Combined effect of boost pressure and injection timing on the performance and combustion of CNG in a DI spark ignition engine

There is an increasing interest in supercharging spark ignition engines operating on CNG (compressed natural gas) mainly due to its superior knock resisting properties. However, there is a penalty in volumetric efficiency when directly injecting the gaseous fuel at early and partial injection timings. The present work reports the combined effects of a small boost pressure and injection timing on performance and combustion of CNG fueled DI (direct injection) engine. The experimental tests were carried out on a 4-stroke DI spark ignition engine with a compression ratio of 14. Early injection timing, when inlet valves are still open (at 300°BTDC), and partial injection timing, in which part of the injection occurs after the inlet valves are closed (at 180°BTDC), were varied at each operating speed with variation of the boost pressure from 2.5 to 10 kPa. A narrow angle injector (NAI) was used to increase the mixing rate at engine speeds between 2000 and 5000 rpm. Similar experiments were conducted on a naturally aspirated engine and the results were then compared with that of the boosting system to examine the combined effects of boost pressure and injection timing. It was observed that boost pressure above 7.5 kPa resulted in an improvement of performance and combustion of CNG DI engine at all operating speeds. This was manifested in the faster heat release rates and mass fraction burned that in turn improved combustion efficiency of the boosting system. An increased in cylinder pressure and temperature was also observed with boost pressure compared to naturally aspirated engine. Moreover, the combustion duration was reduced due to concentration of the heat release near to the top dead center as the result of the boost pressure. Supercharging was also found to reduce the penalty of volumetric efficiency at both the simulated port and partial injection timings.     

Influence of pilot injection on combustion characteristics and emissions in a DI diesel engine fueled with diesel and DME

This work experimentally investigates how the dwell time between pilot injection and main injection influences combustion and emissions characteristics (NOx, CO, THC and smoke) in a single-cylinder DI diesel engine. The experiments were conducted using two fuel injection systems according to the fuel type, diesel or dimethyl ether (DME), due to the different fuel characteristics. The injection strategy is accomplished by varying the dwell time (10°CA, 16°CA and 22°CA) between injections at five main injection timings (?4°CA aTDC, ?2°CA aTDC, 0°CA aTDC, 2°CA aTDC and 4°CA aTDC). Results from pilot-main injection conditions are compared with those shown in single injection conditions to better demonstrate the potential of pilot injection. It was found that pilot injection is highly effective for lowering heat-release rates with smooth pressure traces regardless of the fuel type. Pilot injection also offers high potential to maintain or increase the BMEP; even the combustion-timing is retarded to suppress the NOx emission formation. Overall, NOx emission formation was suppressed more by the combustion phasing retard effect, and not the pilot injection effect considered in this study. Comparison of the emissions for different fuel types shows that CO and HC emissions have low values below 100 ppm for DME operation in both single injection and pilot-main injection. However, NOx emission is slightly higher in the earlier main injection timings (?4°CA aTDC, ?2°CA aTDC) than diesel injections. Pilot injection was found to be more effective with DME for reducing the amount of NOx emission with combustion retardation, which indicates a level of NOx emission similar to that of diesel. Although the diesel pilot-main injection conditions show higher smoke emission than single-injection condition, DME has little smoke emission regardless of injection strategy.     

Effects of the fuel injection ratio on the emission and combustion performances of the partially premixed charge compression ignition combustion engine applied with the split injection method

Currently, due to the severity of world-wide air pollution by substances emitted from vehicles, emission control is being enforced more strictly, and it is expected that the regulation requirements for emission will become even more severe. A new concept combustion technology that can reduce the Nitrogen oxides (NOx) and PM in relation to combustion is urgently required. As a core combustion technology among new combustion technologies for the next generation engine, the homogenous charge compression ignition (HCCI) is expanding its application range by adopting a multiple combustion mode, a catalyst, direct fuel injection and partially premixed charge compression ignition combustion using the split injection method. This paper used a split injection method in order to apply the partially premixed charge compression ignition combustion method without significantly altering engine specifications of the multiple combustion mode and practicality by referring to the results of studies on the HCCI engine. Furthermore, the effects of the ratio of the fuel injection amount on split injection are investigated. From the test results, the adequate combination of the ratio of the fuel injection amount for the split injection method has some benefit on exhaust and fuel economy performance in a naturally aspirated single cylinder diesel engine.     

