Numerical analysis of the combustion process in a four-stroke compressed natural gas engine with direct injection system

In this paper, a numerical study to simulate and analyze the combustion process occurred in a compressed natural gas direct injection (CNG-DI) engine by using a multi-dimensional computational fluid dynamics (CFD) code was presented. The investigation was performed on a single cylinder of the 1.6-liter engine running at wide open throttle at a fixed speed of 2000 rpm. The mesh generation was established via an embedded algorithm for moving meshes and boundaries for providing a more accurate transient condition of the operating engine. The combustion process was characterized with the eddy-break-up model of Magnussen for unpremixed or diffusion reaction. The modeling of gaseous fuel injection was described to define the start and end of injection timing. The utilized ignition strategy into the computational mesh was also explained to obtain the real spark ignition timing. The natural gas employed is considered to be 100% methane (CH₄) with three global step reaction scheme. The CFD simulation was started from the intake valves opening until the time before exhaust valves opening. The results of CFD simulation were then compared with the data obtained from the single-cylinder engine experiment and showed a close agreement. For verification purpose, comparison between numerical and experimental work are in the form of average in-cylinder pressure, engine power as well as emission level of CO and NO. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

A mathematical model for the prediction of the injected mass diagram of a S.I. engine gas injector

A mathematical model of gaseous fuel solenoid injector for spark ignition engine has been realized and validated through experimental data. The gas injector was studied with particular reference to the complex needle motion during the opening and closing phases, which strongly affects the amount of fuel injected. As is known, in fact, when the injector nozzle is widely open, the mass flow depends only on the fluid pressure and temperature upstream the injector: this allows one to control the injected fuel mass acting on the “injection time” (the period during which the injector solenoid is energized). This makes the correlation between the injected fuel mass and the injection time linear, except for the lower injection times, where we experimentally observed strong nonlinearities. These nonlinearities arise by the injector outflow area variation caused by the needle bounces due to impacts during the opening and closing transients [1] and may seriously compromise the mixture quality control, thus increasing both fuel consumption and pollutant emissions, above all because the S.I. catalytic conversion system has a very low efficiency for non-stoichiometric mixtures. Moreover, in recent works [2, 3] we tested the simultaneous combustion of a gaseous fuel (compressed natural gas, CNG, or liquefied petroleum gas, LPG) and gasoline in a spark ignition engine obtaining great improvement both in engine efficiency and pollutant emissions with respect to pure gasoline operation mode; this third operating mode of bi-fuel engines, called “double fuel” combustion, requires small amounts of gaseous fuel, hence forcing the injectors to work in the non-monotonic zone of the injected mass diagram, where the control on air-fuel ratio is poor. Starting from these considerations we investigated the fuel injector dynamics with the aim to improve its performance in the low injection times range. The first part of this paper deals with the realization of a mathematical model for the prediction of both the needle motion and the injected mass for choked flow condition, while the second part presents the model calibration and validation, performed by means of experimental data obtained on the engine test bed of the internal combustion engine laboratory of the University of Palermo.     

Hydraulic characterization of a second-generation common rail injector operating under solo and split injection strategies

In modern compression ignition engines, complex fuel injection strategies are adopted in order to enable a clean and efficient combustion process and an effective combustion noise control algorithm. Multi-injection strategies inject fuel into the combustion chamber several times (e.g. pilot, main, and post injections) during each combustion cycle, while split-injection approach further divides the main injection into different shots, with very short dwell time. Split injection may help to enhance air entrainment into the spray core where the fuel droplets are highly dense and mixing quality is poor. These advanced injection techniques lead to complex hydraulic behaviors including injection instability and eventually affecting fuel metering accuracy, hence detailed investigations are required. Understanding hydraulic characteristics especially during peculiar events like start/end of injection and accurately quantifying the actual injection volume, injection rate, and pressure variations in different locations of the injection system in each single activation of a complex strategy are key targets. In this work, the hydraulic behavior of a second generation common-rail solenoid injector operating under split-injection strategy has been experimentally investigated in terms of injection rate and injected volume. An extensive experiment has been conducted in this study using a state-of-the-art injection system operating on a hydraulic test bench equipped with a Zeuch-method type injection analyzer. It is found that although the standard of deviation of injection rates and injected volume is quite small for isolated injection events, the shot-to-shot deviation for split-injection mode can be significantly higher depending mainly on dwell time, fuel quantity ratio between the two shots and injection pressure level, as an effect of both pressure perturbations in the feeding line and in the injector caused by close actuations, eventually joined to inertial phenomena of the injector needle. The present paper reports an analysis methodology for the quantitative evaluation of systematic inter-cycle deviations, in the effort towards a deeper exploitation of the potential benefits offered by advanced injection strategies.     

