In-cylinder Pressure History Research Articles

Optical diagnostics and chemical kinetic analysis on partially premixed combustion characteristics fueled with methanol and various cetane improvers

The need for decarbonization has inspired the utilization of methanol in transportation industry. Via both experimental and modeling frameworks, this study, for the first time, reveals the ignition, heat release and flame development characteristics of methanol mixed with different cetane improvers, i.e., 2-ethylhexyl nitrate (EHN) and di-tertiary‑butyl peroxide (DTBP), under practical, advanced engine conditions. Particularly, a detailed chemical kinetic model is newly developed for methanol/EHN/DTBP mixtures, incorporating the most updated sub-chemistries for relevant fuel components and critical intermediates, to assist in further analysis on the effects of EHN and DTBP on ignition and emission formation under engine thermodynamic environments derived from recorded in-cylinder pressure histories. Results show that stable combustion of methanol/EHN mixtures can be realized under low direct injection (DI) pressure, high in-cylinder thermodynamic condition, featuring the lowest coefficient of variation of indicated mean effective pressure (COVIMEP) of 1.46 %. Throughout the combustion process, the yellow and white flame dominates. At the same DI timing, methanol/EHN mixtures present shorter ignition delay, faster heat release rate (HRR) and more advanced combustion phasing compared with methanol/DTBP mixtures. At similar combustion phasing, the peak flame/OH* natural luminosity intensity of methanol/EHN mixtures is higher. KL factor, which is utilized to evaluate soot concentration, also shows similar trend, indicating higher sooting tendency of methanol/EHN mixtures. Flux analysis further highlights higher production of soot precursor (i.e., C2H2) with EHN than DTBP, primarily via the pathway of C2H4→ C2H3→C2H2. The active atmosphere created by the decomposition of EHN or DTBP has significant effects on decreasing methanol ignition delay in the early stage of their exothermic process, while the active atmosphere of EHN remains impactful whereas that of DTBP diminishes at later stages. The thermal atmosphere shows greater effects on methanol ignition at later stages of the exothermic process of EHN or DTBP decomposition than the early stages.

Deep Learning for Knock Occurrence Prediction in SI Engines

This research aims to predict knock occurrences by deep learning using in-cylinder pressure history from experiments and to elucidate the period in pressure history that is most important for knock prediction. Supervised deep learning was conducted using in-cylinder pressure history as an input and the presence or absence of knock in each cycle as a label. The learning process was conducted with and without cost-sensitive approaches to examine the influence of an imbalance in the numbers of knock and non-knock cycles. Without the cost-sensitive approach, the prediction accuracy exceeded 90% and both the precision and the recall were about 70%. In addition, the trade-off between precision and recall could be controlled by adjusting the weights of knock and non-knock cycles in the cost-sensitive approach. Meanwhile, it was found that including the pressure history of the previous cycle did not influence the classification accuracy, suggesting little relationship between the combustion behavior of the previous cycle and knock occurrence in the following cycle. Moreover, learning the pressure history up to 10° CA before a knock improved the classification accuracy, whereas learning it within 10° CA before a knock did not noticeably affect the accuracy. Finally, deep learning was conducted using data, including multiple operating conditions. The present study revealed that deep learning can effectively predict knock occurrences using in-cylinder pressure history.

Improved knock detection method for lean burn SI engines

The purpose of this study was to improve the accuracy of knock detection for lean burn SI engines. In this study, we conducted an experiment to observe autoignition in an engine cylinder using a borescope and a high-speed camera under an air excess ratio of 2.1. In addition, the resonance frequency of each vibration mode by knocking and the knock intensity were also calculated from the in-cylinder pressure history. Autoignition was optically observed in all cycles with a knock intensity of more than 20 kPa or more, and in some cycles with that of less than 20 kPa. We compared the vibration caused by combustion and it caused by autoignition. As a result, by distinguishing between the vibration caused by combustion and it caused by autoignition using window functions, a knocking cycle that could not be detected by the conventional index was detected.

Effect of compression ratio, nozzle opening pressure, engine load, and butanol addition on nanoparticle emissions from a non-road diesel engine.

