Showing papers by "Bengt Johansson published in 2007"

Investigation of the Early Flame Development in Spark Assisted HCCI Combustion Using High Speed Chemiluminescence Imaging

Abstract: Auto-ignition with SI-compression ratio can be achieved by replacing some of the fresh charge by hot residuals. In this work an engine is run with a negative valve overlap (NVO) trapping hot residuals. By increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate zone between SI and HCCI as the amount of residuals is increased. Recent research has shown the potential of using spark assistance to aid gasoline HCCI combustion at some operating conditions, and even extend the operating regime into regions where unsupported HCCI combustion is impossible. In this work the influence of the spark is studied in a single cylinder operated engine with optical access. Combustion is monitored by in-cylinder pressure and simultaneous high speed chemiluminescence imaging. It is seen that even for large NVO and thus high residual fractions it is a growing SI flame that interacts with, and governs the subsequent HCCI combustion. Using the spark timing it is possible to phase the combustion timing even when the major part of the released heat is from HCCI combustion. The flame expansion speed is decreases for higher NVO, but prevails also for high residual fractions. A higher spark advance is found to compensate for the slower flame expansion up to a point. The auto-ignition process is found to be stratified for both spark assisted HCCI as well as for pure HCCI. For pure HCCI the initial front spreading velocity is found to be in the same order of magnitude as for the expansion speed of the SI flame. Calculations to estimate the crank angle of auto-ignition are performed based on cylinder pressure information providing good statistics on how the proportion of SI to HCCI behaves for different operating conditions.

Hybrid modelling of homogeneous charge compression ignition (HCCI) engine dynamics - a survey

Abstract: The Homogeneous charge compression ignition (HCCI) principle holds promise to increase efficiency and to reduce emissions from internal combustion engines. As HCCI combustion lacks direct ignition timing control and auto-ignition depends on the operating condition, control of auto-ignition is necessary. Since auto-ignition of a homogeneous mixture is very sensitive to operating conditions, a fast combustion phasing control is necessary for reliable operation. To this purpose, HCCI modelling and model-based control with experimental validation were studied. A six-cylinder heavy-duty HCCI engine was controlled on a cycle-to-cycle basis in real time using a variety of sensors, actuators and control structures for control of the HCCI combustion. Combustion phasing control based on ion current was compared to feedback control based on cylinder pressure. With several actuators for controlling HCCI engines suggested, two actuators were compared, dual fuel and variable valve actuation (VVA). Model-based control s...

Lean burn versus stoichiometric operation with EGR and 3-way catalyst of an engine fueled with natural gas and hydrogen enriched natural gas

Abstract: Engine tests have been performed on a 9.6 liter spark-ignited engine fueled by natural gas and a mixture of 25/75 hydrogen/natural gas by volume. The scope of the work was to test two strategies for low emissions of harmful gases; lean burn operation and stoichiometric operation with EGR and a three-way catalyst. Most gas engines today, used in city buses, utilize the lean burn approach to achieve low NOx formation and high thermal efficiency. However, the lean burn approach may not be sufficient for future emissions legislation. One way to improve the lean burn strategy is to add hydrogen to the fuel to increase the lean limit and thus reduce the NOx formation without increasing the emissions of HC. Even so, the best commercially available technology for low emissions of NOx, HC and CO today is stoichiometric operation with a three-way catalyst as used in passenger cars. The drawbacks of stoichiometric operation are low thermal efficiency because of the high pumping work, low possible compression ratio and large heat losses. The recirculation of exhaust gas is one way to reduce these drawbacks and achieve efficiencies that are not much lower than the lean burn technology. The experiments revealed that even with the 25 vol% hydrogen mixture, NOx levels are much higher for the lean burn approach than that of the EGR and catalyst approach for this engine. However, a penalty in brake thermal efficiency has to be accepted for the EGR approach as the thermodynamic conditions are less ideal. (Less)

Two-dimensional gas-phase temperature measurements using phosphor thermometry

Abstract: A new technique based on phosphor thermometry for measurements of two-dimensional gas-phase temperature, was examined as a new laser diagnostic. Calibration of Dy:YAG phosphor was carried out on the surface of a solid. The data were applicable for gas thermometry since the validation of the line intensity ratios method, showed good agreement with both the lifetime method and thermocouple data in a steady gas flow. Single-shot phosphor thermometry was examined in turbulent combustion in an engine. A reasonable temperature deviation and agreement with calculated data to within 5% precision was achieved in the ignition process of a compression ignition engine. Influencing factors such as chemical luminescence and intrusion into the combustion have also been discussed.