多点电喷天然气汽车发动机进气干涉现象的数值解析

首先采用多维数值模拟的方法解析了从天然气喷射阀喷出的突发天然气喷流的发展过程,并由纹影试验验证了这个数值解析方法的可行性。在此基础上,解析了单进气阀和双进气阀发动机进气歧管内天然气的喷射和反射过程。结果表明,大量天然气与进气阀冲突后向进气歧管入口方向反射并造成进气阀附近气体压力升高;若进气歧管较短时,天然气-空气混合气反射到歧管入口并造成各缸混合气分配不均匀是完全可能的;即使较长的进气歧管时,由于发火顺序不同,各缸间的进气干涉程度也不一样,这将引起各缸实际的进气充量发生变化。     

A study on characteristics and control strategies of cold start operation for improvement of harmful exhaust emissions in SI engines

Emission regulations for automobiles have become more stringent and the improvement of emission during cold start has been a major key issue to meet these regulations. Among many kinds of factors that affect cold start operation, ignition timing is crucial to improve emission characteristics due to the influence on exhaust gas temperature. Recent progress in variable valve timing allows optimized valve event strategies under various ranges of engine operating conditions including cold start. This study investigates effects of ignition and exhaust valve timing on exhaust gas temperature, combustion stability and emission characteristics through cold start bench tests of an SI engine. Experimental results show that exhaust valve timings and ignition timings significantly affect exhaust gas temperature and stability of engine operation under cold start condition. Exhaust valve timing also affects CO and NO_x emission due to changes in residual gas fraction of the combustion chamber. Ignition timing mainly affects exhaust gas temperature and HC emission. A control strategy, advanced exhaust valve timing and retarded ignition, is plausible in order to achieve reduction of exhaust emission while maintaining stability under cold start operation of SI engines.     

Investigation of coal-water slurry fuel combustion in reciprocating,internal combustion engine

Coal-water slurry(CWS) engine tests designed to investigate the ignition and combustion processes of the fuel are described in this paper. The effects of three different parameters, namely, (a) needle lift pressure, (b) fuel injection timing, and (c) percent coal loading in the slurry fuel are studied in detail. Successful operation of the engine using the coal water slurry required modifications to the engine and support systems. The physical trends of combustion under single parametric variations are presented in terms of the cylinder pressure, heat release rates, and cumulative heat release curves. The major conclusions of the work include: (a) higher needle lift pressures led to shorter ignition delay times for the CWS fuel: (b) the ignition delay time of the advanced injection start was little different from that of retarded fuel injection timing due to poor atomization: and (c) dilution of the slurry with water can significantly affect the combustion processes and ease of fuel handling.     

Correlating armature and needle dynamics with voltage waveforms of solenoid-actuated GDI injector

