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A diesel/natural gas (NG) dual-fuel engine is regarded as an appealing option to reduce emissions while maintaining high thermal efficiency. In this study, a reduced n-heptane–n-butylbenzene–NG–polycyclic aromatic hydrocarbon (PAH) mechanism with 746 reactions and 143 species was developed for predicting the combustion characteristics and emission in dual-fuel engines. A mixture of methane, ethane, and propane was used to model the NG, and a mixture of n-heptane and n-butylbenzene was used to model the diesel. This mechanism was based on a reduced n-heptane–PAH mechanism, and the detailed mechanisms of n-butylbenzene and NG were reduced using the methods of directed relation graph with error propagation (DRGEP), rate of production (ROP), and sensitivity analysis. The key kinetic parameters of the model were optimized and adjusted according to the results of sensitivity analysis. The final optimized dual-fuel mechanism was verified against ignition delay, laminar flame speed, and homogenous charge compression ignition (HCCI) engine combustion, and a good prediction was obtained. Finally, the present mechanism was coupled into the CFD software to simulate the combustion characteristics and emission of a dual-fuel engine under four different NG substitution rates. The simulation results are consistent with the experimental data of emissions and combustion characteristics, indicating that the current mechanism can be applied to simulate practical diesel/NG dual-fuel engines.

Physicochemical properties of soot, such as graphitization degree and surface functional groups of dimethoxymethane (DMM)–diesel blends, have been conducted with Raman spectrum, FT-IR (Fourier transform infrared spectroscopy), and XPS (X-ray photoelectron spectroscopy), respectively. The experimental samples are collected in a four-cylinder, turbocharged engine at two different speeds (2200 and 1400 rpm), different loads (1.2 and 0.6 MPa), and different fuels (D100, DMM6.4, and DMM13). The results show that with the increase in loads, the difference of graphitization degree is gradually disappeared with different fuels, and a higher graphitization degree can be obtained. Moreover, the use of DMM reduces the proportion of unsaturated fuels, thereby leading to lower amounts of C H groups. In terms of oxygenated surface functional groups’ content of particulate matter (PM), the C O functional group accounts for 4%-12%, the COO functional group accounts for 1%-4%, and the C O functional group accounts for 3%-8%. Sensitivity analysis of the microscopic properties of soot found that the O/C ratio and sp3/sp2 ratio have the lowest effect on the apparent activation energy (Ea) of soot. And AD1/AG, AD2/AG, and AD4/AG significantly affect Ea.

Dimethyl carbonate (DMC) is a promising alternative fuel for diesel engines. Comparing with the traditional diesel fuel, the DMC application can lead to a different combustion process where the soot is synthesized, yielding variation in chemistry of engine-out soot particles. Soot chemistry is highly related to the concentration and accessibility of radical sites for the reactions regarding to soot oxidation on the exhaust systems or during DPF regeneration. Therefore, the study on the chemical feature of soot emissions is of great necessity. Nevertheless, comprehensive determination on the soot chemical property for DMC-contained fuel and its instinctive explanation is not well understood in the previous studies. In this study, to reveal the effect of DMC addition on the chemical feature of soot emissions, the surface functional groups and their bonding states of soot particles from various DMC-diesel blends were characterized using FT-IR and XPS technologies. Results showed that the DMC addition exerts different influences on the surface oxygen content, oxygenated groups and their bonding states with various DMC content. Therein, DMC addition in large amount can significantly increase the C sp3/sp2 ratio and the concentration of C-O epoxies on soot surfaces. At relatively low engine speed and load conditions, the low-content addition of DMC leads to slight increase in the aromatic C–H/aromatic C C ratio in soot bulk structure. However, the large-content DMC addition distinctly reduces the aromatic C–H/aromatic C C and aliphatic C–H/aromatic C C ratios. The results are useful for the prediction of the subsequent soot oxidation behavior during DPF regeneration on diesel engines.

