Biodiesel combustion in an optical HSDI diesel engine under low load premixed combustion conditions

An optically accessible single-cylinder high-speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes for biodiesel blends under different injection strategies. The experimental results indicated that the heat release rate was dominated by a premixed combustion pattern and the heat release rate peak became smaller with injection timing retardation. The ignition and heat release rate peak occurred later with increasing biodiesel content. Fuel impingement on the wall was observed for all test conditions. The liquid penetration became longer and the fuel impingement was stronger with the increase of biodiesel content. Early and late injection timings result in lower flame luminosity due to improved mixing with longer ignition delay. For all the injection timings, lower soot luminosity was seen for biodiesel blends than pure diesel fuel. Furthermore, NO_x emissions were dramatically reduced for premixed combustion mode with retarded post-TDC injection strategies.     

Effects of injection angles on combustion processes using multiple injection strategies in an HSDI diesel engine

Effects of injection angles and injection pressure on the combustion processes employing multiple injection strategies in a high-speed direct-injection (HSDI) diesel engine are presented in this work. Whole-cycle combustion and liquid spray evolution processes were visualized using a high-speed video camera. NO_x emissions were measured in the exhaust pipe. Different heat release patterns are seen for two different injectors with a 70-degree tip and a 150-degree tip. No evidence of fuel-wall impingement is found for the first injection of the 150-degree tip, but for the 70-degree tip, some fuel impinges on the bowl wall and a fuel film is formed. For the second injection, a large amount of fuel deposition is observed for the 70-degree tip. Weak flame is seen for the first injection of the 150-degree tip while two sorts of flames are seen for the first injection of the 70-degree tip including an early weak flame and a late luminous film combustion flame. Ignition occurs near the spray tip in the vicinity of the bowl wall for the second injection events of the 150-degree tip, however, it is near the injector tip in the central region of the bowl for the 70-degree tip. The flame is more homogeneous for the 150-degree tip with higher injection pressure with little soot formation similar to a premixed-charge-compression-ignition (PCCI) combustion. For other cases, liquid fuel is injected into flames showing diffusion flame combustion. More soot luminosity is seen for the 70-degree tip due to significant fuel film deposition on the piston wall with fuel film combustion for both injection events. Lower NO_x emissions were obtained for the narrow-angle injector due to the rich air–fuel mixture near the bowl wall during the combustion process. Increasing injection pressure leads to increased NO_x emissions for both injection angles because of the relatively leaner and faster combustion process with higher in-cylinder temperature for the increased injection pressure.     

Charge stratification to control HCCI: Experiments and CFD modeling with n-heptane as fuel

An optimized reduced mechanism of n-heptane including 42 species and 58 elementary reactions adapted to charge stratification combustion is developed first in this study. Some engine experiments and a fully coupled CFD and reduced chemical kinetics model with n-heptane as fuel are adopted to investigate the combustion processes of HCCI-like charge stratification combustion aimed at diesel HCCI application. For premixed/direct-injected stratification combustion, the low temperature reaction occurs in the regions with homogeneous fuel first and high temperature reaction begins from high fuel concentration regions involved in the spray process. With the increase of the injection ratio, the high temperature reaction occurs in advance, the pressure rise rate reduces, UHC emissions decrease and CO emissions increase. At larger injection ratio, the onset of the high temperature reaction advances and the maximum pressure rise rate decreases with the retarding of injection timing. UHC and CO emissions have relation to the fuel spray penetration at different injection timings. NO_x emissions increase rapidly with the increase of the stratification degree.     

