Homogeneous Charge Compression Ignition Engine Operation Research Articles

Computational Fluid Dynamics (CFD) Validation and Investigation the Effect of Piston Bowl Geometries Performance on Port Fuel Injection-Homogeneous Charge Compression Ignition (PFI-HCCI) Engines

Computational Fluid Dynamics (CFD) Validation and Investigation the Effect of Piston Bowl Geometries Performance on Port Fuel Injection-Homogeneous Charge Compression Ignition (PFI-HCCI) Engines

Numerical study of piston bowl geometries on PFI-HCCI engine performance

Homogeneous charge compression ignition (HCCI) is an advanced combustion strategy proposed to provide higher efficiency and lower emissions than conventional compression ignition. However, there are still tough challenges in the successful operation of HCCI engines. Among these challenges, homogeneous mixture preparation and combustion phase control plays a vital role in determining the efficiency and emissions. Piston bowl geometry significantly enhances the process by improving the flow, turbulence, and mixing for the combustion. The study utilised experimental and numerical simulation methods to analyse HCCI combustion in port fuel injection (PFI) mode and evaluate the effect of piston geometries on engine performance. For this purpose, the different pistons bowl geometries (Baseline model, SQC, and CCC) with the same volume, compression, and equivalence ratio were numerically tested in a four-stroke, single-cylinder, YANMAR diesel engine. The numerical simulation results provide adequate assurance to proceed with the study with different piston geometries design. Compared to SQC and CCC, the Baseline model produced significantly higher cylinder pressure, temperature, and heat release rate. Different piston shape designs influenced the formation of air-fuel mixing, thereby affecting the time and location of onset combustion. The present investigation offered the significant role of piston geometry for the control mechanism of PFI-HCCI combustion, that is a vital part in demonstrating HCCI combustion.

Performance and emission characteristics of a diesel engine operated on diverse modes using renewable and sustainable fuels derived from dairy scum and municipal solid waste

Abstract Continuous utilization of crude oil in a diesel engine produces detrimental environmental effects. Therefore use of renewable fuels can at least lower the emission levels because such fuels are recyclable, provide energy safety and significantly deal with social, economic and environment. It is well known that diesel engine have several benefits compared to counterpart petrol engine and produce greater smoke and nitric oxide (NOx) levels. As a result, in this current work an effort has been carried out lower both smoke and NOx emission levels. To achieve this, homogeneous charge compression ignition engine (HCCI) is a better option. The influence of producer gas and dairy scum oil methyl ester (DiSOME) on the thermal efficiency and emission character of a compression ignition engine under dual and HCCI mode has been investigated. The HCCI engine was operated at constant air temperature (80 °C) because DiSOME cannot be vaporized at atmospheric temperature. The obtained with DiSOME-producer gas operated dual fuel mode provided amplified thermal efficiency (BTE) and declined hydrocarbon (HC), carbon monoxide (CO) and marginally augmented smoke and NOx levels compared to HCCI operation.

Experimental Investigations on Manifold Injection of Diesel and Biodiesel in a HCCI Engine with Inlet Charge Heating

In the present study, engine tests were conducted on a modified single cylinder four stroke compression ignition (CI) engine operated in homogeneous charge compression ignition (HCCI) mode with injection of diesel and biodiesels through intake manifold. The intake air temperature varied from 50 to 80 °C for diesel and 55 to 85 °C for biodiesel using air pre heater. The coolant temperature varied from 40 to 60 °C for both the diesel and biodiesel operation. For the comparison purpose, CI engine fuelled with diesel was operated at the injection timing of 23°BTDC and an injector pressure of 205 bar. It is seen that HCCI mode of engine operation with diesel and biodiesel resulted into 35-45% lower brake thermal efficiency (BTE) with significant reduction in nitrogen oxide (NOx) emission by 98% and smoke emissions by 65-75%. On the other hand, HCCI engine operation with diesel and biodiesels showed increased hydrocarbon (HC) emissions by 20-25 times and carbon monoxide (CO) emissions by 30-40%. However, peak pressure (PP) and heat release rate (HRR) decreased by 20-25% when compared to CI mode of engine operation.

