Homogeneous Charge Compression Ignition Combustion Research Articles

Multi Cycle Modeling, Simulating and Controlling of a Free Piston Engine with Electrical Generator under HCCI Combustion Conditions

ABSTRACT For distributed electrical power generation, linear engine-alternator systems have the potential to operate at high efficiency. They avoid the frictional losses present in rotating generators and enable the use of efficient combustion modes. However, their design and control optimization are matters of novelty. In this research, a single cylinder, two-stroke, free piston engine model has been built and simulated. The model describes the coupling between the piston motion dynamics and the chemical kinetics of the in-cylinder charge mixture undergoing homogeneous charge compression ignition (HCCI) combustion. The chemical kinetics reactions have been employed to find the net heat release rate and the start of combustion. Piston motion depends on the cylinder pressure as well as the dynamics of the mass-spring-damper system translator. For advanced investigations, the model results are represented by a multi-cycle simulation where 100 engine cycles were simulated with an average frequency of 125 Hz. At near-steady state conditions, the electrical generator converts 47% of the total fuel energy to electrical work at stoichiometric combustion conditions. It is shown that HCCI ignition timing strongly affects the performance of the engine. Combustion timing just before piston top dead center position encourages better engine stability and performance for the given model. The stability of the engine strongly depends on the rate of work done on the translator by the generator, putting the generator thrust force at the core of the engine control scheme. Cycle to cycle stability, with HCCI combustion occurring for each compression stroke, is achieved with the use of a modified proportional-derivative controller which alters the generator thrust force based on the cyclic change in the piston bottom dead center position. The research goal is to develop an optimized design with stable controls that yields a high efficiency conversion of chemical to electrical energy.

Application of Variable Valve Actuation Strategies and Direct Gasoline Injection Schemes to Reduce Combustion Harshness and Emissions of Boosted HCCI Engine

The present study investigates various measures to reduce pressure rise rates (PRRs) in a residual-affected homogeneous charge compression ignition (HCCI) engine. At the same time, the impact of those measures on efficiency and emissions is assessed. Experimental research was performed on a single cylinder engine equipped with a fully flexible valve train mechanism and direct gasoline injection. The HCCI combustion mode with exhaust gas trapping was realized using negative valve overlap (NVO) and fuel reforming, achieved via the injection of a portion of fuel during exhaust recompression. Three measures are investigated for the PRR control under the same reference operating conditions, namely: (i) variable intake and exhaust valve timing, (ii) boost pressure adjustment, and (iii) split fuel injection to control the amount of fuel injected for reforming. Variable exhaust valve timing enabled control of the amount of trapped residuals, and thus of the compression temperature. The reduction in the amount of trapped residuals, at elevated engine load, delays auto-ignition, which results in a simultaneous reduction of pressure rise rates and nitrogen oxides emissions. The effects of intake valve timing are much more complex because they include the variability in the amount of intake air, the thermodynamic compression ratio, as well as the in-cylinder fluid flow. It was found, however, that both early and late intake valve openings (IVOs) delay auto-ignition and prolong combustion. Additionally, the reduction of the amount of fuel injected during exhaust recompression further delays combustion and reduces combustion rates. Intake pressure reduction has by far the largest effect on peak pressure reduction yet is connected with excessive NOX emissions. The research successfully identifies air-path and injection techniques, which allow for the control of combustion rates and emissions under elevated load regime.