The effect of swirl on spark assisted compression ignition (SACI)

Abstract: Autoignition with SI compression ratio can be achieved by retaining hot residuals, replacing some of the fresh charge. In this experimental work it is achieved by running with a negative valve overlap (NVO) trapping hot residuals. The experimental engine is equipped with a pneumatic valve train making it possible to change valve lift, phasing and duration, as well as running with valve deactivation. This makes it possible to start in SI mode, and then by increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate domain between SI and HCCI. The engine is then running in spark-assisted HCCI mode, or spark-assisted compression ignition (SACI) mode that is an acronym that describes the combustion on the borderline between SI and HCCI. In this study the effect of changing the in-cylinder flow pattern by increased swirl is studied. This is achieved by deactivating one of the two intake valves. The effect of the increased turbulence is studied both on the initial slow heat release originating from the spark plug and on the following HCCI combustion. The early SI flame development is highly dependent on the flow field so by increasing the turbulence the flame expansion speed is affected, also at high residual rates. Also, HCCI combustion rate has been shown to slow down as turbulence is increased. As high reaction rate is an issue for HCCI combustion this means that it could be possible to reduce the reaction rate and simultaneously increase the possible usage of SACI combustion by increasing the turbulence. Synchronized simultaneous pressure and high-speed chemiluminescence measurements are conducted making it possible to reproduce fully resolved cycles from the onset of the spark throughout the entire combustion event. From the chemiluminescence images it is possible to calculate a flame expansion speed. The effect on combustion in terms of autoignition timing, combustion duration and the amount of heat released in the different combustion modes is investigated using heat release analysis. LDV measurements are conducted to support the turbulence effects on SACI combustion.

Introductory Study of Variable Valve Actuation for Pneumatic Hybridization

Abstract: Urban traffic involves frequent acceleration and deceleration. During deceleration, the energy previously used to accelerate the vehicle is mainly wasted on heat generated by the friction brakes. If this energy that is wasted in traditional IC engines could be saved, the fuel economy would improve. One solution to this is a pneumatic hybrid using variable valve timing to compress air during deceleration and expand air during acceleration. The compressed air can also be utilized to supercharge the engine in order to get higher load in the first few cycles when accelerating. A Scania D12 single-cylinder diesel engine has been converted for pneumatic hybrid operation and tested in a laboratory setup. Pneumatic valve actuators have been used to make the pneumatic hybrid possible. The actuators have been mounted on top of the cylinder head of the engine. A pressure tank has been connected to one of the inlet ports and one of the inlet valves has been modified to work as a tank valve. The goal has been to test and evaluate 2 different modes – compression mode (CM) where air is stored in an air tank during deceleration and air-motor mode (AM) where the previously stored pressurized air is used for accelerating the vehicle. This paper also includes an optimization of the CM.

Homogeneous charge compression ignition: the future of IC engines?

Abstract: The Homogeneous Charge Compression Ignition Engine, HCCI, has the potential to combine the best of the Spark Ignition and Compression Ignition Engines. With high octane number fuel, the engine operates with high compression ratio and lean mixtures giving CI engine equivalent fuel consumption or better. Owing to pre-mixed charge without rich or stoichiometric zones, the production of soot and NOx can be avoided. This paper presents some results from advanced laser diagnostics showing the fundamental behaviour of the process from a close to homogeneous combustion onset towards a very stratified process at around 20–50% heat released. The need for active combustion control is shown and possible means of control are discussed. Results with multi-cylinder engines using negative valve overlap, variable compression ratio, fast inlet temperature control as well as dual fuel are given.