Experimental study of homogeneous charge compression ignition engine operation fuelled by emulated solid oxide fuel cell anode off-gas

A solid oxide fuel cell (SOFC) hybrid system is a system that combines an SOFC with an additional power generation device to increase the efficiency of the system. The SOFC–gas turbine hybrid system has been primarily investigated for SOFC hybrid systems. However, the current power generation capacity of an SOFC is less than several MWs; for this generation capacity, an internal combustion engine is generally more efficient and economical than a gas turbine. Focusing on this point, recently, the concept of an SOFC–internal combustion engine hybrid system was proposed. However, the operation of this system has not been experimentally studied yet. In this paper, as the first step in an experimental investigation of the hybrid system, an experimental study on the operation of an internal combustion engine fuelled by SOFC anode off-gas was conducted. To successfully combust the SOFC anode off-gas, which includes a large amount of diluents (H2O and CO2), the homogeneous charge compression ignition (HCCI) method was selected instead of spark ignition as the combustion strategy of the internal combustion engine in the hybrid system. For the HCCI engine experiments, a single-cylinder HCCI engine and experimental equipment for emulating SOFC anode off-gas were constructed. Various HCCI engine experiments were performed while varying several system control parameters, e.g., the fuel utilization factor of an SOFC, which primarily affects the composition and flow rate of the engine intake gas. The experiments indicated that, in general system operating condition, HCCI engine operation yields a significant amount of power (w/25–30% gross indicated efficiency) and produces significantly low NOx emissions (<5 ppm @ O2 15%) under stable HCCI combustion (<5% COV IMEPg, which is the coefficient of variance of the gross indicated mean effective pressure). Considering that the experiment was performed using a small single-cylinder engine, these experimental results reveal that the use of an HCCI engine as the bottoming cycle in an SOFC hybrid system is promising. In addition, it has been found how each system control parameter affects HCCI engine operation. It was confirmed that HCCI engine operation was not always stable in all system operating conditions. System operating conditions that induce an exceedingly low engine load (<1.8 bar IMEPg, which is the gross indicated mean effective pressure) should be avoided as it decreases the stability of engine operation. Additionally, system operating conditions that make an engine intake gas with excessive dilution (fuel molar fraction < 0.125) should be avoided to decrease the amount of unburned CO emission and maintain a CO combustion efficiency higher than 90%.

EGR control on operation of a tar tolerant HCCI engine with simulated syngas from biomass

In combined heat and power plants operated with biomass syngas, the removal of condensible tars is a necessary but expensive step (up to one third of the installation and maintenance costs). This step is required because the syngas has to be cooled down to avoid knocking in the spark ignition engines traditionally used in such plants. To remove the tar condensation problem, we developed an alternative system based on an Homogeneous Charge Compression Ignition (HCCI) engine operated at intake temperatures above the tar dew point. To address the challenge of power derating of such engine setups, the current paper focuses on the application of Exhaust Gas Recirculation (EGR) as a control parameter that would indirectly allow the improvement of the engine performance. Based on a conservative estimate of tar dew points, HCCI combustion was studied at an intake temperature of 250°C using synthetic biomass syngas and synthetic EGR on a mono-cylinder HCCI engine operated at 1000 RPM. The effects of charge dilution, thermal and kinetic damping due to the EGR gases were also analysed to understand their main effects. The use of EGR successfully increased the maximum achievable Indicated Mean Effective Pressure from 2.5bar at EGR=0% up to 3.3bar at EGR=25%, through damping the maximum pressure rise rate and allowing higher equivalence ratios.

Identification of the dynamic operating envelope of HCCI engines using class imbalance learning.

