Shorter Combustion Duration Research Articles

Effect of EGR at advanced injection timing on combustion characteristics of diesel engine

In the present work effect of EGR at advanced injection on combustion characteristics of a diesel engine was investigated. A 4.4kW stationary diesel engine was used in this work and test was conducted at different percentage of EGR at advanced fuel injection timing. When compared to standard injection timing longer delay period, higher cylinder peak pressure, higher maximum heat release rate and shorter combustion duration were observed at advanced injection timing. When exhaust gases were recycled into the combustion chamber at advanced injection timing, delay period, peak pressure, MHRR and combustion duration were found to be increased than those of measured without EGR.

Combustion and emission characteristics of diesel engine fueled with diesel/biodiesel/pentanol fuel blends

Abstract Pentanol is one of the next generation biofuels that could potentially help relieve the energy crisis and environmental problems. The objective of this study is to reveal the effects of pentanol addition to diesel and biodiesel fuels in different ratios on the combustion and emission of a single-cylinder direct-injection diesel engine. The tests were conducted at a constant speed (1600 r/min) under different engine loads without exhaust gas recirculation. The indicated thermal efficiency using pentanol blends was found to be higher than that of using pure diesel for all of tested loads from 0.5 to 1.0 MPa indicated mean effective pressure at the test conditions, which is due to its higher maximum heat release rate and shorter combustion duration. An obvious decrease in soot emissions was attained with the addition of pentanol. Moreover, emissions of nitrogen oxides (NO x ) were simultaneously reduced compared with using pure diesel fuel at low to middle loads. Furthermore, diesel engine fueled with oxygenated fuel blends can reduce the carbon monoxide and unburnt total hydrocarbons emissions except for the diesel–pentanol blends at low engine load. Finally, the strategy with 40% diesel–30% biodiesel–30% pentanol showed better combustion, emission characteristics as well as economy performance among all the fuels.

Investigations into the influence of internal and external exhaust gas recirculation on the combustion stability in an optical gasoline spark ignition engine

Combustion stability is the major concern for engines operated in highly diluted conditions, particularly during the mode transition between controlled autoignition and spark ignition. In this research, studies were performed to investigate the influence of the dilution of internal exhaust gas recirculation and the dilution of external exhaust gas recirculation on early flame development and combustion stability in a single-cylinder optical engine. It is found that a higher external exhaust gas recirculation rate slows do‘wn the early flame development, which is responsible for the higher cyclic fluctuation of combustion. The cyclic variation in the normalized flame area matches well the coefficient of variation of the early flame development period, which decreases with development of the flame. The dilution of the internal exhaust gas recirculation shows a more complicated influence on the combustion than the dilution of the external exhaust gas recirculation does. Although more hot residual gas is trapped in the cylinder, the increase in the internal exhaust gas recirculation rate does not contribute to the promotion of the in-cylinder thermal state in the studied cases. However, with a moderate increase in the internal exhaust gas recirculation rate from 9.8% to 13%, faster initial flame kernel formation is observed, which benefits the combustion by accelerating the early flame development, advancing the combustion timing and stabilizing the combustion. With the internal exhaust gas recirculation rate further increased to 19.6%, a sharp decrease in the mean expansion speed of the flame front results in a retarded early flame development and a slower heat release, which leads to severe deterioration in the combustion stability. In addition, by substituting part of the external exhaust gas recirculation with internal exhaust gas recirculation, an advanced combustion timing, a shorter combustion duration and an improved combustion stability can be achieved; this implies that a higher total exhaust gas recirculation tolerance can be achieved with the same limitation of the coefficient of variation in the indicated mean effective pressure. This result can be instructive in optimizing the control strategy of the valve timing and the external exhaust gas recirculation rate during the mode transition between controlled autoignition and spark ignition.

