CAD aTDC Research Articles

Combustion of liquid ammonia and diesel in a compression ignition engine operated in high-pressure dual fuel mode

This study investigates the characteristics of a compression ignition engine fuelled by ammonia and diesel in high-pressure dual fuel mode. The study focuses on the effect of injection strategies on engine performance and important emissions like NH3, NO/NO2 and N2O. The engine was operated with fixed intake air conditions and at an engine speed of 1500 rpm for all operating points, and liquid ammonia was directly injected at 180 bar using a GDI injector. The ammonia energy shares tested were 40 %, 50 % and 60 %. Various combustion modes were achieved by fixing the diesel injection timing at −15 CAD aTDC and varying the ammonia injection timing between -80 and 2.5 CAD aTDC. Injecting ammonia early (-80/-60 CAD aTDC) led to premixed-type combustion with very high ammonia slip (65 %). Delaying the ammonia injection to -30 CAD aTDC, i.e. 15 CAD before the diesel injection, yielded a very long ignition delay due to the strong influence of the ammonia injection on the diesel injection, resulting in premixed combustion of ammonia and diesel. Injecting ammonia simultaneously or after diesel did not affect the diesel ignition, yielding stable combustion where the ammonia burned during the mixing-controlled phase. Temporally overlapping injections of ammonia and diesel resulted in the highest combustion efficiency, lowest ammonia slip, highest NOx emissions, and lowest N2O emissions, i.e., the optimal cases found. For temporally overlapping ammonia and diesel injections, extending the ammonia injection to after the end of diesel injection was found to increase ammonia slip compared to injecting ammonia before diesel, due to the limited time for liquid ammonia to vaporise, mix and combust in the expansion stroke. The NOx and N2O emissions were found to have opposite trends, where the highest NOx and lowest N2O concentrations were achieved for the operating points with the highest combustion efficiency. Delaying the combustion phasing improved the combustion efficiency for the 40 % and 50 % ammonia energy share cases, while for the 60 % cases, it deteriorated, which can be explained by the injection of ammonia after the end of diesel injection for the 60 % ammonia energy share cases.

Numerical investigation on the use of Dimethyl Ether (DME) as an alternative fuel for compression-ignition engines

Dimethyl Ether (DME) is an oxygenated fuel that could favour the transition of the heavy-duty transportation sector to carbon neutrality thanks to its similarities in terms of thermophysical properties with diesel fuel, which will facilitate the retrofitting of existing architectures, and the possibility to achieve good trade-offs between NOx emissions, soot formation and overall combustion efficiency. The possibility of producing it from a multitude of carbon–neutral sources and the low hydrogen-to-carbon ratio would allow for an overall lower CO2 output, making an attractive option in limiting the global warming impact of the heavy-duty transportation sector. In the present work, a numerical analysis of the combustion process of DME is carried out. First, the numerical setup is validated against experimental data available for a constant volume vessel with an initial density of 14.8 kg/m3, discussing the capabilities of a chemistry-based combustion model using tabulated kinetics of homogeneous reactors: the Tabulated Well Mixed (TWM) model. Ignition delay times (IDT) are compared for a wide range of temperatures, from 750 K to 1100 K, and oxygen concentrations, from 15% to 21%. The same setup is then applied in the simulation of a heavy-duty internal combustion engine (ICE). A first validation was done to assess the performance of the numerical methodology in a traditional Mixing Controlled Compression Ignition (MCCI) scenario. Then, two other points were simulated: an MCCI condition with 35% of EGR and a Late-Premixed Charge Compression Ignition (L-PCCI) one, with 35% of EGR and an SOIe of 4 CAD aTDC. Local temperature distributions were compared, analyzing the effect of these technologies in NOx emission mitigation and their impact on gross indicated efficiency (ηg), showing the advantages that using DME can have on a real-world application.