Effects of dimethyl-ether (DME) spray behavior in the cylinder on the combustion and exhaust emissions characteristics of a high speed diesel engine

The purpose of this study was to analyze the exhaust emissions of DME fuel through experimental and numerical analyses of in-cylinder spray behavior. To investigate this behavior, spray characteristics such as the spray tip penetration, spray cone angle, and spray targeting point were studied in a re-entrant cylinder shape under real combustion chamber conditions. The combustion performance and exhaust emissions of the DME-fueled diesel engine were calculated using KIVA-3V. The numerical results were validated with experimental results from a DME direct injection compression ignition engine with a single cylinder.The combustion pressure and IMEP have their peak values at an injection timing of around BTDC 30°, and the peak combustion temperature, exhaust emissions (soot, NO_x), and ISFC had a lower value. The HC and CO emissions from DME fuel showed lower values and distributions in the range from BTDC 25° to BTDC 10° at which a major part of the injected DME spray was distributed into the piston bowl area. When the injection timing advanced to before BTDC 30°, the HC and CO emissions showed a rapid increase. When the equivalence ratio increased, the combustion pressure and peak combustion temperature decreased, and the peak IMEP was retarded from BTDC 25° to BTDC 20°. In addition, NO_x emissions were largely decreased by the low combustion temperature, but the soot emissions increased slightly.     

Effects of ethyl tert-butyl ether addition to diesel fuel on characteristics of combustion and exhaust emissions of diesel engines

The effects of ethyl tert-butyl ether (ETBE) addition to diesel fuel on the characteristics of combustion and exhaust emissions of a common rail direct injection diesel engine with high rates of cooled exhaust gas recirculation (EGR) were investigated. Test fuels were prepared by blending 0, 10, 20, 30 and 40 vol% ETBE to a commercial diesel fuel. Increasing ETBE fraction in the fuel helps to suppress the smoke emission increasing with EGR, but a too high fraction of ETBE leads to misfiring at higher EGR rates. While the combustion noise and NO_x emissions increase with increases in ETBE fraction at relatively low EGR rates, they can be suppressed to low levels by increasing EGR. Though there are no significant increases in THC and CO emissions due to ETBE addition to diesel fuel in a wide range of EGR rates, the ETBE blended fuel results in higher aldehyde emissions than the pure diesel fuel at relatively low EGR rates. With the 30% ETBE blended fuel, the operating load range of smokeless, ultra-low NO_x (<0.5 g/kWi h), and efficient diesel combustion with high rates of cooled EGR is extended to higher loads than with the pure diesel fuel.     

Pilot ignited premixed combustion of dimethyl ether in a turbodiesel engine

This paper describes combustion studies of dimethyl ether in a common rail turbodiesel engine wherein the dimethyl ether was fumigated into the intake air and the conventional diesel injection was used with the intention of igniting the premixed DME-air charge. This combustion process is referred to here as a “mixed mode” process and is similar in some respects to what is commonly referred to as “dual fuel” combustion. In contrast to “dual fuel” combustion, however, in which the gaseous fuel is often natural gas or biogas, in this process with DME the gaseous charge ignites largely independently of the diesel injection. The diesel injection was accomplished with a single, main injection. The engine was operated at a single speed and load. Gaseous and particulate emissions were monitored and heat release analysis was performed to examine how the fuels burn and the impact on emissions formation at various levels of substitution of diesel fuel with fumigated DME, at as high as 44% of the fuel energy from DME. Reductions in NO_x emissions and increases in particulate matter emissions are observed with DME fumigation. The increase in PM emissions is attributed to enrichment of the diesel fuel spray, due to displacement of intake oxygen by the fumigated DME, despite the widely observed soot suppressing effect of DME.     