Ozone-assisted combustion of hydrogen: A comparison with isooctane

Abstract Homogeneous Charge Compression Ignition (HCCI) engines have the potential to achieve low emissions of nitrogen oxides and particulate matter but the use of carbon-based fuels leads to significant emissions of carbon monoxide and unburned hydrocarbon which hinder the widespread development of HCCI. Nonetheless, HCCI engines have also the potential to use various types of fuel such as hydrogen, a carbon free fuel. The present study examined hydrogen combustion in an HCCI engine comparing hydrogen with isooctane. The sensitivities of CA05, CA50 and ringing intensity to intake pressure, intake temperature and ozone were investigated for both fuels. Results highlighted that hydrogen autoignites more easily than isooctane despite its higher-octane number. Ozone seeding was therefore used to shift the combustion region of isooctane in order to perform a viable comparison between the two fuels while starting with the same initial conditions. From this point, all of the three parameters were found to promote the HCCI combustion of both fuels but greater sensitivities were observed for hydrogen than for isooctane. Ozone was observed to have the strongest influence, making it a good strategy for a future control approach. The noise level of hydrogen was very high due its high reactivity. Finally, the use of hydrogen together with ozone should enable a very clean engine cycle to be achieved cycle with only water and nitrogen as engine-out emissions.

Mapping of an HCCI engine using negative valve overlap strategy

ABSTRACTIn order to control combustion phasing and extend operation range of homogeneous charged compression ignition (HCCI) engines, several techniques such as controlling inlet air temperature and pressure, using alternative fuels, variable compression ratio, supercharging, etc. have been applied. Among them, variable valve mechanism received much attention to extend HCCI operating range and control autoignition. Mode switching between spark plug ignition (SI) to HCCI has been started to use widely because of limited operation range of the HCCI mode. To control of an engine switching combustion modes between SI and HCCI, embedded system needs parametrical control maps of the engine. In this study, operation range, air excess ratio, torque and thermal efficiency maps of an HCCI engine using negative valve overlap (NVO) strategy were obtained and additionally the effects of NVO on HCCI combustion in view of in-cylinder temperature, heat release rate, effective thermal efficiency, combustion phasing, and exhaust emissions were investigated. Also, variation of CO, HC, and NOx emissions were evaluated at different valve overlaps. Three different valve overlap including +8, −8, and −16 CA were used in the experiments. Test results showed that in-cylinder pressure and heat release decreased with NVO. The test engine could run without knocking by application of NVO. It was seen that operating range of HCCI extended with NVO. It was also found that effective thermal efficiency increased with −16ºCA valve overlap compared to other valve timings.

Development and experimental validation of a field programmable gate array–based in-cycle direct water injection control strategy for homogeneous charge compression ignition combustion stability

Homogeneous charge compression ignition is a part-load combustion method, which can significantly reduce oxides of nitrogen (NO x) emissions compared to current lean-burn spark ignition engines. The challenge with homogeneous charge compression ignition combustion is the high cyclic variation due to the lack of direct ignition control. A fully variable electromagnetic valve train provides the internal exhaust gas recirculation through negative valve overlap which is required to obtain the necessary thermal energy to enable homogeneous charge compression ignition. This also increases the cyclic coupling as residual gas and unburnt fuel is transferred between cycles through exhaust gas recirculation. To improve combustion stability, an experimentally validated feed-forward water injection controller is presented. Utilizing the low latency and rapid calculation rate of a field programmable gate array, a real-time calculation of residual fuel mass is implemented on a prototyping engine controller. Using this field programmable gate array–based calculation, it is possible to calculate the amount of fuel and the required control interaction during an engine cycle. This controller prevents early rapid combustion following a late combustion cycle using direct water injection to cool the cylinder charge and counter the additional thermal energy from any residual fuel that is transferred between cycles. By cooling the trapped cylinder mass, the upcoming combustion phasing can be delayed to the desired setpoint. The controller was tested at several operating points and showed an improvement in the combustion stability as shown by a reduction in the standard deviation of combustion phasing and indicated mean effective pressure.