Currently, diesel engines are more preferred over gasoline engines due to their higher torque output and fuel economy. However, diesel engines confront major challenge of meeting the future stringent emission norms (especially soot particle emissions) while maintaining the same fuel economy. In this study, nanosize range soot particle emission characteristics of a stationary (non-road) diesel engine have been experimentally investigated. Experiments are conducted at a constant speed of 1500rpm for three compression ratios and nozzle opening pressures at different engine loads. In-cylinder pressure history for 2000 consecutive engine cycles is recorded and averaged data is used for analysis of combustion characteristics. An electrical mobility-based fast particle sizer is used for analyzing particle size and mass distributions of engine exhaust particles at different test conditions. Soot particle distribution from 5 to 1000nm was recorded. Results show that total particle concentration decreases with an increase in engine operating loads. Moreover, the addition of butanol in the diesel fuel leads to the reduction in soot particle concentration. Regression analysis was also conducted to derive a correlation between combustion parameters and particle number emissions for different compression ratios. Regression analysis shows a strong correlation between cylinder pressure-based combustion parameters and particle number emission.

The Effect of Humidity on the Knock Behavior in a Medium BMEP Lean-Burn High-Speed Gas Engine

The effects of air humidity on the knock characteristics of fuels are investigated in a lean-burn, high-speed medium BMEP engine fueled with a CH4 + 4.7 mole% C3H8 gas mixture. Experiments are carried out with humidity ratios ranging from 4.3 to 11 g H2O/kg dry air. The measured pressure profiles at non-knocking conditions are compared with calculated pressure profiles using a model that predicts the time-dependent in-cylinder conditions (P, T) in the test engine (“combustion phasing”). This model was extended to include the effects of humidity. The results show that the extended model accurately computes the in-cylinder pressure history when varying the water fraction in air. Increasing the water vapor content in air decreases the peak pressure and temperature significantly, which increases the measured Knock Limited Spark Timing (KLST); at 4.3 g H2O/kg dry air the KLST is 19 °CA BTDC while at 11 g H2O/kg dry air the KLST is 21 °CA BTDC for the same fuel. Excellent agreement is observed between the calculated knock resistance (using the Propane Knock Index, PKI) and the measured knock resistance (KLST) for the range in water content in air studied in this work. Since the effect of water on autoignition delay time is negligible, the observed increase in knock resistance of the fuel-air mixture is due a decrease in pressure and temperature of the end gas with increasing water content in as a result of changes in the mass burning rate, and thermophysical properties of the fuel-air mixture.

Characterizing Gaseous Fuels for Their Knock Resistance based on the Chemical and Physical Properties of the Fuel

A method is described to characterize the effects of changes in the composition of gaseous fuels on engine knock by computing the autoignition process during the compression and burn periods of the engine cycle. To account for the effects of fuel composition on the in-cylinder pressure and temperature history relevant for knocking, changes in heat capacity of the air-fuel mixture and in the phasing of the combustion process are also incorporated in the method. Comparison between pressure profiles measured in a lean-burn, high-speed medium-BMEP gas engine and the calculated pressure profiles at non-knocking conditions shows that the method accurately computes the in-cylinder pressure history when varying the fuel composition. To characterize gaseous fuels for their resistance to knock, a propane-based scale (Propane Knock Index, PKI) is reported in this study. On this scale, the knock resistance for a given gaseous fuel mixture is expressed as an equivalent fraction of propane in methane under identical engine conditions. The chemical mechanism used to compute the autoignition delay time is optimized based on RCM measurements performed at engine conditions. After optimization, the knock resistance as measured by the knock-limited spark timing (KLST) and calculated using PKI was found to agree within the uncertainty of the measurements (± 0.75 °CA) for all fuels studied, including hydrocarbon mixtures representative for liquefied natural gas (LNG) and natural gas/H2/CO mixtures. In contrast to the method reported here, comparison of KLST for the range of gases studied with methane numbers calculated using the AVL and the MWM methods points to shortcomings in these yardsticks.

