Spark-assisted Homogeneous Charge Compression Ignition Research Articles

Natural gas-fueled HCCI engine performance and emission analysis and comparison with SI and spark-assisted operations

ABSTRACT The homogeneous charge compression ignition (HCCI) engine is a promising technology in terms of both soot and NOx emission. Natural gas (NG) is advantageous especially for HCCI engines with its single stage combustion, low C/H ratio, and high octane number. Since the main challenge of HCCI technology is stability and transient operations, using HCCI for constant speed operations may be advantageous. In this study, a CI diesel engine, which is used as a generator at constant speed, was modified and converted into an NG-fuelled, SI engine. Then, the engine was supplied with NG for SI, HCCI, and spark-assisted HCCI experiments. Spark-assisted operations were executed by using 3, 5, and 7 degree ignition advance values to assist HCCI combustion. In-cylinder pressure variations, exhaust temperature, specific fuel consumption, and emission values were investigated for comparison of different strategies. Experimental results showed that using the HCCI strategy and retardation of ignition advance of spark-assisted HCCI operations reduced maximum pressure. Therefore, these strategies improved NOx emissions, while HC emissions were slightly increased. The spark-assisted HCCI strategy improves the stability of operation while increasing CO emission. The specific fuel consumption value of HCCI was slightly higher than that of SI operation, while it was lower than spark-assisted strategies.

A comparative study of PODE/gasoline and n-heptane/gasoline in a spark-assisted homogeneous charge compression ignition engine with dual-injection strategies and EGR

Homogeneous charge compression ignition (HCCI) has potential to improve thermal efficiency for gasoline engine due to the similar formation of mixture between HCCI and traditional spark ignition mode. High reactivity fuel combined with gasoline is used to achieve HCCI combustion with a relative low compression ratio. Polyoxymethylene dimethyl ethers (PODE) is a kind of low-carbon fuels with unique molecule structure, which is beneficial for carbon neutral. The intermediate products of PODE are different from those of long-chain alkane such as n-heptane and diesel. In this paper, the difference in spark-assisted HCCI (SA-HCCI) combustion combined with spark ignition and EGR between PODE/gasoline and n-heptane/gasoline under different operating conditions were investigated. The results illustrated that PODE/gasoline was more suitable for SA-HCCI combustion due to the positive relation between thermal efficiency and gasoline percentage, which can significantly improve the thermal efficiency while suppressing knock. Although the cetane number of PODE is greater than the n-heptane, the SA-HCCI combustion was more violent fueled with n-heptane/gasoline than that of PODE/gasoline. Low NOx and PM emissions are achieved for SA-HCCI combustion especially for PODE/gasoline. The advancing spark timing reduced the incomplete combustion and shortened the combustion duration, thus reducing the THC and CO emissions, especially fueled with PODE/gasoline. Although a low EGR rate can slightly improve the THC and CO emissions, violent combustion can be significantly improved. In general, high efficiency and low emissions could be achieved by PODE/gasoline and spark ignition combined with EGR. The brake thermal efficiency for SA-HCCI combustion fueled with PODE/gasoline increased by 30% compared with DISI mode under the same loads.

Reduced Reaction Mechanisms for Ethanol under Ultra-lean Conditions in Internal Combustion Engines.

Chemical kinetics models for ethanol under ultra-lean engine conditions were evaluated to couple with CFD multidimensional simulations of a spark-assisted homogeneous charge compression ignition (HCCI) rotary engine. Five reduced reaction sets proper for CFD simulations and two detailed reaction mechanisms for comparison were tested by simulating ignition delay times, laminar flame speeds, and a single-cycle HCCI engine with virtual piston dimensions. The simulated results of the new mechanism with 188 reactions were well-fitted to both experimental ignition delay times for ultra-lean ethanol/air conditions and laminar flame speeds at high pressures. This reaction set resulted in better-simulated ignition delay times at 30 and 40 bar for ultra-lean ethanol/air conditions than other chemical kinetics models. Maximum temperatures and pressures of 2500–2580 K and 280–289 bar, respectively, were observed for hydrous ethanol/air under ultra-lean conditions in HCCI engine. In addition, the simulation results of the HCCI ethanol engine presented high pressure rise rates of 8–26 bar/CAD at 3600 rpm. These results indicated that the engine test should be carried out at 2500 rpm with 2 bar of boost pressure for CFD model calibration with the new optimized reaction mechanism.

