John E. Dec

A Conceptual Model of DI Diesel Combustion Based on Laser-Sheet Imaging*

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: liquidand vapor-fuel zones, fuel/air mixing, autoignition, reaction zones, and soot distributions. By combining all these elements, the schematics show the evolution of a reacting diesel fuel jet from the start of fuel injection up through the first part of the mixing-controlled burn (i.e. until the end of fuel injection). In addition, for a “developed” reacting diesel fuel jet during the mixingcontrolled burn, the schematics explain the sequence of events that occurs as fuel moves from the injector downstream through the mixing, combustion, and emissions-formation processes. The conceptual model depicted in these schematics also gives insight into the most likely mechanisms for soot formation and destruction and NO formation during the portion of the DI diesel combustion event discussed.

Diesel combustion: an integrated view combining laser diagnostics, chemical kinetics, and empirical validation

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.

Isolating the Effects of Fuel Chemistry on Combustion Phasing in an HCCI Engine and the Potential of Fuel Stratification for Ignition Control

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. This involved the use of two novel techniques: 1) alternate-firing operation to remove changes in wall temperature and residuals; and 2) a method for determining the effective intake temperature to remove the effect of heating/cooling during induction. Three fuels were examined. Iso-octane was found to have only a small change in autoignition chemistry with fueling rate; gasoline had a change just slightly larger than iso-octane; and PRF80 had a large change, due to its significant cool-flame chemistry. Comparison of the data with chemical-kinetic modeling showed that the detailed iso-octane mechanism matches the trends well, but that the detailed PRF mechanism does not. The experimental results indicate that engine management becomes more complicated for fuels with cool-flame chemistry. For PRF80, combustion phasing changes immediately with changes in fueling, whereas sudden changes in fueling have little effect on the combustion phasing for iso-octane or gasoline. However, the results also show that the potential for ignition control by fuel stratification is much larger for PRF80. Stratification significantly and rapidly shifts combustion phasing with PRF80, but not with iso-octane. Charge stratification was also found to be effective for improving combustion efficiency at low-load conditions.

Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in HCCI Engines, Based on Multi-Zone Modeling and Experiments

This work investigates the potential of in-cylinder thermal stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between thermal stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (Φ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing. For combustion phasing in the range of 7 - 10°CA after TDC, a linear thermal distribution is optimal since it produces a near-linear pressure rise. For other combustion phasings, non-linear distributions are required to achieve a linear pressure rise. Also, the total thermal width must be greater at higher fueling rates to avoid excessive pressure-rise rates. The study also shows that increasing the natural thermal width of the charge by 50% would allow the equivalence ratio to be increased from 0.44 to 0.60, with an associated increase of the IMEPg from 524 to 695 kPa for naturally aspirated operation. It was also found that the naturally occurring thermal stratification plays a major role in producing the experimentally observed benefit of combustion-timing retard for slowing the combustion rate. Reduced chemical-kinetic rates with combustion retard are found to play a lesser role.

Comparing enhanced natural thermal stratification against retarded combustion phasing for smoothing of HCCI heat-release rates

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. To further exploit the effect, the heat-transfer rates were further enhanced by increasing the in-cylinder air swirl. This led to even longer combustion durations. Unfortunately, the higher heat losses associated with high air swirl decreased the IMEP g . When the fueling rate was increased to compensate, most of the improvements on the heat-release rates were lost. Overall, combustion phasing retard was found to have better potential for smoothing heat-release rates than enhancing the thermal stratification by the means considered in this work. However, operation with highly retarded combustion requires precise control of the ignition timing. Furthermore, it is found that the acceptable intake temperature range narrows rapidly with increasing equivalence ratio. Above a certain fueling rate a steady state operating point cannot be established by setting the intake temperature to a fixed value. This problem is caused by wall heating and the coupling between wall temperature and combustion phasing.

A Parametric Study of HCCI Combustion - the Sources of Emissions at Low Loads and the Effects of GDI Fuel Injection

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. Experimental results show that as fueling is reduced to equivalence ratios (Φ) below 0.20, CO emissions begin to increase substantially, reaching levels corresponding to more than 60% of all fuel carbon at idle loads (Φ = 0.1 -0.12). As this occurs, combustion efficiency falls from 94% to less than 55%. These high CO levels are in very good agreement with those predicted by the model, indicating that the high CO emissions and the associated combustion inefficiencies are due to incomplete bulk-gas reactions. HC emissions also rise, but the increase does not become pronounced until Φ < 0.14. In addition, the model indicates that significant emissions of oxygenated hydrocarbons (e.g., formaldehyde) should occur as bulk-gas reactions become less complete. This prediction is supported by the experimental exhaust carbon balance. Intake temperature significantly affects the onset of incomplete bulk-gas combustion; however, engine speed and compression ratio have only small effects for the fuel studied here. Fuel stratification by late GDI injection was investigated and found to have good potential for improving combustion efficiency at low loads.