Numerical Study on Hydrogen–Gasoline Dual-Fuel Spark Ignition Engine

Hydrogen, as a suitable and clean energy carrier, has been long considered a primary fuel or in combination with other conventional fuels such as gasoline and diesel. Since the density of hydrogen is very low, in port fuel-injection configuration, the engine’s volumetric efficiency reduces due to the replacement of hydrogen by intake air. Therefore, hydrogen direct in-cylinder injection (injection after the intake valve closes) can be a suitable solution for hydrogen utilization in spark ignition (SI) engines. In this study, the effects of hydrogen direct injection with different hydrogen energy shares (HES) on the performance and emissions characteristics of a gasoline port-injection SI engine are investigated based on reactive computational fluid dynamics. Three different injection timings of hydrogen together with five different HES are applied at low and full load on a hydrogen–gasoline dual-fuel SI engine. The results show that retarded hydrogen injection timing increases the concentration of hydrogen near the spark plug, resulting in areas with higher average temperatures, which led to NOX emission deterioration at −120 Crank angle degree After Top Dead Center (CAD aTDC) start of injection (SOI) compared to the other modes. At −120 CAD aTDC SOI for 50% HES, the amount of NOX was 26% higher than −140 CAD aTDC SOI. In the meanwhile, an advanced hydrogen injection timing formed a homogeneous mixture of hydrogen, which decreased the HC and soot concentration, so that −140 CAD aTDC SOI implied the lowest amount of HC and soot. Moreover, with the increase in the amount of HES, the concentrations of CO, CO2 and soot were reduced. Having the HES by 50% at −140 CAD aTDC SOI, the concentrations of particulate matter (PM), CO and CO2 were reduced by 96.3%, 90% and 46%, respectively. However, due to more complete combustion and an elevated combustion average temperature, the amount of NOX emission increased drastically.

Numerical investigation of the effect of injection timing on the in-cylinder activity of a gasoline direct injection engine

GDI (Gasoline direct injection) technology is used in downsized engines for its economy and low emissions. However, if the injection strategy is not set properly, GDI will produce more emissions than conventional gasoline engines. In this paper, the effect of injection timing on GDI engine emissions under optimal phasing conditions was analyzed by using a three-dimensional (3D) computational fluid dynamics (CFD) GDI engine numerical model. The results showed that at medium engine speed and medium load advancing the injection timing from −300 to −290 CAD ATDC resulted in a more efficient and cleaner combustion process, as evidenced by the higher power output, the increased thermal and combustion efficiencies, and the reduced CO, UHC, soot emissions. The raised NOx emissions at advanced injection timing operation corresponded to the high combustion quality. There was a trade-off relation for advancing injection timing strategy. Specifically, an advanced injection timing operation would increase the amount of liquid film formed on the piston and liner, which is not favorable to clean combustion. However, advancing injection timing also provides more time for fuel-air mixing, which is beneficial for the formation of a more homogeneous mixture. The numerical simulations suggested that the advantages of earlier injection timing outweighed the disadvantages and improved engine efficiency, at least for the conditions and the engine investigated here. Moreover, the comparison indicated that changing injection timing also altered the chemical reaction pathways for pollutant species formation. Overall, all of these findings demonstrated that more fundamental work is still needed to understand the effect of injection timing on engine performance.

A comparative study of gasoline skeletal mechanisms under partial fuel stratification conditions using large eddy simulations

Partial fuel stratification (PFS) is a low temperature combustion strategy that can alleviate high heat release rates of traditional low temperature combustion strategies by introducing compositional stratification in the combustion chamber using a split fuel injection strategy. In this study, a three-dimensional computational fluid dynamics (CFD) model with large eddy simulations and reduced detailed chemistry was used to model partial fuel stratification at three different stratified conditions. The double direct injection strategy injects 80% of the total fuel mass at −300 CAD aTDC and the remaining 20% of the fuel mass is injected at three different timings of −160, −50, −35 CAD to create low, medium, and high levels of compositional stratification, respectively. The PFS simulations were validated using experiments performed at Sandia National Laboratories on a single-cylinder research engine that operates on RD5-87, a research-grade E10 gasoline. The objective of this study is to compare the performance of three different reduced chemical kinetic mechanisms, namely SKM1, SKM2, and SKM3, at the three compositional stratification levels and identify the most suitable mechanism to reproduce the experimental data. Zero-dimensional chemical kinetic simulations were also performed to further understand differences in performance of the three reduced chemical kinetic mechanisms to explain variations in CFD derived heat release profiles. The modeling results indicate that SKM3 is the most suitable mechanism for partial fuel stratification modeling of research-grade gasoline. The results also show that the autoignition event progresses from the richer to the leaner compositional regions in the combustion chamber. Notably, the leaner regions that have less mass per unit volume, can contribute disproportionately more toward heat release as there are more cells at leaner equivalence ratio ranges. Overall, this study illuminates the underlying compositional stratification phenomena that control the heat release process in PFS combustion.

