Spray-guided Direct-injection Spark-ignition Engine Research Articles

Combustion characteristics and engine performance of 2-methylfuran compared to gasoline and ethanol in a direct injection spark ignition engine

2-Methylfuran (MF) is considered as a promising second-generation biofuel for spark ignition engines. MF has better anti-knock ability, and the lower heating value of MF is close to that of conventional fuels. The production methods of MF have been largely improved over the last decade. However, the studies of MF on direct injection spark ignition (DISI) engines are quite limited, and little is known about the effects of engine control parameters on MF combustion at typical DISI engine operating conditions. In the present work, the effects of engine control parameters including spark timing, air–fuel ratio, injection timing, and injection pressure on combustion and engine performances of MF were studied at typical medium load and high load conditions on a single-cylinder spray-guided DISI engine, compared to gasoline and ethanol. The results show that the achievable spark timing range considering combustion phasing and knock limit for MF is larger than that of gasoline especially at high load. MF has more robustness to lean burn than gasoline and ethanol. The combustion characteristics and heat release of MF have low sensibility to the variation of injection timings. To achieve the same engine load MF requires up to 25% longer injection durations than gasoline, however, increased injection pressure can mitigate the injection duration differences between MF and gasoline.

Influence of the in-cylinder flow on cycle-to-cycle variations in lean combustion DISI engines measured by high-speed scanning-PIV

In this study, the influence of the three-dimensional (3D) in-cylinder flow on engine's cycle-to-cycle variations (CCV) in a spray-guided direct-injection spark-ignition engine is investigated. The engine is operated at homogeneous lean air–fuel mixture which enhances the sensitivity to CCV due to reduced laminar flame speed. To compensate this, intake velocity is increased by a tumble-flap (TF) in the intake-port. To address the 3D-nature of the temporal evolution of the instantaneous in-cylinder flow for different TF-positions, time-resolved scanning particle image velocimetry (Scanning-PIV) is applied to the engine. The required scan-frequency is provided by an acousto-optical-deflector (AOD) to measure the flow field quasi-simultaneously in the central tumble-plane and both mid-valve-planes. The three planes are 18 mm displaced from each other to capture the variability of the large-scale tumble vortex. The in-cylinder flow measurements are combined with combustion analyses by the in-cylinder pressure-trace and the detection of the location of ignition through the evaluation of the luminous spark-plasma. A correlation-map analysis is conducted to identify coherent flow features responsible for CCV of the combustion parameters. This reveals a strong dependency of the spark position to variations of an upward directed flow pointing onto the spark plug. The variations of the upward flow are due to strong CCV of the bended tumble-axis position. An increased tumble motion caused by the TF results in favorable flow conditions by stabilizing the tumble-axis in the middle of the cylinder which decreases the CCV of the spark position significantly. Further correlation analysis including the combustion process exhibits that flow-structures moving the spark and early flame kernel towards the cylinder center reduces the crank angle of 5% heat release and the combustion duration considerably.

Flow characteristics of natural-gas from an outward-opening nozzle for direct injection engines

In this research article, we present for the first time an experimental investigative study on the flow characteristics of compressed natural-gas (CNG) issued from a direct injector with an outward-opening nozzle operating at a wide range of pressures. High speed schlieren imaging is used to capture the growth of the highly turbulent transient gaseous jet with high spatial and temporal resolution in a quiescent chamber at atmospheric conditions. The existing penetration scaling correlations for gas jets issued from single round nozzles were found to apply to outward-opening nozzle as well. The penetration constant is found to be 1.15 ± 0.05 for fuel pressure ranging from 20 bar to 160 bar. A new method to calculate combined penetration for radially spreading jets such as the one used in this study is presented which can also be used for jets of irregular shape. The data from these tests show that the axial penetration from a 90° conical annular nozzle is roughly about 3 times lower than round nozzles for similar injection and ambient conditions. The lower penetration rate of such annular nozzles can be beneficial for stratified mixture formation in spray-guided direct-injection spark-ignition engines.