Effect of Turbulence on HCCI Combustion

Abstract: This paper presents large eddy simulation (LES) and experimental studies of the combustion process of ethanol/air mixture in an experimental optical HCCI engine. The fuel is injected to the intake port manifolds to generate uniform fuel/air mixture in the cylinder. Two different piston shapes, one with a flat disc and one with a square bowl, were employed to generate different in-cylinder turbulence and temperature field prior to autoignition. The aim of this study was to scrutinize the effect of in-cylinder turbulence on the temperature field and on the combustion process. The fuel tracer, acetone, is measured using laser-induced fluorescence (LIF) to characterize the reaction fronts, and chemiluminescence images were recorded using a high-speed camera, with a 0.25 crank angle degree resolution, to further illustrate the combustion process. Pressure in the cylinder is recorded in the experiments. Spatial and temporal resolved LES was used to gain information on the turbulence mixing, heat transfer and combustion process. It was shown that gas temperature in the piston bowl is generally higher than that in the squish, leading to an earlier ignition in the bowl. Compared to the disc engine, the square bowl engine has a higher temperature inhomogeneity owing to the turbulence wall heat transfer. The experimentally observed higher combustion duration and slower pressure rise rate in the square bowl engine as compared to the disc engine can be explained by the higher temperature inhomogeneity in the square bowl engine.

Detailed Heat Release Analyses with Regard to Combustion of RME and Oxygenated Fuels in an HSDI Diesel Engine

Abstract: Experiments on a modern DI Diesel engine were carried out: The engine was fuelled with standard Diesel fuel, RME and a mixture of 85% standard Diesel fuel, 5% RME and 10% higher alcohols under low load conditions (4 bar IMEP). During these experiments, different external EGR levels were applied while the injection timing was chosen in a way to keep the location of 50% heat release constant. Emission analysis results were in accordance with widely known correlations: Increasing EGR rates lowered NOx emissions. This is explained by a decrease of global air-fuel ratio entailing longer ignition delay. Local gas-fuel ratio increases during ignition delay and local combustion temperature is lowered. Exhaust gas analysis indicated further a strong increase of CO, PM and unburned HC emissions at high EGR levels. This resulted in lower combustion efficiency. PM emissions however, decreased above 50% EGR which was also in accordance with previously reported results. Besides those similar trends, fuel dependent differences in indicated thermal efficiency as well as CO, HC, NOx and especially PM emissions were observed. These differences were evaluated by detailed heat release analysis and explanation models based upon fuel characteristics as fuel viscosity and fuel distillation curve. Fuel spray evaporation and heat release were influenced by these fuel characteristics. Due to these characteristics it was concluded that RME has a higher tendency to form fuel rich zones at low load conditions than the other tested fuel types. Moreover it was found that improved fuel spray vaporisation is an option to improve exhaust emissions at low load conditions. (Less)

Operation strategy of a Dual Fuel HCCI Engine with VGT

Abstract: HCCI combustion is well known and much results regarding its special properties have been published. Publications comparing the performance of different HCCI engines and comparing HCCI engines to conventional engines have indicated special features of HCCI engines regarding, among other things, emissions, efficiency and special feedback-control requirements. This paper attempts to contribute to the common knowledge of HCCI engines by describing an operational strategy suitable for a dual-fuel port-injected Heavy Duty HCCI engine equipped with a variable geometry turbo charger. Due to the special properties of HCCI combustion a specific operational strategy has to be adopted for the engine operation parameters (in this case combustion phasing and boost pressure). The low exhaust temperature of HCCI engines limits the benefits of turbo charging and causes pumping losses which means that “the more the merrier” principle does not apply to intake pressure for HCCI engines. It is desirable not to use more boost pressure than necessary to avoid excessively rapid combustion and/or emissions of NOx. It is also desirable to select a correct combustion phasing which, like the boost pressure, has a large influence on engine efficiency. The optimization problem that emerges between the need for boost pressure to avoid noise and emissions and, at the same time, avoiding an extensive decrease of efficiency because of pumping losses is the topic of this paper. The experiments were carried out on a 12 liter Heavy Duty Diesel engine converted to pure HCCI operation. Individually injected natural gas and n-Heptane with a nominal injection ratio of 85% natural gas and the rest n-Heptane (based on heating value) was used as fuel. The engine was under feedback combustion control during the experiments. (Less)

Study on Combustion Chamber Geometry Effects in an HCCI Engine using High-Speed Cycle-Resolved Chemiluminescence Imaging

