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Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural thermal stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural thermal stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions.

Combustion phasing is one important issue that must be addressed for HCCI operation. The intake temperature can be adjusted to achieve ignition at the desired crank angle. However, heat-transfer during induction will make the effective intake temperature different from the temperature measured in the runner. Also, depending on the engine speed and port configuration, dynamic flow effects cause various degrees of charge heating. Additionally, residuals from the previous cycle can have significant influence on the charge temperature at the beginning of the compression stroke. Finally, direct injection of fuel will influence the charge temperature since heat is needed for vaporization. This study investigates these effects in a systematic manner with a combination of experiment and cycle simulation using WAVE from Ricardo.

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.

An investigation has been conducted to determine the relative magnitude of the various factors that cause changes in combustion phasing (or required intake temperature) with changes in fueling rate in HCCI engines. These factors include: fuel autoignition chemistry and thermodynamic properties (referred to as fuel chemistry), combustion duration, wall temperatures, residuals, and heat/cooling during induction. Based on the insight gained from these results, the potential of fuel stratification to control combustion phasing was also investigated. The experiments were conducted in a single-cylinder HCCI engine at 1200 rpm using a GDI-type fuel injector. Engine operation was altered in a series of steps to suppress each of the factors affecting combustion phasing with changes in fueling rate, leaving only the effect of fuel chemistry.

To gain a better understanding of how the onset of incomplete bulk-gas reactions changes with engine speed and fuel-type, a parametric study of HCCI combustion and emissions has been conducted. The experimental part of the study was performed at naturally aspirated conditions and included fueling sweeps at four engine speeds (600, 1200, 1800 and 2400 rpm) for research grade gasoline, pure iso-octane and two mixtures of the primary reference fuels (i.e. n-heptane and iso-octane) with octane numbers of 80 and 60. Additionally, single-zone CHEMKIN computations with a detailed mechanism for iso-octane were conducted. The results show that there is a strong coupling between the ignition quality of the fuel and the required intake temperature to phase the combustion at TDC. There is also a direct influence of intake temperature on the completeness of combustion. This is the case because the CO-to-CO2 reactions are highly sensitive to the peak combustion temperatures.

A combined experimental and modeling study has been conducted to investigate the sources of CO and HC emissions (and the associated combustion inefficiencies) at low-loads. Engine performance and emissions were evaluated as fueling was reduced from knocking conditions to very low loads (ϕ = 0.28 - 0.04) for a variety of operating conditions, including: various intake temperatures, engine speeds, compression ratios, and a comparison of fully premixed and GDI (gasoline-type direct injection) fueling. The experiments were conducted in a single-cylinder engine (0.98 liters) using iso-octane as the fuel. Comparative computations were made using a single-zone model with the full chemistry mechanisms for iso-octane, to determine the expected behavior of the bulk-gases for the limiting case of no heat transfer, crevices, or charge inhomogeneities.

The homogeneous charge, compression-ignition (HCCI) combustion process has the potential to significantly reduce NOx and particulate emissions, while achieving high thermal efficiency and the capability of operating with a wide variety of fuels. This makes the HCCI engine an attractive technology that can ostensibly provide diesel-like fuel efficiency and very low emissions, which may allow emissions compliance to occur without relying on lean aftertreatment systems.

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.

Two fundamental aspects of HCCI engine combustion have been investigated using a single-zone model with time-varying compression and the full chemical-kinetic mechanisms for iso-octane, a representative liquid-phase fuel. This approach allows effects of the kinetics and thermodynamics to be isolated and evaluated in a well-characterized manner, providing an understanding of the selected fundamental processes. The computations were made using the CHEMKIN-III kinetic-rate code for an 1800 rpm operating condition. The study consists of two parts. First, low-load HCCI operation was investigated to determine the role of bulk-gas reactions as a source for HC and CO emissions. The computations show that as fueling is reduced to equivalence ratios of 0.15 and lower (very light load and idle), the bulk-gas reactions do not go to completion, leading to inefficient combustion and high emissions of HC and CO.

