Effects of premixed diethyl ether (DEE) on combustion and exhaust emissions in a HCCI-DI diesel engine

In this study, the effects of premixed ratio of diethyl ether (DEE) on the combustion and exhaust emissions of a single-cylinder, HCCI-DI engine were investigated. The experiments were performed at the engine speed of 2200 rpm and 19 N m operating conditions. The amount of the premixed DEE was controlled by a programmable electronic control unit (ECU) and the DEE injection was conducted into the intake air charge using low pressure injector. The premixed fuel ratio (PFR) of DEE was changed from 0% to 40% and results were compared to neat diesel operation. The percentages of premixed fuel were calculated from the energy ratio of premixed DEE fuel to total energy rate of the fuels. The experimental results show that single stage ignition was found with the addition of premixed DEE fuel. Increasing and phasing in-cylinder pressure and heat release were observed in the premixed stage of the combustion. Lower diffusion combustion was also occurred. Cycle-to cycle variations were very small with diesel fuel and 10% DEE premixed fuel ratio. Audible knocking occurred with 40% DEE premixed fuel ratio. NO_x-soot trade-off characteristics were changed and improvements were found simultaneously. NO_x and soot emissions decreased up to 19.4% and 76.1%, respectively, while exhaust gas temperature decreased by 23.8%. On the other hand, CO and HC emissions increased.     

Assessing combustion and emission performance of direct use of SVO in a diesel engine by oxygen enrichment of intake air method

This work investigated the effect of the oxygen enrichment in the intake air of diesel engines on the combustion and emissions performance using rape seed oil (RSO) as a fuel. The purpose of the paper is to investigate the potential of oxygen enrichment in the intake air method to restrain the deterioration of particulate emissions of the RSO due to its high viscosity so as to explore the possibility of direct use of SVO (straight vegetable oil) in diesel engines, which can reduce CO₂ emissions and save cost. The combustion parameters such as ignition delay, heat release rate, in-cylinder peak temperature and pressure were determined. Engine out particulate and gaseous emissions of the RSO were measured at oxygen concentrations from 21% (by volume) (no enrichment) to 24% (by volume) and compared to diesel results. The enrichment of the intake air with oxygen decreased the ignition delay and premixed combustion duration, and increased the in-cylinder peak pressure and temperature. The particulate, CO and hydrocarbon emissions were significantly reduced while the NOx emissions increased as the oxygen enrichment rate increased. 22% oxygen enrichment rate was suggested to achieve lower than diesel particulate emissions with the lowest NOx penalty. Increased NOx could be controlled by other methods. The results show that the oxygen enrichment in intake air method enabled direct combustion of SVO in diesel engines with reduced particulate, hydrocarbon and CO emissions.     

Experimental study of various effects of exhaust gas recirculation (EGR) on combustion and emissions of an automotive direct injection diesel engine

Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production and is used on most modern high-speed direct injection (HSDI) diesel engines. However EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease of peak heat release, decrease in O₂ concentration (and thus of global air/fuel ratio (AFR)) and flame temperature, increase of lift-off length, etc.), and thus the influence of EGR on NO_x and particulate matter (PM) emissions is not perfectly understood, especially under high EGR rates. An experimental study has been conducted on a 2.0 l HSDI automotive diesel engine under low-load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NO_x/PM emissions. The increase of inlet temperature with EGR has contrary effects on combustion and emissions, thus sometimes giving opposite tendencies as traditionally observed, as, for example, the reduction of NO_x emissions with increased inlet temperature. For a purely diffusion combustion the ROHR is unchanged when the AFR is maintained when changing in-cylinder ambient gas properties (temperature or EGR rate). At low-load conditions, use of high EGR rates at constant boost pressure is a way to drastically reduce NO_x and PM emissions but with an increase of brake-specific fuel consumption (BSFC) and other emissions (CO and hydrocarbon), whereas EGR at constant AFR may drastically reduce NO_x emissions without important penalty on BSFC and soot emissions but is limited by the turbocharging system.     

Performance and emission characteristics of a diesel engine with Diesel Premixed Compression Ignition and exhaust gas recirculation

In the present work, diesel was used as a premixed fuel along with the conventional injection of diesel with a premixed ratio of 0.25. The premixed charge was burned in the cylinder along with the fuel directly injected into the cylinder by a conventional injection system. To control nitrogen oxide(s) (NO_x) emissions, Exhaust Gas Recirculation (EGR) was adopted and the exhaust gas was varied from 10% to 30% in steps of 10%. The performance and emission characteristics were compared with conventional 100% diesel injection in the main chamber. Based on the experiments conducted on a Compression Ignition Direct Injection (CIDI) engine, it was found that unburnt hydrocarbons, carbon monoxide, and soot emissions increase. Soot emission decreases with up to 20% EGR and increases when EGR was increased beyond 20%. Hence 20% EGR was found to be the optimum use for DPMCI mode with a premixed ratio of 0.25. Due to the lean operation, significant reduction in NO_x was achieved with the DPMCI combustion mode. Brake thermal efficiency was marginally decreased compared to CIDI mode.     