Cycle-by-cycle variations in autonomous and spark assisted homogeneous charge compression ignition combustion of stoichiometric air–fuel mixture

This study investigates cycle-by-cycle variations in a gasoline fuelled, homogeneous charge compression ignition (HCCI) engine with internal exhaust gas recirculation. In order to study the effects of exhaust-fuel reactions occurring prior to the main combustion event fuel was injected directly into the cylinder at two selected timings during the negative valve overlap period. The engine was operated as both autonomous HCCI and spark assisted HCCI (SA-HCCI). The primary interest in this work was the operating region where the engine is switched between HCCI and spark ignition modes, thus operation with stoichiometric air–fuel mixture, which is typical for this region, was considered. Cycle-by-cycle variations in both combustion timing and indicated mean effective pressure (IMEP) were investigated. It was found that long-period oscillations of the IMEP occur when fuel injection is started at early stages of the negative valve overlap period, and that these can be suppressed by delaying the start of injection. This behaviour remained even when fuel injection was split into early and late-negative valve overlap injections. Spark assisted operation allowed eliminating late combustion cycles, thus improving thermal efficiency. However, characteristic patterns of IMEP variations were found to be the same for both HCCI and SA-HCCI operations, irrespective of the adopted negative valve overlap fuel injection strategy, as evidenced by using symbol-sequence statistics.

An assessment of thermodynamic merits for current and potential future engine operating strategies

This work compares the fundamental thermodynamic underpinnings (i.e. working fluid properties and heat release profile) of various combustion strategies with engine measurements. The approach employs a model that separately tracks the impacts on efficiency due to differences in rate of heat addition, volume change, mass addition, and molecular weight change for a given combination of working fluid, heat release profile, and engine geometry. Comparative analysis between the measured and modeled efficiencies illustrates fundamental sources of efficiency reductions or opportunities inherent to various combustion regimes. Engine operating regimes chosen for analysis include stoichiometric spark-ignited combustion and lean compression-ignited combustion including homogeneous charge compression ignition, spark-assisted homogeneous charge compression ignition, and conventional diesel combustion. Within each combustion regime, the effects of engine load, combustion duration, combustion phasing, compression ratio,...

Direct numerical simulation of flame/spontaneous ignition interaction fueled with hydrogen under SACI engine conditions

In spark assisted homogeneous charge compression ignition (SACI) engines a premixed flame is first established; the propagating flame results in compression heating of the end gas and finally auto-ignition of the gas. Two dimensional direct numerical simulation (2D DNS) and one dimensional (1D) detailed simulations of flame/spontaneous ignition interaction in a lean hydrogen/air mixture under a constant volume enclosure relevant to SACI engine conditions are performed using detailed chemistry and detailed transport properties. In a 2D outward propagating spherical flame, thermal-diffusive instability is observed and once auto-ignition starts a low temperature region in the preheat zone of the flame is formed. Subsequently, the ignition in the preheat zone is suppressed. Then 1D flame/ignition interactions in H2/air, syngas/air and methane/air mixtures are studied using detailed numerical simulations. The results reveal that due to preferential diffusion of hydrogen, heat and mass transfer in the preheat zone inhibits the ignition process, yielding a slower reaction rate hence an even lower temperature in the region. It is shown further that the low temperature region is not affected by the domain size and one step chemistry but it will disappear at very low initial temperature conditions. This is due to the absence of auto-ignition under low temperature conditions.