Effect of a 2-stage injection strategy on the combustion and flame characteristics in a PCCI engine

Recently, to reduce environmental pollution and the waste of limited energy resources, there is an increasing requirement for higher engine efficiency and lower levels of harmful emissions. A premixed charge compression ignition (PCCI) engine, which uses a 2-stage type injection, has drawn attention because this combustion system can simultaneously reduce the amount of NOx and PM exhausted from diesel engines. It is well known that the fuel injection timing and the spray angle in a PCCI engine affect the mixture formation and the combustion. To acquire two optimal injection timings, the combustion and emission characteristics of the PCCI engine were analyzed with various injection conditions. The flame visualization was performed to validate the result obtained from the engine test. This study reveals that the optimum injection timings are BTDC 60° for the first injection and ATDC 5° for the second injection. In addition, the injection ratio of 3 to 7 showed the best NOx and PM emission results.     

Effects of diesel fuel characteristics on spray and combustion in a diesel engine

Fuel properties play a dominant role in the spray, mixture formation and combustion process, and are a key to emission control and efficiency optimization. This paper deals with the influence of the fuel properties on the spray and combustion characteristics in a high-pressure and temperature chamber. Light diesel fuel spray and combustion images were taken by using a high-speed video camera and analyzed by their penetration and evaporation characteristics in comparison with current diesel fuel. Then, a single-cylinder DI engine was used to investigate combustion and exhaust characteristics. The mixture formation of the light diesel fuel is faster than that of the current fuel depending on physical properties like boiling point, density, viscosity and surface tension. Engine test results show that smoke is reduced without an increase in other emissions.     

Study on pilot injection of DI diesel engine using common-rail injection system

In recent DI diesel engines designed to achieve high output and meet future exhaust regulation, the pilot injection control using a common-rail injection system is adopted. In this research, we developed visualization equipment for pilot combustion behavior of non-luminous flame to clarify the influence of pilot injection parameters (timing and quantity) on engine performance. As a result of this analysis, we clarified the influence of pilot injection parameters on pilot-main combustion and found the optimum pilot injection controlling method.     

The influence of fuel properties on combustion and emissions in a DI stratified charge engine

The influence of fuel properties on combustion characteristics and exhaust emissions in a direct injection stratified charge SI engine with a “two-stage fuel injection system” was examined. The results showed that this type of DISC combustion system can be used with a wider range of fuels than ordinary homogeneous combustion systems. Lower exhaust emissions and higher thermal efficiency were achieved even with fuels with lower octane numbers and higher distillation temperatures.     

Experimental study on the spray and combustion characteristics of SIDI CNG

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels. In the spark-ignition (SI) engine, direct injection (DI) technology can significantly increase the engine volumetric efficiency and reduce “pumping losses” in engines without a throttle valve. DI allows engine operation with the stratified charge which enables relatively higher combustion efficiency. In this study, a combustion chamber with a visualization system is designed. The spray development and combustion propagation process of spark-ignition direct injection (SIDI) CNG were digital recorded and analyzed. The ignition probability was also examined. The results of this study can contribute important data for the design and optimization of the SIDI CNG engine.     