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

This study investigated the combustion and emissions characteristics of a compression-ignition engine using a dual-fuel approach with ammonia and diesel fuel. Ammonia can be regarded as a hydrogen carrier and used as a fuel, and its combustion does not produce carbon dioxide. In this study, ammonia vapor was introduced into the intake manifold and diesel fuel was injected into the cylinder to initiate combustion. The test engine was a four-cylinder, turbocharged diesel engine with slight modifications to the intake manifold for ammonia induction. An ammonia fueling system was developed, and various combinations of ammonia and diesel fuel were successfully tested. One scheme was to use different combinations of ammonia and diesel fuel to achieve a constant engine power. The other was to use a small quantity of diesel fuel and vary the amount of ammonia to achieve variable engine power. Under the constant engine power operation, in order to achieve favorable fuel efficiency, the preferred operation range was to use 40-60% energy provided by diesel fuel in conjunction with 60-40% energy supplied by ammonia. Exhaust carbon monoxide and hydrocarbon emissions using the dual-fuel approach were generally higher than those of using pure diesel fuel to achieve the same power output, while NO_x emissions varied with different fueling combinations. NO_x emissions could be reduced if ammonia accounted for less than 40% of the total fuel energy due to the lower combustion temperature resulting in lower thermal NO_x. If ammonia accounted for the majority of the fuel energy, NO_x emissions increased significantly due to the fuel-bound nitrogen. On the other hand, soot emissions could be reduced significantly if a significant amount of ammonia was used due to the lack of carbon present in the combination of fuels. Despite the overall high ammonia conversion efficiency (nearly 100%), exhaust ammonia emissions ranged from 1000 to 3000 ppmV and further after-treatment will be required due to health concerns. On the other hand, the variable engine power operation resulted in relatively poor fuel efficiency and high exhaust ammonia emissions due to the lack of diesel energy to initiate effective combustion of the lean ammonia-air mixture. The in-cylinder pressure history was also analyzed, and results indicated that ignition delay increased with increasing amounts of ammonia due to its high resistance to autoignition. The peak cylinder pressure also decreased because of the lower combustion temperature of ammonia. It is recommended that further combustion optimization using direct ammonia/diesel injection strategies be performed to increase the combustion efficiency and reduce exhaust ammonia emissions.     

An investigation of the effects of spray angle and injection strategy on dimethyl ether (DME) combustion and exhaust emission characteristics in a common-rail diesel engine

An experimental investigation was performed on the effects of spray angle and injection strategies (single and multiple) on the combustion characteristics, concentrations of exhaust emissions, and the particle size distribution in a direct-injection (DI) compression ignition engine fueled with dimethyl ether (DME) fuel. In this study, two types of narrow spray angle injectors (θ_spray = 70° and 60°) were examined and its results were compared with the results of conventional spray angle (θ_spray = 156°). In addition, to investigate the optimal operating conditions, early single-injection and multiple-injection strategies were employed to reduce cylinder wall-wetting of the injected fuels and to promote the ignition of premixed charge. The engine test was performed at 1400 rpm, and the injection timings were varied from TDC to BTDC 40° of the crank angle.The experimental results showed that the combustion pressure from single combustion for narrow-angle injectors (θ_spray = 70° and 60°) is increased, as compared to the results of the wide-angle injector (θ_spray = 156°) with advanced injection timing of BTDC 35°. In addition, two peaks of the rate of heat release (ROHR) are generated by the combustion of air-fuel premixed mixtures. DME combustion for all test injectors indicated low levels of soot emissions at all injection timings. The NO_x emissions for narrow-angle injectors simultaneously increased in proportion to the advance in injection timing up to BTDC 25°, whereas BTDC 20° for the wide-angle injector. For multiple injections, the combustion pressure and ROHR of the first injection with narrow-angle injectors are combusted more actively, and the ignition delay of the second injected fuel is shorter than with the wide-angle injector. However, the second combustion pressure and ROHR were lower than during the first injection, and combustion durations are prolonged, as compared to the wide-angle injector. With advanced timing of the first injection, narrow-angle injectors with multiple injections could achieve low NO_x levels and soot levels similar to single-injection cases.     

Biodiesel engine performance and emissions in low temperature combustion

Engine performance and emission comparisons were made between the use of soy, Canola and yellow grease derived B100 biodiesel fuels and an ultra-low sulphur diesel fuel in the high load engine operating conditions. Compared to the diesel fuel engine-out emissions of nitrogen oxides (NO_x), a high-cetane number (CN) biodiesel fuel produced comparable NO_x while the biodiesel with a CN similar to the diesel fuel produced relatively higher NO_x at a fixed start of injection. The soot, carbon monoxide and un-burnt hydrocarbon emissions were generally lower for the biodiesel-fuelled engine. Exhaust gas recirculation (EGR) was then extensively applied to initiate low temperature combustion (LTC) mode at medium and low load conditions. An intake throttling valve was implemented to increase the differential pressure between the intake and exhaust in order to increase and enhance the EGR. Simultaneous reduction of NO_x and soot was achieved when the ignition delay was prolonged by more than 50% from the case with 0% EGR at low load conditions. Furthermore, a preliminary ignition delay correlation under the influence of EGR at steady-state conditions was developed. The correlation considered the fuel CN and oxygen concentrations in the intake air and fuel. The research intends to achieve simultaneous reductions of NO_x and soot emissions in modern production diesel engines when biodiesel is applied.     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.     

