Combustion Mode Switching Research Articles

Co-firing of coal and biomass under pressurized oxy-fuel combustion mode: Experimental test in a 10 kWth fluidized bed

• Pressurized oxy-fuel combustion of coal/biomass in a fluidized bed is successful. • Stable, pressurized, oxy-fuel combustion mode using coal/biomass can be realized. • Increased pressure leads to higher combustion efficiency and CO 2 enrichment. • Increased pressure has the advantages of reducing CO, NO x , and SO 2 emissions. • Increased pressure makes the fly ash particles finer with surface more cracked. Pressurized oxy-fuel combustion (POFC) of solid fuels in fluidized beds possess the potential for CO 2 capture at low cost. However, the practical experience of oxy-coal combustion in pressurized fluidized beds (PFB) is still very limited, and there is a lack of attempts on the co-firing of coal and other fuels. In this study, the co-firing of coal and biomass in a POFC mode at a 10 kW th PFB was tested. The dynamic behaviors of the start-up process and combustion mode switching were investigated. The effects of key operating parameters, including combustion pressure ( P ), biomass blending ratio ( M b ), and excess oxygen coefficient ( α ), on the temperature distributions, CO 2 enrichment and conversion, pollutant emissions (CO, NO X , SO 2 ), and solid residues were methodically studied. The results show that the stable, pressurized, and oxy-fuel combustion mode with coal and biomass mixtures as fuels can be successfully realized in a fluidized bed. Increasing P and M b not only conduces to better temperature distribution, more CO 2 enrichment in flue gas, and higher combustion efficiency but also has the advantage of reducing NO x and SO 2 emissions by over 30%. In the oxy-fuel PFB, the positive effect of α on the combustion performance is more significant than that under atmospheric conditions. As P increases, the fly ash surface is more cracked, and the particle size distribution of fly ash decreases, while the bottom slag surface is smoother. Besides, the increase in P results in a decreased specific surface area and cumulative pore volume but an increased average pore diameter in fly ash.

Research on Homogeneous Charge Compression Ignition Combustion of Intake Port Exhaust Gas Recirculation Based on Cam Drive Hydraulic Variable Valve Actuation Mechanism

The thermal efficiency of an efficient gasoline engine is only about 40% and it will produce a large number of harmful products. Curbing harmful emissions and enhancing thermal efficiency have always been the goals pursued and emission regulations are also being tightened gradually. As one of the main consumers of fossil fuels, automobile engines must further reduce fuel consumption and emissions to comply with the concept of low-carbon development, which will also help them compete with electric vehicles. Homogeneous charge compression ignition (HCCI) combustion combined with variable valve actuation (VVA) technology is one of the important ways to improve engine emissions and economy. HCCI combustion based on VVA can only be realized at small and medium loads. The actual application on the entire vehicle needs to be combined with spark ignition (SI) combustion to achieve full working condition coverage. Therefore, HCCI combustion needs fast valve response characteristics; however, the valve lift and timing of the existing VVA mechanisms are mostly controlled separately, resulting in poor valve response. In order to solve this problem, the cam driven hydraulic variable valve actuation (CDH-VVA) mechanism was designed. The valve lift and timing can be adjusted at the same time and the switching of valve lift and timing can be completed in 1~2 cycles. A set of combustion mode switching data is selected to show the response characteristics of the CDH-VVA mechanism. When switching from spark ignition (SI) to HCCI, it switches to HCCI combustion after only one combustion cycle and it switches to stable HCCI combustion after two combustion cycles, which proves the fast response characteristics of the CDH-VVA mechanism. At the same time, the CDH-VVA mechanism can form the intake port exhaust gas recirculation (EGR), as one type of internal EGR. This paper studies the HCCI combustion characteristics of the CDH-VVA mechanism in order to optimize it in the future and enable it to realize more forms of HCCI combustion. At 1000 rpm, if the maximum lift of the exhaust valve (MLEV) is higher than 5.0 mm or lower than 1.5 mm, HCCI combustion cannot operate stably, the range of excess air coefficient (λ) is largest when the MLEV is 4.5 mm, ranging from 1.0~1.5. Then, as the MLEV decreases, the range of λ becomes smaller. When the MLEV drops to 1.5 mm, the range of λ shortens to 1.0~1.3. The maximum value of the MLEV remains the same at the three engine speeds (1000 rpm, 1200 rpm and 1400 rpm), which is 5.0 mm. The minimum value of the MLEV gradually climbs as the engine speed increase, 1000 rpm: 1.5 mm, 1200 rpm: 2.0 mm, 1400 rpm: 3.0 mm. With the increase of engine speed, the range of indicated mean effective pressure (IMEP) gradually declines, 3.53~6.31 bar (1000 rpm), 4.11~6.75 bar (1200 rpm), 5.02~6.09 bar (1400 rpm), which proves that the HCCI combustion loads of the intake port EGR are high and cannot be extended to low loads. The cyclic variation of HCCI combustion basically climbs with the decrease of the MLEV and slightly jumps with the increase of the engine speed. At 1000 rpm, when the MLEV is 5.0 mm, the cyclic variation range is 0.94%~1.5%. As the MLEV drops to 1.5 mm, the cyclic variation range rises to 3.5%~4.5%. Taking the maximum value of the MLEV as an example, the cyclic variation range of 1000 rpm is 0.94%~1.5%, 1200 rpm becomes 1.5%~2.3% and 1400 rpm rises to 2.0%~2.5%.