Effect of the fuel injection strategy on first-cycle firing and combustion characteristics during cold start in a TSDI gasoline engine

The first firing cycle is very important during cold-start for all types of spark ignition engines. In addition, the combustion characteristics of the first firing cycle affect combustion and emissions in the following cycles. However, the first-cycle fuel-air mixing, combustion and emissions generation within the cylinder of a two-stage direct-injection (TSDI) engine during cold start is not completely understood. Based on the total stoichiometric air-fuel ratio and local richer mixture startup strategy, the first-cycle firing and combustion characteristic at cold start were investigated in a two-stage direct injection (TSDI) gasoline engine. In addition, the effects of the first injection timing, second injection timing, 1st and 2nd fuel injection proportion and total excess air ratio on the in-cylinder pressure, heat release rate and accumulated heat release were analyzed on the basis of a cycle-by-cycle analysis. It is shown that a larger 2nd fuel injection amount and later 2nd injection timing are more beneficial to the firing of the first cycle in the case of a total excess air ratio of 1.0. The optimum 1st and 2nd injection timing fuel injection proportions are 120°CA ATDC during the intake stroke, 60°CA BTDC during the compression stroke and 1:1. In addition, the firing boundary is a 2nd injection timing later than 90°CA BTDC during the compression stroke in the case of the 1st injection timing from 60°CA to 180°CA ATDC during an intake stroke and involves a 1st and 2nd fuel injection proportion of 1:1 and an excess air ratio of 1.0. The study provides a detailed understanding of cold-start combustion characteristics and a guide for optimizing the reliable first-cycle firing at cold start.     

Comparison of fuel injection systems and a new combustion method for a direct injection two-stroke-cycle automobile engine

A study of a direct fuel injection 2-stroke engine as a new generation power unit was performed and concluded as follows.

• 1.(1) A comparison of fuel injection system candidates was made and the one-fluid high-pressure type was chosen.
• 2.(2) Adopting the high-pressure fuel system to a single-cylinder engine, stratified charge combustion was realized using the late injection.
• 3.(3) It was found that homogeneous charge combustion using the early injection would enable high power output.
• 4.(4) The “2-zone combustion” was also achieved by injecting the fuel twice within one cycle.

     

Influence of injection parameters on the transition from PCCI combustion to diffusion combustion in a small-bore HSDI diesel engine

In this paper, the influence of injection parameters on the transition from Premixed Charge Combustion Ignition (PCCI) combustion to conventional diesel combustion was investigated in an optically accessible High-Speed Direct-Injection (HSDI) diesel engine using multiple injection strategies. The heat release characteristics were analyzed using incylinder pressure for different operating conditions. The whole cycle combustion process was visualized with a high-speed video camera by simultaneously capturing the natural flame luminosity from both the bottom of the optical piston and the side window, showing the three dimensional combustion structure within the combustion chamber. Eight operating conditions were selected to address the influences of injection pressure, injection timing, and fuel quantity of the first injection on the development of second injection combustion. For some cases with early first injection timing and a small fuel quantity, no liquid fuel is found when luminous flame points appear, which shows that premixed combustion occurs for these cases. However, with the increase of first injection fuel quantity and retardation of the first injection timing, the combustion mode transitions from PCCI combustion to diffusion flame combustion, with liquid fuel being injected into the hot flame. The observed combustion phenomena are mainly determined by the ambient temperature and pressure at the start of the second injection event. The start-of-injection ambient conditions are greatly influenced by the first injection timing, fuel quantity, and injection pressure. Small fuel quantity and early injection timing of the first injection event and high injection pressure are preferable for low sooting combustion.     

