Magnus Sjöberg

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.

Partial Fuel Stratification to Control HCCI Heat Release Rates: Fuel Composition and Other Factors Affecting Pre-Ignition Reactions of Two-Stage Ignition Fuels

Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock.

Spatial Analysis of Emissions Sources for HCCI Combustion at Low Loads Using a Multi-Zone Model

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 f = 0.26 to very low loads (f = 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. The results of this paper contribute to the understanding of the high emission behavior of HCCI engines at low equivalence ratios and are important for characterizing this previously little explored, yet important range of operation.

Modeling Iso-octane HCCI Using CFD with Multi-Zone Detailed Chemistry; Comparison to Detailed Speciation Data Over a Range of Lean Equivalence Ratios

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 headgasket 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. Results show that combustion is well characterized, as demonstrated by good agreement between calculated and measured pressure traces. In addition, excellent quantitative agreement between the model and experiment is achieved for specific exhaust species components, such as unburned fuel, formaldehyde, and many other intermediate hydrocarbon species. Some calculated trace intermediate hydrocarbon species do not agree as well with measurements, highlighting areas needing further investigation for understanding fundamental chemistry processes in HCCI engines.

PIV examination of spray-enhanced swirl flow for combustion stabilization in a spray-guided stratified-charge direct-injection spark-ignition engine

Practical implementation of spray-guided stratified-charge direct-injection spark-ignition engines can be inhibited by combustion instability, in particular the occurrence of misfire and partial burns. Performance testing in an all-metal spray-guided stratified-charge direct-injection spark-ignition engine shows that increasing the engine speed from 1000 to 2000 r/min can cause a deterioration of the combustion stability for operation without intake-generated swirl. Introducing swirl to the in-cylinder air charge motion maintains combustion stability while the speed is increased. To gain understanding how swirl reduces cycle-to-cycle variability of the flow, two-dimensional Particle Image Velocimetry (PIV) measurements were made in a horizontal swirl plane near the top of the piston bowl and in a central vertical tumble plane. Tests with and without injection were conducted at 1000 and 2000 r/min for operation both with and without swirl. The results demonstrate that the swirl creates flow patterns in each cycle that are more similar to the ensemble-averaged cycle, and with decreased variability. Furthermore, the fuel injection causes a redistribution of angular momentum resulting from spray–swirl interaction. The gas-phase swirl flow is redistributed by the spray to create a very repeatable vortex with enhanced angular momentum close to the spray centerline. This decreases the cycle-to-cycle variability of the flow patterns. Quantified changes in the stability of the flow patterns with swirl and engine speed are consistent with the combustion-instability trends.

Development of the RIOT web service and information technologies to enable mechanism reduction for HCCI simulations

New approaches are being explored to facilitate multidisciplinary collaborative research of Homogeneous Charge Compression Ignition (HCCI) combustion processes. In this paper, collaborative sharing of the Range Identification and Optimization Toolkit (RIOT) and related data and models is discussed. RIOT is a developmental approach to reduce the computational of detailed chemical kinetic mechanisms, enabling their use in modeling kinetically controlled combustion applications such as HCCI. These approaches are being developed and piloted as a part of the Collaboratory for Multiscale Chemical Sciences (CMCS) project. The capabilities of the RIOT code are shared through a portlet in the CMCS portal that allows easy specification and processing of RIOT inputs, remote execution of RIOT, tracking of data pedigree, and translation of RIOT outputs to a table view and to a commonly-used mechanism format.

Combined effects of intake flow and spark-plug location on flame development, combustion stability and end-gas autoignition for lean spark-ignition engine operation using E30 fuel

Lean or dilute spark-ignition engine operation can provide efficiency improvements relative to that of traditional well-mixed stoichiometric spark-ignition operation. However, to maintain a sufficiently short burn duration with the direct-injection spark-ignition engine hardware of the current study, mixed-mode combustion is required for operation with ϕ < 0.6. Such mixed-mode combustion uses a combination of deflagration and end-gas autoignition whereby the pressure rise of the deflagration-based combustion compresses the end-gas reactants to the point of autoignition. For better understanding of the transition from deflagration to autoignition, it is desirable to apply optical diagnostics. However, with the use of a single centrally located spark plug, the end-gas is found at the periphery of the combustion chamber, where it is difficult to examine optically. To overcome this, two additional spark plugs were mounted in the pent-roof gables (called East and West). Performance testing was performed for five different spark strategies: Central Only, East-West, ALL Three, East Only, and West Only. The five spark strategies are combined with swirl or no-swirl operation for a total of 10 ϕ-sweeps. A high-octane E30 fuel is used here, and intake heating is used to promote both lean combustion stability and end-gas autoignition. The best lean combustion stability is found for the ALL Three spark strategy, followed by the East-West and Central spark strategies, enabling stable mixed-mode spark-ignition combustion for ϕ down to 0.50 and 0.55, respectively. Here, operation without swirl provides the most stable combustion. High-speed imaging of ultra-lean operation without swirl at ϕ = 0.55 using the East-West spark strategy reveals that the transition from deflagration to end-gas autoignition frequently occurs within the view offered by the small piston-bowl window. These results encourage future optical investigations of fuel effects on this transition process, but a larger piston-bowl window is recommended.