Effect of biofuels combustion on the nanoparticle and emission characteristics of a common-rail DI diesel engine

This study was performed to investigate the effect of biogas-biodiesel fuel combustion on the emissions reduction and nanoparticle characteristics in a direct injection (DI) diesel engine. In order to apply the two biofuels, biogas was injected into a premixed chamber during the intake process by using two electronically controlled gas injectors, and biodiesel fuel was directly injected into combustion chamber by a high-pressure injection system. The in-cylinder pressure and rate of heat release (ROHR) were investigated under various fuel conditions for single-fuel (biodiesel) and dual-fuel (biogas-biodiesel) combustions. To evaluate the engine performances and exhaust emissions characteristics, the indicated mean effective pressure (IMEP) and exhaust emissions were also investigated under various test conditions. Furthermore, the particle number concentration and the size distribution of nanoparticles were analyzed by using a scanning mobility particle sizer (SMPS).In the case of dual-fuels, the peak combustion pressure and ROHR were gradually decreased with the increase of the biogas fraction in the dual-fuels. As the premixed ratios increased, ignition delay and combustion durations were prolonged compared to single-fuel mode. The dual-fuels combustion showed that the IMEP decreased slightly and maintained similar levels up to 20° BTDC due to the retarded combustion phase. The concentrations of NO_x emissions were decreased for all injection timings as the premixed ratio (r_p) increased. The soot emissions in dual-fuel operations were significantly lower than those in the single-fuel mode (r_p = 0), and decreased gradually as the premixed ratio increased, regardless of injection timing. A lower nanoparticle size distribution was observed at all premixed ratios for dual-fuel combustion compared to those of the single fuel mode. The number distribution of both nuclei and accumulation modes also decreased with an increase in the biogas fraction. A slight reduction in the total particle number and total volume for all premixed ratios was observed as the injection timing increased from TDC up to 20° BTDC.     

Evaluation of formulation strategies to eliminate the biodiesel NOx effect

In this paper, we explore the efficacy of (1) reducing the iodine value of soy-derived biodiesel fuels through increasing the methyl oleate (methyl ester of oleic acid) content and (2) addition of cetane improvers, as strategies to combat the biodiesel NO_x effect: the increase in NO_x emissions observed in most studies of biodiesel and biodiesel blends. This is accomplished by spiking a conventional soy-derived biodiesel fuel with methyl oleate or with cetane improver. The impact on bulk modulus of compressibility, fuel injection timing, cetane number, combustion, and emissions were examined. The conventional B20 blend produced a NO_x increase of 3–5% relative to petroleum diesel, depending on injection timing. However, by using a B20 blend where the biodiesel portion contained 76% methyl oleate, the biodiesel NO_x effect was eliminated and a NO_x neutral blend was produced. The bulk modulus of petroleum diesel was measured to be 2% lower than B20, yielding a shift in fuel injection timing of 0.1–0.3 crank angle. The bulk modulus of the high methyl oleate B20 blend was measured to be 0.5% lower than B20, not enough to have a measurable impact on fuel injection timing. Increasing the methyl oleate portion of the biodiesel to 76% also had the effect of increasing the cetane number from 48.2 for conventional B20 to 50.4, but this effect is small compared to the increase to 53.5 achieved by adding 1000 ppm of 2-ethylhexyl nitrate (EHN) to B20. For the particular engine tested, NO_x emissions were found to be insensitive to ignition delay, maximum cylinder temperature, and maximum rate of heat release. The dominant effect on NO_x emissions was the timing of the combustion process, initiated by the start of injection, and propagated through the timing of maximum heat release rate and maximum temperature.     