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.

Enhanced heat release analysis for advanced multi-mode combustion engine experiments

Advanced combustion strategies, such as Homogeneous-Charge Compression-Ignition (HCCI) and Spark-Assisted HCCI or Spark-Assisted Compression-Ignition (SACI) hold considerable promise for improving engine efficiencies while maintaining low pollutant emissions, yet few models exist that accurately include the important chemical and physical mechanisms of these advanced combustion strategies. Further, experimental data from advanced combustion engine experiments are not well represented using conventional spark ignited analytical tools. This paper presents new methods for advanced combustion analysis that integrate previous analytical methods with new algorithms to capture the unique features of advanced combustion strategies like SACI.The new analytical capabilities were incorporated into a program which was designated the Advanced Combustion Engine Heat Release Analysis (ACE-HRA) tool. The models developed and applied in ACE-HRA were assessed by comparison with high fidelity engine simulations of HCCI and SACI. The high fidelity simulations provided data sets with detailed predictions of heat release rates, temperatures, auto-ignition timing, flame speeds and other key parameters not resolved or measured in engine experiments. The sensitivity of ACE-HRA estimates to model input data was quantified for important engine performance parameters. The sensitivity analysis showed that estimates for in-cylinder masses have the largest overall impact on the ACE-HRA results (e.g. ±10% variation led to changes on the order of ±5–10% in peak rate of heat release, burn duration and peak temperature). Noticeable differences in peak heat release rate and ringing intensity were also observed when comparing cycle-by-cycle analysis against ensemble-average analysis, which has implications on how the results are interpreted and applied in modeling work.After validating ACE-HRA with the high-fidelity simulations, ACE-HRA was applied to interpret the data from a recent experimental study of SACI combustion. The ACE-HRA methods were used to infer the effects of flame propagation on in-cylinder gradients and cycle-to-cycle variability, and to provide quantitative estimates for the associated changes in the end-gas burn rate. The trends observed, such as the decrease in burn rate for later auto-ignition and higher burn fraction by flame, provide more in-depth understanding of SACI combustion and demonstrate the insight that can be revealed by the new ACE-HRA tool.

Optical Investigation of the Effects of Ethanol/Gasoline Blends on Spark-Assisted HCCI

Spark assist (SA) has been demonstrated to extend the operating limits of homogeneous charge compression ignition (HCCI) modes of engine operation. This experimental investigation focuses on the effects of 100% indolene and 70% indolene/30% ethanol blends on the ignition and combustion properties during SA HCCI operation. The spark assist effects are compared to baseline HCCI operation for each blend by varying spark timing at different fuel/air equivalence ratios ranging from Φ = 0.4–0.5. High speed imaging is used to understand connections between spark initiated flame propagation and heat release rates. Ethanol generally improves engine performance with higher net indicated mean effective pressure (IMEPn) and higher stability compared to 100% indolene. SA advances phasing within a range of ∼5 crank angle degrees (CAD) at lower engine speeds (700 rpm) and ∼11 CAD at higher engine speeds (1200 rpm). SA does not affect heat release rates until immediately (within ∼5 CAD) prior to auto-ignition. Unlike previous SA HCCI studies of indolene fuel in the same engine, flames were not observed for all SA conditions.