REMOVED: Experimental investigation of impacts of engine hardware, operating parameters and combustion performance on particulate emissions in a DISI engine

With increasing concern over ultrafine Particulate Matter (PM) emissions from Direct Injection Spark Ignition (DISI) engines and ever tightening emissions legislation, substantial research work is required to obtain a better understanding of influence of advanced engine technologies on particulate formation and oxidation. In this work, experimental investigation of engine-out particulate emissions were carried out on a single cylinder spray-guided DISI engine. With regard to effects of engine hardware on PM emissions; impacts of fuel spray angle, fuel flow rate and intake valves’ lift and camshaft phasing were examined over a wide start of injection sweep window. Injector tests highlighted a complex relation between injector characteristics and engine operating parameters with PM emissions as a result of fuel impingement and poor mixture preparation. Valvetrain tests indicated some benefits from engine throttling by valve lift and intake camshaft phasing in particulate number concentration reduction at low load. This was associated with increased intake flow turbulence and improved mixture preparation. Number-weighted particle size distribution was also affected by intake valve modes at low load. Nevertheless, at high load, impacts of variable valve lift and camshaft phasing on particulate number concentration and size distribution became negligible. Furthermore, combustion phase indicators were studied in order to probe impacts of combustion performance on PM emissions. While no satisfactory correlation of initial flame development with PM emissions was identified; shorter rapid-burning angles were observed with lower particulate number concentrations.

Analysis of the repeatability of fuel spray indexes in a spray-guided direct-injection spark-ignition engine

The development and research works on liquid fuel injection in spark-ignited direct injection (SIDI) engines, apart from so common in recent years simulation methods, still have a significant cognitive substrate. This is related to experimental research on repeatability of combustion process using multi- and mono-cylinder test engines and Rapid Compression Machines. The repeatability of preparation and delivery processes has immediate impact on repeatability of combustion process. Except for the necessity of obtaining the repeatability of fuel amounts, the repeatability of injected fuel spray is required. The penetration range and spray area in combustion chamber have direct impact on mixture creation and formation. The optical research on fuel injection has been made in order to determinate its repeatability. The research on unrepeatability of fuel spray propagation has been conducted using piezoelectric injectors of outward-opening type, being primary elements of the spray-guided combustion systems. The results of research were presented in the form of index of variation of the selected parameters. The evaluation of the results of the optical research concerns radial spray penetration and fuel spray velocity. Unrepeatability has been presented with coefficient of variation of radial penetration in relation to the time of injection duration. It has been observed that the coefficients of various parameters are lower with longer times of fuel injection.

Effects of a split injection in a spray-guided direct-injection spark ignition engine under lean stratified operation

The effects of a split injection were investigated in a spray-guided direct-injection spark ignition engine under lean stratified operation. In-cylinder pressure analysis, exhaust emissions measurement and visualization of the spray and combustion were applied. From the results using a single-injection strategy, stratified combustion showed advanced and rapid combustion characteristics. However, the low work conversion efficiency and the high nitrogen oxide emissions were obstacles to stratified combustion. Furthermore, the smoke emissions were high as a result of the dominant mixing-controlled combustion. A dual-injection strategy was applied to control the level of stratified combustion by modifying the dwell time and the split ratio. With a split injection, the nitrogen oxide emissions and the smoke emissions were reduced because of the moderated combustion speed and the enhanced local homogeneity. The liquid phase of the stratified mixture and the luminous sooting flame were reduced significantly as well. However, a low combustion efficiency and a low indicated mean effective pressure resulted owing to the over-mixing effect when the portion of lean premixed combustion was increased. Via moderately controlled stratified combustion, a split injection could be an effective way to reduce the nitrogen oxide emissions and the smoke emissions with a reasonable indicated mean effective pressure and a reasonable combustion efficiency.