The injector voltage hump that appears near the needle closing has been used for the real-time monitoring and feedback control of fuel injection duration in modern engines. This voltage hump has been thought to result from the abrupt change in electromagnetic induction by the stoppage of needle motion but detailed electromagnetic processes and associated armature and needle dynamics during the needle closing have not been thoroughly investigated in a wide range of injection conditions, which knowledge is crucial for the delicate control of fuel injection based on the voltage hump. The current study analyzes the transient armature and needle dynamics of a solenoid-actuated gasoline direct injection injector using an X-ray phase-contrast imaging technique. Then, the results are correlated with voltage waveforms during the needle closing transient under various injection pressures, injection pulse durations, and dwell times of split injections. The time derivatives of voltage waveforms showed lower and upper peaks in order in the regime of the voltage hump. Inconsistent with conventional understandings, the lower peak timing of the voltage derivative did not match with the timing of needle closing (end of injection) but rather matched with the abrupt descent timing of the armature and needle. The inflection timing and upper peak timing of the voltage derivative matched with the timings of actual needle closing and armature closing respectively. The amplitude of the voltage hump was near linearly dependent on the needle closing speed. The needle closing speed decreased upon the decrease of injection pulse duration and injection pressure which made it difficult to detect the voltage humps in ballistic injection regimes and low injection pressures. In split injection conditions, the voltage hump of the first injection was not detectable if the dwell time was shorter than the needle closing delay, the time from the current cut-off to the actual needle closing.     

Model-based optimization of injection strategies for SI engine gas injectors

A mathematical model for the prediction of the mass injected by a gaseous fuel solenoid injector for spark ignition (SI) engines has been realized and validated through experimental data by the authors in a recent work [1]. The gas injector has been studied with particular reference to the complex needle motion during the opening and closing phases. Such motion may significantly affect the amount of injected fuel. When the injector nozzle is fully open, the mass flow depends only on the upstream fluid pressure and temperature. This phenomenon creates a linear relationship between the injected fuel mass and the injection time (i.e. the duration of the injection pulse), thus enabling efficient control of the injected fuel mass by simply acting on the injection time. However, a part of the injector flow chart characterized by strong nonlinearities has been experimentally observed by the authors [1]. Such nonlinearities may seriously compromise the air-fuel mixture quality control and thus increase both fuel consumption and pollutant emissions (SI engine catalytic conversion systems have very low efficiency for non-stoichiometric mixtures). These nonlinearities arise by the injector outflow area variation caused by needle impacts and bounces during the transient phenomena, which occur in the opening and closing phases of the injector. In this work, the mathematical model previously developed by the authors has been employed to study and optimize two appropriate injection strategies to linearize the injector flow chart to the greatest extent. The first strategy relies on injection pulse interruption and has been originally developed by the authors, whereas the second strategy is known in the automotive engine industry as the peak and hold injection. Both injection strategies have been optimized through minimum injection energy considerations and have been compared in terms of linearization effectiveness. Efficient linearization of the injector flow chart has been achieved with both injection strategies, and a similar increase in injector operating range has been observed. The main advantage of the pulse interruption strategy lies on its ease of implementation on existing injection systems because it only requires a simple engine electronic control unit software update. Meanwhile, the peak and hold strategy reveals a substantial lack of robustness and requires expressly designed injectors and electronic components to perform the necessary voltage commutation.     

NUMERICAL ANALYSIS AND VISUALIZATION OF NATURAL GAS JET WITH MULTI-POINT INJECTION SYSTEM

Aiming at the change in intake air flow caused by the injection of natural gas in intake manifold if one simply replaces the gasoline injector with natural gas injector with the installing position of injector in intake manifold unchanged,and also the reflection of gas toward intake manifold inlet resulted from the impingement between the injected large volumetric natural gas jet and intake valve,an impulsively started natural gas jet injected from a gas injector is characterized as a three-dimensional unsteady compressible viscous turbulent flow,based on which its transient development process is numerically analyzed using general-purpose CFD codes.The predicted velocity vector maps show a vortex,which indicates the occurrence of an unsteady state jet region,is formed downstream of the jet.A schlieren apparatus is utilized to get several groups of visible schlieren photographs of natural gas jets.In the experiment,photographs of natural gas jets taken by a CCD camera are laid in a portrait processor where the shapes,tip penetration distance and injection angles of the gas jets are investigated.Comparisons between predicted results and measurements indicate an excellent agreement between simulations and experimental results.     