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.

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.

Over the past decade, laser diagnostics have improved our understanding of many aspects of diesel combustion. However, interactions between the combusting fuel jet and the piston-bowl wall are not well understood. In heavy-duty diesel engines, with typical fuels, these interactions occur with the combusting vapor-phase region of the jet, which consists of a central region containing soot and other products of rich-premixed combustion, surrounded by a diffusion flame. Since previous work has shown that the OH radical is a good marker of the diffusion flame, planar laser-induced fluorescence (PLIF) imaging of OH was applied to an investigation of the diffusion flame during wall interaction. In addition, simultaneous OH PLIF and planar laser-induced incandescence (PLII) soot imaging was applied to investigate the likelihood for soot deposition on the bowl wall.

Empirical and analytical data on the minimum possible flame temperatures for combustion processes rapid enough to be effective for engine operation are presented. The fundamental basis for these minimum temperatures is explored with chemical kinetic analysis. The combination of these minimum temperatures and the time scales associated with engine processes yield minimum possible levels of in-cylinder NOx production for both diesel and spark-ignition engines. These minimum NOx levels are identified and validated empirically. Legislated NOx levels lower than those indicated will require exhaust aftertreatment in addition to in-cylinder combustion control.

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.

This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the two-stage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors. A framework is also proposed for understanding the heat release and emission formation processes.

Chemiluminescence imaging has been applied to a parametric investigation of diesel autoignition. Time-resolved images of the natural light emission were made in an optically accessible DI diesel engine of the heavy-duty size class using an intensified CCD video camera. Measurements were obtained at a base operating condition, corresponding to a motored TDC temperature and density of 992 K and 16.6 kg/m3, and for TDC temperatures and densities above and below these values. Data were taken with a 42.5 cetane number blend of the diesel reference fuels for all conditions, and measurements were also made with no. 2 diesel fuel (D2) at the base condition. For each condition, temporal sequences of images were acquired from the time of first detectable chemiluminescence up through fully sooting combustion, and the images were analyzed to obtain quantitative measurements of the average emission intensity.

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.

NO formation during direct-injection (DI) diesel combustion has been investigated using planar laser-induced fluorescence (PLIF) imaging. Measurements were made at a typical medium-speed operating condition in a heavy-duty size-class engine modified for optical access. By combining a unique laser system with a particular spectroscopic scheme, single-shot NO images were obtained at realistic operating conditions with negligible O2 interference. Temporal sequences of NO PLIF images are presented along with corresponding images of combined elastic scattering and natural luminosity. These images show the location and timing of the NO formation relative to the other components of the reacting fuel jet. In addition, total NO formation was examined by integrating the NO PLIF signal over a large fraction of the combustion-chamber volume.

A phenomenological description, or “conceptual model,” of how direct-injection (DI) diesel combustion occurs has been derived from laser-sheet imaging and other recent optical data. To provide background, the most relevant of the recent imaging data of the author and co-workers are presented and discussed, as are the relationships between the various imaging measurements. Where appropriate, other supporting data from the literature is also discussed. Then, this combined information is summarized in a series of idealized schematics that depict the combustion process for a typical, modern-diesel-engine condition. The schematics incorporate virtually all of the information provided by our recent imaging data including: liquid- and vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions.

Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location on and nature of the flame zones within the combusting fuel jet have been largely unstudied. To address this issue, planar laser-induced fluorescence (PLIF) imaging of the OH radical has been applied to the reacting fuel jet of a direct-injection diesel engine of the “heavy-duty” size class, modified for optical access. An Nd:YAG-based laser system was used to pump the overlapping Q19 and Q28 lines of the (1,0) band of the A→X transition at 284.01 nm, while the fluorescent emission from both the (0,0) and (1,1) bands (308 to 320 nm) was imaged with an intensified video camera. This scheme allowed rejection of elastically scattered laser light, PAH fluorescence, and laser-induced incandescence. OH PLIF is shown to be an excellent diagnostic for diesel diffusion flames.