Study of the characteristic of diesel spray combustion and soot formation using laser-induced incandescence (LII)

In this paper, the planar images of diesel spray combustion flame and soot formation were measured and analyzed by using LII, in a constant volume combustion vessel. The effects of combustion flame and fuel–air mixing characteristics on soot formation and distribution of soot concentration were studied at different conditions. The result indicates that, with increase in ambient temperature and pressure, the ignition delay of diesel fuel is shorter. The increase of ambient temperature and pressure and the reduction of injection pressure shorten the diesel flame lift-off length. The lower the ambient temperature and pressure, the weaker LII signal intensity. At the same ambient temperature and pressure condition, the higher the diesel injection pressure, the smaller the soot production in diesel jet spray, and soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel jet.     

Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation)

The effects of biodiesel (rapeseed methyl ester, RME) and different diesel/RME blends on the diesel engine NO_x emissions, smoke, fuel consumption, engine efficiency, cylinder pressure and net heat release rate are analysed and presented. The combustion of RME as pure fuel or blended with diesel in an unmodified engine results in advanced combustion, reduced ignition delay and increased heat release rate in the initial uncontrolled premixed combustion phase. The increased in-cylinder pressure and temperature lead to increased NO_x emissions while the more advanced combustion assists in the reduction of smoke compared to pure diesel combustion. The lower calorific value of RME results in increased fuel consumption but the engine thermal efficiency is not affected significantly. When similar percentages (% by volume) of exhaust gas recirculation (EGR) are used in the cases of diesel and RME, NO_x emissions are reduced to similar values, but the smoke emissions are significantly lower in the case of RME. The retardation of the injection timing in the case of pure RME and 50/50 (by volume) blend with diesel results in further reduction of NO_x at a cost of small increases of smoke and fuel consumption.     

Hydrogen addition to acetylene-air laminar diffusion flames: Studies on soot formation under different flow arrangements

Axisymmetric co-flowing acetylene/air laminar diffusion flames have been experimentally investigated to study the effect of hydrogen addition on soot formation and soot morphology. An acetylene-hydrogen jet burning in co-flowing air at atmospheric pressure has been studied under different flow arrangements, i.e., premixed and with separate addition of acetylene and hydrogen. Thermophoretic sampling and analysis by transmission electron microscopy are employed for soot diagnostics. Soot microstructure, primary particle size, soot volume fraction, and fractal geometry results are reported. The effect of hydrogen addition on the temperature field is moderate (maximum increase ∼100 K), the effect being greater when hydrogen is premixed with acetylene. Soot volume fraction decreases with hydrogen addition. A shift was noted in the soot volume fraction peak with change in the Reynolds and Froude numbers at the burner exit. The primary soot particle diameter is in the range of 20-35 nm. Soot particles are larger in size close to the burner for the pure acetylene flame. A reverse trend is observed with hydrogen addition. The fractal dimension of the soot aggregates is about 1.7-1.8. It is unaffected by hydrogen addition and location in the flame. Soot aggregate size tends to decrease with hydrogen addition. The results of the present study on the effect of hydrogen addition on soot volume fraction and mean primary particle size are in good correlation with the results of other investigators for ethylene-, propane-, and butane-air flames, which have been described with regard to the HACA mechanism of soot nucleation and growth and enhanced soot oxidation in fuel-rich flames by increased OH radical concentration.     

Experimental study of the performances of a modified diesel engine operating in homogeneous charge compression ignition (HCCI) combustion mode versus the original diesel combustion mode

Homogeneous charge compression ignition (HCCI) combustion mode provides very low NO_x and soot emissions; however, it has some challenges associated with hydrocarbon (HC) emissions, fuel consumption, difficult control of start of ignition and bad behaviour to high loads. Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NO_x production in diesel and HCCI combustion mode. However EGR has different effects on combustion and emissions, which are difficult to distinguish. This work is intended to characterize an engine that has been modified from the base diesel engine (FL1 906 DEUTZ-DITER) to work in HCCI combustion mode. It shows the experimental results for the modified diesel engine in HCCI combustion mode fueled with commercial diesel fuel compared to the diesel engine mode. An experimental installation, in conjunction with systematic tests to determine the optimum crank angle of fuel injection, has been used to measure the evolution of the cylinder pressure and to get an estimate of the heat release rate from a single-zone numerical model. From these the angle of start of combustion has been obtained. The performances and emissions of HC, CO and the huge reduction of NO_x and smoke emissions of the engine are presented. These results have allowed a deeper analysis of the effects of external EGR on the HCCI operation mode, on some engine design parameters and also on NO_x emission reduction.     