Planar laser Rayleigh scattering for quantitative vapor-fuel imaging in a diesel jet

Abstract Quantitative images of vapor-phase fuel concentrations were obtained in an evaporating and combusting diesel jet using planar laser Rayleigh scattering. The diagnostic has been calibrated, evaluated, and successfully applied to an optically accessible direct-injection diesel engine for fired and nonfired operating conditions. The measurements were obtained in the leading portion of the diesel jet (the zone beyond 27 mm from the injector nozzle), where the fuel is entirely evaporated, and which corresponds to the main combustion zone in this engine. The technique was shown to be effective for quantitative imaging of the fuel-vapor concentration before ignition, with high spatial and temporal resolution. Additionally, images of the fuel-vapor concentration were further reduced to imagers of the equivalence ratio using an adiabatic mixing assumption to model the local temperature of the evaporating diesel jet. This procedure also yielded temperature distribution images. The results show that, at 4.5° crank angle (0.63 ms) after the start of injection, which corresponds to the time just before the first indicated heat release, the fuel and air are relatively well mixed in the leading portion of the diesel jet. At this crank angle, the equivalence ratio in the majority of the jet ranges from 2 to 4. The edges of the jet are well defined, with the signal level rising sharply from the background level up to levels corresponding to equivalence ratios in the jet. The temperature of the richest mixture regions in the jet is as low as 700 K, with the ambient air temperature at 1000 K. Finally, comparisons of Rayleigh images of the reacting and nonreacting jet show that the initial breakdown of the fuel, indicated by a significant decrease in the Rayleigh signal intensity, occurs throughout the cross section of the leading portion of the diesel jet.

A computational study of the effect of fuel type on ignition time in homogenous charge compression ignition engines

The homogeneous charge, compression ignition (HCCI) engine has advantages in terms of efficiency and reduced emissions in comparison to conventional internal combustion engines. One of the distinguishing characteristics of an HCCI engine is that the ignition is controlled by the chemical kinetics, unlike the diesel or spark ignition engines, for which ignition time can be controlled externally by the fuel injection or spark time. As a consequence of being controlled by chemical kinetics, the HCCI ignition time can vary significantly with changes in the operating conditions, and this variation can limit the practical range of operation of the engine. Using a single-zone combustion model and established reaction rate mechanisms, the influences of the compression ratio, intake temperature, equivalence ratio, engine speed, and exhaust gas recirculation on the ignition time of two fuels, normal heptane and iso-octane, were studied. The model simulated the environment in the engine combustion chamber by assuming adiabatic compression and expansion. The sensitivity of the ignition time to changes in operating conditions was found to be dependent on the type of fuel. The results indicate that the use of fuels with a characteristics two-stage ignition (e.g., n -heptane) exacerbates the problem of ignition control compared with the use of fuels with a single-stage ignition (e.g., iso-octane).

OH radical imaging in a DI diesel engine and the structure of the early diffusion flame

Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location 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 Q{sub 1}9 and Q{sub 2}8 lines of the (1,0) band of the A{yields}X transition at 284.01 nm, while the fluorescent emission from both the (0,O) and (1, I) 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. The signal is strong, and it is confined to a narrow region about the flame front because the threebody recombination reactions that reduce high flame-front OH concentrations to equilibrium levels occur rapidly at diesel pressures. No signal was evident in the fuel-rich premixed flame regions where calculations and burner experiments indicate that OH concentrations will be below detectable limits. Temporal sequences of OH PLIF images are presented showing the onset and development of the early diffusion flame up to the time that soot obscures the images. These images show that the diffusion flame develops around the periphery of the-downstream portion of the reacting fuel jet about half way through the premixed burn spike. Although affected by turbulence, the diffusion flame remains at the jet periphery for the rest of the imaged sequence.

The effect of TDC temperature and density on the liquid-phase fuel penetration in a D.I. diesel engine

A parametric study of the liquid-phase fuel penetration of evaporating Diesel fuel jets has been conducted in a directinjection 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/m{sup 3} 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 corresponding apparent heat release rate plots are presented and the results of the liquid-phase fuel penetration are discussed with respect to the ignition delay and premixed bum fraction.