A Transported Livengood–Wu Integral Model for Knock Prediction in Computational Fluid Dynamics Simulation

This work describes the development of a transported Livengood-Wu (L-W) integral model for computational fluid dynamics (CFD) simulation to predict auto-ignition and engine knock tendency. The currently employed L-W integral model considers both single-stage and two-stage ignition processes, thus can be generally applied to different fuels such as paraffin, olefin, aromatics and alcohol. The model implementation is first validated in simulations of homogeneous charge compression ignition combustion for three different fuels, showing good accuracy in prediction of auto-ignition timing for fuels with either single-stage or two-stage ignition characteristics. Then, the L-W integral model is coupled with G-equation model to indicate end-gas auto-ignition and knock tendency in CFD simulations of a direct-injection spark-ignition engine. This modeling approach is about 10 times more efficient than the ones that based on detailed chemistry calculation and pressure oscillation analysis. Two fuels with same Research Octane Number (RON) but different octane sensitivity are studied, namely Co-Optima Alkylate and Co-Optima E30. Feed-forward neural network model in conjunction with multi-variable minimization technique is used to generate fuel surrogates with targets of matched RON, octane sensitivity and ethanol content. The CFD model is validated against experimental data in terms of pressure traces and heat release rate for both fuels under a wide range of operating conditions. The knock tendency — indicated by the fuel energy contained in the auto-ignited region — of the two fuels at different load conditions correlates well with the experimental results and the fuel octane sensitivity, implying the current knock modeling approach can capture the octane sensitivity effect and can be applied to further investigation on composition of octane sensitivity.

Study on the knock tendency and cyclical variations of a HCCI engine fueled with n-butanol/n-heptane blends

The contribution of intermediate-temperature heat release to octane sensitivity

Study of dimethyl ether homogeneous charge compression ignition combustion process using a multi-dimensional computational fluid dynamics model

This paper presents some experimental operating and combustion properties of homogeneous charge compression ignition (HCCI) combustion. HCCI operating range, start of combustion, burn duration, indicated mean effective pressure, indicated specific emissions, and indicated specific fuel consumption are evaluated as charge dilution and octane number are varied. Primary reference fuels with octane numbers of 20, 40, and 60 are used in this study. The autoignition properties of the air-fuel mixture are varied by changing the fuel octane number, percentage of exhaust gas recirculation (EGR), and air-fuel ratio, while holding the intake temperature, engine speed, and compression ratio constant. Results show that both the start of combustion and the burn duration are sensitive to mixture dilution (excess air or EGR). The fuel octane number is not an effective method of controlling the start of combustion or the burn duration but can be used to increase the load range of the HCCI engine. Both the NO x emissions and the indicated specific fuel consumption increase as the octane number is increased due to lower dilution and higher peak temperatures. Correct amounts of dilution are critical in controlling HCCI combustion. Separating dilution into EGR and excess air, it is found that a given amount of EGR is more effective at controlling the start of combustion and the burn duration than the same amount of excess air.

Homogenous charge compression ignition (HCCI) combustion in diesel engines can provide for cleaner operation with ultra-low NOx and soot emissions. While HCCI combustion has generated significant attention in the last decade, however, to date, it has seen very limited application in production diesel engines. HCCI combustion is typically characterized by earlier than top-dead-center (pre-TDC) phasing, very high pressure rise rates, short combustion durations and minimal control over the timing of the combustion event. To offset the high reactivity of the diesel fuel, large amounts of EGR (30 to 60%) are usually applied to postpone the initiation of combustion, shift the combustion towards TDC and alleviate to some extent, the high pressure rise rates and the reduced energy efficiency. In this work, a detailed analysis of HCCI combustion has been carried out on a high-compression ratio, single-cylinder diesel engine. The effects of intake boost, EGR quantity/temperature, engine speed, injection scheduling and injection pressure on the operability limits have been empirically determined and correlated with the combustion stability, emissions and performance metrics. The empirical investigation is extended to assess the suitability of common alternate fuels (n-butanol, gasoline and ethanol) for HCCI combustion. On the basis of the analysis, the significant challenges affecting the real-world application of HCCI are identified, their effects on the engine performance quantified and possible solutions to overcome these challenges explored through both theoretical and empirical investigations. This paper intends to provide a comprehensive summary of the implementation issues affecting HCCI combustion in diesel engines.