Effect of fuel injection strategies on the combustion process in a PFI boosted SI engine

A low-cost solution based on fuel injection strategies was investigated to optimize the combustion process in a boosted port fuel injection spark ignition (PFI SI) engine. The goal was to reduce the fuel consumption and pollutant emissions while maintaining performance. The effect of fuel injection was analyzed for the closed and open valve conditions, and the multiple injection strategies (MIS) based on double and triple fuel injection in the open-valve condition. The tests were performed on an optical accessible single-cylinder PFI SI engine equipped with an external boost device. The engine was operated at full load and with a stoichiometric ratio equivalent to that of commercial gasolines. Optical techniques based on 2D-digital imaging were used to follow the flame propagation from the flame kernel to late combustion phase. In particular, the diffusion-controlled flames near the valves and cylinder walls, due to fuel deposition, were studied. In these conditions, the presence of soot was measured by two-color pyrometry, and correlated with engine parameters and exhaust emissions measured by conventional methods. The open valve fuel injection strategies demonstrated better combustion process efficiency than the closed ones. They provided very low soot levels in the combustion chamber and engine exhaust, and a reduction in specific fuel consumption. The multiple injection strategies proved to be the best solution in terms of performance, soot concentration, and fuel consumption.     

高压喷油对柴油机燃烧的影响

在直喷式柴油机上,提高喷油压力能改善其混合气形成过程和燃烧。但应该查明,转速范围在2000r/min内,使用常规喷油系统(泵-管-喷嘴),使喷油压力约至200MPa,喷油持续期为22℃A,每循环喷油量为530mm~3是否可行;进而考虑到指示效率和燃料的不完全混合,这种改善的可能性是否存在。在上述基础上则应确定喷油压力的一个合理的上限值。 通过模拟计算来辅助确定高压喷油系统的设计参数时,必须对诸如柴油机燃油中的音速、高压油管中的衰减、柱塞偶件上的泄漏以及喷油泵的总形等基础数据予以确定及评估。 通过高压力的喷油能改善指示效率,减少黑烟排放,并在调整其余的混合气形成参数时降低氮氧化物的排放。由此得出16OMPa是喷油压力合理的上限值,是油耗、烟度和氮氧化物之间的合理折衷。     

Evaporating spray and flame distributions in the combustion chamber of a direct injection diesel engine

The characteristics of an evaporating diesel spray and the flame distribution in a combustion chamber of a D.I. diesel engine were investigated by using the laser light technique. The technique used was based on the extinctions of two wavelengths of ultraviolet and visible laser light. The transmitted laser light absorbed and scattered by the vapor, drops, soot and combustion products in the spray flame were separated into two wavelengths and captured. Further, the light radiated from the flame was imaged using the same measuring system by modifying the optical filters and the timing of the camera shutter.     

Effect of injection timing,injector opening pressure,injector nozzle geometry,and swirl on the performance of a direct injection,compression-ignition engine fuelled with honge oil methyl ester (HOME)

Increasing petroleum prices, increasing threat to the environment from exhaust emissions and global warming have generated intense international interest in developing renewable and alternative non-petroleum fuels for engines. Evolving feasible technology and recurring energy crisis necessitated a continued investigation into the search for sustainable and clean-burning renewable fuels. In this investigation, Honge oil methyl ester (HOME) was used in a four stroke, single cylinder diesel engine. Tests were carried out to study the effect of fuel injection timing, fuel injector opening pressure (IOP) and injector nozzle geometry on the performance and combustion of CI engine fuelled with HOME. Injection timing was varied from 19°bTDC (before top dead centre) to 27°bTDC in incremental steps of 4°bTDC; injector opening pressure was varied from 210 bar to 240 bar in steps of 10 bar. Nozzle injectors of 3, 4 and 5 holes, each of 0.2, 0.25 and 0.3 mm size were selected for the study. It was concluded that retarded injection timing of 19°bTDC, increased injector opening pressure of 230 bar and 4 hole nozzle injector of 0.2 mm size resulted in overall better engine performance with increased brake thermal efficiency (BTE) and reduced HC, CO, smoke emissions. Further air-fuel mixing was improved using swirl induced techniques which enhanced the engine performance as well.     