Direct numerical simulations of HCCI/SACI with ethanol

Two and three dimensional direct numerical simulations (DNS) of an autoignitive premixture of air and ethanol in Homogeneous Charge Compression Ignition (HCCI) mode have been conducted. A special feature of these simulations is the use of compression heating through mass source/sink terms to emulate the compression and expansion due to piston motion. Furthermore, combustion phasing is adjusted such that peak heat release occurs after Top Dead Center (TDC) during the expansion stroke, as in a real engine. Zero dimensional simulations were first conducted to identify important parameters for the higher dimensional simulations. They showed that for ethanol, temperature and dilution are the parameters the problem is most sensitive to. One set of two dimensional simulations were conducted with a uniform mixture composition and different levels of temperature stratification, both with and without compression heating. Another set of simulations varied the mixture stratification with constant temperature stratification. Both sets showed considerable differences in ignition delay, heat release and peak temperature and peak pressure. Compression heating was also found to have a significant effect on the heat release profile. A three dimensional simulation was conducted for Spark-Assisted HCCI (SACI). It was initiated with a small spark kernel, which evolved into a premixed flame. The entire mixture eventually underwent autoignition. Distance function based analysis showed a strongly attenuating flame. Analysis of scalar mixing frequencies shows that differential diffusion and reaction induced mixing play an important role in predicting the mixing of reactive scalars. This has significant implications for mixing models for reactive flows. Chemical explosive mode analysis (CEMA) was applied to the 3D simulation and showed promise in identifying the transition from flame propagation to autoignition.

Multi-cylinder spark-assisted homogeneous charge compression ignition engine balancing control

Cylinder-to-cylinder variations (CTCVs) are regarded as one of the most important obstacles to put HCCI combustion into practical application in multi-cylinder engines. In this paper, HCCI combustion is realised in a four cylinder naturally aspired PFI gasoline HCCI engine equipped with variable valve actuation systems through negative valve overlap strategy. CTCVs are observed and explained, and the according balancing strategy is developed. Firstly, cylinder individual lambda control is used for IMEP balancing, by aid of intake air mass model derived from system identification. For improvement, a dynamic lambda lean limit searching strategy is designed through combustion stability observation, thanks to which stable combustion is maintained without the negative effect from IMEP balancing. Secondly, regarding CA50 balancing, a mid-ranging control strategy is proposed for coordinated control of intake-valve closing-timing and spark timing. And finally, this control structure is validated in experiment and proved to work well.

Large Eddy Simulation of Turbulent Combustion in a Spark-Assisted Homogenous Charge Compression Ignition Engine

A large eddy simuation (LES) model is presented for simulation of spark-assisted homogeneous charge compression ignition combustion. The model is based on tabulated chemical kinetic rate for ignition and flame surface density for flame propagation, taking into account interaction between flame propagation introduced by the spark and auto-ignition due to charge compression. The model is used to simulate the combustion process in an experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. The model is shown to be able to predict the combustion behavior observed in previous engine experiments. With low initial temperature, the SI flame mode prevails; with high initial temperature, the HCCI mode prevails. With moderate initial temperature, the SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of the observed high cyclic variation in the experiments.

The Effects of Fuel Characteristics on Stoichiometric Spark-Assisted HCCI

The characteristics of fuel lean homogeneous charge compression ignition (HCCI) operation using a variety of fuels are well known and have been demonstrated using different engine concepts in the past. In contrast, stoichiometric operation of HCCI is less well documented. Recent studies have highlighted the benefits of operating at a stoichiometric condition in terms of load expansion combined with the applicability of three way catalyst technology to reduce NOx emissions. In this study the characterization of stoichiometric HCCI using gasoline-like fuels was undertaken. The fuels investigated are gasoline, a 50% volume blend of iso-butanol and gasoline (IB50), and an 85% volume blend of ethanol and gasoline (E85). A single cylinder engine operating with direct injection (DI) and spark assist combined with a fully variable hydraulic valve actuation system allowed a wide range of operating parameters to be studied. The resultant fuel properties, which differed in terms of octane rating, fuel oxygenation, and heat of vaporization, show that stoichiometric HCCI is possible using a range of fuels but that these fuel characteristics do have some effect on the combustion characteristics. How these fuel properties can enable an increased engine operating envelope to be achieved, in comparison with both fuel lean HCCI and conventional spark ignited combustion, is then discussed.