Influence of hydrogen as a fuel additive on combustion and emissions characteristics of a free piston engine

It has been shown that using fuel additives play an important role in enhancing the combustion characteristics in terms of efficiency and emissions. In addition, free piston engines have shown capable in reducing energy losses and presenting more efficient and reliable engines. In this context, the objective of the present work is to investigate the effect of using hydrogen as a fuel additive in natural gas homogeneous charge compression ignition free piston engine. To this aim, two models have been iteratively coupled: the combustion model that is used to calculate the heat release of the combustion and the scavenging model that is employed to determine the in-cylinder mixture state after scavenging in terms of its homogeneity and species mass fractions and to obtain the finial pressure and temperature of the in-cylinder mixture. In the former model, the 0-D approach through Cantera toolkit has been considered due to the fact that homogeneous charge compression ignition combustion is very rapid and the fuel-air mixture is well-homogenous, whereas in the latter model, 3-D-CFD approach through AN-SYS FLUENT software is considered to ensure precise calculations of the species exchange at the end of each engine cycle. The effect of hydrogen as a fuel additive has been quantified in terms of the combustion characteristics (e. g., ignition delay, heat release rate, engine overall efficiency and emissions, etc.). It has been shown that hydrogen addition reduces ignition delay time, decreases the in-cylinder peak pressure, while allowing the engine to operate with higher mechanical efficiency as it has high heat release rate, increases the NOx emission levels of the engine, but decreases the CO levels

In-cycle control for stabilization of homogeneous charge compression ignition combustion using direct water injection

Homogeneous charge compression ignition offers a high potential for the reduction of CO2 and NOx raw emissions; however, its use entails problems that are associated with low combustion stability, especially at the limits of the operating range. The recirculation of exhaust gases inside the combustion chamber by using a negative valve overlap leads to a strong coupling of consecutive cycles. The cyclic coupling induces phases of unstable operation after the occurrence of stochastic outlier cycles with misfire or incomplete combustion. These unstable phases are marked by reduced efficiency and increased emissions.Two in-cycle closed-loop control algorithms, which focus on the heat release in the intermediate compression, are presented in this article. To control the combustion process, direct water injection is used to ensure a direct influence on the temperature level in the combustion chamber; subsequently this influences combustion phasing. The decoupling of consecutive cycles serves to reduce deviations in the indicated mean effective pressure and crank angle position of 50% mass fraction burned. To develop a suitable controller, a first-order autoregressive model of homogeneous charge compression ignition combustion is split into intermediate compression and main combustion phases. Moreover, unstable sequences are analyzed in the time domain to identify appropriate in-cycle control concepts.The control concepts are developed based on the heat release in the intermediate compression as a strong correlation factor for consecutive cycles. To realize fast control interventions, a real-time cylinder pressure analysis as well as the control algorithms are implemented on a field-programmable gate array.The control algorithms are validated on a single-cylinder research engine and compared with conventional operation without in-cycle control. Results show a significant increase in the stability of combustion phasing and load by means of in-cycle control.

Mathematical modeling and validation of mass transfer phenomenon in homogeneous charge compression ignition engines based on a thermodynamic multi zone model

ABSTRACTThe main purpose of the current study is mathematical modelling and validation of mass transfer phenomenon in homogeneous charge compression ignition engines. A validated multi-zone model coupled to a semi-detailed chemical kinetics is used to predict homogeneous charge compression ignition combustion and emissions. Heat and Mass transfer submodels are linked to the multi-zone model. Bulk flow and diffusion mass transfer between zones are considered. The results indicate that the diffusion mass transfer is negligible in homogeneous charge compression ignition engines. Bulk flow mass transfer plays a critical role in homogeneous charge compression ignition simulation and applying it in the multi-zone model leads to accurate prediction of the start of combustion, peak pressure and exhaust emissions. The results show that the maximum error changes from 90% to 5% in carbon monoxide prediction and from 98% to 14% in unburned hydrocarbons prediction, using the mass transfer submodel.