Gasoline-diesel dual fuel intelligent charge compression ignition (ICCI) combustion: Conceptual model and comparison with other advanced combustion modes

The internal combustion engines can remain the advantage over competitor technologies for automotive driven, especially the engine efficiency, exceeded 50% while maintaining ultra-low emissions. In this paper, a novel combustion mode characterized by dual high-pressure common-rail direct injection systems, denoted as intelligent charge compression ignition (ICCI) combustion, is proposed to realize high efficiency and clean combustion in wide engine operating ranges. Specifically, commercial gasoline and diesel, which are considered to be complementary in physical and chemical properties, are directly injected into the cylinder by the two independent injection systems, respectively. Through this unique design, the in-cylinder air-fuel mixtures can be flexibly adjusted by regulating injection timing and duration of different fuels, consequently obtaining suitable combustion phase and heat release rate. The ICCI mode can widely run from indicated mean effective pressure 2 bar to 16 bar with an utterly controllable cylinder pressure rising rate, around 50% indicated thermal efficiency and low NOx emissions. A series of experiments were carried out to compare the combustion and emissions of ICCI with other combustion modes (including conventional diesel combustion, reactivity-controlled compression ignition, partially premixed combustion, and gasoline compression ignition). The results show that at the medium engine loads, ICCI mode can reach much high indicated thermal efficiency, especially up to 52% along with extremely low NOx emissions. Prospectively, ICCI mode can realize real-time adjustments of in-cylinder mixture stratification and instantaneous combustion mode switch in one cycle at any operating conditions, and has an excellent commercial application prospect for energy conservation and environmental improvement.

A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y′) and wavelength (0.01 mm<λ<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y′, λ) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion–reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called β-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y′, λ) parameter space while the β-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the β-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified β-curve for turbulent cases. The practical output of the paper is the β-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations.

Performance and emission characteristics of conventional diesel combustion/partially premixed charge compression ignition combustion mode switching of biodiesel-fueled engine

In this experimental study, a production grade engine was modified to operate in two combustion modes, namely conventional diesel combustion (CDC) and premixed charge compression ignition (PCCI) combustion, depending on the engine load. For mode switching, an open electronic control unit was programmed to operate the engine in PCCI combustion mode up to medium engine loads and then automatically switching it to CDC mode at higher engine loads, by varying the fuel injection parameters and the exhaust gas recirculation rate. For performance and emission characterization in the entire load range (idling-to-full load) of the test engine, a test cycle of 300 s was used, which included CDC mode, PCCI combustion mode, and transition between these two modes. Results showed that both mineral diesel and B20 (20% biodiesel blended with mineral diesel, v/v) fueled PCCI combustion resulted in significantly lower NOx and particulate emissions compared to baseline CDC. Relatively lower exhaust gas temperature in PCCI combustion mode led to slightly inferior engine performance and higher concentration of unregulated emission species such as SO2, HCHO, and so on. B20-fueled engine resulted in relatively lower unregulated emission species and particulates compared to the mineral diesel–fueled engine in both the combustion modes. In CDC mode, contributions of accumulation mode particles were significantly higher compared to nucleation mode particles. Relatively lower emission of aromatic compounds in PCCI combustion mode compared to CDC mode was another important finding of this study; however, B20-fueled engines resulted in slightly higher emissions of aromatic compounds.