Impacts of acetone on the spray combustion of Acetone–Butanol–Ethanol (ABE)-Diesel blends under low ambient temperature

Acetone–Butanol–Ethanol (ABE), an intermediate product in the ABE fermentation process for producing bio-butanol, is considered a promising alternative fuel because it not only preserves the advantages of oxygenated fuels which present lower soot emissions compared to diesel, but also lowers the cost of fuel recovery for individual component during fermentation. The advantageous characteristics of ABE–Diesel include higher oxygen content which promotes soot oxidation; longer ignition delay and flame lift-off length allowing more air entrainment upstream of the spray jet thus providing better air–fuel mixing. With the development of advanced ABE fermentation technology, the volumetric percentage of acetone, butanol and ethanol in the bio-solvents can be precisely controlled. In this respect, it is desirable to estimate the performance of different ABE blends to determine the best blend and optimize the production process accordingly. In this paper, ABE fuels with different component volumetric ratio (A:B:E of 6:3:1; 3:6:1; 0:10:0), were blended with 80% vol. diesel and injected into a constant volume chamber. The combustion characteristics of these blends were experimentally investigated under LTC conditions (ambient temperature of 800K with varying ambient oxygen content). Time-resolved broadband luminosities from the combustion process were captured by a high speed camera and flame lift-off lengths were also acquired. The impacts of acetone and butanol content on the combustion characteristics of the blends were addressed in detail. ABE20(6:3:1) displayed combustion characteristics very similar to those of pure diesel, displaying a much shorter ignition delay than that of the other ABE–Diesel blends due to a high fraction of acetone. At the same time, ABE20(6:3:1) presented a much lower natural flame luminosity (lower soot formation) compared to diesel, owing to its shorter combustion duration and stronger premixed combustion.

An Experimental Study of Flame Kernel Evolution in Lean and Diluted Methane-Air Mixtures at Engine-Like Conditions

An experimental study was performed to investigate the flame kernel propagation and combustion characteristics of methane (CH4)-air mixture at spark-ignition (SI) engine-like conditions. Tests were performed in a constant-volume combustion vessel (CV) with a product engine spark plug mounted at the top to simulate the engine combustion chamber. In this work, the equivalence ratio of 0.7 for lean methane-air mixture was used as test fuel, and different levels of inert gas were introduced to simulate exhaust gas recirculation (EGR) as in actual engine operations. The visualization of flame kernel formation and flame propagation was performed with a schlieren optical system and flame propagation speed was measured and analyzed accordingly. Global combustion characteristics are acquired and analyzed by utilizing the pressure history of combustion events recorded by high speed and fast response pressure measurements. The flame kernel expansion rates have been reported with quiescent and turbulent conditions. Global combustion characteristics, such as heat release rate, cumulative heat release, and mass fraction burned (MFB) as functions of time, are also reported. The suppression effect of pressure on flame propagation showed the flame slowdown while turbulence played a significant role in changing the combustion mode of the flame kernel and shortened combustion duration. These sets of experimental work and measurements are expected to be useful for development and validation of computational fluid dynamic (CFD) codes that simulate combustion processes for combustion ignition processes inside SI engines.

Experimental Investigation of Spark-Ignited Combustion with High-Octane Biofuels and EGR. 2. Fuel and EGR Effects on Knock-Limited Load and Speed

The present study experimentally investigates spark-ignited combustion with 87 AKI E0 gasoline in its neat form and in midlevel alcohol–gasoline blends with 24% vol/vol isobutanol–gasoline (IB24) and 30% vol/vol ethanol–gasoline (E30). A single-cylinder research engine is used with an 11.85:1 compression ratio, hydraulically actuated valves, laboratory intake air, and was capable of external exhaust gas recirculation (EGR). Experiments were conducted with all fuels to full-load conditions with λ = 1, using both 0% and 15% external-cooled EGR. Higher octane number biofuel blends exhibited increased stoichiometric torque capability at this compression ratio, where the unique properties of ethanol enabled a doubling of the stoichiometric torque capability with E30 as compared to that of 87AKI, up to 20 bar IMEPg (indicating mean effective pressure gross) at λ = 1. The results demonstrate that for all fuels, EGR is a key enabler for increasing engine efficiency but is less useful for knock mitigation with E30 than for 87AKI gasoline or IB24. Under knocking conditions, 15% EGR is found to offer 1°CA of CA50 timing advance with E30, whereas up to 5°CA of CA50 advance is possible with knock-limited 87AKI gasoline. Compared to 87AKI, both E30 and IB24 are found to have reduced adiabatic flame temperature and shorter combustion durations, which reduce knocking propensity beyond that indicated by the octane number. However, E30+0% EGR is found to exhibit the better antiknock properties than either 87AKI+15% EGR or IB24+15% EGR, expanding the knock limited operating range and engine stoichiometric torque capability at high compression ratio. Furthermore, the fuel sensitivity (S) of E30 was attributed to reduced speed sensitivity of E30, expanding the low-speed stoichiometric torque capability at high compression ratio. The results illustrate that intermediate alcohol–gasoline blends exhibit exceptional antiknock properties and performance beyond that indicated by the octane number tests, particularly E30.