Method for determining gasoline surrogate component proportions and development of reduced chemical kinetics model of the determined surrogate fuel

Response surface method is used to build models for predicting an octane number and determining the component proportions of a gasoline surrogate fuel. The fuel is synthesized using toluene, iso-octane, and n-heptane and is referred to as toluene reference fuel. The built models include second-order model and third-order model. Both models can excellently predict the octane number of the toluene reference fuel with known component proportions. Moreover, the third-order model is more accurate than second-order model in determining the component proportions of the toluene reference fuel, and the relative error is less than 8%. Therefore, the third-order model can accurately predict the octane number and determine the component proportions of the toluene reference fuel. Moreover, a new reduced mechanism of the toluene reference fuel is proposed and validated by using shock tube ignition delay and in-cylinder pressure in a homogeneous charge compression ignition engine. The toluene reference fuel mechanism coupled with third-order model is used to simulate the ignition delay of American gasoline (RD387) and the homogeneous charge compression ignition combustion behaviors of European gasoline (ULG95). Both cases are simulated thoroughly.

Development of a compact and robust Polyoxymethylene Dimethyl Ether 3 reaction mechanism for internal combustion engines

With high oxygen content, Polyoxymethylene Dimethyl Ether 3 (PODE3) is a potential fuel additive to reduce soot emissions. However, reaction mechanisms to describe PODE3 combustion are not yet compact enough for 3-D numerical simulations. Therefore, the current work aims to develop a small yet reliable PODE3 reaction mechanism. Based on sensitivity analysis, the major reaction pathway is identified to construct the PODE3 sub-mechanism. Thereafter, it is integrated with a primary reference fuel (PRF) mechanism to create a PRF-PODE3 mechanism containing 61 species and 190 reactions. The major reaction pathway of the PODE3 sub-mechanism consists of only 11 species and 17 reactions. Furthermore, the new mechanism has been well validated with experimental results in terms of ignition delay times (rapid compression machine at pressures = 1.0 MPa and 1.5 MPa, equivalence ratios = 0.5, 1.0 and 1.5), laminar flame speeds (Pin = 1 atm, Tin = 408 K), flame species concentrations (pressure = 33.3 mbar, equivalence ratio = 1) and homogeneous charge compression ignition (HCCI) combustion (equivalence ratios = 0.18 and 0.34). Overall, this highly compact yet robust PRF-PODE3 mechanism is demonstrated to be suitable for internal combustion engine simulations. In addition, with good agreement in terms of fundamental combustion validations, the proposed PRF-PODE3 mechanism can reasonably be applied to other practical applications such as simulations in jet engines, pulse detonation engines, boilers and furnaces.

The effect of fuel composition and additive packages on deposit properties and homogeneous charge compression ignition combustion

Homogeneous charge compression ignition combustion is highly dependent on in-cylinder thermal conditions that are favorable to auto-ignition, and the presence of deposits can dramatically impact the in-cylinder environment. Because fuels available at the pump can differ considerably in composition, and fuel composition and the included additive package directly affect how deposits accumulate in a homogeneous charge compression ignition engine, strategies intended to bring homogeneous charge compression ignition to market must account for this fuel and additive variability. In order to investigate this impact, two oxygenated refinery stream test fuels with two different additives were run in a single cylinder homogeneous charge compression ignition engine. The two fuels had varying chemical composition; one represents a “dirty” fuel with high aromatic content that was intended to simulate a worst-case scenario for deposit growth, while the other represents a California Reformulated Gasoline Blendstock for Oxygenate Blending fuel, which is the primary constituent of pump gasoline at fueling stations across the state of California. The additive packages are typical of technologies that are commercially available to treat engine deposits. Both fuels were run in an experimental, single-cylinder homogeneous charge compression ignition engine in a passive conditioning study, during which the engine was run at steady state over a period of time in order to track changes in the homogeneous charge compression ignition combustion event as deposits accumulated in-cylinder. Both the composition and the additive influenced the structure of the combustion chamber deposit layer, but more importantly, both the rate at which the layer developed and the equilibrium thickness it achieved. The overall thickness of the combustion chamber deposit layer was found to have a significant impact on homogeneous charge compression ignition combustion phasing.