Effect of exhaust gas recirculation (EGR) temperature for various EGR rates on heavy duty DI diesel engine performance and emissions

DI diesel engines are well established today as the main powertrain solution for trucks and other relevant heavy duty vehicles. At the same time emission legislation (mainly for NO_x and particulate matter) becomes stricter, reducing their limit to extremely low values. One efficient method to control NO_x in order to achieve future emissions limits is the use of rather high exhaust gas recirculation (EGR) rates accompanied by increased boost pressure to avoid the negative impact on soot emissions. The method is based on the reduction of gas temperature level and O₂ availability inside the combustion chamber, but unfortunately it has usually an adverse effect on soot emissions and brake specific fuel consumption (bsfc). The use of high EGR rates creates the need for EGR gas cooling in order to minimize its negative impact on soot emissions especially at high engine load were the EGR flow rate and exhaust temperature are high. For this reason in the present paper it is examined, using a multi-zone combustion model, the effect of cooled EGR gas temperature level for various EGR percentages on performance and emissions of a turbocharged DI heavy duty diesel engine operating at full load. Results reveal that the decrease of EGR gas temperature has a positive effect on bsfc, soot (lower values) while it has only a small positive effect on NO. As revealed, the effect of low EGR temperature is stronger at high EGR rates.     

Investigation of reverse flow distributed combustion for gas turbine application

In this paper reverse flow modes of colorless distributed combustion (CDC) have been investigated for application to gas turbine combustors. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation reverse flow modes consisting of three configurations at thermal intensity of 28 MW/m³-atm and five configurations at thermal intensity of 57 MW/m³-atm have been investigated and these high thermal loadings represent characteristic gas turbine combustion conditions. In all the configurations the air injection port is positioned at the combustor exit end, whereas the location of fuel injection ports is changed to give different configurations. The results are presented on the exhaust emissions and radical emissions using experiments, and evaluation of flowfield using numerical simulations. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes investigated here. Cross-flow configuration, wherein the fuel is injected at high velocity cross stream to the air jet resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from closer to diffusion mode to distributed regime. This feature is beneficial for part load operation where higher stability limit is desirable.     

Homogeneous Charge Compression Ignition (HCCI) combustion: Implementation and effects on pollutants in direct injection diesel engines

Homogeneous Charge Compression Ignition (HCCI) combustion is a combustion concept which offers simultaneous reductions in both NO_x and soot emissions from internal combustion engines. In light of increasingly stringent diesel emissions limits, research efforts have been invested into HCCI combustion as an alternative to conventional diesel combustion. This paper reviews the implementation of HCCI combustion in direct injection diesel engines using early, multiple and late injection strategies. Governing factors in HCCI operations such as injector characteristics, injection pressure, piston bowl geometry, compression ratio, intake charge temperature, exhaust gas recirculation (EGR) and supercharging or turbocharging are discussed in this review. The effects of design and operating parameters on HCCI diesel emissions, particularly NO_x and soot, are also investigated. For each of these parameters, the theories are discussed in conjunction with comparative evaluation of studies reported in the specialised literature.     

Effect of advancing the closing angle of the intake valves on diffusion-controlled combustion in a HD diesel engine

An experimental investigation has been performed on the modification of in-cylinder gas thermodynamic conditions by advancing the intake valve closing angle in a HD diesel engine. The consequences on the diffusion-controlled combustion process have been analysed in detail, including the evolution of exhaust emissions and engine efficiency. This research has been carried out at full load (100%) and low engine speed (1200 rpm) with the aim of generating a long and stable diffusion-controlled combustion process. The intake oxygen mass concentration was kept at 17.4% to obtain low NO_x levels in all cases. The required flexibility on intake valve motion has been attained by means of an electro-hydraulic variable valve actuation system. The results obtained from advancing the intake valve closing angle (IVC) have shown an important reduction on in-cylinder gas pressure and density, whereas the gas temperature showed less sensitivity. Consequently, the diffusion-controlled combustion process is slowed down mainly due to the lower in-cylinder gas density and oxygen availability. Important effects of advancing IVC have also been observed on pollutant emissions and engine efficiency. Where NO_x production decreases, soot emissions increase. Finally, the results of pollutant emissions and engine efficiency have been compared with those obtained retarding the start of injection.     

