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Particulate matter (PM) emitted from compression ignition (CI) engines (diesel engines) has a detrimental effect on human health and the environment. The physical and chemical characteristics of PM emitted from CI-engines are influenced by engine operating conditions and fuel properties. The morphology, nanostructure, and chemical composition of PM affect its toxicity and interaction with the environment. From automotive industry perspective, these parameters influence the design of diesel particulate filters. This study presents a review of the physical and chemical characteristics of particulate emissions from the CI-engine. The present study commences with a brief description about the composition of PM emitted from CI-engine and the PM formation mechanism in CI-engine. Later on, the detailed review of PM's physical and chemical characteristics and the effect of engine operating parameters and alternative fuels on the particle number concentration, morphology, nano-structure, and oxidative reactivity of PM is presented. Online and offline methods of diesel particulate characterization and emerging chemical characterization techniques such as X-ray photoelectron spectroscopy and X-ray absorption fine structure (EXAFS) are also discussed briefly. Correlation between physical and chemical properties, and oxidative reactivity of PM is also discussed. It was found that engine operating parameters affect the physical and chemical properties of PM. Use of alternative fuels changes the diesel particulate morphology, nanostructure, and chemical composition which enhances the oxidative reactivity of PM. [ABSTRACT FROM AUTHOR]

Alcohol fuels, primarily ethanol and methanol, have emerged as one of the important alternatives for sustainable transportation and power generation applications, due to the overall lower carbon dioxide (CO2) emissions. The higher octane number of alcohol makes it suitable for spark ignition (SI) engines while lower blend ratios can be used for compression ignition (CI) engines as well. Since significant work and exploration have already been performed for the application of alcohol in SI engines, the present study is primarily focused on alcohol utilization in CI engines. This review majorly consists of three parts: first, a discussion on the physical and chemical properties of ethanol and methanol from the fuel perspective, second, combustion, and engine performance of CI engines fueled with alcohol and lastly, emissions characterization of alcohol as fuel. A summary of this review is provided which highlights the potential of alcohol utilization in the form of blend or under dual-fuel combustion modes, as well as neat alcohol fueled CI engine. Alcohol-fueled CI engine improves the soot-NOx trade-off characteristic in comparison to conventional diesel combustion, this fuel could be an enabler to meet future emissions regulations. Overall lower CO2 emissions (up to 15% lower compared to diesel) by utilizing alcohol as fuel make it suitable for sustainable transportation. [ABSTRACT FROM AUTHOR]

This work investigates the influence of fuel injection events on the nanoparticle emission characteristics of light-duty gasoline-diesel RCCI engine. The formation of nanoparticle emissions strongly depends on fuel injection events. The present study experimentally investigates the influence of diesel injection pressure, injection timing, and port-injected gasoline mass on the nanoparticles emitted from the gasoline-diesel RCCI engine. For this purpose, the engine is tested at different engine speeds and a fixed load of 1.5 bar BMEP without exhaust gas recirculation. The particle-size and number distribution (PSD) and total particle number (total PN) concentration are measured using a differential mobility spectrometer. The results depict that at higher diesel injection pressure (IP) operation, the peak of the NMP increases while the AMP peak decreases for neat diesel operation as well as RCCI engine. Nucleation, as well as accumulation mode particles, increases with advanced diesel injection timing in RCCI combustion. An increase in port fuel injected mass also leads to an increase in the total particle concentration and total unburned hydrocarbon (THC) emissions. [ABSTRACT FROM AUTHOR]

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 1500 rpm 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 1000 nm 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. [ABSTRACT FROM AUTHOR]

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 (CA 50 ), higher inlet temperature, and equivalence ratio. To control the ringing operation, the CA 50 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 CA 50 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. [ABSTRACT FROM AUTHOR]