The effects of ethanol addition with waste pork lard methyl ester on performance, emission and combustion characteristics of a diesel engine

In the recent research, as a result of depletion of world petroleum reserves, considerable attention has been focused on the use of different alternative fuels in diesel engines. The present work aims to ensure the possibility of adding ethanol as an additive with animal fat biodiesel that is tested as an alternative fuel for diesel in a CI engine. In this study, biodiesel is obtained from waste pork lard by base-catalyzed transesterification with methanol when potassium hydroxide as catalyst. 2.5%, 5% and 7.5% by volume of ethanol is blended with neat biodiesel in order to improve performance and combustion characteristics of a diesel engine. The experimental work is carried out in a 3.7 kW, single cylinder, naturally aspirated, water cooled, direct injection diesel engine for different loads and at a constant speed of 1500 rpm. The performance, emission and combustion characteristics of biodiesel-ethanol blends are investigated by comparing them with neat biodiesel and standard diesel. The experimental test results showed that the combustion and performance characteristics improved with the increase in percentage of ethanol addition with biodiesel. When compared to neat biodiesel and standard diesel, an increase in brake thermal efficiency of 5.8% and 4.1% is obtained for BEB7.5 blend at full load of the engine. With the increase in percentage of ethanol fraction in the blends, peak cylinder pressure and the corresponding heat release rate are increased. Biodiesel-ethanol blends exhibit longer ignition delay and shorter combustion duration when compared to neat biodiesel. Optimum reduction in carbon monoxide, unburned hydrocarbon and smoke emission are attained while using BEB5 blend at full load of the engine. However, there is an adverse effect in case of nitrogen oxide emission.

Effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions in a spark ignition engine at part loads

The effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions were experimentally investigated in a spark ignition engine in this study. A low pressure loop EGR system and a variable tumble valve in intake ports were employed, by which wide range of EGR ratio and tumble ratio could be achieved. All the tests were performed at part loads, and the equivalence ratio was kept constant at 1.0. The results show that the combination of EGR and enhanced tumble motion produces a significant improvement in fuel economy (13.1–19.5%) and a reduction in the coefficient of variations of indicated mean effective pressure (CoVIMEP) due to the significantly shortened combustion duration. By optimizing the combustion process with EGR and enhanced tumble, NOx (nitrogen oxides) emissions drastically decrease, while HC (hydrocarbons) emissions slightly rise. For CO (carbon monoxide) emissions, the enhanced tumble takes a positive effect.

Performance, combustion and emission characteristics of n-butanol additive in methanol–gasoline blend fired in a naturally-aspirated spark ignition engine

The aim of the study was to compare the effects of dual alcohols (n-butanol and methanol) with single alcohol (methanol) blended in gasoline fuel (GF) against performance, combustion and emission characteristics. Problems arise in the fuel delivery system when using the highly volatile methanol-gasoline blends. This problem is reduced by adding n-butanol to methanol-gasoline blends. However, the satisfactory engine performance of the dual alcohol–gasoline blends need to be proved. The test fuels were GF, blend M53b17 (53 % methanol, 17 % n-butanol and 30% GF by volume), M20, and M70. The blend M53b17 was selected to match the vapor pressure (VP) of GF, whereas M70 to match the total alcohol content in the blend. The test fuels were a lean mixture with excess-air ratio of λ=1.1. The experiments were conducted on a naturally-aspirated, spark ignition engine. The brake thermal efficiency (BTE) improved whereas the exhaust gas temperature (EGT) of the blends reduced, which is a benefit that reduces compression work. The regulated emissions were also reported. The blend M53b17 was recommended in preference to M70 because the former had shortened combustion duration, high-energy content and its VP was selectively matched to that of GF's.