The chemistry of soot particles has high affinity with the concentration and accessibility of potential radical sites for the reactions regarding to soot growth and oxidation behaviors during in-cylinder combustion, and thus affects the particle emissions of the engines. The addition of methanol in diesel fuel is widely believed to transform the soot formation and oxidation behaviors. However, the instinctive explanation on these transformation by soot chemistry is not well understood. In this study, the chemical feature of soot emissions from a methanol-diesel blend fueled diesel engine was characterized, using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), respectively. Results showed that epoxy, ether and C vacancy as well as ester groups were the dominant surface species, while the carboxyl and carbonyl groups exerted less contribution amongst the carbon–oxygen groups on soot surfaces for both of pure diesel fuel and methanol-diesel blends. At low engine load conditions, the increase in methanol blending content led to an increase–decrease-increase behavior of C-O epoxy concentration, which was opposite with the C vacancy contribution. At the higher engine load, increasing methanol content in fuel blends led to higher increment in the hydroxyl species with respect to the epoxy groups on soot surfaces, especially for the 5% and 10% methanol addition. The predominant groups within the bulk of soot particles consisted of aliphatic C–H (methyl and methylene), carbonyl-like C O, aromatic C–H groups and C–O bonds in phenols, anhydrides, esters and ether-like groups for both diesel and methanol blending fuels. Relative to the aromatic C C, the concentration of aliphatic C–H reduced with the increasing methanol content in the fuel blends. The lower-volume-proportion addition of methanol at low engine load led to an increase in the aromatic C–H with respect to aromatic C C. While different, the addition of methanol exerted no significant influence on the aromatic C–H concentration within the soot particles at high engine load condition.

Since the technology of n-butanol/diesel blending fuel coupled with post-injection (PI) strategy increases the complexity of engine combustion process, the disputes that the interpretation of the formation mechanism of pollutants are still existed. Moreover, studies on the effects of this coupling-strategy on fine-particles (DP

Dimethoxymethane (DMM) is considered a potential oxygenated fuel due to its high oxygen content (42.1%) and considerable heat value. However, little research emphasizes the impact of oxygenated fuels on small-particles (DP

In high altitude regions (>4000 m), atmosphere pressure and thus in-cylinder ambient density is decreased significantly, leading to longer spray penetration. Affected by liquid spray impingement on cylinder wall, heavy-duty diesel engines operating at high loads may encounter abnormal combustion like diesel knock. However, with limited research data, the detailed combustion mechanism is not fully understood. To explore the origins of diesel knock and its correlations with spray impingement, optical experiments in a rapid compression machine were employed, and diesel spray impingement and subsequent combustion processes were investigated. High-speed direct photography and simultaneous pressure acquisition were synchronically performed to understand combustion evolutions and pressure characteristics. Meanwhile, the role of injection parameters, ambient conditions, and cylinder sizes were considered. The results show that early injection timing and spray impingement can lead to long ignition delay time. Depending on the premixed mixture formation within longer ignition delay, diesel knock manifesting supersonic detonation-like reaction front propagation can be observed, which results in strong pressure waves with an amplitude approaching hundreds of atmosphere. Further analysis on the critical conditions show that long ignition delay time does not necessarily result in diesel knock. It lies in whether there is sufficient premixed mixture formation before main combustion. The prevalence of diesel knock seems sensitive to large fuel injection pressure and small combustion chamber. Besides, depending on the ambient conditions, there are four kinds of combustion modes under spray impingement conditions.