Technical feasibility study of butanol–gasoline blends for powering medium-duty transportation spark ignition engine

Amongst primary alcohols, butanol is considered as the most promising alternative fuel candidate because of its favourable chemical and physical properties, which are quite similar to gasoline. Butanol is completely miscible with gasoline in any proportion and forms a stable blend. It is not hygroscopic in nature therefore does not absorb moisture from the atmosphere similar to ethanol and methanol, which makes it a superior alternate fuel. Experiments are conducted on 5, 10, 20, 50 and 75% butanol–gasoline blends for evaluating their engine performance, emissions and combustion characteristics in a medium-duty transportation spark ignition (SI) engine. The engine was suitably instrument for the experiments. Engine performance was evaluated by finding performance parameters such as brake specific fuel consumption (BSFC), power output and torque, thermal efficiency and exhaust gas temperature of butanol–gasoline blends vis-a-vis baseline gasoline. Regulated emissions were compared for butanol–gasoline blends vis-a-vis baseline gasoline and an attempt was made to find the reason for these variations. Combustion characteristics of butanol–gasoline blends were evaluated for parameters such as in-cylinder pressure history, heat release rate, rate of pressure rise, mass burn fractions and combustion duration. Overall, butanol–gasoline blends showed performance, emissions and combustion characteristics similar to gasoline.

Investigation on effect of equivalence ratio and engine speed on homogeneous charge compression ignition combustion using chemistry based CFD code

Combustion in a large-bore natural gas fuelled diesel engine operating under Homogeneous Charge Compression Ignition mode at various operating conditions is investigated in the present paper. Computational Fluid Dynamics model with integrated chemistry solver is utilized and methane is used as surrogate of natural gas fuel. Detailed chemical kinetics mechanism is used for simulation of methane combustion. The model results are validated using experimental data by Aceves, et al. (2000), conducted on the single cylinder Volvo TD100 engine operating at Homogeneous Charge Compression Ignition conditions. After verification of model predictions using in-cylinder pressure histories, the effect of varying equivalence ratio and engine speed on combustion parameters of the engine is studied. Results indicate that increasing engine speed provides shorter time for combustion at the same equivalence ratio such that at higher engine speeds, with constant equivalence ratio, combustion misfires. At lower engine speed, ignition delay is shortened and combustion advances. It was observed that increasing the equivalence ratio retards the combustion due to compressive heating effect in one of the test cases at lower initial pressure. Peak pressure magnitude is increased at higher equivalence ratios due to higher energy input.

Investigation of Boundary Condition and Field Distribution Effects on the Cycle-to-Cycle Variability of a Turbocharged GDI Engine Using LES

The paper reports some preliminary results of a numerical activity aiming at characterizing cycle-to-cycle variability of a highlydownsized turbocharged DISI (Direct Injection Spark Ignition) engine for high-performance car applications, using a customized version of the commercial software Star-CD licensed by CD-adapco. During experimental investigations at the engine testbed, high cycle to cycle dispersion was detected even for relatively stable peak-power/full-load operations of the engine, thus limiting the overall engine performance. Despite the complex architecture of the V-8 engine, the origin of such cyclic variability could not be related to cyclic fluctuations of the gas-dynamics within the intake and exhaust pipes. Several subsequent acquisitions of the instantaneous pressure traces were measured at both the intake port entrance and exhaust port junction, showing almost negligible differences in terms of both amplitude and phasing compared to those within the cylinder. In order to explore the potentials of the LES application to the analysis and understanding of the cycle-to-cycle variability, which notoriously strongly limits the overall engine performance, a numerical activity is carried out using full-cycle LES simulations over several subsequent engine cycles. Despite the very early stage of the investigation, two main issues are addressed in the paper: the analysis of the possible causes originating the high cycle to cycle variability and the influence of the boundary conditions on the predicted cyclic dispersion. Concerning the former aspect, a detailed investigation of local and global instantaneous fields is carried out aiming at identifying both a possible hierarchy of responsibilities on one side and limitations and possible improvements of the adopted numerical procedure on the other side. Concerning the latter aspect, a set of simulations, performed applying cycle-independent averaged experimental conditions, is compared to those resulting from the application of cycle-specific variable pressure traces at both intake and exhaust sides, in order to analyze the influence of port fluctuations on the in-cylinder pressure history. Results are also qualitatively compared to those resulting from a multi-cycle RANS simulation using the same grid-size, in order to better highlight the superior LES potentials.