Experimental Analysis of Partially Premixed Charge in a Diesel Engine with Jatropha Oil Methyl Ester and Diesel Blends

Homogeneous charge compression ignition (HCCI) is an alterna -tive combustion process which received considerable interest to meetthe stringent emission norms. One such technology developed is par -tially premixed charge compression ignition (PCCI) combustion modea variant of HCCI combustion mode. In this work an attempt has beenmade to experimentally analyze the performance and emission char-acteristics with Premixed Charge Compression Ignition (PCCI) modeassisted with pilot injection. Experiments were conducted in a modifiedsingle cylinder water-cooled diesel engine employing a conceptual sys -tem known as transient state fuel induction with different modes suchas diesel and biodiesel conventional mode, diesel PCCI–diesel, biodieselPCCI-diesel and biodiesel PCCI-biodiesel modes. In the present work,diesel and jatropha methyl ester was used premixed fuel along with con-ventional injection of either diesel or 20% jatropha oil methyl ester blendwith diesel. The results indicate that brake thermal efficiency decreasesmarginally as the methyl ester in the conventional and PCCI modes andHC, CO emissions are higher and soot levels are lower in the case ofbiodiesel PCCI-biodiesel mode.

HCCI and SICI combustion modes analysis with simultaneous PLIF imaging of formaldehyde and high-speed chemiluminescence in a rapid compression machine

Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (Φ = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition – e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow.

Effect of engine size, speed, and dilution method on thermal stratification of premixed homogeneous charge compression–ignition engines: A large eddy simulation study

High heat release rates limit the operating range of homogeneous charge compression–ignition engines to low and medium loads. Thermal stratification has been shown to stagger autoignition, lower heat release rates, and extend the operating range of homogeneous charge compression–ignition engines. However, the dependence of naturally occurring thermal stratification on the engine size, speed, and internal residual dilution is not fully understood. A three-dimensional computational fluid dynamics model with large eddy simulations and detailed chemical kinetics was developed using CONVERGE. This model was used to simulate two different engines: (1) a light-duty 2.0 GM Ecotec Engine modified for homogeneous charge compression–ignition combustion in one of the cylinders and (2) a medium-duty Cummins B-series engine modified for homogeneous charge compression–ignition combustion in one of the cylinders. For the light-duty engine, five consecutive modeled cycles were compared with experimental data from 300 consecutive cycles using residual gas dilution at 2000 r/min. For the medium-duty engine, five consecutive modeled cycles were compared with experimental data from 100 consecutive cycles using air dilution with intake heating at 1200 r/min. In the light-duty engine, it was found that incomplete mixing between fresh charge and residual gas increased thermal stratification early in the compression stroke for residual dilution compared to air dilution. Residual stratification at the onset of ignition was small and not directly coupled with thermal stratification. Heat losses to the walls were the dominant source of thermal stratification at the onset of ignition. The reduced oxygen concentration due to residual dilution, increased the temperature requirement for autoignition, which increased heat transfer losses and increased the thermal stratification around top dead center. The thermal stratification before ignition reduced when the engine speed increased because of the lower heat transfer losses. The light-duty engine was found to have larger portion of the fuel energy lost to heat transfer than the medium-duty engine, which resulted in larger thermal stratification before ignition.

N-Butanol reactivity modulation with early fuel injections for low NOx compression ignition combustion