Abstract: The aim of this study is to see how geometry generated turbulence affects the Rate of Heat Release (ROHR) in an HCCI engine. HCCI combustion is limited in load due to high peak pressures and too fast combustion. If the speed of combustion can be decreased the load range can be extended. Therefore two different combustion chamber geometries were investigated, one with a disc shape and one with a square bowl in piston. The later one provokes squish-generated gas flow into the bowl causing turbulence. The disc shaped combustion chamber was used as a reference case. Combustion duration and ROHR were studied using heat release analysis. A Scania D12 Diesel engine, converted to port injected HCCI with ethanol was used for the experiments. An engine speed of 1200 rpm was applied throughout the tests. The effect of air/fuel ratio and combustion phasing was also studied. The behavior of the heat release was correlated with high speed chemiluminescence imaging for both combustion chamber geometries. Optical access was enabled from beneath by a quartz piston and a 45 degree mirror. It was found that the square bowl in piston generates higher turbulence levels resulting in half the ROHR and twice as long combustion duration as the disc shaped combustion chamber. By using a resolution of 3 images per CAD, the fast gas movements during the entire HCCI combustion process could be studied inside the bowl.

Intracardiac thrombus in adults with the Fontan circulation.

Abstract: OBJECTIVES To determine the treatment and outcomes of a cohort of adults with the Fontan circulation diagnosed with intracardiac thrombus. BACKGROUND Formation of thrombus is common after the Fontan operation, albeit little has been published on the treatment and outcomes of these patients once they have developed an intracardiac thrombus. METHODS We identified all patients who had been converted to the Fontan circulation from the cardiology database at the Toronto Congenital Cardiac Centre for Adults, Toronto, and The Royal Brompton Hospital, London, studying the period from 1981 to 2003. We then reviewed the relevant echocardiograms and medical records. RESULTS Intracardiac thrombus was identified in 28 of 235 patients with the Fontan circulation, the patients having an average age of 27 plus or minus 9 years. Of the patients, 21 were initially medically treated, 19 with heparin or warfarin, and 2 with thrombolysis, whereas 7 patients underwent immediate surgical removal of the clot. Overall mortality was 18%, with residual clots seen in 39% of surviving patients at 1 year of follow up. At presentation, the haemodynamic stability of each patient with intracardiac thrombus dictated initial strategies for management, with 17% of those with stable presentations undergoing immediate surgical treatment, as opposed to 75% of those with unstable presentations (p-value equals 0.04), as well as correlating with ultimate clinical outcome. The rate of death was 8% in haemodynamically stable patients, versus 75% in haemodynamically unstable patients (p value equals 0.01). CONCLUSION Formation of intracardiac thrombus is not rare in adults with the Fontan circulation, and carries a significant risk of death, especially in clinically unstable patients. Emphasis on prevention of formation of the clot is warranted.

A Study of a Glow Plug Ignition Engine by Chemiluminescence Images

Abstract: An experimental study of a glow plug engine combustion process has been performed by applying chemiluminescence imaging. The major intent was to understand what kind of combustion is present in a glow plug engine and how the combustion process behaves in a small volume and at high engine speed. To achieve this, images of natural emitted light were taken and filters were applied for isolating the formaldehyde and hydroxyl species. Images were taken in a model airplane engine, 4.11 cm₃, modified for optical access. The pictures were acquired using a high-speed camera capable of taking one photo every second or fourth crank angle degree, and consequently visualizing the progress of the combustion process. The images were taken with the same operating condition at two different engine speeds: 9600 and 13400 rpm. A mixture of 65% methanol, 20% nitromethane and 15% lubricant was used as fuel. The experiments show that glow plug combustion is a propagating autoignition combustion and that the homogeneity of the oxidation process increases with the engine speed. It was also observed that at low speed, the low temperature reactions start together with the rate of heat release and once they are over the high temperature reactions appear. On the other hand at high speed there is no time for low temperature reactions followed by high temperature reactions. This means that formaldehyde formation is partially skipped and hydroxyl shows up almost at the same time but not in the same location as formaldehyde. (Less)

High-Speed PLIF Imaging for Investigation of Turbulence Effects on Heat Release Rates in HCCI Combustion