Experimental investigation on the influence of engine operating conditions on combustion and nanoparticle emission characteristics of a small DI diesel engine

The influence of variations in engine speed, injection pressure, injection timing, and multiple injection strategies on the combustion and nanoparticle characteristics of a small Direct injection (DI) diesel engine was experimentally investigated. To measure the size distribution and number concentration of particle emissions, a rotating disk thermo-diluter (dilution system), a Condensation particle counter (CPC), and a Scanning mobility particle sizer (SMPS) were used. The injection pressure was changed from 60 MPa to 120 MPa, at an engine speed of 1200 rpm. Injection timing was varied from Before top dead center (BTDC) 40˚ to Top dead center (TDC). To investigate the effect of multiple-injection strategies, the injection strategies consisted of two pulse signals with different dwell time. The experimental results show that the peak combustion pressure and Rate of heat release (ROHR) profile are increased and ignition delay is shortened with the increase of injection pressure from 60 MPa to 120 MPa. The concentration of soot emission for 120 MPa is lower than that of 60 MPa at advanced injection timing from TDC up to BTDC 25°. As the injection timing advances to over BTDC 30°, soot emissions rapidly increase and the high injection pressure case (120 MPa) creates more emissions than the 60 MPa case. The overall trends of total particle number are relatively increased with high injection pressure for single injection conditions. In the advanced injection timings of over BTDC 30°, the trend of total particle number is high for all injection pressures. For multiple injections, the peak combustion pressures and ROHR of multiple-injection strategies are slightly lower compared with those of single-combustion results. Comparing the multiple injection strategies, soot emission is reduced with the retard of second injection timing (-30°+5°). The overall trends of particle size and total number for the 7 mg+3 mg case revealed the lowest level compared with other cases, which is 50% lower than that for the 5 mg+5 mg case. When compared with single injection results, the total particle number and Dp of multiple injection cases were eventually lower.

     

Effects of different piezo-acting mechanism on two-stage fuel injection and CI combustion in a CRDi engine

In this study, the effects of two piezo injectors operated by different mechanisms on multi-injection and Compression ignition (CI) combustion were investigated. High-pressure injectors for CI engines are divided into two categories according to the actuator: Solenoid and piezo injectors. It is commonly known that both injectors have a hydraulic circuit for fuel injection; thus, the performance of the injector is highly dependent on not only hydraulic characteristics such as volume of internal chambers and nozzle geometry, but also the actuation mechanism. Specially, the direct needle-Driven piezo injector (DPI) is introduced in this study and compared with the indirectacting Piezo injector (PI) to investigate the injection characteristics and influences on CI combustion performance by using spray visualization, injection rate measurement, and single cylinder diesel engine experiments, as well as numerical simulation for injection rate modeling of DPI. In the spray visualization experiment, a high-speed camera was used to examine spray tip penetration length and spray speed with respect to each injector. Also, in order to investigate injection rate information, which is a significantly dominant factor in combustion characteristics, the Bosch-tube method was adapted under the condition of a back pressure of 4.5 MPa, corresponding to engine motoring pressure. Also, a single-cylinder CRDi (Common-rail direct-injection) engine experiment was carried out to determine the effects of different piezo-acting mechanisms on two-stage fuel injection and CI combustion. From the key results obtained by this study, the direct needle-driven piezo injector has a faster SOI (Start of injection) and EOI (End of injection). In addition, the overall shape of the injection rate of DPI was narrow and the injection had a higher spray speed than that of PI. Also, DPI has a higher heat release rate and peak pressure, as verified by the engine experiment. In particular, it was found that DPI showed the possibility of combustion improvement when applying a pilot injection strategy.

     

Experimental analysis of emission reduction by the split injection strategy using close post injection with a double-row nozzle in heavy EGR conditions

As EURO-6 regulations will be enforced in 2014, simultaneous reduction of NO_x and PM emissions becomes an important issue in recent diesel engine research. New combustion concepts, such as LTDC and pHCCI, have been introduced to overcome the NO_x and PM trade-off relation. However, these novel combustion concepts are usually implemented with a high EGR rate and by advancing the main injection timing which cause high CO and THC emissions along with poor fuel consumption due to low combustion efficiency. Therefore, the split injection strategy, which was consisted of applying post injection close to the main injection, was carried out in this experiment. Specifically, two different nozzles — a 7-hole conventional nozzle and a 12-hole double-row nozzle — were evaluated to determine the effects of nozzle configurations on engine-out emissions. The result shows that CO emission was reduced by the close post injection strategy regardless of the nozzle configuration. However, THC and PM emissions were reduced only when the 12-hole double-row nozzle was used. Thus, the use of close post injection with the 12-hole double-row nozzle could increase the combustion efficiency in heavy EGR conditions.     