Study on the application of DME/diesel blends in a diesel engine

In this paper fuels, based on various DME to diesel ratios are investigated. Physical and chemical properties of DME and diesel display mutual solubility at any ratio. The vapor pressure of DME/diesel blends is lower than that of pure DME at the same temperatures and it decreases with an increase of diesel mass fraction in blends, which is beneficial to the elimination of vapor lock in the fuel supply system on CI engines. Performance, emission and other features of three kinds of DME/diesel blend fuels and diesels are evaluated in a four-cylinder test engine. By taking relative advantages of DME and diesel, the DME/diesel blends could achieve satisfactory properties in lubricity and atomization, which contributed to improvements in spray and combustion characteristics. Simultaneously, smoke emission could be reduced significantly with a little penalty on CO and HC emissions for DME/diesel blended engine at high loads, in comparison to diesel engine. NO_x emissions of the engine powered by DME/diesel blends are decreased somewhat. Moreover, the power output would be improved a little and NO_x emission could be reduced further if the fuel supply advance angle is retarded appropriately.     

Mitigation of NOx emissions from a jatropha biodiesel fuelled DI diesel engine using antioxidant additives

Biodiesel offers cleaner combustion over conventional diesel fuel including reduced particulate matter, carbon monoxide and unburned hydrocarbon emissions. However, several studies point to slight increase in NO_x emissions (about 10%) for biodiesel fuel compared with conventional diesel fuel. Use of antioxidant additives is one of the most cost-effective ways to mitigate the formation of prompt NO_x. In this study, the effect of antioxidant additives on NO_x emissions in a jatropha methyl ester fuelled direct injection diesel engine have been investigated experimentally and compared. A survey of literature regarding the causes of biodiesel NO_x effect and control strategies is presented. The antioxidant additives L-ascorbic acid, α tocopherol acetate, butylated hydroxytoluene, p-phenylenediamine and ethylenediamine were tested on computerised Kirloskar-make 4 stroke water cooled single cylinder diesel engine of 4.4 kW rated power. Results showed that antioxidants considered in the present study are effective in controlling the NO_x emissions of biodiesel fuelled diesel engines. A 0.025%-m concentration of p-phenylenediamine additive was optimal as NO_x levels were substantially reduced in the whole load range in comparison with neat biodiesel. However, hydrocarbon and CO emissions were found to have increased by the addition of antioxidants.     

A procedure to reduce pollutant gases from Diesel combustion during European MVEG-A cycle by using electrical intake air-heaters

While intake air heating is a widespread technology used in heavy duty diesel engines, it rarely appears in the field of HSDI diesel engines. Nevertheless, intake air heating is currently under study as a possible alternative to glow plugs for cold start assistance in HSDI diesel engines. Since the use of intake air heaters involves a cylinder-head simplification, this solution presents both economical and combustion advantages. Despite of the negative impact of the high intake temperatures on NO_x emissions, intake air heating reduces significantly HC and CO emissions during warm up due to combustion improvement. While the potential of intake air heating technology to assist cold start is proved, the present paper is devoted to the evaluation of the intake air heating technology potential to reduce pollutant emissions from Diesel combustion. To deal with this objective a deep analysis of both glow plug and intake air heating technology behaviour during the MVEG-A cycle is done. Experimental tests are carried out on a small turbocharged direct injection Diesel engine in an engine-in-the-loop (EIL) approach. Results prove that an optimal intake air heating strategy together with an adapted air loop control calibration can improve HC, CO and NO_x emissions with only a scarce PM increase.     