An experimental investigation of the sensitivity of the ignition and combustion properties of a single-cylinder research engine to spark-assisted HCCI

Spark-assisted homogeneous charge compression ignition (HCCI) combustion may be a method to improve the operation of HCCI engines. In the current study, the impact of spark assist on the fundamental properties of ignition and combustion was investigated in a single cylinder, optically-accessible research engine. Early port fuel injection and air preheating were used with indolene fuel in the study. The effects of a range of air preheat (Tin = 256–281 °C), fuel/air equivalence ratio (ϕ = 0.38–0.62) and spark assist timing (10°−90° before top dead centre) conditions on maximum in-cylinder pressure and timing, cycle variability, indicated mean effective pressure (IMEP) and heat release rate were investigated. Additionally, high-speed imaging was used to capture the piston-view ignition and combustion events during spark-assisted and unassisted HCCI operation. Methods were developed and applied to the imaging sequences to quantify the physical characteristics (e.g. location of autoignition sites) and the rate of propagation of the reaction fronts formed during spark-assisted and unassisted HCCI operation. The imaging data show that autoignition sites appear with increasing frequency as air preheat temperature is increased. The addition of spark assist led to the formation of reaction fronts at all conditions that propagated outward from the spark electrode at average speeds between 1.9 and 4.3 m/s. The imaging data indicate the effects of spark assist are due to compression heating of the unburned gases by the propagating reaction fronts which also leads to more consistent location of autoignition. Comparison of the imaging and engine data show the initial formation of the reaction fronts are not significant sources of heat release. While the engine data show that spark assist can affect phasing, heat release rate, IMEP and engine stability at the marginal HCCI operating conditions studied, the results also indicate spark assist has a narrow temperature range where the changes will be significant compared to the effects of the inherent thermal stratification of the HCCI fuel/air charge.

An experimental investigation of the sensitivity of the ignition and combustion properties of a single-cylinder research engine to spark-assisted HCCI

Spark-assisted homogeneous charge compression ignition (HCCI) combustion may be a method to improve the operation of HCCI engines. In the current study, the impact of spark assist on the fundamental properties of ignition and combustion was investigated in a single cylinder, optically-accessible research engine. Early port fuel injection and air preheating were used with indolene fuel in the study. The effects of a range of air preheat ( Tin = 256–281 °C), fuel/air equivalence ratio (ϕ = 0.38–0.62) and spark assist timing (10°−90° before top dead centre) conditions on maximum in-cylinder pressure and timing, cycle variability, indicated mean effective pressure (IMEP) and heat release rate were investigated. Additionally, high-speed imaging was used to capture the piston-view ignition and combustion events during spark-assisted and unassisted HCCI operation. Methods were developed and applied to the imaging sequences to quantify the physical characteristics (e.g. location of autoignition sites) and the rate of propagation of the reaction fronts formed during spark-assisted and unassisted HCCI operation. The imaging data show that autoignition sites appear with increasing frequency as air preheat temperature is increased. The addition of spark assist led to the formation of reaction fronts at all conditions that propagated outward from the spark electrode at average speeds between 1.9 and 4.3 m/s. The imaging data indicate the effects of spark assist are due to compression heating of the unburned gases by the propagating reaction fronts which also leads to more consistent location of autoignition. Comparison of the imaging and engine data show the initial formation of the reaction fronts are not significant sources of heat release. While the engine data show that spark assist can affect phasing, heat release rate, IMEP and engine stability at the marginal HCCI operating conditions studied, the results also indicate spark assist has a narrow temperature range where the changes will be significant compared to the effects of the inherent thermal stratification of the HCCI fuel/air charge.