Detailed Injection Strategy Analysis of a Heavy-Duty Diesel Engine Running on Rape Methyl Ester

Using biodiesel fuel in diesel engines for heavy-duty transport is important to meet the stringent emission regulations. Biodiesel is an oxygenated fuel and its physical and chemical properties are close to diesel fuel, yet there is still a need to analyze and tune the fuel injection parameters to optimize the combustion process and emissions. A four-injections strategy was used: two pilots, one main and one post injection. A highly advanced SOI decreases the NOx and the compression work but makes the combustion process less efficient. The pilot injection fuel mass influences the combustion only at injection close to the top dead center during the compression stroke. The post injection has no influence on the compression work, only on the emissions and the indicated work. An optimal injection strategy was found to be: pilot SOI 19.2 CAD BTDC, pilot injection fuel mass 25.4%; main SOI 3.7 CAD BTDC, main injection fuel mass 67.3% mg; post SOI 2 CAD ATDC, post injection fuel mass 7.3% (the injection fuel mass is given as a percentage of the total fuel mass injected). This allows the indicated work near the base case level to be maintained, the pressure rise rate to decrease by 20% and NOx emissions to decrease by 10%, but leads to a 5% increase in PM emissions.

Statistical study on engine knock oscillation and heat release using multiple spark plugs and pressure sensors

Engine knock has been an inherent problem that limits engine efficiency improvement and threatens engine service life. To trigger the controllable knock, a specialized liner with installing four side spark plugs mounted on the cylinder head are used in this study to produce various in-cylinder flame propagation. Various spark strategies (e.g., spark timing, spark number, spark location) are applied to generate different auto-ignition sites and knock characteristics. Up to five channels of pressure signal are collected to analyze the knock intensities regarding different spark strategies. To investigate the knock-induced fluctuations and heat release, we use the multiple correlations to evaluate the maximum amplitude of pressure oscillation (MAPO) respecting different influential factors and building a multiple linear regression (MLR) model for MAPO prediction and validate it against the experiment results. Besides, the Wiebe function and curve-fitting techniques are applied to estimate the energy released by auto-ignition in each cycle, and its relations with knock intensity are assessed. The results show that the average growth of pre-oscillation plays an important role in MAPO value (correlation coefficient, r_s = 0.965), the established MLR model possesses high accuracy for MAPO prediction. The pressure oscillation regarding the 1st resonance mode is extracted from the band-pass filtered signal, representing the main pressure oscillating process. Compared with the single spark ignition, triggering more spark plugs could boost the fuel consumption rate, while shortening the knocking combustion duration and reducing the heat release fraction contributed by auto-ignition. At the same CA50 (9 CAD aTDC), the four spark ignition leads to the least heat release fluctuations than other spark strategies. With advancing the CA50, the heat release fraction produced by knock increases at first then reduces due to the thermodynamic conditions and the flame propagation. Activating two or three spark plugs, the rising rate of the heat release rate goes up, but the knock-induced heat release fraction decreases. Moreover, the primary acoustic resonance mode (1, 0) gives more regular knock vibrations than the mixing form (including mode (1, 0), (0, 1), (2, 0), etc.), the average traveling distance of acoustic wave in the time gap of receiving two continuous signals is very close to the cylinder bore. The results give insights into the pressure wave propagation during knock and the relations between knock conditions, pressure oscillations, and heat release.