SI/HCCI Mode Switching Optimization in a Gasoline Direct Injection Engine Employing Dual Univalve System

Homogeneous charge compression ignition (HCCI) is a feasible combustion mode meeting future stringent emissions regulations, and has high efficiency and low NOX and particle emissions. As the narrow working condition range is the main challenge limiting the industrialization of HCCI, combustion mode switching between SI and HCCI is necessary when employing HCCI in mass production engines. Based on a modified production gasoline direct injection (GDI) engine equipped with dual UniValve system (a fully continuously variable valvetrain system), SI/HCCI mode switching under low load condition is investigated. According to the results, combustion mode switching from SI to HCCI is more complicated than from HCCI to SI. As HCCI requires strict boundary conditions for reliable and repeatable fuel auto-ignition, abnormal combustion easily appears in transition cycle, especially when combustion switches from SI to HCCI. Timing control strategies can optimize the combustion of transition cycles. With the optimization of timing control, the mode switching from SI to HCCI can be completed with only two transition cycles of late combustion, and abnormal combustion can be avoided during the mode switching from HCCI to SI. Under the low load condition, the indicated efficiency reaches 39% and specific NOX emissions drop down to around 1 mg/L/s when the combustion mode is switched to HCCI mode. Compared to SI mode, the indicated efficiency is increased by 10% and the specific NOX emissions are reduced by around 85%.

Potential of RCCI Series Hybrid Vehicle Architecture to Meet the Future CO2 Targets with Low Engine-Out Emissions

Reactivity controlled compression ignition (RCCI) combustion has been shown to provide simultaneous ultra-low NOx and soot emissions with similar or better thermal efficiencies than conventional diesel combustion (CDC). Nonetheless, RCCI still has several challenges that restrict its operating range and limit its practical application. The dual-mode operation, which involves switching between different combustion modes, has been found as a promising alternative to operation in the whole engine map. However, the combustion mode switching requires difficult engine control, particularly during transient operation. The series hybrid vehicle (SHV) architecture allows the thermal engine to operate in a limited operating range by decoupling it from the drivetrain. Therefore, it could be an interesting alternative to the dual-mode concept. This work explores the potential of the RCCI series hybrid vehicle architecture to provide low engine-out emissions and CO2 by means of vehicle systems simulations. The results show the influence of the main parameters and control strategies of the SHV vehicle on its efficiency and emissions under different driving cycles. Finally, the optimal RCCI-SHV configuration is compared to CDC and dual-mode combustion strategies, confirming its potential as a future vehicle architecture for high efficiency and low emissions.

Combustion Mode Switching Characteristics of a Medium-Duty Engine Operated in Compression Ignition/PCCI Combustion Modes

Premixed charge compression ignition (PCCI) combustion is a novel combustion concept, which reduces oxides of nitrogen (NOx) and particulate matter (PM) emissions simultaneously. However, PCCI combustion cannot be implemented in commercial engines due to its handicap in operating at high engine loads. This study is focused on the development of hybrid combustion engine in which engine can be operated in both combustion modes, namely, PCCI and compression ignition (CI). Up to medium loads, engine was operated in PCCI combustion and at higher loads, the engine control unit (ECU) automatically switched the engine operation to CI combustion mode. These combustion modes can be automatically switched by varying the fuel injection parameters and exhaust gas recirculation (EGR) by an open ECU. The experiments were carried out at constant engine speed (1500 rpm) and the load was varied from idling to full load (5.5 bar brake mean effective pressure (BMEP)). To investigate the emission and particulate characteristics during different combustion modes and mode switching, continuous sampling of the exhaust gas was done for a 300 s cycle, which was specifically designed for this study. Results showed that PCCI combustion resulted in significantly lower NOx and PM emissions compared to the CI combustion. Lower exhaust gas temperature (EGT) in the PCCI combustion mode resulted in slightly inferior engine performance. Slightly higher concentration of unregulated emission species such as sulfur dioxide (SO2) and formaldehyde (HCHO) in PCCI combustion mode was another important observation from this study. Lower concentration of aromatic compounds in PCCI combustion compared to CI combustion reflected relatively lower toxicity of the exhaust gas. Particulate number-size distribution showed that most particulates emitted in PCCI combustion mode were in the accumulation mode particle (AMP) size range, however, CI combustion emitted relatively smaller sized particles, which were more harmful to the human health. Overall, this study indicated that mode switching has significant potential for application of PCCI combustion mode in production grade engines for automotive sector, which would result in relatively cleaner engine exhaust compared to CI combustion mode engines.