Hydrogen is a potential alternative and renewable fuel for homogenous charge compression ignition (HCCI) engine to achieve higher efficiency and zero emissions of CO, unburned hydrocarbons as well as other greenhouse gases such as CO 2 and CH 4 . In this study, a detailed hydrogen oxidation mechanism with NO x was developed by incorporating additional species and NO x reactions to the existing hydrogen combustion mechanism (10 species and 40 reactions). The detailed hydrogen combustion mechanism used in this study consists of 39 species and 311 reactions. A reduced mechanism consisting 30 species and 253 reactions was also developed by using directed relation graph (DRG) method from detailed mechanism. Developed mechanisms were validated with experimental data by HCCI engine simulation using stochastic reactor model. Sensitivity analysis was performed to identify the most important reactions in hydrogen combustion and NO x formation in HCCI engine. Pathway analysis was also performed to analyze the important reaction pathways at different temperatures. Results revealed that H2 + HO2 [=] H + H2O2 and O2 + NNH [=] N2 + HO2 are the most significant reactions in the hydrogen HCCI combustion and NO x formation respectively. Detailed parametric study of HCCI combustion was conducted using developed chemical kinetic model. Numerical simulations are performed at different engine operating condition by varying engine speed (1000–3000 rpm), intake air temperature (380–460 K), and compression ratio (16–18) at different relative air fuel ratios (λ). The HCCI operating range was determined for different compression ratios and results show that operating range expands with increase in compression ratio. The effect of intake temperature, engine speed and equivalence ratio on cylinder pressure and heat release rate were investigated. Maximum thermal efficiency of 46% and maximum combustion efficiency of 98% was observed among all the test conditions. Parametric study of NO x emissions was also conducted and it was found that NO x emissions decrease exponentially from higher to lower engine loads. [ABSTRACT FROM AUTHOR]

Ethanol fuelled homogenous charge compression ignition engine offers a better alternative to tackle the problems of achieving higher engine efficiency and lower emissions using renewable fuel. Present study computationally investigates the HCCI operating range of ethanol at different compression ratios by varying inlet air temperature and engine speed using stochastic reactor model. A newly developed reduced ethanol oxidation mechanism with NO x having 47 species and 272 reactions is used for simulation. HCCI operating range for compression ratios 17, 19 and 21 are investigated and found to be increasing with compression ratio. Simulations are conducted for engine speeds ranging from 1000 to 3000 rpm at different intake temperatures (range 365–465 K). Parametric study of combustion and emission characteristics is conducted and engine maps are developed at most efficient inlet temperatures. HCCI operating range is defined using combustion efficiency (>85%) and maximum pressure rise rate (<5 MPa/ms). In HCCI operating range, higher efficiency is found at higher engine loads and lower engine speeds. Emission characteristics of species (NO x , N 2 O, CO, CH 4 , C 2 H 4 , C 2 H 6 , CH 3 CHO, and HCHO) found in significant amount is also analysed for ethanol fulled HCCI engine. Emission maps for different species are presented and discussed for wide range of speed and load conditions. Some of unregulated species such as aldehydes are emitted in significantly higher quantities from ethanol fuelled HCCI engine at higher load and speed conditions. [ABSTRACT FROM AUTHOR]

Ethanol is considered a potential biofuel for internal combustion engines. In this study, homogeneous charge compression ignition (HCCI) simulations of ethanol engine experiments were performed using stochastic reactor model (SRM). Detailed ethanol oxidation mechanism is developed by including NO x reaction in existing detailed oxidation mechanism with 57 species and 383 reactions. Detailed ethanol mechanism with NO x used in this study contains 76 species and 495 reactions. This mechanism was reduced by direct relation graph (DRG) method, which was validated with the experimental results. Existing Lu’s 40-species skeletal mechanism with NO formation were also compared with detailed and reduced mechanisms for predicting maximum cylinder pressure, maximum heat release rate and crank angle position of maximum cylinder pressure in HCCI engine. Reduced mechanism developed in this study exhibited the best resemblance with the experimental data. This reduced mechanism was also validated by measured engine cylinder pressure curves and measured ignition delays in constant volume reactors. The results showed that reduced mechanism is capable of predicting HCCI engine performance parameters with sufficient accuracy. Sensitivity analysis was conducted to determine the influential reactions in ethanol oxidation. Results also show that detailed and reduced mechanism was able to predict NO x emission in good agreement with the corresponding experimental data. [ABSTRACT FROM AUTHOR]

Abstract: The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for in reciprocating engines. The HCCI combustion engine offers significant benefits in terms of its high efficiency and ultra low emissions. In this investigation, port injection technique is used for preparing homogeneous charge. The combustion and emission characteristics of a HCCI engine fuelled with ethanol were investigated on a modified two-cylinder, four-stroke engine. The experiment is conducted with varying intake air temperature (120–150°C) and at different air–fuel ratios, for which stable HCCI combustion is achieved. In-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. In this study, effect of intake air temperature on combustion parameters, thermal efficiency, combustion efficiency and emissions in HCCI combustion engine is analyzed and discussed in detail. The experimental results indicate that the air–fuel ratio and intake air temperature have significant effect on the maximum in-cylinder pressure and its position, gas exchange efficiency, thermal efficiency, combustion efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operation points, NO x emissions are lower than 10ppm however HC and CO emissions are higher. [Copyright &y& Elsevier]