NO2-Assisted Soot Regeneration Behavior in a Diesel Particulate Filter with Heavy-Duty Diesel Exhaust Gases

A major concern in operating a diesel engine is how to reduce the soot emission from the exhaust gases, as soot has a negative effect on both human health and the environment. More stringent emission regulations make the diesel particulate filter (DPF) an indispensable after-treatment component to reduce diesel soot from exhaust gases. The most important issue in developing an effective DPF, however, is regeneration technology to oxidize the diesel soot trapped in the filter, either periodically or continuously, during regular engine operations. Various methods exist for regenerating diesel soot captured by the filter. Of these, NO₂ is widely used for continuous regeneration of diesel soot since it can oxidize diesel soot at lower temperatures than the conventional oxidizer O₂ In this work, after introducing governing equations for trapping and regenerating diesel soot in the DPF, regeneration behavior is examined by changing such various parameters as exhaust gas temperature and O₂ concentration. Numerical investigation is then performed in order to find the optimum NO₂/soot ratio required for continuous regeneration of the soot deposited in the DPF.     

Investigation on soot emissions from diesel-CNG dual-fuel

The burning of diesel and compressed natural gas (CNG) is attractive compared to diesel fuel because of the reduction of CO₂ emissions and particulate matter (PM) emissions. While soot emissions from the diesel-CNG combustion can be tested in a real-world single-cylinder engine, the soot formation characteristics cannot be tested in the same way. Therefore, to understand the mechanisms behind soot formation in diesel-CNG combustion, soot evolution must be investigated using a simulation model. In this study, the soot evolution is investigated under different CNG substitution ratios with single and split fuel injection. An AVL 5402 single-cylinder diesel engine was modified to run diesel/CNG dual-fuel to investigate the combustion and soot emissions. A new soot model using KIVA-3V R2 code and integrated with a reduced heptane/methane PAH (polycyclic aromatic hydrocarbons) mechanism was used to simulate soot behavior. For the combustion, the results show that the ignition delay gets extended, the combustion duration gets shorter and the peak pressure can be improved when CNG substitution ratio is increased both with single and split injection. Additionally, a slight increase of pressure is observed when the split injection is used. This is because the split injection is an effective strategy to change the distribution and vaporization of fuel, which results in an incremental increase in combustion efficiency and increase pressure. As the CNG substitution ratio is increased, soot emissions get drastically reduced. The reason is the equivalence ratio distribution of air-fuel becomes more homogenous and the local fuel-rich region shrinks with increasing of CNG substitution ratios. Pyrene is an important intermediate specie to generate soot particles. The results show that pyrene distribution decreases, leading to a reduced generation of soot precursors. As a result, the soot mass of CNG70 is less than the other two cases. The basic reason is the prolonged ignition delay allowed for more time for fuel−air mixing, which reduces soot mass formation.     

Effect of Exhaust Gas Recirculation (EGR) on performance,emissions, deposits and durability of a constant speed compression ignition engine

To meet stringent vehicular exhaust emission norms worldwide, several exhaust pre-treatment and post-treatment techniques have been employed in modern engines. Exhaust Gas Recirculation (EGR) is a pre-treatment technique, which is being used widely to reduce and control the oxides of nitrogen (NO_x) emission from diesel engines. EGR controls the NO_x because it lowers oxygen concentration and flame temperature of the working fluid in the combustion chamber. However, the use of EGR leads to a trade-off in terms of soot emissions. Higher soot generated by EGR leads to long-term usage problems inside the engines such as higher carbon deposits, lubricating oil degradation and enhanced engine wear. Present experimental study has been carried out to investigate the effect of EGR on soot deposits, and wear of vital engine parts, especially piston rings, apart from performance and emissions in a two cylinder, air cooled, constant speed direct injection diesel engine, which is typically used in agricultural farm machinery and decentralized captive power generation. Such engines are normally not operated with EGR. The experiments were carried out to experimentally evaluate the performance and emissions for different EGR rates of the engine. Emissions of hydrocarbons (HC), NO_x, carbon monoxide (CO), exhaust gas temperature, and smoke opacity of the exhaust gas etc. were measured. Performance parameters such as thermal efficiency, brake specific fuel consumption (BSFC) were calculated. Reduction in NO_x and exhaust gas temperature were observed but emissions of particulate matter (PM), HC, and CO were found to have increased with usage of EGR. The engine was operated for 96 h in normal running conditions and the deposits on vital engine parts were assessed. The engine was again operated for 96 h with EGR and similar observations were recorded. Higher carbon deposits were observed on the engine parts operating with EGR. Higher wear of piston rings was also observed for engine operated with EGR.     