Homogeneous charge compression ignition (HCCI) is a futuristic automotive engine technology that can significantly improve fuel economy and reduce emissions. HCCI engine operation is constrained by combustion instabilities, such as knock, ringing, misfires, high-variability combustion, and so on, and it becomes important to identify the operating envelope defined by these constraints for use in engine diagnostics and controller design. HCCI combustion is dominated by complex nonlinear dynamics, and a first-principle-based dynamic modeling of the operating envelope becomes intractable. In this paper, a machine learning approach is presented to identify the stable operating envelope of HCCI combustion, by learning directly from the experimental data. Stability is defined using thresholds on combustion features obtained from engine in-cylinder pressure measurements. This paper considers instabilities arising from engine misfire and high-variability combustion. A gasoline HCCI engine is used for generating stable and unstable data observations. Owing to an imbalance in class proportions in the data set, the models are developed both based on resampling the data set (by undersampling and oversampling) and based on a cost-sensitive learning method (by overweighting the minority class relative to the majority class observations). Support vector machines (SVMs) and recently developed extreme learning machines (ELM) are utilized for developing dynamic classifiers. The results compared against linear classification methods show that cost-sensitive nonlinear ELM and SVM classification algorithms are well suited for the problem. However, the SVM envelope model requires about 80% more parameters for an accuracy improvement of 3% compared with the ELM envelope model indicating that ELM models may be computationally suitable for the engine application. The proposed modeling approach shows that HCCI engine misfires and high-variability combustion can be predicted ahead of time, given the present values of available sensor measurements, making the models suitable for engine diagnostics and control applications.

Homogeneous charge compression ignition (HCCI) combustion: Mixture preparation and control strategies in diesel engines

Abstract At present achieving fuel economy and reducing emissions are the two main targets set by the automotive industries. Homogeneous charge compression ignition (HCCI) technology is believed to be a promising one to be applied in both spark ignition (SI) and compression ignition (CI) engines in the near future. However, some challenges such as compromise combustion phase control, controlled auto-ignition, operating range, homogeneous charge preparation, cold start and emissions of unburned hydro carbon (UHC), and carbon monoxide (CO) need to be overcome for successful operation of HCCI engine. Extensive research on HCCI combustion with a homogeneous fuel–air mixture preparation is going on throughout the world. This paper reviews the strategies of different external and in-cylinder mixture preparation methods which were adopted and proposed in the recent years. The different strategies of controlled auto-ignition by HCCI combustion are also discussed in this paper.

Understanding and detecting misfire in an HCCI engine fuelled with ethanol

Homogeneous charge compression ignition (HCCI) with ethanol as a renewable fuel offers a promising solution to tackle some of major challenges before realizing green powertrains. Misfire limits HCCI engine operation and can damage exhaust after treatment system. This article aims to understand the effect of misfire on the operation of an ethanol fuelled HCCI engine. The experimental data from a 0.3 liter converted-diesel HCCI engine was used to investigate the effect of misfire on exhaust emissions, in-cylinder pressure trace, indicated mean effective pressure (IMEP), heat release and combustion phasing metrics. It was found that variation of combustion parameters such as start of combustion (SOC) and crank angle of maximum in-cylinder pressure are not effective parameters for HCCI misfire detection. However, there is a strong correlation between the occurrence of misfire and variation of cylinder pressure at 5, 10, 15 and 20 CAD aTDC. These experimental findings were then used to design an artificial neural network (ANN) model to detect misfire in the HCCI engine. The model was tested on the experimental data for a mix of 7800 normal and misfire cycles. The results indicated that the ANN misfire detection (AMD) model can detect HCCI misfire with 100% accuracy. In addition, the AMD model was found to be capable of successfully detecting the onset of the transition from normal to misfire operation region.