In-Cylinder Polycyclic Aromatic Hydrocarbons Sampled during Diesel Engine Combustion.

Polycyclic aromatic hydrocarbons (PAHs) are potentially carcinogenic pollutants emitted by diesel engines, both in the gas phase and adsorbed onto the surface of particulate matter (PM). There remains limited understanding of the complex and dynamic competing mechanisms of PAH formation, growth and oxidation in the gas phase, and their adsorption onto soot and how these processes impact on the abundance and composition of exhaust PAH. Therefore, this paper presents analysis of gas and particulate samples taken from the cylinder and exhaust of a diesel engine during combustion of fossil diesel with the 16 US-EPA priority PAH species identified and quantified. In-cylinder results showed that gas-phase PAHs were more abundant than soot-bound PAHs in the engine cylinder. The in-cylinder PAHs included 2- to 6-ring PAHs; however, 6-ring PAHs were not observed in the soot samples collected from the engine exhaust. Levels of both PM and the total in-cylinder PAHs decreased following a peak at 10 CAD ATDC but subsequently increased significantly during the late combustion phase. The B[a]P equivalence of PM in the engine cylinder increased during the period of early diffusion to late combustion phase, following an initial decrease during the period of premixed to early diffusion combustion.

Effects of pilot diesel injection strategies on combustion and emission characteristics of dual-fuel engines at part load conditions

The high level of UHC and CO emissions and poor performance are the Achilles heel of the dual-fuel engines at part load condition. In this research, the KIVA-3V code has been used to simulate, and analyze the combustion process of a dual-fuel diesel engine at part loads. To resolve these problems, the effects of three injection strategies including single and split injection timing, and the number of injector nozzle holes have been investigated on engine combustion and emission characteristics. Results indicate that by advancing the baseline injection timing of the diesel fuel from −17 to −70 CAD ATDC, the maximum pressure value increases from 46 bars to 84.2 bars, UHC and CO emissions decrease 98% and 99%, respectively, and NOx emissions increase 339%. In the split injection, the optimum injection timings of pilot diesel fuel are −80 and −60 CAD ATDC for the first and second pulses. In addition, UHC and CO emissions decrease 98% and 99.9%, respectively. Finally, in the third strategy, with increasing injector nozzle holes, ignition and combustion start earlier, and the maximum pressure value, NOx and CO emissions increase, but UHC emissions decrease.

Effect of Compression Ratio on the Performance and Emission Characteristics, and Cycle-to-Cycle Combustion variations of a Spark-Ignition Engine Fueled with Bio-methane Surrogate

Low-carbon fuels such as bio-methane, obtained from biomass gasification consisting 96–98% of methane by volume, is a promising alternative fuel for internal-combustion engines. In the present work, methane is used as a surrogate of bio-methane for studying the engine characteristics. First, a comparative study was performed on a single-cylinder, water-cooled, variable compression ratio, spark-ignition engine operating under methane and gasoline fuels with compression ratio (CR) of 8.5:1. Subsequently, the effect of compression ratio was investigated on the performance, combustion including cycle-to-cycle combustion variations through statistical analysis, and emissions, when engine was operated over a wide load range at 8.5:1, 10:1 and 12:1 CRs under methane. It was found that the brake specific energy consumption and emissions were reduced for methane compared to gasoline at all operating loads. Results showed that when the CR was increased from 8.5:1 to 12:1 at low operating load of 5 N-m, brake specific fuel consumption was reduced by 7.2% and the average location of peak in-cylinder pressure (θpmax) was shifted towards the TDC, i.e. 17.4 to 13.5 CAD ATDC, with a decrease in cycle-to-cycle combustion variations. In addition, θpmax showed linear and quadratic relationships with peak in-cylinder pressure and IMEP, respectively.

Homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) in compression ignition engine with low octane gasoline

The present study investigated the in-cylinder combustion for low octane 70 primary reference fuel (PRF70) by the method of the flame index during the transition from homogeneous charge compression ignition (HCCI) combustion to partially premixed combustion (PPC). Full cycle engine simulations were performed using CONVERGE™, coupled with chemical kinetics. Good agreements between the simulations and experiments were achieved at HCCI and PPC combustion modes. The fully premixed HCCI mode was achieved at the earliest injection timing of −180 CAD aTDC with the combustion temperature below 1600 K, where the formation of soot and NOx can be successfully avoided. For the injection timing of −100 CAD aTDC, the premixed charge compression ignition (PCCI) was achieved where the premixed combustion clouds were mainly distributed in the piston top-land zone and were surrounded by the diffused combustion that occurs in the piston bowl and the periphery of piston top. Less premixed flames were formed in piston top and surrounded by more diffusion mixtures at PPC mode. The in-cylinder HO2 evolution profile displayed two bumps which were distributed in low temperature zone and high temperature zone respectively. The spatial and temporal evolution of HO2 is very similar to the distribution of premixed flames.

In-Cylinder Combustion and Soot Evolution in the Transition from Conventional Compression Ignition (CI) Mode to Partially Premixed Combustion (PPC) Mode

The present study intends to explore the in-cylinder combustion and evolution of soot emission during the transition from conventional compression ignition (CI) combustion to partially premixed combustion (PPC) under low load conditions. In-cylinder combustion images and engine-out emissions were measured in an optical engine fueled with low octane heavy naphtha fuel (RON = 50). Full cycle engine simulations were performed using a three-dimensional computational fluid dynamics code CONVERGE, coupled with gas-phase chemical kinetics, turbulence, and a particulate size mimic soot model. The simulations were performed under low load conditions (indicated mean effective pressure (IMEP) of ∼2–3 bar) at an engine speed of 1200 rpm. The start of injection (SOI) was advanced from late (−10 CAD aTDC) to early fuel injection timings (−40 CAD aTDC) to realize the combustion transition from CI combustion to PPC. The simulation results of combustion and emission are compared with the experimental results in both CI an...

Numerical investigation of injector geometry effects on fuel stratification in a GCI engine

Injectors play an important role in direct injection (DI) gasoline compression ignition (GCI) engines by affecting the in-cylinder mixture formation and stratification, which in turn impacts combustion and emissions. In this work, the effects of two different injector geometries, a 7-hole solid-cone injector and an outwardly opening hollow-cone injector, on fuel mixture stratification in a GCI engine were investigated by computational simulations. Three fuels with similar autoignition kinetics, but with different physical properties, were studied to isolate the effect of the combustion chemistry on combustion phasing. In addition, start of injection (SOI) sweeps relevant to low-load engine operating conditions were performed. The results show that physical properties of the fuel do not have significant influence when using a hollow-cone injector. Richer mixtures were observed at all the studied SOI (−40 to −14 CAD aTDC) cases, which can be attributed to the nature of the hollow cone spray. At later SOIs (−18 and −14 CAD aTDC), the richer mixtures are accompanied by lower mean in-cylinder temperature due to the charge cooling effect, which surpasses the equivalence ratio effect. The effect of fuel physical properties on combustion phasing was evident in multi-hole injection cases, which can be attributed to the differences in mixture stratification and equivalence ratio distribution at the time of ignition.

Numerical simulation of combustion and soot under partially premixed combustion of low-octane gasoline

In-cylinder combustion visualization and engine-out soot particle emissions were investigated in an optical diesel engine fueled with low octane gasoline. Single injection strategy with an early injection timing (−30 CAD aTDC) was employed to achieve partially premixed combustion (PPC) condition. A high-speed color camera was used to record the combustion images for 150 cycles. The regulated emission of carbon dioxide, carbon monoxide, nitrogen oxides and soot mass concentration were measured experimentally. Full cycle engine simulations were performed using CONVERGE™ and the simulation results matched with the experimental results. The in-cylinder soot particle evolution was performed by coupling a reduced toluene reference fuel mechanism including the PAHs formation/oxidation reactions with particulate size mimic model. The results showed that PPC presents typical stratified combustion characteristics, which is significantly different from the conventional diesel spray-driven combustion. The in-cylinder temperature and equivalence ratio overlaid with soot-NOX formation regime revealed that PPC operating condition under study mostly avoided the main sooting conditions throughout the entire combustion. The evaluation of temperature distribution showed formaldehyde could be regarded as an indicator for low temperature reactions, while hydroxyl group represents the high temperature reactions. Soot evolution happened during the combustion process, hydroxyl radicals promoted the soot oxidation.