Abstract: Combustion in HCCI engines is a controlled auto ignition of well-mixed fuel, air and residual gas. Since onset of HCCI combustion depends on the auto ignition of fuel/air mixture, there is no direct control on the start of combustion process. Therefore, HCCI combustion becomes unstable rather easily, especially at lower and higher engine loads. In this study, cycle-to-cycle variations of a HCCI combustion engine fuelled with ethanol were investigated on a modified two-cylinder engine. Port injection technique is used for preparing homogeneous charge for HCCI combustion. The experiments were conducted at varying intake air temperatures and air–fuel ratios at constant engine speed of 1500rpm and P-θ diagram of 100 consecutive combustion cycles for each test conditions at steady state operation were recorded. Consequently, cycle-to-cycle variations of the main combustion parameters and performance parameters were analyzed. To evaluate the cycle-to-cycle variations of HCCI combustion parameters, coefficient of variation (COV) of every parameter were calculated for every engine operating condition. The critical optimum parameters that can be used to define HCCI operating ranges are ‘maximum rate of pressure rise’ and ‘COV of indicated mean effective pressure (IMEP)’. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanoparticle and carbonyl emission from methanol-diesel dual fuel was investigated. • Health risk assessment for nanoparticle and carbonyl emissions was conducted. • Methanol-diesel engines carbonyl emission has a higher cancer risk potential. • Diesel engine nanoparticles showed higher lung load and longer retention period. • At high loads, Methanol in diesel engines can reduce health risks significantly. Engine-emitted nanoparticles pose a threat to respiratory health, prompting the exploration of solutions like low carbon alternative fuels (methanol) and reactivity-controlled compression ignition (RCCI) engines. However, employing methanol in RCCI engines releases carcinogenic carbonyl compounds. The present study aims to experimentally investigate nanoparticle and carbonyl emissions from conventional diesel (CDC), gasoline-diesel RCCI (GD-RCCI) and methanol-diesel RCCI (MD-RCCI) engines. This study focuses on predicting lung loading caused by emitted nanoparticles and the potential cancer risk associated with carbonyl compounds. The investigation explores the impact of engine load (lower and medium) and fuel premixing ratio (r p) on particle and carbonyl emissions in RCCI engines. Findings reveal that MD-RCCI reduces total particle number (TPN) emissions by 60% compared to CDC. Results indicate that the concentration of nucleation mode particles (NMPs) decreases as engine load increases, while accumulation mode particles (AMPs) increase for both CDC and MD-RCCI combustion modes. With an increase in r p , the NMPs in the MD-RCCI and GD-RCCI engines are increased. Furthermore, the emission of HCHO and CH 3 CHO decreases with an increase in engine load. As the r p increases, HCHO and CH 3 CHO increase significantly. The forecast for lung loading reveals that nanoparticles emitted by CDC show a 30 % higher lung load compared to MD-RCCI. Additionally, as engine load increases, lung loading due to particles increases in CDC and decreases in MD-RCCI. In all test conditions, RCCI combustion demonstrates lower lung retention compared to CDC engine. The lung loading of particles of MD-RCCI decreases significantly with an increase in premixing ratio for a constant load as compared to GD-RCCI. The MD-RCCI engine shows higher cancer risk potential as compared to CDC and GD-RCCI engines. Operating the MD-RCCI engine at medium loads is advantageous for environmental and human health considerations. [ABSTRACT FROM AUTHOR]