A DNS study of ignition characteristics of a lean iso-octane/air mixture under HCCI and SACI conditions

The effect of thermal stratification, spark-ignition, and turbulence on the ignition of a lean homogeneous iso-octane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 99-species reduced kinetic mechanism developed for very lean mixtures from a detailed mechanism. Two-dimensional DNS are performed in a fixed volume with two-dimensional isotropic velocity spectrums, temperature fluctuations, and an ignition source superimposed on the initial scalar fields. The influence of variations in the initial temperature field imposed by changing the variance of temperature, the ignition-timing by changing the time at which the ignition source is superimposed, and the turbulence intensity and length scale on ignition of a lean iso-octane/air mixture is elucidated. The mean heat release rate increases more slowly and ignition delay decreases with increasing thermal stratification under homogeneous charge compression-ignition (HCCI) conditions since the present mean temperature lies far outside of the negative temperature coefficient (NTC) regime. The spark-ignition induces relatively short ignition delay under spark-assisted compression ignition (SACI) conditions while mildly spreading out the mean heat release rate. For SACI combustion, high turbulence intensity decreases the ignition delay more by increasing the turbulent flame area. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. For SACI combustion, the heat release rate occurs solely by deflagration prior to the occurrence of the maximum heat release rate and subsequently by mixed modes of deflagration and spontaneous ignition. These results suggest that thermal stratification provides smooth operation of HCCI engines and moreover, spark-ignition can precisely control the ignition timing for SACI combustion.

Use of a single-zone thermodynamic model with detailed chemistry to study a natural gas fueled homogeneous charge compression ignition engine

A single zone thermodynamic model with detailed chemical kinetics was used to simulate a natural gas fueled homogeneous charge compression ignition (HCCI) engine. The model employed Chemkin and used chemical kinetics for natural gas with 53 species and 325 reactions. This simulation was used to complete analyses for a modified 0.4 L single cylinder engine. The engine possessed a compression ratio of 21.5:1, and had a bore and stroke of 86 and 75 mm, respectively. Several sets of parametric studies were completed to investigate the minimal initial temperature, engine performance, and nitric oxide emissions of HCCI engine operation. The results show significant changes in combustion characteristics with varying engine operating conditions. Effects of varying equivalence ratios (0.3–1.0), engine speeds (1000–4000 RPM), EGR (0–40%), and fuel compositions were determined and analyzed in detail. In particular, every 0.1 increase in equivalence ratio or 500 rpm increase in engine speed requires about a 5 K higher initial temperature for complete combustion, and leads to around 0.7 bar increase in IMEP.

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.

Ignition control of homogeneous-charge compression ignition (HCCI) combustion through adaptation of the fuel molecular structure by reaction with ozone

The present paper describes a method of controlling the time of ignition in homogeneous-charge compression ignition (HCCI) combustion. In the described experiments some control of ignition timing in HCCI combustion is achieved through alteration of the fuel molecular structure using a chemical reaction of the fuel with ozone, prior to introduction of the fuel into the combustion chamber. Controlling ignition timing is essential, in achieving high thermal efficiency and low pollutant emission in HCCI engine operation. To this end, ignition should occur in the vicinity of piston top-dead-centre (TDC), the point of maximum compression of the fuel–air charge. The present paper proposes a method of controlling the time of ignition of the fuel–air charge by adapting the ignitability of the fuel through prior chemical reaction of the fuel with ozone. Ozone can be readily produced using air in conjunction with a corona discharge ozoniser and may be brought into contact with the fuel in a reaction chamber before its injection into the engine. It was shown through experiments that an acetal fuel which has undergone treatment with ozone, ignites earlier during the engine cycle in HCCI combustion, than fuel which has not undergone treatment with ozone, as a result of changes in its molecular structure prior to combustion. The observed changes in molecular structure consisted primarily in the formation of peroxides within the fuel. This method can be used to operate an engine in HCCI combustion mode with some control over the point of ignition of the fuel–air charge by varying the proportions of fuel previously treated with ozone and fuel not treated with ozone. The experiments showed that the time of ignition could be controlled, whilst keeping other parameters such as the load and speed of the engine, and pressure and temperature of the intake air and fuel, constant.