Optimize combustion of compressed natural gas engine by improving in-cylinder flows

In order to solve the problem of slow flame propagation in a spark-ignition engine fueled with compressed natural gas (CNG), the influence of in-cylinder flows on combustion process was investigated in CA6SE3-21E4N CNG-engine by means of numerical simulation and experiment. The status of in-cylinder flows from intake to expansion stroke was described by computational fluid dynamic tool, which revealed that the in-cylinder flows were one of the main reasons of slow burning rate. Therefore, a special-shaped combustion chamber called Cross was used to improve the in-cylinder flows. The results showed that peak turbulent kinetic energy of Cross was 43.9% higher than that of original combustion chamber called Cylinder during the late compression period at 1450 rpm 100% load. The combustion parameters, brake specific fuel consumption (BSFC) and regulated emissions were obtained by means of experiment. At 1450rpm 25%, 50%, 75% and 100% load conditions, the ignition delay of Cross was longer than that of Cylinder, moreover, the Cross produced averagely 5.75°CA shorter combustion duration. The BSFC of Cross was on an average of 4.3% reduction at 1450 rpm as well as the HC and CO emissions were reduced whereas the NOx emissions were significantly increased.

Fuel combustion under asymmetric piston motion: Tested results

This paper presents the measured characteristics of a single piston hydraulic free-piston engine prototype. The detailed results about the piston motion and the fuel combustion are presented. The results show that there is a strong interaction between the piston motion and the fuel combustion in the free-piston engine. The gas force plays a leading role in the innate asymmetric piston motion around top dead centre. The small moving piston inertia makes the asymmetric characteristics more visible. The combustion energy conversion of the single piston hydraulic free-piston engine is fast. Most of the fuel injected burns in 1.0 ms in the pre-mixed combustion phase. Because of the asymmetric characteristics, the mixing-controlled combustion phase in the single piston hydraulic free-piston engine is much shorter than the one in the conventional diesel engine. The short combustion duration decreases the rated power of the free-piston engine. The asymmetric piston motion is unfavourable for the rated power increase of the free-piston engine. The smaller rated power has a negative effect on the free-piston engine application development.

Combustion and emissions characteristics of a dual fuel engine operated on alternative gaseous fuels

Among the different efforts towards the reduction in pollutant emissions from direct injection (DI) diesel engines, the use of gaseous fuels as a partial supplement for diesel fuel has been proposed by many researchers. An experimental investigation was performed to investigate the influence of dual-fuel combustion on the performance and exhaust emissions of a DI diesel engine fueled with natural gas (NG) and biogas (BG). The engine was operated at a constant speed of 1750rpm and at two different loads: low (∼3Nm) and high (∼28Nm), which were about 10% and 85% respectively of the rated torque output of the engine at 1800rpm. In this work, the combustion pressure and the rate of heat release were evaluated experimentally in order to analyze the combustion characteristics and their effects on exhaust emissions including particulate matter (PM) for single-fuel (diesel) and dual fuel combustion modes. In dual fuel mode, the peak cylinder pressure was found to be similar to diesel at 75% of the rated output of the engine. About 27–30% higher maximum net heat release rates were obtained for NG and biogas fueling respectively compared to diesel fueling. Longer ignition delays but shorter combustion durations were characterized for dual fueling operations. Specific NOx emissions for dual fueling was always lower than diesel fueling case. Significantly lower specific PM emissions but sharply increased unburned hydrocarbons (UHC) emissions were measured for biogas–diesel dual fuel operations as compared to diesel fueling.