PDA research on a novel pulverized coal combustion technology for a large utility boiler

In this paper, a new technology for a tangential firing pulverized coal boiler, high efficiency and low NO_x combustion technology with multiple air-staged and a large-angle counter flow fuel-rich jet (ACCT for short) is proposed. To verify the characteristics of this technology, experiments of two combustion technologies, ACCT and CFS-1 (Concentric Firing System-1), are carried out under a cold model of a 1025 t/h tangential firing boiler with a PDA (particle dynamics anemometer). The distributions of velocity, particle concentration, particle diameters and the particle volume flux of primary air and secondary air are obtained. The results show that the fuel-rich primary air of ACCT can go deeper into the furnace and mix with the main flow better, which means that the counter flow of fuel-rich jets in ACCT can realize stable combustion, low NO_x emission and slagging prevention.     

Optical diagnostics for soot and temperature measurement in diesel engines

This paper reviews the optical techniques for in-cylinder combustion temperature measurement, particularly soot measurements in diesel engines. The review starts with the two-colour method for in-cylinder soot and combustion temperature measurement. The principle and implementation of the two-colour technique are described in detail. Both signal point and full-field temperature and soot measurements by the two-colour method are considered. In the second part, the soot diagnostics based on light scattering, especially the light extinction method for in-cylinder soot concentration measurements, are discussed. In the third part, optical techniques for spatially resolved two-dimensional measurements of soot particles in diesel engines are introduced. Since laser induced incandescence (LII) is a relatively new technique and is particularly suitable for the two-dimensional imaging of soot distribution, the operating principle and implementation of LII are discussed in detail. At the end of each part, examples are given to illustrate the understanding gained about diesel combustion as a result of the application of these optical techniques. This paper provides a comprehensive review for those who are interested in using optical diagnostics for in-cylinder soot and combustion temperature measurement in diesel engines.     

柴油机部分均质预混燃烧模式下混合与化学控制参数对指示热效率的影响

基于部分均质预混燃烧(PPC)的柴油机研究开发和优化了一种混合燃烧控制策略,在平均指示压力(IMEP)高达1.1,MPa的负荷范围内实现了高的指示热效率以及超低排放.燃烧过程中的混合与化学控制参数包括了喷油定时、喷油模式(如多脉冲喷射)、增压压力、EGR率以及进气气门关闭定时等,通过优化耦合以上控制参数可以优化控制当量比与温度的变化路径,从而避开NOx与碳烟(Soot)生成区.基于热力学第一定律,通过能量平衡的分析方法研究了混合与化学控制参数对热效率的影响.研究表明,相对于排放而言,热效率受控制参数的影响更加敏感.     

电控高压共轨柴油机排气微粒的测量及数值模拟

利用改进后的全气缸取样系统,对不同工况下高压共轨电控柴油机燃烧过程中的微粒生成历程进行测量.同时,采用三维CFD数值模拟方法分析了微粒的生成机理.结果表明,微粒质量浓度曲线呈单峰状,微粒生成质量随负荷的增大而增加;发动机排出微粒是碳烟粒子生成和氧化这两个过程综合作用的结果.加强缸内的气流运动和合理组织燃烧过程,有助于减少微粒的生成.     

Combustion,performance and emissions of a diesel engine with H2, CH4 and H2–CH4 addition

Experiments were conducted to investigate the combustion and emission characteristics of a diesel engine with addition of hydrogen or methane for dual-fuel operation, and mixtures of hydrogen–methane for tri-fuel operation. The in-cylinder pressure and heat release rate change slightly at low to medium loads but increase dramatically at high load owing to the high combustion temperature and high quantity of pilot diesel fuel which contribute to better combustion of the gaseous fuels. The performance of the engine with tri-fuel operation at 30% load improves with the increase of hydrogen fraction in methane and is always higher than that with dual-fuel operations. Compared with ULSD–CH₄ operation, hydrogen addition in methane contributes to a reduction of CO/CO₂/HC emissions without penalty on NO_x emission. Dual-fuel and tri-fuel operations suppress particle emission to the similar extent. All the gaseous fuels reduce the geometry mean diameter and total number concentration of diesel particulate. Tri-fuel operation with 30% hydrogen addition in methane is observed to be the best fuel in reducing particulate and NO_x emissions at 70 and 90% loads.