A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO_x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO_x emissions while reducing the tendency to knock     

Hydrogen as an ignition-controlling agent for HCCI combustion engine by suppressing the low-temperature oxidation

Homogeneous charge compression ignition (HCCI) combustion enables internal combustion engines to achieve higher thermal efficiency and lower _NOx${\mathrm{NO}}_x$ emission than with conventional combustion systems. Controlling the ignition timing in accordance with the operating conditions is crucial for utilizing HCCI combustion engines. Adding hydrogen-containing gas is known to retard the autoignition of dimethyl ether (DME) considerably. The effective ignition control by hydrogen can expand the operation range of equivalence ratios and engine loads in HCCI combustion. This research investigated the mechanisms in the ignition control by the chemical kinetics analysis. The results show that the retarded ignition can be attributed to a consumption of OH by hydrogen during low-temperature oxidation of DME. The decreased OH concentration leads to retarded heat release and delays the onset of the high-temperature oxidation.     

Three-stage autoignition of gasoline in an HCCI engine: An experimental and chemical kinetic modeling investigation

The alternative HCCI combustion mode presents a possible means for decreasing the pollution with respect to conventional gasoline or diesel engines, while maintaining the efficiency of a diesel engine or even increasing it. This paper investigates the possibility of using gasoline in an HCCI engine and analyzes the autoignition of gasoline in such an engine. The compression ratio that has been used is 13.5, keeping the inlet temperature at 70 °C, varying the equivalence ratio from 0.3 to 0.54, and the EGR (represented by N₂) ratio from 0 to 37 vol%. For comparison, a PRF95 and a surrogate containing 11 vol% n-heptane, 59 vol% iso-octane, and 30 vol% toluene are used. A previously validated kinetic surrogate mechanism is used to analyze the experiments and to yield possible explanations to kinetic phenomena. From this work, it seems quite possible to use the high octane-rated gasoline for autoignition purposes, even under lean inlet conditions. Furthermore, it appeared that gasoline and its surrogate, unlike PRF95, show a three-stage autoignition. Since the PRF95 does not contain toluene, it is suggested by the kinetic mechanism that the benzyl radical, issued from toluene, causes this so-defined “obstructed preignition” and delaying thereby the final ignition for gasoline and its surrogate. The results of the kinetic mechanism supporting this explanation are shown in this paper.     

A multiscale combustion model formulation for NOx predictions in hydrogen enriched jet flames

The present paper investigates the role of combustion models and kinetic mechanisms on the prediction of NO_x emissions in a turbulent combustion system where conventional and unconventional routes are equally important for NO_x formation. To this end, a lab-scale combustion system working in Moderate and Intense Low-oxygen Dilution (MILD) conditions, namely the Adelaide Jet in Hot Co-flow (JHC) burner, is targeted. The Eddy Dissipation Concept (EDC) and the Partially-Stirred Reactor (PaSR) models are used for turbulence-chemistry interactions. The KEE and GRI2.11 chemical mechanisms are employed. The results show that the choice of the combustion model has a higher impact than the selection of the kinetic mechanism for the investigated cases, indicating that biases in the turbulent reactive flow closure are as important, if not more, as the level of the accuracy of the chemical scheme employed. Moreover, the sensitivity of the NO emissions to the uncertain kinetic parameters of the rate-limiting reactions of the NNH pathway is found to be significant when a detailed kinetic mechanism is used. An engineering modification of the PaSR combustion model is proposed to account for the different chemical time scales of fuel oxidation reactions and NO_x formation pathways. It shows an equivalent impact on the emissions of NO than the uncertainty in the NNH pathway kinetics. At the cost of introducing a negligible mass imbalance, the adjustment leads to improved predictions of NO. The investigation establishes a possibility for the engineering modeling of NO formation in turbulent flames with a finite-rate chemistry combustion model that can incorporate a detailed mechanism at an affordable computational cost.     

Methodology development of a time-resolved in-cylinder fuel oxidation analysis: Homogeneous charge compression ignition combustion study application

A technique was developed and applied to understand the mechanism of fuel oxidation in an internal combustion engine. This methodology determines the fuel and concentrations of various intermediates during the combustion cycle. A time-resolved measurement of a large number of species is the objective of this work and is achieved by the use of a sampling probe developed in-house. A system featuring an electromagnetically actuated sampling valve with internal N₂ dilution was developed for sampling gases coming from the combustion chamber. Combustion species include O₂, CO₂, CO, NO_x, fuel components, and hydrocarbons produced due to incomplete combustion of fuel. Combustion gases were collected and analyzed with the objectives of analysis by an automotive exhaust analyzer, separation by gas chromatography, and detection by flame ionization detection and mass spectrometry. The work presented was processed in a homogeneous charge compression ignition combustion mode context.     