Abstract: High-speed laser diagnostics was utilized for single-cycle resolved studies of the fuel distribution in the combustion chamber of a truck-size HCCI engine. A multi-YAG laser system consisting of four individual Nd:YAG lasers was used for planar laser-induced fluorescence (PLIF) imaging of the fuel distribution. The fundamental beam from the lasers at 1064 nm was frequency quadrupled in order to obtain laser pulses at 266 nm suitable for excitation of acetone that was used as fuel tracer. Bursts of up to eight pulses with very short time separation were produced, allowing PLIF images with high temporal resolution to be captured within one single cycle event. The system was used together with a high-speed framing camera employing eight ICCD modules, with a frame-rate matching the laser pulse repetition rate. The combustion evolution was studied in terms of spatial distribution and rate of fuel consumption for different engine hardware configurations as well as operating conditions, e.g., different stoichiometries and combustion phasing. Two different piston crown geometries were used for altering the degree of turbulence in the combustion chamber. In addition to the optical investigations, the impact of turbulence effects was also studied by calculating the rate of heat release and combustion phasing from the pressure trace. (Less)

Formaldehyde and Hydroxyl Radicals in an HCCI Engine - Calculations and LIF-Measurements

Abstract: Concentrations of hydroxyl radicals and formaldehyde were calculated using homogeneous (HRM) and stochastic reactor models (SRM), and the result was compared to LIF measurements from an optically accessed iso-octane/n-heptane-fuelled homogeneous charge compression ignition (HCCI) engine. The comparison was at first conducted from averaged total concentrations/signal strengths over the entire combustion volume, which showed a good qualitative agreement between experiments and calculations. Time- and the calculation-inlet-temperature-resolved concentrations of formaldehyde and hydroxyl radicals obtained through HRM are presented. Probability density plots (PDPs) through SRM calculations and LIF measurements are presented and compared, showing a very good agreement considering their delicate and sensitive nature. Thus it is concluded that SRM is a valid model for these purposes, justifying the use of SRM in order to extend the evaluated concentration ranges of the analyzed species beyond the detection/separation level. It is shown that formaldehyde concentration increases slowly, contrary to hydroxyl which is fast developed. Formaldehyde is locally fast consumed once high temperature chemistry has started, and the highest maximum concentrations of formaldehyde are found in cases where low-temperature chemistry was never transitioned to high-temperature ignition. The PDP's from SRM calculations give increased insight of the occurrence and development of autoignition. During the onset of ignition, the regions with the highest formaldehyde concentrations also have the highest concentrations of hydroxyl radicals. The low-temperature heat release (LTHR) maximum occurs before maximum of formaldehyde, and the regions of (for the LTHR regime relatively) high hydroxyl concentrations gradually becomes fewer until they cease to exist; this occurs after the LTHR peak but before formaldehyde maximum. During the transition state all regions have similar formaldehyde concentrations but varying concentrations of hydroxyl. (Less)

Mini High Speed HCCI Engine Fueled with Ether: Load Range, Emission Characteristics and Optical Analysis

Abstract: Power supply systems play a very important role in everyday life applications. There are mainly two ways of producing energy for low power generation: electrochemical batteries and small engines. In the last few years, many improvements have been carried out in order to obtain lighter batteries with longer durations but unfortunately the energy density of 1 MJ/kg seems to be an asymptotic value. An energy source constituted of an organic fuel with an energy density around 29 MJ/kg and a minimum overall efficiency of only 3.5% could surpass batteries. Nowadays, the most efficient combustion process is HCCI combustion which has the ability to combine a high energy conversion efficiency with low emission levels and a very low fuel consumption. The present paper describes an investigation carried out on a modified model airplane engine, on how a pure HCCI combustion behaves in a small volume, Vd = 4.11 Cm₃, at very high engine speeds (up to 17,500 [rpm]). Using ether like fuel, for the first part of the experiments the behavior of the engine was characterized by studying the variation of IMEP, indicated power, main combustion features (temperature, duration, phasing, residuals, efficiency and heat release) and emissions. The characterization was carried out between 7,500 and 17,500 [rpm], and the speed was changed by using six different propellers. In the second part of the experiment, an optical study was performed; the main intent was to characterize the combustion structure and its transient behavior in a small volume at high engine speed. In order to achieve this, chemiluminescence images were acquired together with images of hydroxyl, formaldehyde and C2. Two sets of measurements were performed, i.e., at 6,500 and 14,000 [rpm], and in each test the behavior of the low and high temperature reactions, as well as that of C2 was studied together with the natural emitted light. This was done to understand the degree of homogeneity of the combustion and the boundary layer behavior. The third part of this paper describes the analysis of the behavior of the optical and the metal engine in order to provide an understanding to whether the use of the optical window affected the combustion event.