For researching the influence of n-butanol proportion in diesel fuel on homogeneous charge compression ignition (HCCI) combustion of the free-piston diesel engine generator (FPDEG), a three-dimensional (3D) moving mesh computational fluid dynamics (CFD) simulation model of a FPDEG prototype was developed. A detailed chemical reaction mechanism of diesel fuel was selected as the HCCI combustion mechanism and coupled in the established HCCI combustion simulation model of the FPDEG prototype. The validity of the established HCCI combustion simulation model is proved by comparing the simulation and experimental pressure curves under the condition of pure diesel fuel. The simulation results of different n-butanol proportions in diesel fuel showed that as the n-butanol proportion increased from 0 to 60%, the maximum heat release rate decreased to 59.6 J/deg, the calculated indicated thermal efficiency augmented to 4.6%, the calculated indicated mean effective pressure increased to 0.057 MPa, and the final NOx and CO content decreased to 0.239 and 0.57 g/kg fuel, respectively, but the final soot content increased to 0.000562 g/kg fuel. Therefore, the n-butanol proportion of diesel fuel played a vital role in combustion and emission progress of the FPDEG.

Homogeneous Charge Compression Ignition (HCCI) combustion in a small-scale engine (25 cc) was experimentally examined. Historically, HCCI combustion has been studied in engines sized for passenger vehicles and trucks (approximately 500 cc and larger). HCCI combustion in large-scale engines is characterized by higher efficiency than SI and CI combustion. High rates of heat flux, due to large surface area to volume ratio and engine body material, initially prevented the small-scale engine from operating in HCCI mode. The high level of heat transfer was overcome and sustained small-scale HCCI operation was achieved with n-heptane fuel. Large-scale HCCI engines utilize ultralow equivalence ratios to achieve high efficiencies. The small-scale HCCI engine could not operate with equivalence ratios lower than 0.73. Performance characteristics including power, efficiency, and NOx emissions of the small-scale HCCI engine were poorer than when operated in SI mode. Recommendations to overcome high rates of heat flux and increase small-scale HCCI engine efficiency are presented.

Addition of nitroalkanes to gasoline is shown to reduce the octane quality. The reduction in the Motor Octane Number (MON) is greater than the reduction in the Research Octane Number (RON). In other words addition of nitroalkanes causes an increase in octane sensitivity. The temperature of the compressed air/fuel mixture in the MON test is higher then in the RON test. Through chemical kinetic modelling, we are able to show how the temperature dependence of the reactions responsible for break-up of the nitroalkane molecule can lead to an increase in octane sensitivity. Results are presented from an Homogenous Charge Compression Ignition (HCCI) engine with a homogeneous charge in which the air intake temperature was varied. When the engine was operated on gasoline-like fuels containing nitroalkanes, it was observed that the combustion phasing was much more sensitive to the air intake temperature. This suggests a possible means of controlling combustion phasing for HCCI.

Homogeneous charge compression ignition (HCCI) combustion, when applied to a gasoline engine, offers the potential for a noticeable improvement in fuel economy and dramatic reductions in NOx emissions. In this study, Computational Fluid Dynamic (CFD) is used coupled with detailed chemical mechanism (38 species and 69 reactions) for simulation of HCCI combustion of iso-octane and transitional flow inside the combustion chamber of a 2-stroke engine. Results show that increasing the overall gas temperature significantly advances the HCCI combustion timing. Concerning the equivalence ratio, by increasing it the ignition timing has been advanced and the maximum cylinder pressure has been increased. When equivalence ratio increases to more than 0.5 , NOx emissions significantly increases and go beyond 1000 [ppm].