Nitric oxide interactions with hydrocarbon oxidation in a jet-stirred reactor at 10 atm

The purpose of the present work is to better define the influence of trace amounts of NO on the oxidation of model fuels such as n-heptane, iso-octane, toluene, and methanol. This information is of interest for understanding and modeling autoignition whether for engine knock or for engines operating under compression ignition modes such as HCCI (homogeneous charge compression ignition) or CAI^TM (controlled autoignition). The experiments were performed in a jet-stirred reactor at 10 atm over a temperature range of 550 to 1180 K with a residence time of 1 s for stoichiometric mixtures highly diluted in nitrogen. The carbon content was about 1 molar percent and the added NO ranged from 25 to 500 ppmv. The effects of NO vary with the temperature regime. At the lowest temperatures NO inhibits the reaction. As temperature rises beyond 675 K, NO can considerably accelerate the reactivity of all fuels to an extent that can supercede the NTC behavior in the case of n-heptane. Modeling work indicates that in this temperature region at 10 atm the promoting effect of NO is largely due to the catalyzed production of OH, involving the dissociation of HONO, with the latter formed from reactions between NO₂ and HO₂, CH₃O, or CH₂O. In the intermediate temperature regime the intensity of the accelerating effects is observed to rise with the octane number of the fuel, with the exception of methanol. For toluene, the onset of oxidation drops down from 900 to 800 K with as little as 50 ppmv NO.     

Effect of ignition timing retard strategy on NOx reduction in hydrogen-compressed natural gas blend engine with increased compression ratio

Hydrogen-compressed natural gas blend (HCNG) engines can extend the lean burn limit because of the wide flammability range of hydrogen. Lean combustion helps facilitate high efficiency and fundamentally reduces NO_x emission. Increasing the compression ratio (CR) of an HCNG engine was reported to improve its thermal efficiency. However, the high risk of knock occurrence and the increase in NO_x emission can hinder CR increase.     

Experimental study of the kinetic interactions in the low-temperature autoignition of hydrocarbon binary mixtures and a surrogate fuel

The oxidation and autoignition of five undiluted stoichiometric mixtures, n-heptane/toluene, isooctane/toluene, isooctane/1-hexene, 1-hexene/toluene, and isooctane/1-hexene/toluene, has been studied in a rapid compression machine below 900 K. Ignition delay times of two- and one-stage autoignition have been measured and compared to those for pure hydrocarbons. The largest influence of mixing is in the region of the negative temperature coefficient. Intermediate products have been analyzed. The main reaction paths of low-temperature co-oxidation are discussed according to current knowledge of the oxidation paths of pure hydrocarbons. The influence of toluene on the temperature coefficient of the first stage of ignition of isooctane cannot be accounted for by the current theories of low-temperature autoignition. Each hydrocarbon generates a pool of radicals whose reactivity and selectivity toward further attack changes with temperature and with the family of hydrocarbons. The overall behavior of mixtures may result from changing competition for HO₂ and OH as temperature increases during the delay time. Termination reactions between stable radicals seem to have a minor impact at low temperature.     

Investigating various effects of reformer gas enrichment on a natural gas-fueled HCCI combustion engine

Homogenous charge compression ignition (HCCI) combustion has the potential to work with high thermal efficiency, low fuel consumption, and extremely low NO_x-PM emissions. In this study, zero-dimensional single-zone and quasi-dimensional multi-zone detailed chemical kinetics models were developed to predict and control an HCCI combustion engine fueled with a natural gas and reformer gas (RG) blend. The model was validated through experiments performed with a modified single-cylinder CFR engine. Both models were able to acceptably predict combustion initiation. The result shows that the chemical and thermodynamic effects of RG blending advance the start of combustion (SOC), whereas dilution retards SOC. In addition, the chemical effect was stronger than the dilution effect, which was in turn stronger than the thermal effect. Furthermore, it was found that the strength of the chemical effect was mainly dependent on H₂ content in RG. Moreover, the amount of RG and concentration of species (CO–H₂) were varied across a wide range of values to investigate their effects on the combustion behavior in an HCCI engine. It was found that the H₂ concentration in RG has a more significant effect on SOC at lower RG percentages in comparison with the CO concentration. However, in higher RG percentages, the CO mass concentration becomes more effective than H₂ in altering SOC.     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

A study of spray strategies on improvement of engine performance and emissions reduction characteristics in a DME fueled diesel engine