Fundamental Study of Spray and Partially Premixed Combustion of Methane/Air Mixture

This study presents fundamentals of spray and partially premixed combustion characteristics of directly injected methane in a constant volume combustion chamber (CVCC). The constant volume vessel is a cylinder with inside diameter of 135 mm and inside height of 135 mm. Two end of the vessel are equipped with optical windows. A high speed complementary metal oxide semiconductor (CMOS) camera capable of capturing pictures up to 40,000 frames per second is used to observe flow conditions inside the chamber. The injected fuel jet generates turbulence in the vessel and forms a turbulent heterogeneous fuel–air mixture in the vessel, similar to that in a compressed natural gas (CNG) direct-injection (DI) engine. The fuel–air mixture is ignited by centrally located electrodes at a given spark delay timing of 1, 40, 75, and 110 ms. In addition to the four delay times, a 5 min waiting period was used in order to make sure of having laminar homogeneous combustion. Spray development and characterization including spray tip penetration (STP), spray cone angle (SCA), and overall equivalence ratio were investigated under 30–90 bar fuel pressures and 1–5 bar chamber pressure. Flame propagation images and combustion characteristics were determined via pressure-derived parameters and analyzed at a fuel pressure of 90 bar and a chamber pressure of 1 bar at different stratification ratios (S.R.) (from 0% to 100%) at overall equivalence ratios of 0.6, 0.8, and 1.0. Shorter combustion duration and higher combustion pressure were observed in direct injection-type combustion at all fuel air equivalence ratios compared to those of homogeneous combustion.

Performance, emission and combustion characteristics of an indirect injection (IDI) multi-cylinder compression ignition (CI) engine operating on neat jatropha and karanj oils preheated by jacket water

Renewable non-edible plant oils such as jatropha and karanj have potential to substitute fossil diesel fuels in CI engines. A multi-cylinder water cooled IDI type CI engine has been tested with jatropha and karanj oils and comparisons made against fossil diesel. The physical and chemical properties of the three fuels were measured to investigate the suitability of jatropha and karanj oils as fuels for CI engines. The engine cooling water circuit and fuel supply systems were modified such that hot jacket water preheated the neat plant oil prior to injection. Between jatropha and karanj there was little difference in the performance, emission and combustion results. Compared to fossil diesel, the brake specific fuel consumption on volume basis was around 3% higher for the plant oils and the brake thermal efficiency was almost similar. Jatropha and karanj operation resulted in higher CO2 and NOx emissions by 7% and 8% respectively, as compared to diesel. The cylinder gas pressure diagram showed stable engine operation with both plant oils. At full load, the plant oils gave around 3% higher peak cylinder pressure than fossil diesel. With the plant oils, cumulative heat release was smaller at low load and almost similar at full load, compared to diesel. At full load, the plant oils exhibited 5% shorter combustion duration. The study concludes that the IDI type CI engine can be efficiently operated with neat jatropha (or karanj) oil preheated by jacket water, after small modifications of the engine cooling and fuel supply circuits.

Experimental investigations of comparative performance, emission and combustion characteristics of a cottonseed biodiesel-fueled four-stroke locomotive diesel engine

A large-bore, four-stroke, medium-speed, compression-ignition railway traction locomotive engine was fueled with cottonseed methyl ester (Biodiesel). The cottonseed methyl ester was stored for 6 months under ordinary storage conditions, and various fuel properties of this seasoned cottonseed methyl ester were evaluated. Various blends of cottonseed methyl ester (B10, B20, B50 and cottonseed methyl ester) were evaluated for engine performance, emissions and combustion characteristics of the locomotive engine compared to baseline diesel. Correlation of the fuel-injection performance parameters with the physical properties of biodiesel has been carried out. The engine was able to operate on cottonseed methyl ester without noticeable power loss. With cottonseed methyl ester, the thermal efficiency decreased marginally by 0.7% and the brake-specific energy consumption increased by 0.1 MJ/kWh at the rated power. Due to the lower calorific value of biodiesel, brake specific fuel consumption (BSFC) increased by 13.4% at eighth engine notch, nitrogen oxide emissions increased by 8% and particulate matter emissions decreased by 32% vis-a-vis mineral diesel. Nitrogen oxide emissions were found to be a function of injection timings, global oxygen/carbon ratios, time of the maximum mean in-cylinder temperatures and apparent heat release. Particulate matter emissions were found to depend on the air–fuel mixture’s oxygen/carbon ratio, fuel bound oxygen and fuel-injection pressure. The experiments were carried out to evaluate the in-cylinder pressure, heat-release rate, cumulative heat release, fuel-injection pressure, needle lift and fuel-injection velocities. Increased fuel-injection pressures (1000 bar compared to 900 bar for mineral diesel), advanced fuel-injection timings, shorter combustion duration, advanced in-cylinder pressures and higher heat-release rates were observed for biodiesel and blends. The findings of the present study have provided further insights into the combustion of biodiesel in locomotive engines.