Experimental investigation of methanol compression ignition in a high compression ratio HD engine using a Box-Behnken design

Methanol is an alternative fuel offering a lower well-to-wheel CO2 emission as well as a higher efficiency, given that the fuel is derived from biomass. In addition to reduced CO2, methanol does not emit soot particles when combusted which is a great advantage when attempting to reduce NOX levels due to the effectively non-existing NOX-soot trade-off. The engine setup used was a Scania D13 engine modified to run on one cylinder, utilizing a high compression piston with a rc of 27:1. This study analyzes the effects of four control parameters on gross indicated efficiency and the indicated specific emissions; CO, THC and NOX. The control parameters chosen in this work was common rail pressure (PRAIL), EGR, λ and CA50, running at 6bar IMEPG and 1200rpm. The effects of the control parameters on performance and emissions was analyzed using a surface response method of the Box-Behnken type. Predictive mathematical models were obtained from regression analysis performed on the responses from the experiments. The highest gross indicated efficiency achieved was ∼53%, when a high level of EGR was applied together with the combustion phasing set to its low level at CA50=6 CAD ATDC. The control parameters influencing the CO emissions are λ and the interaction between PRAIL and λ, while THC is only controlled by PRAIL and EGR. NOX emissions was, as expected, influenced mainly by EGR and λ, although PRAIL and CA50 also had minor effects. The effect of increased PRAIL, increased THC emissions which in its turn reduced the gross indicated efficiency. Throughout the experiment, THC concentration never decreased below ∼150ppm due to utilization of high rc in combination with the volatility of methanol. It was also concluded that a rc=27 is rather high if operation flexibility is required, especially at the higher load range.

Real-time modeling of ringing in HCCI engines using artificial neural networks

Intense ringing operation is one of the major drawbacks associated with homogeneous charge compression ignition (HCCI) engines at high load conditions that limits HCCI operation range and can damage engine parts. This study uses HCCI experimental data to investigate combustion noise and ringing operation in a 0.3 L converted-diesel HCCI engine. A novel method is utilized to operate the HCCI engine in the ringing region by periodic variation in injected fuel amounts. Combustion noise level (CNL) is investigated along with main HCCI combustion parameters and emissions. A strong correlation is found between the CNL and variation of in-cylinder pressure at 10, 15, 20 CAD aTDC (P10, P15 and P20) and maximum in-cylinder pressure (Pmax). These experimental findings are then used to design an artificial neural network (ANN) model to predict CNL for identifying normal and ringing regions. A large amount of experimental data at cyclic and steady-state operating conditions are used to evaluate the ANN noise level (ANL) model. The results indicate that the real-time ANL model can predict CNL with less than 0.5% error for the HCCI engine. The ANL model is of utility to identify engine operating limits to avoid the ringing operation.

An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, [Formula: see text] and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

Physical and chemical effects of low octane gasoline fuels on compression ignition combustion