This paper investigated the impact of injection angle and advance injection timing on combustion, emission, and performance characteristics in a dimethyl ether (DME) fueled compression ignition engine through experimentation on spray behaviors and the use of numerical methods. To achieve this aim, a visualization system and two injectors with different injection angles were used to analyze spray characteristics. The combustion, emission, and performance characteristics were analyzed by numerical methods using a detailed chemical kinetic DME oxidation model. Each of five injection angles and timings were selected to examine the effect of injection angle and timing. It was revealed that the injected spray with narrow injection angles was impinged on the bottom wall after the SOI of BTDC 60°, and most of the fuel spray and evaporation with the wide injection angles had a distribution at the crevice region when the injection timing was advanced. In addition, NO_x emissions at the SOI of BTDC 20° and TDC had higher values, and the soot emission quantities were extremely small. The narrow injection angles had good performance at the advanced injection timing, and the injection timing over a range of BTDC 40-20° showed superiority in performance characteristics.     

Experimental and modeling study of performance and emissions of SI engine fueled by natural gas–hydrogen mixtures

Based on CFD software and reaction kinetics software, multi-dimensional CFD Model coupled with detail reaction kinetics is built to study the combustion process in H₂/CNG Engine. Detail reaction mechanism is used to simulate the chemistry of combustion and a combustion model considering the turbulent mixing effects was also applied. To reduce the computation time, the coupled software is reprogrammed to have the function of parallel computing and the revised software is computed in a Massively Parallel Processor. The model is validated using the experiment data from a modified diesel engine. The results show: cylinder pressure from simulation has a good agreement with experiment data and CO and NO_x emission is well predicted by the model in a wide range.     

Low NOx combustion technologies for high temperature applications

Because of the high process temperature and the high temperature to which the combustion air is preheated, NO_x emissions from glass melting furnaces are extremely high. Even at these high temperatures, NO_x emissions could be reduced drastically by using advanced combustion techniques such as staged combustion or flameless oxidation, as experimental work has shown. In the case of oxy-fuel combustion, the NO_x emissions are also very high if conventional burners are used. Staged combustion achieves similar NO_x reductions.     

Effects of compression ratio on performance and emission characteristics of heavy-duty SI engine fuelled with HCNG

The wide range of hydrogen's flammable limits enables ultra-lean combustion. A lean burn reduces the combustion temperature, increases thermal efficiency, and reduces knock, which is a serious problem in a spark ignition (SI) engine. The anti-knock improvement from hydrogen addition makes it feasible to increase the compression ratio (CR) and further improve the thermal efficiency. Herein, the effects of the CR on performance and emission characteristics were investigated using an 11-L heavy-duty SI engine fuelled with HCNG30 (CNG 70 vol%, hydrogen 30 vol%) and CNG. These fuels were used to operate an engine with CRs of 10.5 and 11.5. The results showed that thermal efficiency improved with an increased CR, which significantly decreased CO₂ emission. On the other hand, the NO_x emission was largely increased. Nevertheless, for HCNG30, a CR of 11.5 improved thermal efficiency by 6.5% and decreased NO_x emission by over 75%, as compared to a conventional CNG engine.     

A review of ammonia as a compression ignition engine fuel

During the past decades, the diesel engine has been through times of upheaval, boom and bust. At the beginning of the century, almost 50% of the new vehicle registrations in the European market were diesel-powered. However, the news of deadly diesel NO_x emissions supported by the diesel emission scandals caused a shock to the diesel engine market, and the sustainability of the diesel engine is currently in dispute.Recently major automotive manufacturers announced the development of diesel-powered vehicles with negligible NO_x emissions. Moreover, the NO_x emissions production is of lower concern for heavy-duty, marine or power generations applications where the implementation of advanced aftertreatment systems is feasible. However, despite the tackle of NO_x emissions, the decarbonisation of the automotive, marine and power generation markets is mandatory for meeting greenhouse gas emissions targets and limiting global warming.The decarbonisation of the diesel engine can be achieved by the implementation of a carbon-free fuel such as ammonia. This paper provides a detailed overview of ammonia as a fuel for compression ignition engines. Ammonia can be combusted with diesel or any other lower autoignition temperature fuel in dual-fuel mode and lead to a significant reduction of carbon-based emissions. The development of advanced injection strategies can contribute to enhanced performance and overall emissions improvement. However, ammonia dual-fuel combustion currently suffers from relatively high unburned ammonia and NO_x emissions because of the fuel-bound nitrogen. Therefore, the implementation of aftertreatment systems is required. Hence, ammonia as a compression ignition fuel can be currently seen as a feasible solution only for marine, power generation and possibly heavy-duty applications where no significant space constraints exist.     