Soot Evolution With Cyclic Crank-Angle-Resolved Two-Color Thermometry in an Optical Diesel Engine Fueled With Biodiesel Blend and ULSD

Biodiesel is a desirable alternative fuel for the diesel engine due to its low engine-out soot emission tendency. When blended with petroleum-based diesel fuels, soot emissions generally decrease in proportion to the volume fraction of biodiesel in the mixture. While comparisons of engine-out soot measurements between biodiesel blends and petroleum-based diesel have been widely reported, in-cylinder soot evolution has not been experimentally explored to the same extent. To elucidate the soot emission reduction mechanism of biodiesel, a single-cylinder optically-accessible diesel engine was used to compare the in-cylinder soot evolution when fueled with ultra-low sulfur diesel (ULSD) to that using a B20 biodiesel blend (20% vol./vol. biodiesel ASTM D6751-03A). Soot temperature and KL factors are simultaneously determined using a novel two-color optical thermometry technique implemented with a high-speed CMOS color camera having wide-band Bayer filters. The crank-angle resolved data allows quantitative comparison of the rate of in-cylinder soot formation. High-speed spray images show that B20 has more splashing during spray wall impingement than ULSD, distributing rebounding fuel droplets over a thicker annular ring interior to the piston bowl periphery. The subsequent soot luminescence is observed by high-speed combustion imaging and soot temperature and KL factor measurements. B20 forms soot both at low KL magnitudes over large areas between fuel jets, and at high values among remnants of the fuel spray, along its axis and away from the bowl edge. In contrast, ULSD soot luminescence is observed exclusively as pool burning on the piston bowl surfaces resulting from spray wall impingement. The soot KL factor evolution during B20 combustion indicates earlier and significantly greater soot formation than with ULSD. B20 combustion is also observed to have a greater soot oxidation rate, which results in lower late-cycle soot emissions. For both fuels, higher fuel injection pressure led to lower late-cycle soot KL levels. The apparent rate of heat release (ARHR) analysis under steady skip-fire conditions indicates that B20 combustion is less sensitive to wall temperature than that observed with ULSD due to a lesser degree of pool burning. B20 was found to have both a shorter ignition delay and shorter combustion duration than ULSD.

Performance and emissions of a direct-injection diesel engine operating on emulsions of corn stalk bio-oil in diesel

The experimental bio-oil produced from corn stalk through a fast pyrolysis process is mainly composed of oxygenated organics and water, which makes it unsuitable for direct use as fuel. However, the use of bio-oil in diesel engines can be realized by developing stable emulsions of bio-oil in diesel. In this paper, two emulsions with 10% and 20% bio-oil by mass fraction in No. 0 diesel, represented by BO10 and BO20 respectively, were prepared using an ultrasonic emulsification method after determining the optimum hydrophilic and lipophilic balance value of the emulsifier for the emulsions. Then, the performance and emissions of an unmodified direct-injection diesel engine operating with the two emulsions were studied. The results show that the two emulsions display longer ignition delays, exhibit higher peak values of the premixed burning rate and pressure rise rate, demonstrate lower peak values of the in-cylinder pressure and combustion temperature and have a shorter combustion duration than No. 0 diesel does. In comparison with BO10, BO20 has a longer ignition delay, while it exhibits lower peak values of the premixed burning rate, the pressure rise rate, the in-cylinder pressure and the combustion temperature. In addition, the fuel economy of BO10 is comparable with that of No. 0 diesel, while the fuel economy of BO20 is poorer than that of No. 0 diesel. Nitrogen oxide emissions of the emulsions are lower but carbon monoxide emissions are higher than those of No. 0 diesel, and these trends are more marked with increasing bio-oil mass fraction. The hydrocarbon and smoke emissions of BO10 are lower than those of No. 0 diesel, but the hydrocarbon and smoke emissions of BO20 are higher.