Multimode combustion in a mild hybrid electric vehicle. Part 1: Supervisory control

This is the first of a two-part simulation study that discusses the application of a multimode combustion engine in a mild hybrid electric vehicle (HEV). The torque assist, offered by the electric motor, can be used to extend the residence time in the homogeneous charge compression ignition (HCCI) regime, before returning to spark-ignition (SI) combustion. To enable multimode operation in the HEV, the supervisory control strategy has to maintain the battery's state-of-charge while accounting for the SI/HCCI combustion mode switch. In this study four supervisors are discussed which extend the baseline equivalent consumption minimization strategy by the mode switching decision.

Investigation of Cold Starting and Combustion Mode Switching as Methods to Improve Low Load RCCI Operation

Reactivity controlled compression ignition (RCCI) is an engine combustion strategy that utilizes in-cylinder fuel blending to produce low NOx and particulate matter (PM) emissions while maintaining high thermal efficiency. The current study investigates RCCI and conventional diesel combustion (CDC) operation in a light-duty multicylinder engine (MCE) using a transient capable engine test cell. The main focus of the work uses engine experiments to investigate methods which can improve low load RCCI operation. The first set of experiments investigated RCCI operation during cold start conditions. The next set of tests investigated combustion mode switching between RCCI and CDC. During the cold start tests, RCCI performance and emissions were measured over a range of engine coolant temperatures (ECTs) from 48 °C to 85 °C. A combination of open- and closed-loop controls enabled RCCI to operate at a 1500 rpm, 1 bar BMEP operating point over this range of coolant temperatures. At a similar operating condition, i.e., 1500 rpm, 2 bar BMEP, the engine was instantaneously switched between CDC and RCCI combustion using the same open- and closed-loop controls as the cold start testing. During the mode switch tests, emissions and performance were measured with high-speed sampling equipment. The tests revealed that it was possible to operate RCCI down to 48 °C with simple open- and closed-loop controls with emissions and efficiency similar to the warm steady-state values. Next, the mode switching tests were successful in switching combustion modes with minimal deviations in emissions and performance in either mode at steady state.

Accounting for combustion mode switch dynamics and fuel penalties in drive cycle fuel economy

A methodology is introduced to analyze the drive cycle fuel economy of a vehicle equipped with a multimode combustion engine, utilizing commercial cam phasers and two-step cam profile switching. The analysis is based on a longitudinal vehicle model with manual transmission. The engine model employs a finite state machine describing mode switches between two distinct combustion modes, namely, spark-ignited and homogeneous charge compression ignition. Preliminary combustion mode switch experiments were used to parameterize the model. The influence of mode switch fuel penalties on drive cycle fuel economy was quantified through application of the model to the federal test procedure (FTP-75), the highway fuel economy test (HWFET) and the US06 supplemental federal test procedure. The mode switches were analyzed individually in terms of their benefit on fuel economy and a distinction was made between harmful and beneficial mode switches. Mode switches were defined as harmful whether their fuel penalty is greater than the benefits originating from spending time in the subsequent homogeneous charge compression ignition mode. A parametric study was conducted to investigate the impact of harmful mode switches on fuel economy as a function of the fuel penalties during the switch. In the case of high fuel penalties, supervisory control becomes an important tool for minimizing the number of harmful mode switches. One possible supervisory strategy discussed is a smoothing strategy, in which a mode switch is delayed by introducing a dwell time.