Using a transmission electron microscopy (TEM), the morphological characteristic variations of exhaust soot particles have been studied on a modern diesel engine fuelled with a commercial diesel fuel (D100, as a baseline fuel) and three different oxygenated fuel blends with the same oxygen content, including a 11.5% methanol blend (M11.5), a 8.3% dimethyl carbonate blend (DMC8.3), and a 13% dimethyl methane blend (DMM13). The results showed that oxygenated fuel blends emitted smaller soot primary particles with respect to neat diesel, with the most significant decrease in M11.5 followed by DMM13 and DMC8.3. Similar variations with primary particle diameter were found in the decrease of the mean radius of gyration, mean primary particle number and estimated mass for an agglomerate when making comparisons among diesel and the three oxygenated fuel blends. Aggregates shape analysis showed that M11.5 soot had the minimal trend to form a rather circular structure and maximal boundary irregularity among the four fuels, while all soot particles showed larger deviation to the sphere (i.e. higher elongation) regardless of fuel type used. In addition, both mass and perimeter fractal dimension showed consistent results for diesel and the three oxygenated fuel blends, with the particle aggregates being the least compacted morphology for M11.5, followed by DMM13, DMC8.3 and diesel. Therefore, compared to diesel, the exhaust soot particles from the oxygenated fuel blends (especially for M11.5) are more prone to be trapped (lowest fractal dimension) and display a higher reactivity towards oxidation (smallest primary particle size).

Biodiesel has a high viscosity, which limits its application to engines. However, n-pentanol has low viscosity and high volatility, which is considered a promising additive. This study examined the influence of n-pentanol/biodiesel/diesel blends on the performance and particulate emissions of a diesel engine at different loads, under the condition of coupling exhaust gas recirculation (EGR) (0–30%). The three test fuels included pure diesel (D100); 80% diesel and 20% biodiesel blended fuel (BD20); 64% diesel, 16% biodiesel, and 20% n-pentanol blended fuel (BDP20). The results indicated that the influence of EGR on the NOx emissions between fuels was greater than that of the physicochemical properties of the fuels. For the same EGR rate, the heat release rate (HRR), peak in-cylinder pressure (IP), and brake thermal efficiency (BTE) increased with the engine load increased. After adding n-pentanol to BD20, the HRR increased. The peak IP and BTE values of BDP20 were similar to those of D100 and BD20. With the addition of n-pentanol, the total particulate number concentration (TPNC), soot, and total particulate mass concentration (TPMC) emissions decreased. Furthermore, the trade-off relationship between TPMC and NOx was improved, and the geometric mean diameter (GMD) of the particles decreased.

This paper compared the soot reactivity and its primarily related factors including soot structure, graphitization degree and surface functional groups, when using mineral diesel fuel (DF) and methanol blended fuel (MBF) at two typical low and high engine loads. Results showed that the soot reactivity increases with increasing methanol concentration in the MBF. Additionally, with the constant methanol blend ratio of methanol, the reactivity of MBF soot degrades as the engine load increases. Correspondingly, the MBF soots had lower aggregate compactness, smaller primary particle size, more disordered structure and more active aliphatic C-H groups in comparison with the DF soot, providing higher concentration and accessibility of active sites for oxidation reactions. The addition of methanol creates variations in combustion kinetics and fuel formulation, which exerts reverse effects on the soot structure and chemistry. Therefore, the factors related to these properties show non-monotonic variations when alter the methanol blend ratio and engine operating mode. Especially, when the methanol blend ratio increases from 10% to 15%, the soot structure and chemistry transform onto the features that are reluctant for soot oxidation.

A gasoline compression ignition (GCI) engine would cause difficulties of ignition stability or even misfired phenomenon at low loads. Internal exhaust gas recirculation (IEGR) technology is used to broaden the operating range and provide a GCI engine with combustion stability. In this paper, the effects of the IEGR rate (10–90%) and excess air ratio (EAR) (1.0–4.0) on the combustion performance and emissions from a GCI were investigated on a fully variable valve engine, with a particular focus on fuel economy and emissions from a GCI within less than or equal to 5% of the coefficient of variation of the indicated mean effective pressure (COVIMEP). And based on the condition of COVIMEP = 5%, the difference values between maximum and minimum cycle pressure and knock are analyzed in detail. The study finds that excessive EGR hinders combustion but improves the ignition stability of a GCI. With the EGR rate from 10% to 60%, and the EAR is 1.5 to 3.0, the combustion efficiency that greater than 90% can be obtained. Moreover, the different values of the peak of maximum and minimum pressure from the 200 continuous cycles decrease as the EGR increases. In addition, the frequency of knock occurred is further greater than that of misfire even though the low loads with the COVIMEP ranges around 5%.