Detailed chemistry flamelet modeling of mixed-mode combustion in spark-assisted HCCI engines

A detailed chemistry mixed-mode flamelet model for the prediction of combustion in spark-assisted homogeneous charge compression ignition (HCCI) engines is presented in this paper. The complex phenomena of spark-channel processes (turbulent corrugation, multiple restrikes) and of early flame kernel propagation (localized flame kernel formation, non-spherical early flame shapes), both induced by spray-guided spark-ignition combustion initiation, are captured by the recently introduced SparkCIMM ignition model. In this paper, laminar burning velocities and flame thicknesses are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelet calculations to appropriately consider locally rich, highly diluted, and auto-igniting stratified mixtures. Flame extinction criterions are formulated and incorporated into an extended G-equation flame front tracking scheme. Auto-ignition processes in the unburnt mixture, controlled by the flame-induced pressure and temperature increase, are captured by a recently developed multi-zone flamelet model that accounts for detailed chemistry and effects of scalar mixing on turbulent combustion. The laminar burning velocity is shown to increase significantly as the flame propagates into the chemically reacting mixture within the first stage of auto-ignition. After the initiation of the thermal runaway, however, flame extinction occurs rapidly. These interactions of mixed-mode combustion processes are captured by the presented flamelet model. It is developed based on a time and length scale analysis, revealing a scale separation between the ignition delay of diffusion combustion and the flame time of flame propagation. The analysis of the mixture preparation process, along with the simulation of turbulent flame front propagation and its extinction, demonstrates that the prediction of combustion in spark-assisted HCCI engines requires the on hand comprehensive mixed-mode combustion model. A comparison of simulation results from this new model with data from experiments and combustion models of reduced physical complexity proofs its qualification.

Analysis of cyclic variability in spark-assisted HCCI combustion using a double Wiebe function

A heuristic algorithm based on a double Wiebe function is proposed for estimating the relative importance of distinct combustion modes (propagating flame and compression ignition) occurring within individual combustion cycles as an engine is transitioned from conventional spark-ignited (SI) combustion to homogeneous charge compression ignition (HCCI). The proposed algorithm automates the analysis and categorization of pressure measurements from large numbers of individual cycles, providing new insight into the unstable combustion processes occurring during mode transition. Similar techniques could potentially be utilized for on-line diagnostics and control of the balance between SI and HCCI combustion in spark-assisted HCCI.

Modeling Cyclic Variability in Spark-Assisted HCCI

Spark assist appears to offer considerable potential for increasing the speed and load range over which homogeneous charge compression ignition (HCCI) is possible in gasoline engines. Numerous experimental studies of the transition between conventional spark-ignited (SI) propagating-flame combustion and HCCI combustion in gasoline engines with spark assist have demonstrated a high degree of deterministic coupling between successive combustion events. Analysis of this coupling suggests that the transition between SI and HCCI can be described as a sequence of bifurcations in a low-dimensional dynamic map. In this paper, we describe methods for utilizing the deterministic relationship between cycles to extract global kinetic rate parameters that can be used to discriminate multiple distinct combustion states and develop a more quantitative understanding of the SI-HCCI transition. We demonstrate the application of these methods for indolene-containing fuels and point out an apparent HCCI mode switching not previously reported. Our results have specific implications for developing dynamic combustion models and feedback control strategies that utilize spark assist to expand the operating range of HCCI combustion.

Study of the Effect of Spark Ignition on Gasoline HCCI Combustion

Homogeneous charge compression ignition (HCCI) has challenges with regard to ignition control. A spark-assisted gasoline HCCI combustion strategy was used to control ignition of HCCI combustion. In this paper, the effects of spark ignition (SI) on HCCI combustion were investigated on a gasoline direct injection (GDI) engine. The experimental results show that SI improves the stability of HCCI combustion at HCCI critical status (HCCI-CS). At the transition from SI to HCCI mode, the transition fluctuations can be smoothed by the SI in SI/HCCI hybrid combustion engine. Local SI can trigger entire HCCI in the cylinder at sub-HCCI-CS. Without SI, misfire occurs at sub-HCCI-CS. The reason is that local high temperature and pressure owing to local heat release from the spark electrode leads to increase of temperature and pressure in the entire cylinder. When the temperature and concentration of mixture are below sub-HCCI-CS, SI has little effect on HCCI ignition.