Improving Ion Current Feedback for HCCI Engine Control

Abstract: In HCCI you do not have the same control of the combustion like in SI and Diesel engines. Controlling the start of a combustion event is a difficult task and requires feedback from previous cycles. This feedback can be retrieved from ion current measurements. By applying a voltage over the spark gap, ions will lead a current and a signal that represents the combustion in the cylinder will be retrieved. Voltages of 450 V were used. The paper describes a new method to enhance the combustion phasing from the Ion current trace in HCCI engines. The method is using the knowledge of how the signal should look. This is known due to the fact that the shape of the ion current signal is similar from cycle to cycle. This new observation is shown in the paper. Also the correlation between the ion current and CA50 was studied. Later the signals have been used for combustion feedback. (Less)

Influence of the compression ratio on the performance and emissions of a mini HCCI engine fuelled with diethyl ether

Abstract: Power supply systems play a very important role in applications of everyday life. Mainly, for low power generation, there are two ways of producing energy: electrochemical batteries and small engines. In the last few years many improvements have been carried out in order to obtain lighter batteries with longer duration but unfortunately the energy density of 1 MJ/kg seems to be an asymptotic value. If the energy source is an organic fuel with an energy density of around 29 MJ/kg and a minimum overall efficiency of only 3.5%, this device can surpass the batteries. Nowadays the most efficient combustion process is HCCI combustion which is able to combine high energy conversion efficiency and low emission levels with a very low fuel consumption. In this paper, an investigation has been carried out concerning the effects of the compression ratio on the performance and emissions of a mini, Vd me 4.11 [cmu3], HCCI engine fueled with diethyl ether. Because of its high reactivity, autoignition of the mixture was achieved only using compression energy. The compression ratio was changed by altering the squish distance: 0.25, 0.50, 0.75, 1.00 and 1.25 [mm]. For each compression ratio, three sets of measurements were performed: 3000, 7000 and 12000 [rpm]. The study showed that diethyl ether was only slightly affected by quenching problems when the squish distance was 0.25 and 0.50 [mm] at 7000 [rpm]. It was also demonstrated that the performance improved when decreasing the compression ratio to an optimum point and subsequently dropped to zero when the highest spacer, 1.25 [mm], was used. Due to a very low combustion and thermodynamic efficiencies, the specific emissions of CO and HC were one order of magnitude higher than for a normal car/truck engine, whereas NOx emissions were comparable to those of a conventional diesel engine. Finally, the study rendered it possible to understand how much an HCCI engine fueled with diethyl ether could be scaled down since it was shown that this fuel was not very sensitive to quenching, with a squish distance of 0.25 [mm]. (Less)

Investigation of the Early Flame Development in Spark Assisted HCCI Combustion Using High Speed Chemiluminescence Imaging

Abstract: Auto-ignition with SI-compression ratio can be achieved by replacing some of the fresh charge by hot residuals. In this work an engine is run with a negative valve overlap (NVO) trapping hot residuals. By increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate zone between SI and HCCI as the amount of residuals is increased. Recent research has shown the potential of using spark assistance to aid gasoline HCCI combustion at some operating conditions, and even extend the operating regime into regions where unsupported HCCI combustion is impossible. In this work the influence of the spark is studied in a single cylinder operated engine with optical access. Combustion is monitored by in-cylinder pressure and simultaneous high speed chemiluminescence imaging. It is seen that even for large NVO and thus high residual fractions it is a growing SI flame that interacts with, and governs the subsequent HCCI combustion. Using the spark timing it is possible to phase the combustion timing even when the major part of the released heat is from HCCI combustion. The flame expansion speed is decreases for higher NVO, but prevails also for high residual fractions. A higher spark advance is found to compensate for the slower flame expansion up to a point. The auto-ignition process is found to be stratified for both spark assisted HCCI as well as for pure HCCI. For pure HCCI the initial front spreading velocity is found to be in the same order of magnitude as for the expansion speed of the SI flame. Calculations to estimate the crank angle of auto-ignition are performed based on cylinder pressure information providing good statistics on how the proportion of SI to HCCI behaves for different operating conditions. (Less)