels. Temporal and spatial sequences of LII images were made which show the extent of the soot distribution within the optically accessible portion the combusting spray plume.

This work explores the potential of partial fuel stratification to smooth HCCI heat-release rates at high load. A combination of engine experiments and multi-zone chemical-kinetics modeling was used for this. The term “partial” is introduced to emphasize that care is taken to supply fuel to all parts of the in-cylinder charge, which is essential for reaching high power output. It was found that partial fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, partial fuel stratification has the potential to increase the high-load limits for HCCI/SCCI operation. However, for the technique to be effective the crank-angle phasing of the “hot” ignition has to be sensitive to the local ϕ. Sufficient sensitivity was observed only for fuel blends that exhibit low-temperature heat release (like diesel fuel).

Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel stratification to increase the in-cylinder thermal stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel stratification, the strong fuel-vaporization cooling produces thermal stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber.

Multi-zone CFD simulations with detailed kinetics were used to model iso-octane HCCI experiments performed on a single-cylinder research engine. The modeling goals were to validate the method (multi-zone combustion modeling) and the reaction mechanism (LLNL 857 species iso-octane) by comparing model results to detailed exhaust speciation data, which was obtained with gas chromatography. The model is compared to experiments run at 1200 RPM and 1.35 bar boost pressure over an equivalence ratio range from 0.08 to 0.28. Fuel was introduced far upstream to ensure fuel and air homogeneity prior to entering the 13.8:1 compression ratio, shallow-bowl combustion chamber of this 4-stroke engine. The CFD grid incorporated a very detailed representation of the crevices, including the top-land ring crevice and head-gasket crevice. The ring crevice is resolved all the way into the ring pocket volume. The detailed grid was required to capture regions where emission species are formed and retained.

A tracer-based single-line PLIF imaging technique using a unique optical configuration that allows simultaneously viewing the bulk-gas and the boundary layer region has been applied to an investigation of the naturally occurring thermal stratification in a HCCI engine. Thermal stratification is critical for HCCI engines, because it determines the maximum pressure rise rate which is a limiting factor for high-load operation. The investigation is based on the analysis of temperature maps that were derived from PLIF images, using the temperature sensitivity of fluorescence from toluene introduced as tracer in the fuel. Measurements were made in a single-cylinder optically accessible HCCI engine operating under motored conditions with a vertical laser-sheet orientation that allows observation of the development of thermal stratification from the cold boundary layers into the central region of the charge.

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NOx-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NOx. These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NOx emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NOx emissions become unacceptable at IMEPg = 473 kPa.

A combination of optical imaging diagnostics has been applied to the fuel jet of a direct-injection diesel engine to study the ignition and early soot formation processes. Measurements were made in an optically accessible direct-injection diesel engine of the “heavy-duty” size class at a representative medium speed (1200 rpm) operating condition. Two fuels were used, a 42.5 cetane number mixture of the diesel reference fuels and a new low-sooting fuel (needed to reduce optical attenuation at later crank angles) that closely matches both the cetane number and boiling point of the reference fuel mixture. The combustion and soot formation processes are found to be almost identical for both fuels. Ignition and early combustion were studied by imaging the natural chemiluminescence using a calibrated intensified video camera. The early soot development was investigated via luminosity imaging and simultaneous planar imaging of laser-induced incandescence (LII) and elastic scattering.

The objective of this research was to characterize the effects of fuel component volatility on gasoline direct injection (GDI) engine cold start. Three different fuel components, representing gasoline light end, mid-point and heavy end components, were used to form three fuel blends of different volatility. Performance tests and in-cylinder fuel distribution imaging tests using these fuel blends were carried out in a firing single-cylinder optically-accessible engine following a simulated cold start test schedule. Performance results, based on in-cylinder pressure and engine-out hydrocarbon measurements, during the initial transient phase of GDI cold start showed significantly degraded performance with the low volatility fuel blend, while the high volatility blend showed slightly improved performance. Neither the low nor high volatility fuel, however, showed a discernable effect on the quasi-steady state cold start performance.