This paper investigates pilot injection strategies to improve the combustion controllability of a premixed n-butanol cylinder charge while achieving ultra-low NOx emissions comparable to homogenous charge compression ignition (HCCI). Detailed insights are given for the ignition processes controlled by fuel and reactivity stratification. The n-butanol port injection is applied to form a homogeneous cylinder charge, an early pilot direct injection of either n-butanol or diesel is generated to control the ignition and combustion. In dual-fuel reactivity stratification combustion (RSC) strategy with an early diesel pilot injection, the prompt control of combustion phasing relies on fuel energy ratio adjustment. Under heavy EGR, the RSC can lower the NOx emissions to the level of 10 ppm and retain the combustion completeness, however, at the cost of increased pressure rise rate. EGR plays a key role in suppressing the pressure rise rate for load extension of RSC combustion. In order to maintain a moderate pressure rise rate when the engine load increases, the combustion phasing must be retarded through heavy EGR together with increased PFI n-butanol energy ratio, which leads to an efficiency penalty. In the single-fuel strategy, the early pilot injection of n-butanol results in fuel stratification in the cylinder which is responsible for the combustion initiation. The single-fuel strategy retains effective ignition controllability although the start of combustion manifests less sensitivity to the n-butanol pilot injection timing. In the tested low to medium engine load range, the single-fuel strategy can achieve NOx emissions level same to HCCI combustion, while the NOx emissions of dual-fuel RSC strategy are slightly higher because of the reactivity gradient and more advanced combustion phasing.

Effect of hydrogen peroxide additive on the combustion and emission characteristics of an n-butanol homogeneous charge compression ignition engine

The homogeneous charge compression ignition (HCCI) combustion was conducted on a modified engine to investigate the effect of hydrogen peroxide (H2O2) additive on the combustion and emission characteristics, such as the indicated thermal efficiency (ηi), the total hydrocarbon (THC), the carbon monoxide (CO) and the nitrogen oxides (NOx) emissions. A simulation using CHEMKIN software was also performed to analyze the experimental data. The results indicate that the addition of H2O2 facilitates the formation of hydroxyl (OH) radical and hydroperoxyl (HO2) radical in the cylinder under HCCI mode, which greatly affects the combustion and emission characteristics of the n-butanol HCCI engine. By adding H2O2, ηi increases, NOx emission remains at an ultra-low level, but THC and CO emissions reduce at almost all the test conditions. Especially at the low intake temperature (Tin) or low load condition, the improvements are very obvious, which suggests that addition of H2O2 may extend the low Tin and low load limits of HCCI engines.

Combustion and Emission Performance of an HCCI Engine Fuelled by n-Heptane/Toluene Blends at a Low-Load Operating Condition

Homogeneous charge compression ignition (HCCI) engine technology offers high fuel efficiency and extra low nitrogen oxide (NOx) and particulate matter (PM) emissions, which makes it a potential alternative combustion mode to conventional diesel engines. A diesel fuel is usually composed of many classes of hydrocarbons among which aromatic compounds have attracted special attention due to their specific combustion characteristics. Understanding the combustion and emission characteristics of different classes of hydrocarbons is crucial for identifying appropriate diesel fuels suitable for HCCI combustion. Toluene is a typical aromatic compound in a diesel fuel. A study of toluene content may provide implications of the effect of aromatic in a diesel fuel on HCCI combustion. In this paper, the combustion and emission performance of an HCCI engine fuelled by n-heptane/toluene blends at a low load operating condition was investigated by experiment and numerical simulation. A modified Cooperative Fuel Research (CFR) engine and a in-house-developed multi-zone model were employed. The engine was operated at the condition of engine speed of 900 rpm, relative air/fuel ratio of 3.5 and without external exhaust gas recirculation. The investigated fuel blends covered a range from pure n-heptane to 70% toluene by volume. Both experimental and numerical results showed that an increase in toluene fraction in the fuel blend retarded combustion phasing. As a result, the optimal compression ratio, at which thermal efficiency reached its maximum for a fuel blend, increased with increasing toluene fraction. The maximum thermal efficiency increased as the toluene fraction increased from 0 to 50%, but then decreased with further increasing toluene fraction to higher values. The peak pressure rise rate also increased with increasing toluene fraction at a constant combustion phasing. An increase in toluene fraction resulted in an increase in unburned hydrocarbon emissions but had little effect on NOx emissions.