Analysis of compression ignition combustion in a schnurle-type gasoline engine comparison of performance between direct injection and port injection systems-

A two-stroke Schnurle-type gasoline engine was modified to enable compression-ignition in both the port fuel injection and the in-cylinder direct injection. Using the engine, examinations of compression-ignition operation and engine performance tests were carried out. The amount of the residual gas and the in-cylinder mixture conditions were controlled by varying the valve angle rate of the exhaust valve (VAR) and the injection timing for direct injection conditions. It was found that the direct injection system is superior to the port injection system in terms of exhaust gas emissions and thermal efficiency, and that almost the same operational region of compression-ignition at medium speeds and loads was attained. Some interesting combustion characteristics, such as a shorter combustion period in higher engine speed conditions, and factors for the onset of compression-ignition were also examined.     

Optical investigation of the fuel injector influence in a PFI spark ignition engine for two-wheel vehicles

This paper describes the results obtained in a port fuel injection spark-ignition (PFI SI) engine by optical diagnostics during the fuel injection and the combustion process. A research optical engine was equipped with the fuel injection system, the head and the exhaust device of a commercial 250 cc engine for scooters and small motorcycles. Two injectors were tested: standard 3-hole injector that equipped the real reference engine and a 12-hole injector. The intake manifold was modified to allow the visualization of the fuel injection using an endoscopic system coupled with CCD camera. Size and number of the fuel droplets were evaluated through an image processing procedure. The cycle resolved visualization and chemiluminescence allowed to follow the combustion process from the spark ignition to the exhaust phase. All the optical data were correlated with engine parameters and exhaust emissions. The effect of the fuel injector type on deposits formed by fuel accumulation and dripping on the intake valves steams and seats was investigated. In particular, the evolution of diffusion-controlled flames due to the fuel deposits burning was analyzed. These flames were principally located near the intake valves, and they persisted well after the normal combustion event. The consequences were the formation and emission of soot and unburned hydrocarbons. The multi-hole injector helped reducing wall wetting and deposit formation so that the emission characteristic can be improved. The use of 12-hole injector allowed a more homogeneous distribution for a lower time of fuel droplets in the intake manifold than the 3-hole injector. This study also investigated the detailed physical/chemical phenomena to figure out reasons for the improvement using optical measurements.     

电控单体泵柴油机燃油喷射控制策略

电控单体泵由电控泵喷嘴发展而来,由于在电控泵与喷油器之间加入了高压油管,使电控单元从发出喷油信号到燃油喷入汽缸的时间延迟加长.针对该特点,以及考虑到程序计算时间、电磁阀响应特性等因素对喷油正时的影响,设计了喷油正时控制策略.试验结果表明,电控单元可按喷油正时控制策略准确地、柔性地控制喷油正时和喷油量.     

Effect of water induction on the performance and exhaust emissions in a diesel engine (II)

This study was to investigate the effects of water induction through the air intake system on the characteristics of combustion and exhaust emissions in an IDI diesel engine. The fuel injection timing was also controlled to investigate a method for the simultaneous reduction of smoke and NOx when water was injected into the combustion chamber. The formation of NOx was significantly suppressed by decreasing the gas peak temperature during the initial combustion process because the water played a role as a heat sink during evaporating in the combustion chamber, while the smoke was slightly increased with increased water amount. Also, NOx emission was significantly decreased with increase in water amount. A simultaneous reduction in smoke and NOx emissions was obtained when water was injected into the combustion chamber by retarding more 2°CA of the fuel injection timing than without water injection.     