Effect of EGR and injection timing on combustion and emission characteristics of split injection strategy DI-diesel engine fueled with biodiesel

In this study, the effect of injection timing and EGR rate on the combustion and emissions of a Ford Lion V6 split injection strategy direct injection diesel engine has been experimentally investigated by using neat biodiesel produced from soybean oil. The results showed that, with the increasing of EGR rate, the brake specific fuel combustion (BSFC) and soot emission were slightly increased, and nitrogen oxide (NO_x) emission was evidently decreased. Under higher EGR rate, the peak pressure was slightly lower, and the peak heat release rate kept almost identical at lower engine load, and was higher at higher engine load. With the main injection timing retarded, BSFC was slightly increased, NO_x emission was evidently decreased, and soot emission hardly varied. The second peak pressure was evidently decreased and the heat release rate was slightly increased.     

Performance, combustion and emissions of a diesel engine operated with reformed EGR. Comparison of diesel and GTL fuelling

In this work, the effects of a standard ultra-low sulphur diesel (ULSD) fuel and a new, ultra-clean synthetic GTL (gas-to-liquid) fuel on the performance, combustion and emissions of a single-cylinder, direct injection, diesel engine were studied under different operating conditions with addition of simulated reformer product gas, referred to as reformed EGR (REGR). For this purpose various levels of REGR of two different compositions were tested. Tests with standard EGR were also carried out for comparison. Experiments were performed at four steady state operating conditions and the brake thermal efficiency, combustion process and engine emission data are presented and discussed. In general, GTL fuel resulted in a higher brake thermal efficiency compared to ULSD but the differences depended on the engine condition and EGR/REGR level and composition. The combustion pattern was significantly modified when the REGR level was increased. Although the extent of the effects of REGR on emissions depended on the engine load, it can be generally concluded that an optimal combination of GTL and REGR significantly improved both NO_x and smoke emissions. In some cases, NO_x and smoke emission reductions of 75% and 60%, respectively, were achieved compared to operation with ULSD without REGR. This offers a great potential for engine manufacturers to meet the requirements of future emission regulations.     

NOx and SOx emissions of a high sulfur self-retention coal during air-staged combustion

NO_x and SO_x emissions of air-staged combustion were investigated in a 1 MW tangentially-fired furnace combusting a high sulfur self-retention coal. Two variables including the air stoichiometric ratio of primary combustion zone and the relative location of over-fire air (OFA) injection ports were studied. These results suggest that NO_x reduction efficiency monotonically increases with increasing the relative location of OFA injection ports, and the lowest NO_x emissions are achieved when the air stoichiometric ratio of primary combustion zone is 0.85. In the meantime, SO_x emissions can be effectively reduced when the air stoichiometric ratio of primary combustion zone is 0.85 or 0.95, and SO_x emissions monotonically decrease with increasing the relative location of OFA injection ports.     

Improved emission characteristics of HCCI engine by various premixed fuels and cooled EGR

This work investigates partial HCCI (homogeneous charge compression ignition) combustion as a control mechanism for HCCI combustion. The premixed fuel is supplied via a port fuel injection system located in the intake port of DI diesel engine. Cooled EGR is introduced for the suppression of advanced autoignition of the premixed fuel. The premixed fuels used in this experiment are gasoline, diesel, and n-heptane. The results show that with diesel premixed fuel, a simultaneous decrease of NO_x and soot can be obtained by increasing the premixed ratio. However, when the inlet charge is heated for the improved vaporization of diesel fuel, higher inlet temperature limits the operational range of HCCI combustion due to severe knocking and high NO_x emission at high premixed ratios. Gasoline premixing shows the most significant effects in the reductions of NO_x and soot emissions, compared to other kinds of premixed fuels.     

Emissions from large-scale medium-speed diesel engines: 3. Influence of direct water injection and common rail

The influence of direct water injection (DWI) on emissions from a multivariable large-scale (6–18 cyl, ~ 1 MW/cyl) diesel engine is reported, using a combined injection valve and nozzle that allows for injection of water and fuel oil into the cylinder. This method allows for injecting a relatively large amount of water without derating the engine power and NO_x emissions can be more than halved by DWI. Indeed DWI decreases combustion temperatures and NO_x emissions, but it gives somewhat increased (yet not problematic) emissions of CO, HC, soot (smoke) and particulate matter (PM), depending on the water injection timing and degree of incomplete combustion.