To meet the increasingly stringent emission regulations on CI engines, a new theory that combines post-injection technology and uses dimethyl carbonate (DMC) and a certain amount of EGR to achieve ultra-low emissions of soot and nitrogen oxides (NOX) is proposed. However, DMC causes a delay in combustion phase of the mixed fuel due to its smaller cetane number (CN) value. Because the combustion phase affects thermal efficiency, economic efficiency, and emission performance of compression ignition (CI) engines, 2-ethylhexyl nitrate (EHN) can be used as a CN enhancer, to improve the combustion performance parameters of the mixed fuel. This study verifies this theory by controlling the post-injection rate (0, 5%, 10%, 15%, and 20%) of different experimental fuels for a diesel engine with four-cylinder and turbocharged. The five fuels were pure diesel and a mixture of 80% diesel with 20% DMC (DMC20). As well, EHN was added to DMC20 with a proportion of 0.5%, 1%, and 2%. Through this study, the use of 0.5% EHN allows maximum in-cylinder pressure (MINP) and maximum heat rise rate (MHRR) of the DMC/diesel blend to achieve results similar to those of diesel. And the use of EHN reduced the ignition delay time, but leading to a slightly increase of soot. As the post-injection rate increased, both NOX and carbon monoxide (CO) decreased and soot significantly decreased. The addition of 0.5% EHN with the post-injection rate of >15% has a good combustion and emission effect and extends the trade-off limits of NOX and soot.

Interactions between fuel and engine are basic issues for gasoline compression ignition (GCI) engine, and recent engine experiment results show that the gasoline fuel with low-octane number RON = 60–80 is optimal for GCI engines. However, gasoline fuel with low-octane ratings exhibits significant low-temperature combustion behavior, which will significantly affect the entire combustion process. In this study, different toluene primary reference fuels (TPRF) with identical research octane number (RON) but various octane sensitivities were employed. Optimized start of injection (SOI) was obtained based on indicated mean effective pressure (IMEP), combustion stability limits and engine emissions. Under the optimized SOI, the role of fuel sensitivity in combustion processes was investigated. With addressing low-temperature heat release (LTHR) before main ignition, the effect of intake conditions and fuel sensitivity on combustion phasing was clarified. The results indicate that the LTHR only occurs when the SOI is sufficiently advanced, and an earlier injection timing can achieve stable combustion, higher IMEP and lower NOX emission. More obvious LTHR promotes the advancement of combustion phasing, and the magnitude of LTHR is negatively correlated with intake temperature but positively correlated with intake pressure. The effect of increasing LTHR magnitude on combustion phasing is similar to that of increasing intake temperature. Finally, the effect of octane sensitivity on LTHR was also studied, which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing, and the onset of LTHR is more advanced for low sensitivity fuels.

Natural gas (NG) dual-fuel engines can attain a similar thermal efficiency as that of diesel engines while achieving lower emissions. However, the trade-off relationship between CH4 and NOX emissions limits the development of dual-fuel (DF) engines. In order to resolve this problem, the effects of injection pressure (IP) and exhaust gas recirculation (EGR) ratio on the combustion and emission of diesel/NG dual-fuel engines are investigated in this study. The results show that the diesel/NG dual fuel has a distinct three-stage heat release characteristic in the high-temperature combustion process. As the injection pressure increases, the flame propagation speed of methane and the indicated thermal efficiency (ITE) increase. However, the methane in the crevice region and cylinder wall cannot be ignited because of low temperature; these are the main sources of methane emissions. When the EGR rate increases, the indicated thermal efficiency first increases and there after decreases, and diesel is cleaved through the reaction chain, PC4H9 → C2H4 → CH4, to produce CH4; this becomes one of the reasons for CH4 emissions. When the EGR rate is small ( 30%), CO and CH4 emissions can be significantly reduced by increasing the IP. Accordingly, when the injection pressure is 160 MPa and the EGR rate is 20%, the diesel/NG DF engine can achieve higher ITE and lower emissions.