Comparison of the effects of multiple injection strategy on the emissions between moderate and heavy EGR rate conditions: part 2-post injections

To draw a comparison of the effect of multiple injection strategy on the engine-out emissions under two different EGR rate conditions, the effect of pilot injection on emissions and combustion was evaluated and discussed in part 1. Thus, in the second research as part 2, the effects of post injection on the engine-out emissions were systemically evaluated for two different EGR rate conditions (30 % and 60 %). Since the behavior of diesel combustion is significantly different as EGR rate is changed, the characteristics of post injection was different between two EGR rate conditions. This research was investigated as varying injection parameters such as the timing and quantity of the post injection. The results show that the close post injection with injection interval as 10 degree has the potential to reduce PM emission, regardless of EGR rate. However, the reason of reduction of PM emission is different for each case. For a moderate EGR rate condition, close post injection with interval 10 degree enhances the fuel at bottom of bowl. Thus, the distribution of fuel can be improved. On the other hand, for a heavy EGR rate condition, close post injection with interval 10 degree has the charge cooling effect to prolong the ignition delay, rather than well-matched injection targeting. Especially, there is an effect to oxidize PM emission under moderate EGR rate condition as post injection is applied. However, post injection for late cycle of combustion under heavy EGR rate condition does not oxidize PM emission due to low oxygen concentration (～ 10%).     

被动预燃室发动机低温冷起动及低负荷试验研究

预燃室射流点火是改善汽油发动机热效率的有效手段,为了研究和改善被动预燃室低温冷起动及低负荷时的燃烧稳定性,设计了不同容积、孔面积、材料、喷孔结构的被动预燃室装置,安装在一台涡轮增压汽油发动机上,进行了低温冷起动试验,以及低速、低负荷燃烧稳定性试验。研究结果表明,被动预燃室容积、孔面积、材料、喷孔结构对低温冷起动性能有显著影响。预燃室容积较小时,预燃室内部淬熄层占预燃室容积的比例大,预燃室内部混合气少。较小的孔径或孔面积减少了预燃室内残余废气的排出。旋转孔使得预燃室内部废气分层,火花塞附近废气比例大。较高的导热率使预燃室冷起动时预燃室散热较快。因此,小容积、小孔径、高导热率材料以及旋转喷孔等均不利于发动机冷起动。优化结构的被动预燃室在-20℃～-8℃的冷起动工况下能实现发动机稳定着火起动。点火角和排气VVT对发动机的燃烧稳定性影响较小。进气VVT对预燃室燃烧稳定性影响较大,进气门开起时刻推迟,着火上止点附近缸内湍动能变强;另一方面实际压缩比变大,主燃烧压入预燃室内部的新鲜混合气比例提高,预燃室点火燃烧稳定性显著改善。     

车用柴油机燃烧和热流分析的数值研究

利用商用软件STAR—CD及ES—ICE对某D6114柴油机在的缸内燃烧过程进行了数值模拟计算,分析和比较了不同喷油提前角对缸内燃烧过程和燃烧室表面热流的影响。研究结果表明：喷油提前角提前,柴油机缸内的燃烧效果优于喷油提前角推迟,燃烧过程中缸内的压力和温度比推迟喷油提前角时要大,同时缸内的最高燃烧压力和最高温度也高;喷油提前角对缸盖和活塞顶壁面平均热流的影响与其对缸内平均温度的影响相似,对缸套壁面的影响是喷油提前角提前越早,传给缸套的热流越小。数值计算结果为高功率、高强化和低热损的柴油机设计提供理论依据。．     

A study on the effect of operating conditions for the stability at idle

A gasoline engine with an electronically controlled fuel injection system has substantially better fuel economy and lower emissions than a carburetted engine. In general, the stability of engine operation is improved with fuel injector, but the stability of engine operation at idle is not improved compared with a carburetted gasoline engine. In addition, the increase in time that an engine is at idle due to traffic congestion has an effect on the engine stability and vehicle reliability. Therefore, in this research, we will study the influence of fuel injection timing, spark timing, dwell angle, and air-fuel ratio on engine stability at idle.