Numerical Study on the Hydrogen Fueled SI Engine Combustion Optimization through a Combined Operation of DI and PFI Strategies

As the practicability of a hydrogen-fueled economy emerges, intermediate technologies would be necessary for the transition between hydrocarbon fueled internal combustion engines and hydrogen powered fuel cells. In the present study, the hydrogen engine efficiency and the load control are the two main parameters that will be improved by using the combined operation of in-cylinder direct fuel injection (DI) and port fuel injection (PFI) strategies to obtain maximum engine power outputs with acceptable efficiency equivalent to gasoline engines. Wide open throttle (WOT) operation has been used to take advantage of the associated increase in engine efficiency, in which the loads have been regulated with mixture richness (qualitative control) instead of volumetric efficiency (quantitative control). The capabilities of a 3D-CFD code have been developed and employed to simulate the whole engine physicochemical process which includes the hydrogen injection through the intake manifold (PFI) and/or the hydrogen DI in the engine compression stroke. Conditions with simulated PFI, PFI + DI and DI have been analyzed to study the effects of mixture preparation behaviors on the hydrogen ignition and its flame propagation inside the engine combustion chamber. Numerically, the CFD code has been intensively validated against experimental engine data which provided remarkable agreement in terms of in-cylinder pressure history evaluation.

Effects of fuel constituents and injection timing on combustion and emission characteristics of a compression-ignition engine fueled with diesel-DMM blends

The effects of DMM addition and fuel injection timing on combustion characteristics, fuel efficiency and emissions of a compression-ignition engine fueled with diesel-dimethoxymethane (DMM) blends are investigated experimentally in this study. Three diesel-DMM blends with 15%, 30% and 50% volume fraction of DMM addition respectively are tested at different engine loads and engine speeds. Not only HC, CO, smoke and NOx emissions, but also particle-size distribution and number concentration in exhaust gas have been measured. According to the measured in-cylinder pressure history, the in-cylinder combustion process is promoted by using diesel-DMM blends and can be further improved with early fuel injection. We find that using diesel-DMM blends can improve thermal efficiency and is beneficial to the reduction of smoke and CO emissions as well as particle number of both nanoparticles and ultrafine particles in exhaust gas with slightly increased NOx emission. Both fuel efficiency and thermal efficiency are improved with advanced fuel injection timing. Advancing fuel injection timing reduces smoke emission and particle number at the cost of increased NOx emission. We find that early fuel injection can either increase or decrease nanoparticles in exhaust gas. When advancing fuel injection from 20 to 23 CA BTDC, the number of nanoparticles is reduced; the further advanced fuel injection timing from 23 to 26 CA BTDC produces more nanoparticles. In this study, the lowest nanoparticle number in exhaust gas was achieved by injecting diesel-DMM blends with 50% DMM addition at 23 CA BTDC.

Combustion and emissions characteristics of compression-ignition engine using dual ammonia-diesel fuel

This study investigated the combustion and emissions characteristics of a compression-ignition engine using a dual-fuel approach with ammonia and diesel fuel. Ammonia can be regarded as a hydrogen carrier and used as a fuel, and its combustion does not produce carbon dioxide. In this study, ammonia vapor was introduced into the intake manifold and diesel fuel was injected into the cylinder to initiate combustion. The test engine was a four-cylinder, turbocharged diesel engine with slight modifications to the intake manifold for ammonia induction. An ammonia fueling system was developed, and various combinations of ammonia and diesel fuel were successfully tested. One scheme was to use different combinations of ammonia and diesel fuel to achieve a constant engine power. The other was to use a small quantity of diesel fuel and vary the amount of ammonia to achieve variable engine power. Under the constant engine power operation, in order to achieve favorable fuel efficiency, the preferred operation range was to use 40–60% energy provided by diesel fuel in conjunction with 60–40% energy supplied by ammonia. Exhaust carbon monoxide and hydrocarbon emissions using the dual-fuel approach were generally higher than those of using pure diesel fuel to achieve the same power output, while NO x emissions varied with different fueling combinations. NO x emissions could be reduced if ammonia accounted for less than 40% of the total fuel energy due to the lower combustion temperature resulting in lower thermal NO x . If ammonia accounted for the majority of the fuel energy, NO x emissions increased significantly due to the fuel-bound nitrogen. On the other hand, soot emissions could be reduced significantly if a significant amount of ammonia was used due to the lack of carbon present in the combination of fuels. Despite the overall high ammonia conversion efficiency (nearly 100%), exhaust ammonia emissions ranged from 1000 to 3000 ppmV and further after-treatment will be required due to health concerns. On the other hand, the variable engine power operation resulted in relatively poor fuel efficiency and high exhaust ammonia emissions due to the lack of diesel energy to initiate effective combustion of the lean ammonia-air mixture. The in-cylinder pressure history was also analyzed, and results indicated that ignition delay increased with increasing amounts of ammonia due to its high resistance to autoignition. The peak cylinder pressure also decreased because of the lower combustion temperature of ammonia. It is recommended that further combustion optimization using direct ammonia/diesel injection strategies be performed to increase the combustion efficiency and reduce exhaust ammonia emissions.