Fuel Property Effects on PCCI Combustion in a Heavy-Duty Diesel Engine

An experimental study was performed to investigate fuel property effects on premixed charge compression ignition (PCCI) combustion in a heavy-duty diesel engine. A matrix of research diesel fuels designed by the Coordinating Research Council, referred to as the Fuels for Advanced Combustion Engines (FACE), was used. The fuel matrix design covers a wide range of cetane numbers (30 to 55), 90% distillation temperatures (270 to 340 °C) and aromatics content (20 to 45%). The fuels were tested in a single-cylinder Caterpillar diesel engine equipped with a common-rail fuel injection system. The engine was operated at 900 rpm, a relative air/fuel ratio of 1.2 and 60% exhaust gas recirculation (EGR) for all fuels. The study was limited to a single fuel injection event starting between −30° and 0 °CA after top dead center (aTDC) with a rail pressure of 150 MPa. The brake mean effective pressure (BMEP) ranged from 2.6 to 3.1 bar depending on the fuel and its injection timing. The experimental results show that cetane number was the most important fuel property affecting PCCI combustion behavior. The low cetane number fuels had better brake specific fuel consumption (BSFC) due to more optimized combustion phasing and shorter combustion duration. They also had a longer ignition delay period available for premixing, which led to near-zero soot emissions. The two fuels with high cetane number and high 90% distillation temperature produced significant soot emissions. The two fuels with high cetane number and high aromatics produced the highest brake specific NOx emissions, although the absolute values were below 0.1 g/kW-h. Brake specific HC and CO emissions were primarily a function of the combustion phasing, but the low cetane number fuels had slightly higher HC and lower CO emissions than the high cetane number fuels.

Effects of engine operating conditions on the size and nanostructure of diesel particles

Diesel particulate is composed of spherical or nearly spherical primary particles. In this study, the primary particle size and nanostructure were investigated at five engine operating conditions for a medium-duty direct injection diesel engine, using a high-resolution transmission electron microscope (TEM). The diesel particles were collected from diluted exhaust gas and an ultrasonic extraction method was used for obtaining the TEM samples. The TEM images reveal that primary particle size and nanostructure depend on the engine operating conditions. The mean diameter of the primary particles is in the range 23.8–28.5nm, depending on the engine speed and engine load. The nanostructure could reflect the particle formation processes. The larger primary particles are formed at low engine speed and high engine load. They have irregular shape and distinguishable trace of pre-formed particle, indicating the combined effect of particle coalescence and surface growth. Primary particles with core–shell structure are common at high engine load with different engine speeds. The particles exhibit disorder structure at low engine load and high engine speed due to lower in-cylinder gas temperature and shorter combustion duration. Quantitative analysis of the nanostructures indicates that graphitization of primary particles is affected by engine load but not by engine speed.

Investigation on the combustion characteristics of a diesel engine fueled with diesel–azides blends

In order to solve the problems associated with rapid combustion process under high engine speeds, benzyl azides were chosen as a fuel additive blended with diesel fuel. The physical and chemical properties of the fuel blends were analyzed. A single-cylinder engine was built and the in-cylinder working process was studied thoroughly for the new diesel–azides blends. Results show that, compared with pure diesel fuel, diesel–azides blends lead to higher heat release rate peak of premixed combustion, shorter combustion duration, and improved cycle efficiency. This indicates that diesel–azides blends have the potential for promoting rapid combustion. The current study provides a new way to improve the diesel combustion process and has laid a foundation for increasing engine power.