<div class="htmlview paragraph">The reduction of NOx emissions required by the future Euro 6 standards leads engine manufacturers to develop Diesel Homogeneous Charge Compression Ignition (HCCI) combustion processes. Because this concept allows reducing both NOx and particulates simultaneously, it appears as a promising way to meet the next environmental challenges.</div> <div class="htmlview paragraph">Unfortunately, HCCI combustion often increases CO and HC emissions. Conventional oxidation catalyst technologies, currently used for Euro 4 vehicles, may not be able to convert these emissions because of the saturation of active catalytic sites. As a result, such increased CO and HC emissions have to be reduced under standard levels using innovative catalysts or emergent technologies.</div> <div class="htmlview paragraph">The work reported in this paper has been conducted within the framework of the PAGODE project (PSA, IFP, Chalmers University, APTL, CRF, Johnson Matthey and Supelec) and financed by the European Commission. The objectives are to develop advanced new catalyst formulation for efficient CO/HC conversion at low temperature, possibly coupled with new emerging technologies based on non-thermal plasma concept and adapted to the specific HCCI exhaust emissions.</div> <div class="htmlview paragraph">This paper presents one of the first steps of the study: the definition of the oxidation catalyst boundary conditions under HCCI combustion. For this, we perform a detailed characterisation of the CO and HC emissions on a NADI™ dual mode engine (HCCI at low load and switch to conventional combustion at high load), based on a 2.2-liter, 4-cylinder for passenger car.</div> <div class="htmlview paragraph">The regulated pollutant emissions (CO, THC) have been measured with standard gas analyzer devices. Moreover, hydrocarbon speciation has been done with a C1-C9 gas chromatograph (GC) coupled with a FID system. In addition, aldehydes and ketones emissions have been analyzed by means of a High Pressure Liquid Chromatograph (HPLC).</div> <div class="htmlview paragraph">The results show that HCCI combustion tends to form higher methane emissions than conventional combustion. Moreover, high EGR rates lead to higher oxygenated emissions, in particular formaldehyde. If aldehydes are easily converted when the catalyst is warm, methane is not oxidized at these temperatures.</div> <div class="htmlview paragraph">The next steps of the program will be the definition, the fabrication and the tests of an HCCI-dedicated efficient oxidation catalyst to comply with CO and HC emissions standards. Non-thermal plasma reactor will be also mounted in the exhaust line to help the oxidation reaction.</div>

Detection of low Temperature heat release (LTHR) in the standard Cooperative Fuel Research (CFR) engine in both SI and HCCI combustion modes

Fuel design real-time to control HCCI combustion

<div class="section abstract"><div class="htmlview paragraph">The purpose of this paper is to find a way to extend the high load limit of homogeneous charge compression ignition (HCCI) combustion. This paper presents the effect of in-cylinder flow and stratified mixture on HCCI combustion by experiments and three-dimensional computer fluid dynamics coupled with a detailed chemical reaction calculation. The first study was conducted using a rapid compression and expansion machine (RCEM) equipped with a flow generation plate to create in-cylinder turbulent flow and with a control unit of in-cylinder wall temperature to create in-cylinder temperature distribution. The study assesses the effect of the turbulent flow and the temperature distribution on HCCI combustion. In the second study, the numerical simulation of HCCI combustion was conducted using large eddy simulation coupled with a detailed chemical reaction calculation. The study analyzes the interaction between in-cylinder turbulent flow and mixture distribution and HCCI combustion. The result shows that turbulent flow effects a change in the distribution of chemical species which is a trigger of combustion initiation.</div></div>

The Homogeneous Charge Compression Ignition (HCCI) combustion has potential to radically reduce emissions, especially NOx, but is inherently difficult to achieve and control, especially when natural gas is used as fuel. Hydrogen addition, particularly from fuel reforming was successfully used as an aid. Methane reforming with exhaust gas and air in a monolith catalytic reactor is presented here as a source of hydrogen. An open-loop fuel exhaust gas reforming system has been developed and tested; experimental results are compared with two different models of basic thermodynamic equilibria calculations. The experimental results have shown that for low temperature of engine exhaust gas, typical for HCCI conditions, additional air is needed to produce the quantities of H2 required to enhance the feasibility and range of HCCI combustion. At the low reactor inlet temperatures needed for the HCCI application the simplified three-reaction thermodynamic model is in broad agreement with experimental results, while the high hydrogen yields predicted from the multi-component equilibrium model for medium temperature reforming are difficult to achieve in a practical reformer.