A chemical mechanism for low to high temperature oxidation of n-dodecane as a component of transportation fuel surrogates

Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species [G. Blanquart, P. Pepiot-Desjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607], and several substituted aromatic species [K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898], ideally suited as a base to model surrogates, has now been extended to describe the oxidation of n-dodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of n-dodecane is extracted from the detailed scheme of Sarathy et al. [S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2-methylalkanes from C₇ to C₂₀, Combust. Flame 158 (12) (2011) 2338–2357] and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.     

Characterization of ringing intensity in a hydrogen-fueled HCCI engine

Homogeneous charge compression ignition (HCCI) is a promising alternative combustion strategy having higher thermal efficiency while maintaining the NO_x and soot emissions below the current emissions mandates. The HCCI combustion engine has typically lower operating load range in comparison to conventional engines. The HCCI combustion is constrained by various operational limits such as combustion instability limit, combustion noise limits, emission limits and peak cylinder pressure limit. High load limit of HCCI combustion is typically limited by very high heat release rate, which leads to ringing operation. Intense ringing operation leads to very high combustion noise, and heavy ringing operation can also damage the engine parts. Thus, it is important to investigate the characteristics of ringing intensity (RI) in HCCI engine. Hydrogen fueled HCCI engine combines the potential advantages of alternative fuel as well as the alternative combustion strategy. This study presents the RI characterization and prediction using chemical kinetics and artificial neural network (ANN) for hydrogen-HCCI operation. In the first part of the study, the effect of equivalence ratio (φ), inlet temperature (T_ivc), and engine speed on ringing intensity is investigated using chemical kinetics model. Based on ringing operation characteristics of hydrogen HCCI engine, ANN model is used to predict the ringing intensity (RI) for different engine operating conditions (i.e., φ T_ivc, engine speed) and different combustion parameters. The result indicates that RI increases with advanced combustion phasing (CA₅₀), higher inlet temperature, and equivalence ratio. To control the ringing operation, the CA₅₀ position needs to be retarded by optimizing the T_ivc and φ. Maximum engine operating range is found for lower engine speed (i.e., 1000 rpm) and reduces with increase in the engine speed. The results showed that the RI is strongly correlated to the CA₅₀ position with a correlation coefficient of 0.99 at constant inlet temperature. The ANN results also show that ANN model predicts RI with sufficient accuracy. The ANN model predicts RI with engine operating conditions as well as combustion parameters with a correlation coefficient of 0.97 and 0.95 respectively.     

An analytic study of applying Miller cycle to reduce NOx emission from petrol engine

An analytic investigation of applying Miller cycle to reduce nitrogen oxides (NO_x) emissions from a petrol engine is carried out. The Miller cycle used in the investigation is a late intake valve closing version. A detailed thermodynamic analysis of the cycle is presented. A comparison of the characters of Miller cycle with Otto cycle is presented. From the results of thermodynamic analyses, it can be seen that the application of Miller cycle is able to reduce the compression pressure and temperature in the cylinder at the end of compression stroke. Therefore, it lowers down the combustion temperature and NO_x formation in engine cylinder. These results in a lower exhaust temperature and less NO_x emissions compared with that of Otto cycle. The analytic results also show that Miller cycle ratio is a main factor to influence the combustion temperature, and then the NO_x emissions and the exhaust temperature. The results from the analytic study are used to analyse and to compare with the previous experimental results. An empirical formula from the previous experimental results that showed the relation of NO_x emissions with the exhaust temperature at different engine speed is presented. The results from the study showed that the application of Miller cycle may reduce NO_x emissions from petrol engine.     

Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation)

The effects of biodiesel (rapeseed methyl ester, RME) and different diesel/RME blends on the diesel engine NO_x emissions, smoke, fuel consumption, engine efficiency, cylinder pressure and net heat release rate are analysed and presented. The combustion of RME as pure fuel or blended with diesel in an unmodified engine results in advanced combustion, reduced ignition delay and increased heat release rate in the initial uncontrolled premixed combustion phase. The increased in-cylinder pressure and temperature lead to increased NO_x emissions while the more advanced combustion assists in the reduction of smoke compared to pure diesel combustion. The lower calorific value of RME results in increased fuel consumption but the engine thermal efficiency is not affected significantly. When similar percentages (% by volume) of exhaust gas recirculation (EGR) are used in the cases of diesel and RME, NO_x emissions are reduced to similar values, but the smoke emissions are significantly lower in the case of RME. The retardation of the injection timing in the case of pure RME and 50/50 (by volume) blend with diesel results in further reduction of NO_x at a cost of small increases of smoke and fuel consumption.