Prediction of the combustion process and emission formation of a bi-fuel s.i. engine

A thermodynamic model is developed and validated for the prediction of the combustion process and pollutant formation in s.i. engines, fuelled by gasoline and compressed natural gas. Attention is focused on the main physical and chemical phenomena to allow a reliable evaluation of the burning rate and of the specie concentrations, including intermediates such as CO, O, H, and OH. A new correlation for laminar flame speed of methane–air mixtures is derived by interpolating more than 1000 different conditions at high pressure and temperature, computed by a detailed chemical approach. Successively an extended dissertation about the fundamental mechanisms which govern the pollutant formation in the turbulent premixed combustion which characterizes the s.i. engines is given. The conclusion of such analysis is the definition of a new reduced chemical scheme, based on the application of partial-equilibrium and steady-state assumptions for the radicals and the solution of a transport equation for each specie which is kinetically controlled. Finally the proposed schemes and formulations were embedded into the developed quasi-D model and into a CFD code, to simulate a s.i. engine fuelled by gasoline and CNG, allowing a deeper understanding of the reliability of the simplifications made in the quasi-dimensional model and a comprehensive investigation of several physical and chemical properties, whose experimental measurement is not usually available. Computed results were compared with the available experimental data of in-cylinder pressure histories and engine emissions for two different engine configurations.

Analysis of a natural gas fuelled homogeneous charge compression ignition engine with exhaust gas recirculation using a stochastic reactor model

Combustion and emissions formation in a Volvo TD 100 series diesel engine running in a homogeneous charge compression ignition (HCCI) mode and fuelled with natural gas is simulated and compared with measurements for both with and without external exhaust gas recirculation (EGR). A new stochastic approach is introduced to model the convective heat transfer, which accounts for fluctuations and fluid-wall interaction effects. This model is included in a partially stirred plug flow reactor (PaSPFR) approach, a stochastic reactor model (SRM), and is applied to study the effect of EGR on pressure, autoignition timing and emissions of CO and unburned hydrocarbons (HCs). The model accounts for temperature inhomogeneities and includes a detailed chemical mechanism to simulate the chemical reactions within the combustion chamber. Turbulent mixing is described by the interaction by exchange with the mean (IEM) model. A Monte Carlo method with a second-order time-splitting technique is employed to obtain the numerical solution. The model is validated by comparing the simulated in-cylinder pressure history and emissions with measurements taken from Christensen and Johansson (SAE Paper 982454). Excellent agreement is obtained between the peak pressure, ignition timing and CO and HC emissions predicted by the model and those obtained from the measurements for the non-EGR, 38 per cent EGR and 47 per cent EGR cases. A comparison between the pressure profiles for the cases studied reveals that the ignition timing and the peak pressure are dependent on the EGR. With EGR, the peak pressure reduces and the autoignition is delayed. The trend observed in the measured emissions with varying EGR is also predicted correctly by the model.

Fluctuation Analysis of Diesel Combustion Process by means of the Optical Fiber Thermometer. Comparison of Combustion Fluctuation between Conventional Injection and Pilot Injection.

In order to clarify the characteristics of combustion fluctuation in diesel engines quantitatively, the fluctuations of pressure and flame temperature were measured, and the root mean square value, the autocorrelation coefficient and the power spectrum density of the measured fluctuations were analyzed by comparing the cases with and without pilot injection. The following concluding remarks are obtained. (1) The frequency power spectrum of the in-cylinder pressure history is reduced markedly by pilot injection in two frequency ranges from 0.2 to 2.0 kHz and from 2 to 5 kHz. (2) The fluctuation of combustion pressure is dependent on the maximum rate of pressure rise, which is dependent on the ignition delay. (3) The maximum rms of the flame temperature fluctuation during diffusion combustion has a clear correlation with that of the pressure fluctuation in the initial combustion. (4) The fluctuation of the flame temperature during diffusion combustion has the characteristics of isotropic turbulence.