Construction and Validation of a Five-Component Fuel Simplification Mechanism for Homogeneous Charge Compression Ignition Engine

The detailed chemical kinetics of ethanol was reduced by means of path analysis and sensitivity analysis. The important components and reaction pathways of ethanol combustion were sorted out, and a reduced chemical kinetics model of ethanol containing 37 components and 78 reactions was developed. On the basis of the reduced mechanism of the gasoline surrogate [iso-octane, n-heptane, toluene, and diisobutylene (DIB)], the reduced mechanism of ethanol was then coupled. A reduced chemical kinetic model of a five-component (iso-octane, n-heptane, toluene, DIB, and ethanol) fuel containing 124 components and 244 reactions and suitable for homogeneous charge compression ignition (HCCI) combustion was finally obtained. Simulation results of the five-component reduced mechanism are compared with the ignition delay of four different components of gasoline surrogate under a shock tube. These results well matched the experimental value. At the same time, the simulation results of the model well matched the experimen...

Development of a reduced polyoxymethylene dimethyl ethers (PODEn) mechanism for engine applications

Polyoxymethylene dimethyl ethers (PODEn) features high oxygen content (>45 wt%) and cetane number (>60) and is a promising alternative fuel for diesel engines. In the current study, a reduced primary reference fuel (PRF)-PODEn chemical kinetic mechanism consisting of 145 species and 585 reactions is proposed for combustion and soot formation prediction. The PODEn sub-mechanism is reduced from a comprehensive mechanism using an iterative reduction procedure with different mechanism reduction methods (directed relation graph with error propagation, sensitivity analysis and isomer lumping), and is then incorporated into the base reduced PRF-PAH mechanism. This reduced PRF-PODEn mechanism has been extensively validated against experimental results including ignition delay times, laminar flame speeds, species concentration profiles and homogeneous charge compression ignition (HCCI) combustion. At specific operating conditions, PODE3 HCCI combustion exhibits a three-stage heat release, which is found to be caused by PODE3 decomposition, CH2O-to-CO reactions and CO-to-CO2 oxidation, respectively. Furthermore, the present mechanism is further applied in three-dimensional simulations of the engine direct injection compression ignition (DICI) combustion fueled with pure diesel (D100), gasoline/diesel blends (GD50) and gasoline/diesel/PODEn blends (GDP30). Compared to D100, the soot emissions are reduced with GD50 and GDP30 by 75% and 96%, respectively, due to the higher ratio of premixed combustion and higher fuel oxygen content. Additionally, 0-D modeling investigations with the present mechanism are conducted to clarify the chemical effects on soot formation. The A4 formation of PODE3 is three orders of magnitude smaller than that of PRF since no CC bonds exist in PODE3′s molecules. The decline of A4 formation in the n-heptane/iso-octane/PODE3 blend illustrates that oxygenated fuels like PODEn are beneficial to the soot reduction.

A predictive 0-D HCCI combustion model for ethanol, natural gas, gasoline, and primary reference fuel blends

Homogeneous Charge Compression Ignition (HCCI) is a promising advanced combustion concept with high thermal efficiency and low exhaust emissions. This work purposes a computationally-efficient, zero-dimensional (0-D) HCCI combustion model that does not need to solve any differential equations. To develop the burn rate correlations, experimental HCCI data of ethanol, natural gas, E10-gasoline, and Primary Reference Fuel (PRF) blends was collected on a CFR engine. The burn rate model is built based on the individual cycle mass fraction burned (MFB) curves calculated from the experimental data. CA0 can be predicted from an ignition delay correlation. Once CA0 is known, CA50 and CA90 can be predicted based on CA0 and the charge-mass equivalence ratio ϕ′. Then, with CA0, 50 and 90 known, a Wiebe function can be constructed to represent the MFB curve and burn rate. For PRF blends, low-temperature and high-temperature heat releases (LTHR and HTHR) are modeled by two Wiebe functions with ϕ′ and PRF number dependence. The fitted model has high accuracy compared to the experimental data with R2 values generally greater than 0.97. Finally, various ignition delay correlations from the literature are also tested against the experimentally collected data and some modifications to the correlations are suggested.