Fuel Economy of a Multimode Combustion Engine With Three-Way Catalytic Converter

Highly diluted, low temperature homogeneous charge compression ignition (HCCI) combustion leads to ultralow levels of engine-out NOx emissions. A standard drive cycle, however, would require switches between HCCI and spark-ignited (SI) combustion modes. In this paper we quantify the efficiency benefits of such a multimode combustion engine, when emission constraints are to be met with a three-way catalytic converter (TWC). The TWC needs unoccupied oxygen storage sites in order to achieve acceptable performance. The lean exhaust gas during HCCI operation, however, fills the oxygen storage and leads to a drop in NOx conversion efficiency. If levels of tailpipe NOx become unacceptable, a mode switch to a fuel rich combustion mode is necessary in order to deplete the oxygen storage and restore TWC efficiency. The resulting lean-rich cycling leads to a penalty in fuel economy. Another form of penalty originates from the lower combustion efficiency during a combustion mode switch itself. In order to evaluate the impact on fuel economy of those penalties, a finite state model for combustion mode switches is combined with a longitudinal vehicle model and a phenomenological TWC model, focused on oxygen storage. The aftertreatment model is calibrated using combustion mode switch experiments from lean HCCI to rich spark-assisted HCCI (SA-HCCI) and back. Fuel and emission maps acquired in steady-state experiments are used. Different depletion strategies are compared in terms of their influence on drive cycle fuel economy and NOx emissions. It is shown that even an aggressive lean-rich cycling strategy will marginally satisfy the cumulated tailpipe NOx emission standards under warmed-up conditions. More notably, the cycling leads to substantial fuel penalties that negate most of HCCI's efficiency benefits.

Fuel injection and EGR control strategy on smooth switching of CI/HCCI mode in a diesel engine

Homogeneous charge compression ignition(HCCI) combustion has been demonstrated abilities to lower engine emissions due to its nature of even mixture and low temperature combustion. This combustion mode is only applicable in part load and low speed area so far. This range limitation results in frequently combustion mode switching from CI mode to HCCI mode(CI–HCCI) and HCCI mode to CI mode(HCCI–CI) in real applications. Large variation of IMEP and combustion phase occurs during mode switching process. This is unfavorable to the performance of engine. In the research, diesel HCCI realized by early injection with negative valve overlap(NVO) is investigated.Fuel injection control strategy is studied to improve the smoothness of IMEP during mode switching firstly. Based on the concluded fuel injection control strategy, EGR strategy is studied to improve the smoothness of CA50 during mode switching. In the analysis, fuel compensation coefficient(η) and dimensionless cycle number(σ) are defined. Results of this study show fuel compensation of transition cycle is necessary in CI–HCCI mode switching, but direct mode switching is adequate in HCCI–CI mode switching. The optimum fuel compensation coefficient for CI–HCCI mode switching is at about 1.06, and the COVIMEP is about 5.6. Too late and early switching of EGR are both unfavorable to the smoothness of CA50 during mode switching. The optimum dimensionless cycle number of CI–HCCI and HCCI–CI mode switching are 4.8 and 0.6 respectively.

Effects of fuel compensation in transitional cycles on the smoothness of combustion mode switching in a diesel engine

Because indicated mean effective pressure (IMEP) fluctuation often occurs in diesel homogeneous charge compression ignition (HCCI) engines during the conventional compression-ignition (CI) combustion mode and HCCI combustion mode switching process, in this paper, the effects of a fuel injection compensation strategy on the smoothness in the CI-HCCI combustion mode switching process at equal load were studied. The fuel compensation in one transitional cycle, two transitional cycles and three transitional cycles were discussed, and the return maps of the IMEP variation were analyzed. The results showed that the fuel increase of the first transitional cycle can enhance the combustion of this cycle and prevent the IMEP from decreasing. More transitional cycles with decreased fuel compensation ratio can improve the smoothness of the IMEP. However, the magnitude of the decreasing trend of the IMEP fluctuation ratio decreases and the fuel control strategy becomes complicated as the number of transitional cycles increases. The fuel injection compensation strategy for the diesel engine is optimized when two transitional cycles and fuel ratios of 1.07 for the first cycle and 1.03 for the second cycle are used, and the fluctuation of the IMEP decreased by 68% compared to the direct combustion mode switching process.