• Determinism in a computationally cheap parameter is investigated for a CNG-diesel RCCI engine. • Conventional dual-fuel to RCCI mode shifting is coupled with the onset of periodic-2 bifurcations. • A combination of higher CNG mass and advanced diesel injection timing leads to a higher periodic (3 and 4) nature of combustion dynamics. • Deterministic features of location of maximum pressure and combustion phasing are comparable in the RCCI regime. • A strong correlation and phase synchronization between location of maximum pressure and combustion phasing in RCCI regime. This study investigates the changing deterministic features of the cycle-resolved location of maximum pressure and its correlation with combustion phasing for dynamical transitions in the combustion of a CNG-Diesel RCCI engine. The investigation is performed using nonlinear dynamical and chaotic methods such as return maps, recurrence, and cross recurrence plots. The experiments are performed on a single-cylinder automotive engine operated in RCCI mode with the aid of the development ECU. The experiments are conducted by running the engine at a fixed engine speed of 1500 rpm and a load of 3 bar BMEP. Diesel fuel is injected directly into the cylinder by following a double injection strategy using an equal amount of fuel in both the pilot and main injections. The effect of the start of main injection timing of diesel on combustion dynamics is investigated at two different masses of CNG. The predominance of deterministic periodic features is discovered in the cycle-resolved dynamics of the engine combustion during the RCCI regime. Results show that with advancement in diesel injection timing, the mode of combustion shifts from conventional dual fuel to RCCI, and this shift is coupled to the onset of noisy periodic-2 bifurcations., The periodic-2 behavior even transforms to periodic-3 and 4 with an increasing advancement in diesel injection timing for engines operating with a higher CNG mass. For most of the operating conditions, the deterministic features in the location of maximum pressure are comparable with that of combustion phasing. Recurrence and cross recurrence plots-based methodology advocates for the existence of strong correlations or at least a phase synchronization between the location of maximum pressure and combustion phasing when the engine operates in the RCCI regime, irrespective of diesel injection timing and amount of port-injected CNG fuel. The presence of similar deterministic features in the location of maximum pressure and combustion phasing and a strong relationship between these two at intermediate diesel injection timings in the RCCI regime for both the CNG masses makes this regime most suitable for using the location of maximum pressure as a feedback/controlled parameter for model predictive control of the engine. [ABSTRACT FROM AUTHOR]

• Nonlinear dynamical and chaotic methods used investigate the cyclic combustion dynamics of RCCI engine. • Symbol sequence analysis, return maps, recurrence plots (RPs), RQA, and 0–1 test used for analysis. • The dominance of deterministic characteristics are found in the cyclic combustion dynamics at advanced SOI timings. • Laminarity is found to be the most robust RQA parameter to investigate any qualitative changes in combustion dynamics. • The onset of RCCI combustion mode is accompanied by the onset of a noisy period-2 bifurcation. This study uses nonlinear dynamical and chaotic methods to investigate the cyclic combustion dynamics of a CNG-Diesel reactivity control compression ignition (RCCI) engine. The experiments of RCCI combustion mode are conducted on a single-cylinder automotive diesel engine with the development ECU. Low reactivity fuel (i.e., CNG) is injected into the intake manifold, and high reactivity fuel (i.e., diesel) is injected directly into the engine cylinder during the RCCI experiments. Mass of fuel injection per cycle and their injection events are controlled by using ECU. The engine is tested for fixed engine load and speed of 3 bar BMEP and 1500 rpm, respectively. A double diesel injection strategy is used for injecting the diesel fuel into the engine cylinder. In-cylinder pressure is measured using a piezoelectric pressure transducer on a crank angle basis with a resolution of 0.1 CAD. Combustion parameters are calculated on a cyclic basis from measured in-cylinder pressure. In this study, the effect of diesel injection timing (SOI-2) on the behavior of combustion dynamics for two different masses of port-injected CNG fuel (m c) is investigated. The CA50 time series of 1000 consecutive engine cycles is analyzed for combustion dynamics analysis using symbol sequence analysis, return maps, recurrence plots (RPs), recurrence quantitative analysis (RQA), and 0–1 test. The peaks in symbol sequence histograms and the corresponding symbol series depict the period-2 nature of the cyclic combustion dynamics at the intermediate SOI-2 of 30° and 40° bTDC. Return maps also confirm the onset of noisy period-2 bifurcation at 30° bTDC SOI-2. The dominance of deterministic period-2 characteristics is found in the cyclic combustion dynamics at 30° and 40° bTDC SOI-2 using recurrence plots. The 0–1 test method depicts stronger chaotic cyclic combustion dynamics at 20° and 50° bTDC SOI-2. In summary, the present study supports the existence of a periodic window in between the chaotic combustion for the intermediate values of SOI-2. The dominance of the deterministic characteristics makes this regime of intermediate SOI-2 values suitable for a better cycle resolved control. [ABSTRACT FROM AUTHOR]