An investigation of hydrogen-fuelled HCCI engine performance and operation

This paper presents the findings of an experimental investigation into the operation of a compression ignition (CI) engine in homogeneous charge compression ignition (HCCI) mode using hydrogen fuel. Factors that were investigated include engine efficiency, emissions and mechanical loads. Hydrogen was found to be a possible fuel for operation of a CI engine in HCCI mode. The heat release rate is extremely high, which leads to high ignition timing control requirements. The ignition timing was controlled using heating of the inlet air, and satisfactory performance was demonstrated using this method. Some cycle-to-cycle variations were, however, observed due to the difficulty in achieving accurate control of ignition timing. Use of hydrogen in HCCI engines allows operation with very lean air–fuel mixtures, giving extremely low emissions of nitrogen oxides and other pollutants. Operation with an excess air ratio of 6 was demonstrated, and a maximum fuel efficiency of 45% was achieved with an excess air ratio of 3. The maximum power of the engine in HCCI mode is, however, quite modest, being limited by the need for inlet air heating to ensure autoignition, which reduces the air mass flow through the engine. There are also mechanical limitations to the operation of the HCCI hydrogen engine at high loads due to higher rates of pressure rise and in-cylinder peak pressures compared to conventional diesel mode.

A computational study of an HCCI engine with direct injection during gas exchange

We present a new probability density function (PDF)-based computational model to simulate a homogeneous charge compression ignition (HCCI) engine with direct injection (DI) during gas exchange. This stochastic reactor model (SRM) accounts for the engine breathing process in addition to the closed-volume HCCI engine operation. A weighted-particle Monte Carlo method is used to solve the resulting PDF transport equation. While simulating the gas exchange, it is necessary to add a large number of stochastic particles to the ensemble due to the intake air and EGR streams as well as fuel injection, resulting in increased computational expense. Therefore, in this work we apply a down-sampling technique to reduce the number of stochastic particles, while conserving the statistical properties of the ensemble. In this method some of the most important statistical moments (e.g., concentration of the main chemical species and enthalpy) are conserved exactly, while other moments are conserved in a statistical sense. Detailed analysis demonstrates that the statistical error associated with the down-sampling algorithm is more sensitive to the number of particles than to the number of conserved species for the given operating conditions. For a full-cycle simulation this down-sampling procedure was observed to reduce the computational time by a factor of 8 as compared to the simulation without this strategy, while still maintaining the error within an acceptable limit. Following the detailed numerical investigation, the model, intended for volatile fuels only, is applied to simulate a two-stroke, naturally aspirated HCCI engine fueled with isooctane. The in-cylinder pressure and CO emissions predicted by the model agree reasonably well with the measured profiles. In addition, the new model is applied to estimate the influence of engine operating parameters such as the relative air–fuel ratio and early direct injection timing on HCCI combustion and emissions. The qualitative trends observed in the parametric variation study match well with experimental data in literature.

Characteristics of HCCI engine operation for additives, EGR, and intake charge temperature while using iso-octane as a fuel

This work investigates the effects of exhaust gas recirculation (EGR) and operation parameters including engine speed, equivalence ratio, coolant-out temperature, and intake charge temperature on the basic characteristics of a single-cylinder homogeneous charge compression ignition (HCCI) engine powered with reformulated iso-octane fuels. The running range of iso-octane HCCI engine can be expanded to lower temperature and more load by adding di-tertiary butyl peroxide (DTBP) in the fuel. The combustion timing advances with the increase of DTBP concentrations, coolant temperature and equivalence ratio. The effects of EGR on the combustion and emissions are remarkable when the EGR rate is higher than 25%, and the combustion phase is sharply postponed and the UHC and CO emissions deteriorate. The intake charge temperature has a moderate effect on combustion and emissions when it is lower than 35°C; but the combustion timing advances, the combustion duration shortens, and sometimes it leads to knock combustion when the intake charge temperature increases to above 35°C.