Gasoline compression ignition (GCI) engines running on low octane gasoline fuels are considered an attractive alternative to traditional spark ignition engines. In this study, three fuels with different chemical and physical characteristics have been investigated in single cylinder engine running in GCI combustion mode at part-load conditions both experimentally and numerically. The studied fuels are: Saudi Aramco light naphtha (SALN) (Research octane number (RON)=62 and final boiling point (FBP)=91°C), Haltermann straight run naphtha (HSRN) (RON=60 and FBP=140°C) and a primary reference fuel (PRF65) (RON=65 and FBP=99°C). Injection sweeps, where the start of injection (SOI) is changed between −60 and −11 CAD aTDC, have been performed for the three fuels. Full cycle computational fluid dynamics (CFD) simulations were executed using PRFs as chemical surrogates for the naphtha fuels. Physical surrogates based on the evaporation characteristics of the naphtha streams have been developed and their properties have been implemented in the engine simulations. It was found that the three fuels have similar combustion phasings and emissions at the conditions tested in this work with minor differences at SOI earlier than −30 CAD aTDC. These trends were successfully reproduced by the CFD calculations. The chemical and physical effects were further investigated numerically. It was found that the physical characteristics of the fuel significantly affect the combustion for injections earlier than −30 CAD aTDC because of the low evaporation rates of the fuel because of the higher boiling temperature of the fuel and the colder in-cylinder air during injection.

Numerical comparison of PCCI combustion and emission of diesel and biodiesel fuels at low load conditions using 3D-CFD models coupled with chemical kinetics

LTC (Low Temperature Combustion) has demonstrated abilities to lower diesel engine emissions while achievement of better fuel economy in comparison with conventional combustion. One of the major LTC methods is PCCI (Premixed Charge Compression Ignition) combustion strategy in which high levels of EGR (exhaust gas recirculation) is utilized to reduce overall combustion temperatures and to lengthen the ignition delay. Increase in ignition delay provides time for fuel evaporation and reduces in-homogeneities in the reactant mixture, thus NOx formation from local temperature spikes and Soot formation from locally rich mixtures can be reduced. However, as dilution is increased to the limits, Soot, HC and CO can significantly increase. One of the ways to control these emissions in PCCI combustion is utilizing biodiesel fuel due to its different physical and chemical properties compared to petroleum-based diesel fuel. Hence, in the present work, a numerical study is performed to compare the combustion, performance and emission characteristics of neat biodiesel and diesel fuel surrogates under the condition of PCCI combustion mode in a light-duty, single-cylinder diesel engine by KIVA-CHEMKIN code. The operating conditions are at the engine speed of 2000rpm and load of 5.5bar IMEP. Simulations are capable of capturing, not only the bulk combustion characteristics, as well as the first and second-stage combustion processes, but also the details of the important reactive species and emission formation. For both diesel and biodiesel fuels, it has been tried to find optimum cases which are only based on sweep of injection timing and do the comparisons at those optimum states. The results indicate that for both diesel and biodiesel fuels, −30 CAD ATDC SOI timing can be regarded as an optimum SOI timing for the lowest NOx+THC and CO emissions. For the optimum case, NOx, CO, THC, thermal efficiency and ISFC for the biodiesel case are lowered by 13%, 27%, 12%, 9.5% and increased by 6%, respectively compared to the diesel case.

Identification of ringing operation for low temperature combustion engines

High-efficiency and low-emission low temperature combustion modes including homogeneous charge compression ignition (HCCI) are limited at high load conditions due to rapid pressure rise rate, short combustion duration and ringing operation. This study uses two different HCCI engines to investigate combustion-generated ringing at a number of HCCI engine operating points between misfire and ringing zones for ethanol and n-heptane fuels. Ringing intensity (RI) is investigated along with main HCCI combustion parameters and engine-out emissions. The results show the RI generally increases by advancing crank angle of 50% fuel burnt (CA50) and also decreasing burn duration (BD). It is found that adjusting CA50 can provide a control knob for the RI since all the extreme noisy data points have CA50<9 CAD aTDC. In-cylinder pressure at 5, 10, 15 CAD aTDC (P5,P10 and P15) and maximum in-cylinder pressure (Pmax) show strong correlation with RI. To this end, P5,P10 and P15 and Pmax are used to develop an artificial neural network (ANN) model to predict RI. Experimental data at 155 steady-state points are used to evaluate the ANN model for two totally different HCCI engines running with high and low octane fuels. The validation results indicate that the ANN model can predict RI with less than 4.2% error. The ANN model can be used to identify HCCI ringing operation for combustion control applications.