Biodiesel is a environmentally friendly, biodegradable and renewable alternative fuel, which is widely used around the world. However, the higher viscosity and NOX emissions are its main drawbacks. n-Pentanol has lower viscosity, higher oxygen content and volatility as compared with biodiesel, which is a promising additive for biodiesel/diesel blends. In this work, the influences of pilot injection (PI) strategies included pilot-main interval (PMI) and pilot injection rate (PIR), and n-pentanol additive on emission characteristics and combustion behaviors of a diesel engine at medium load were investigated. The three test fuels including diesel (D100), a mixture of 20% biodiesel and 80% diesel (BD20), and 20% n-pentanol, 16% biodiesel and 64% diesel (BDP20), respectively. The results indicated that, compared with single injection (SI), the peak of main injection heat release rate reduced and brake thermal efficiency (BTE) increased when using PI strategies. Adding n-pentanol to biodiesel/diesel (BD) blends resulted in an increase in maximum in-cylinder pressure, but BTE decreased. After utilizing PI strategies, the NOX emissions decreased significantly, but the soot, CO and THC emissions increased. As the PIR and PMI increased, the accumulation mode particles and total particle mass concentrations increased. Compared with SI, the ratio of sub-25 nm particles of BDP20 decreased with the use of PI strategies. The combination of n-pentanol additive and small PMI strategies could achieve lower total particles number concentration and smaller geometric mean diameter when compared with pure diesel.

Dimethyl carbonate (DMC) has high oxygen content (53.3%) and a non-toxic preparation, which is a promising additive for diesel fuel. Exhaust gas recirculation (EGR) technology inhibits the formation of NOX. The combination of DMC with EGR is considered to be a practical means of meeting emissions regulations. In this study, a four-cylinder diesel engine was used to study the effects of DMC and EGR on engine emissions and performance. The three test fuels included a pure diesel (D100), 10% DMC and 90% diesel (DMC10), and 20% DMC and 80% diesel (DMC20). The results show that the DMC20 mixtures created a longer ignition delay, and resulted in the highest pressure and heat release rate. However, the use of DMC shortened the duration of the combustion. The brake specific fuel consumption of the DMC blends was higher than that of diesel, and the brake thermal efficiency of the DMC10 was higher than diesel at the same EGR. Through an energy balance analysis, DMC20 has a significant potential for exhaust gas energy recovery. Overall, DMC20 had the best combustion performance when it was used in combination with EGR rates of 20%−30%. When EGR = 40%, the NOX emission of DMC20 was reduced by 64.9%, and the soot emission was reduced by 80%. The nucleation mode and aggregation mode particulate matter (PM) emissions were less affected by EGR, the total PM concentration of DMC20 reduced to less than 20% of D100, and the mass concentration was reduced by approximately 70%.

In the present study, the impact of exhaust gas recirculation (EGR) rates, from 0% to 15%, and compression ratio (CR) of 8, 9, and 10 on the combustion characteristics and emission performance of 2-methylfuran (MF) and gasoline were studied. Experiments were carried out on a Ricardo E6 single-cylinder sparkignition (SI) research engine, under stoichiometric conditions, MF could produce higher cylinder pressure, knocking intensity, combustion temperature, and nitrogen oxides (NOx) emissions than gasoline at higher CRs. However, an appropriate level of cool EGR improved the combustion and emissions, particularly through knock suppression and reduced NOx emissions. When the cooled EGR rate reached 15%, the NOx emissions from the gasoline at a compression ratio of 10 was reduced by about 20.6 g/kW h (>72.5%) compared with 0% EGR. With a low EGR rate, there was only a slight improvement in the indicated thermal efficiency; however, when the EGR reaches 15%, the MF results in 31.2% higher indicated thermal efficiency when compared to gasoline with a CR of 10. This work further advances the knowledge of how to improve the overall performance of MF as an alternative fuel for internal combustion engines. 2014 Elsevier Ltd. All rights reserved.