Fuel stratification has been investigated as a means of improving the low-load combustion efficiency in an HCCI engine. Several stratification techniques were examined: different GDI injectors, increased swirl, and changes in injection pressure, to determine which parameters are effective for improving the combustion efficiency while maintaining NOx emissions below U.S. 2010 limits. Performance and emission measurements were obtained in an all-metal engine. Corresponding fuel distribution measurements were made with fuel PLIF imaging in a matching optically accessible engine. The fuel used was iso-octane, which is a good surrogate for gasoline. For an idle fueling rate (ϕ = 0.12), combustion efficiency was improved substantially, from 64% to 89% at the NOx limit, using delayed fuel injection with a hollow-cone injector at an injection pressure of 120 bar.

The effects of reduced fuel injection pressure on the cold start performance of a GDI engine have been studied in a single-cylinder, optically-accessible research engine. Two Delphi Automotive Systems DI-G injectors, with included spray cone angles of 60° and 80° respectively, were studied. Both injectors are designed to operate at a nominal fuel line pressure of 10 MPa. For the study they were operated at several fuel feed pressures between 10 MPa and 2 MPa. Two start of injection timings (50° and 100° ATDC) were examined. Cold start performance was characterized by measurements of the GIMEP, COV of GIMEP, and total engine out UHCs. Simultaneous Planar Laser Induced Fluorescence (PLIF) and Mie Scattering images of the fuel spray were used to observe spray penetration, mixing, and in-cylinder fuel distribution throughout the intake and compression strokes. Ultimately these images were used to explain observed performance differences.

The combustion process in diesel engines deposits soot on the in-cylinder surfaces. Previous works have suggested that these soot deposits eventually break off during cylinder blow-down and the exhaust stroke and contribute significantly to exhaust soot emissions. In order to better understand this potential pathway to soot emissions, the authors recently investigated combusting fuel-jet/wall interactions in a diesel engine. This work, published as a companion paper, showed how soot escaped from the combusting fuel jet and was brought in close proximity to the wall so that it could become a deposit. The current study extends this earlier work with laser-extinction measurements of the soot-deposition rate in the same single-cylinder, heavy-duty DI diesel engine. Measurements were made by passing the beam of a CW-diode laser through a window in the piston bowl rim that was in-line with one of the fuel jets.

The effects of intake-generated swirl and tumble on cold start performance have been investigated in a firing single-cylinder Gasoline Direct-Injection (GDI) engine. The engine utilizes a Ford Zetec cylinder head modified for GDI operation and a fused silica cylinder which provides extensive optical access to the combustion chamber. Uniquely designed port-inserts were positioned in the intake ports to generate enhanced swirling or tumbling motion inside the cylinder. Experiments were conducted using a constant speed (∼ 900 rpm) simulated cold start procedure, where the engine is motored for approximately 40 cycles, after which fuel injection and spark ignition commence and continue for 190 cycles and then the engine is stopped. Measurements were made of the various engine temperatures, engine-out total hydrocarbon emissions, and in-cylinder pressure throughout the test period.

To better understand the factors affecting soot formation in diesel engines, in-cylinder soot and diffusion flame lift-off were measured in a heavy-duty, direct-injection diesel engine. Measurements were obtained at two operating conditions using two commercial diesel fuels and a range of oxygenated paraffinic fuel blends. A line-of-sight laser extinction diagnostic was improved and employed to measure the relative soot concentration within the jet (“jet-soot”) and the rates of soot-wall deposition on the piston bowl-rim. An OH chemiluminescence imaging technique was developed to determine the location of the diffusion flame and to measure the lift-off lengths of the diffusion flame to estimate the amount of oxygen entrainment in the diesel jets. Both the jet-soot and the rate of soot-wall deposition were found to decrease with increasing fuel oxygen-to-carbon ratio (O/C) over a wide range of O/C.