Effects of valve timing and intake flow motion control on combustion and time-resolved HC & NOx formation characteristics

In SI engines, valve events have a major influence on volumetric efficiency, fuel economy and exhaust emissions. Moreover, swirl and tumble motions in the intake charge also improve combustion speed and quality by stratifying the mixture as well as intensifying the mixing rate of air and fuel. This paper investigates the behaviors of an engine and the combustion phenomenon for various intake valve timings and intake charge motions using CVVT system and port masking schemes. Test condition includes a part load and a cold idle condition inclusive of a cold start of the engine. Time-resolved HC and NOx emissions were also measured at an exhaust port to examine their formation mechanisms and behaviors with fast response HC/NOx analyzers. In conclusion, the fast burning of fuel and improved combustion quality by enhanced charge motions reduced unburned HC emissions, and advancing the intake valve opening reduced HC as well as NOx. Furthermore, HCs during the cold transient phase and idle conditions decreased with recalibrated start parameters such as lean air-fuel ratio and spark retardation via the enhancement of intake charge motions.     

Impact of SME blended fuel combustion on soot morphological characteristics in a diesel engine

The soot morphological study and NOx emissions of soybean oil methyl ester (SME) in a passenger diesel vehicle were investigated experimentally. The soot morphological characteristics were conducted at various injection pressures, engine speeds and engine loads. Soot sampling and image processing analysis with a scanning electron microscope (SEM) were used to investigate the influence on particulate morphologies. The dimensions of average primary particles and the size of the radius of gyration were gradually decreased as injection pressures increased at all operating engine conditions. The average radius of gyration was increased with increasing engine load, while the average primary particle size decreased. NOx emissions were gradually increased with the increasing injection pressure at all operating engine conditions.     

Effect of the number of fuel injector holes on characteristics of combustion and emissions in a diesel engine

The diesel combustion process is highly dependent on fuel injection parameters, and understanding fuel spray development is essential for proper control of the process. One of the critical factors for controlling the rate of mixing of fuel and air is the number of injector holes in a diesel engine. This study was intended to explore the behavior of the formation of spray mixtures, combustion, and emissions as a function of the number of injector hole changes; from this work, we propose an optimal number of holes for superior emissions and engine performance in diesel engine applications. The results show that increasing the number of holes significantly influences evaporation, atomization, and combustion. However, when the number of holes exceeds a certain threshold, there is an adverse effect on combustion and emissions due to a lack of the air entrainment required for the achievement of a stoichiometric mixture.     

Ignition and combustion characteristics of two-stage injection diesel spray

Ignition and combustion characteristics of a two-stage injection diesel spray were experimentally investigated. A constant volume combustion chamber was filled with air which was controlled at 3.0 MPa and 743–923 K. In order to measure the ignition delay and the ignition position, a high speed video system was used. A 306 nm interference filter and an image intensifier system were attached to the camera for detecting the OH radical emission. The results show that the ignition delay of a two-stage injection spray becomes shorter compared with that of a single injection spray. The ignition positions of two-stage injection spray are observed nearer to the nozzle than that of single injection spray. Also, the temperature limit of complete combustion on a two-stage injection spray becomes lower than that of a single injection spray.     

Effect of injection condition and swirl on D.I. diesel combustion in a transparent engine system

The objective of this work was to investigate the effects of injection conditions and swirl on D.I. diesel combustion using a transparent engine system. The test engine is equipped with a common rail injection system to control injection conditions and to obtain split injection characteristics. A combustion analysis and steady flow test were conducted to measure the heat release rate due to cylinder pressure and the swirl ratio. In addition, spray and diffusion flame images were obtained using a high speed camera. The LII & LIS methods were also used to obtain 2-D soot and droplet distributions. High injection pressure was found to shorten ignition delay, as well as to enhance peak pressure. The results also revealed that the heat release rate in the premixed combustion region was markedly reduced through the use of pilot injection, while the soot distribution and the heat release rate in the diffusion combustion region were increased. The swirl effect was found to shorten ignition delay at certain injection timings, and to enhance the heat release rate in all experimental conditions.