Combustion mode switching control in a HCCI diesel engine

HCCI (Homogeneous Charge Compression Ignition) combustion has the potential to significantly reduce NOx and PM emission of diesel engines due to its nature of even mixture and low temperature combustion. This combustion mode is only applicable in part load and low speed area so far. This range limitation results in frequently combustion mode switching from HCCI to CI and vice versa in real applications. HCCI realized by late injection with heavy EGR rate is investigated, which is also named PCCI (Premixed Charged Compression Ignition) sometimes. By changing control parameters (including injection timing, injection pressure and EGR rate) individually, the influences of these parameters on HCCI combustion and emissions are identified. Based on the evaluation of the influences, one pre-defined step-by-step air–fuel coordination strategy is presented. Additionally one hysteresis combustion mode determination method is proposed to avoid target mode fluctuation due to variation of engine speed or engine load in a small range. Torque deficiency occurs in acceleration transients as a result of fuel limitation to reduce soot emission, especially during HCCI to CI transitions. One ISG (Integrated Starter Generator) motor is applied to perform dynamic torque compensations in transient operations. Engine friction torque is estimated by means of cylinder pressure information from in-cylinder pressure sensors. On the basis of torque estimation, the difference between desired engine brake torque and actual engine brake torque is obtained. The torque output of ISG motor accounts for this torque gap and the torque pre distributed to ISG motor. The strategy is verified by engine tests. Experimental results indicate that, with the finely defined air–fuel coordination, less unstable combustion occurs and the target modes are established more quickly during transitions both in steady states and in transients. The torque deficiency during acceleration operations could be well compensated by the ISG motor.

Study on Judgment Method of Combustion Mode on Dual-mode Scramjet

It was very significant to judge the combustion mode of engine correctly, because the distributions of fuels and other parameters of combustion mode switching depended on it. This paper proposed to a judgment method of combustion mode of engine, namely artificial neural network method, based on the dual-mode scramjet experiments and analysis results. It made use of experimental measurement values, and the parameters along engine were obtained through one dimensional analysis. This method provided the basis for the combustion mode design of engine in the future.

Investigation of SI-HCCI Hybrid Combustion and Control Strategies for Combustion Mode Switching in a Four-Stroke Gasoline Engine

Homogeneous charge compression ignition (HCCI) combustion, also known as controlled autoignition (CAI) combustion, has been realized over a range of engine speed and load conditions in a single cylinder four-stroke gasoline engine equipped with a 4VVAS mechanical variable intake and exhaust valve lift and timing devices system. The engine has the capability to operate either in the spark ignition (SI) or HCCI combustion in order to cover the complete engine operational range. Therefore, smooth switching between SI and HCCI modes is required. In this work, a systematic investigation has been carried out to study the transition between the conventional spark ignition combustion and HCCI combustion and develop the control strategies for optimized mode switch. Results show that dynamic transitions between HCCI and SI can be stably achieved around the boundaries of HCCI operation region through the rapid and effective control of the in-cylinder residual gas concentration by a Four Variable Valve Actuation System (4VVAS). It is found that switching from HCCI to SI operation is less problematic than that from SI to HCCI combustion operation, and certain regions of residual concentrations should be avoided. During the switching process, the spark timing and exhaust valve closing timing have the largest effect and can be optimized for mode switching. Two major approaches have been developed and applied to controlling the HCCI-SI transitions, through the dynamic management of residual gas and the management of hybrid heat release processes.

Characteristic Response of a Production Diesel Oxidation Catalyst Exposed to Lean and Rich PCI Exhaust

Although low-temperature premixed compression ignition (PCI) combustion in a light-duty diesel engine offers dramatic and simultaneous reductions in nitric oxides (NOx) and soot, associated increases in unburned hydrocarbons (HC) and carbon monoxide (CO) become unacceptable. Production diesel oxidation catalysts (DOCs) are effective in oxidizing the increased levels of HC and CO under lean combustion conditions. However, the low-temperature∕high CO combination under rich PCI conditions, designed as a lean NOx trap (LNT) regeneration mode, generally renders the DOC ineffective. The objectives of this study are to characterize the oxidizing efficiency of a production DOC under lean and rich PCI conditions, and attempt to identify probable causes for the observed ineffectiveness under rich PCI. The study uses several tests to characterize the behavior of the DOC under lean PCI and rich PCI combustion conditions, including (1) steady-state feed gas characterization, (2) transient feed gas characterization, (3) air injection (4) insulated air-fuel sweep, and (5) combustion mode switching. The DOC never becomes effective under rich PCI for any of the tests, suggesting that the platinum-based catalyst may be incorrect for use with rich PCI. Furthermore, combustion mode switching between lean PCI and rich PCI (mimicking LNT loading and regeneration) demonstrates diminishing effectiveness of the DOC during and after continuous mode transitioning.