Characterization of the ignition and combustion processes of spray injected by a hole-type nozzle for a direct-injection spark ignition engine

The spray-guided direct-injection spark ignition engine needs a stable ignition and combustion system for favourable operation. In this study, the effects of the ignition position (e.g. upstream, middle or downstream of the spray) and the timing (e.g. before, at or after the end of injection) respectively on the flame development and combustion processes of gasoline spray injected by a hole-type nozzle were investigated in a quiescent constant-volume vessel. OH* chemiluminescence was used to characterize the temporal and spatial distributions of the combusting activity and flame evolution. The soot formation process was clarified by using the high-speed imaging of two-colour pyrometry. At all the ignition positions, a moderate jet-to-jet variation in the maximum integrated OH* intensity was present as the fuel was ignited in the middle region of the spray. The over-lean mixture at the spark gap led to early flame decay and enhanced the variation in the combustion intensity. When the flame started to propagate, the decreased equivalence ratio distribution downstream of the ignition position tended to result in incomplete combustion with a pronounced long narrow flame shape and lower OH* emissions. The flame was impeded owing to the lean unburned mixture and the high flow velocity greater than the flame speed above the ignition position, causing it to propagate towards upstream of the spray. An over-rich mixture produced more soot during combustion, in accordance with the KL factor distribution. A potentially favourable condition for development of the flame and the combustion process was found when the fuel was ignited in the middle region of the spray, and the time when the injection was ended.

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.

In-Cylinder Flow and Fuel Spray Interactions in a Stratified Spray-Guided Gasoline Engine Investigated by High-Speed Laser Imaging Techniques

Spray-guided direct injection spark-ignition engines operated in stratified charge mode have a high potential for improved fuel economy. As fuel is injected late in the compression stroke mixture preparation is crucial for reliable ignition. Multiple injections per cycle have proven to increase the overall combustion stability. Nevertheless cycle-to-cycle variations (ccv) are observed whose origin is not well understood. Strong impact of in-cylinder flows and spray-induced turbulence of preceding injections upon subsequent spray development and mixture formation is one possible reason for ccv. In this work mutual interactions of in-cylinder charge motion and sprays from multiple injections were investigated. Time resolved particle image velocimetry (PIV) and Mie scattering of fuel droplets at 16 kHz was used to simultaneously measure the temporal evolution of in-cylinder flow fields and spray formation. The data revealed significant spray-induced vortices perturbing the tumble flow. Sprays from subsequent injections were disturbed and showed greatly enhanced ccv compared to the first injection. A distinct upwards fluid flow impinging the cylinder head at the injector’s location (termed funnel flow) was identified as primary origin of spray deformation for second and third injections.

성층희박연소 운전조건에서 분사시기에 따른 분무유도식 직접분사 가솔린엔진의 분무 및 화염특성

분무유도식 DISI엔진의 성층연소운전조건에서 분무 및 화염특성에 대한 실험적 연구가 수행되었다. 연소가시화를 통하여 성층연소 DISI의 연소는 희박 예혼합 연소와 확산연소의 성격을 모두 가지고 있는 것으로 확인되었다. 분사시기에 따른 혼합기 형성특성이 연소의 특성을 결정하는 중요한 인자임을 관찰하였다. 분무와 혼합기 가시화를 통해 낮은 분위기압에서의 over-mixing , 높은 분위기압에서의 under-mixing이 발생하는 것을 확인하였으며 이러한 혼합기 형성과정의 결과에 따라 화염특성, 연소효율 등의 연소특성이 결정되는 것을 살펴볼 수 있었다. 또한, NOx, IMEP도 분사시기에 따른 경향성을 보였으며, 분사시기에 따른 연소상의 변화에 의한 영향임을 확인하였다.

Effects of the injection timing on spray and combustion characteristics in a spray-guided DISI engine under lean-stratified operation

An experimental study was carried out to investigate the effects of injection timing on spray and combustion characteristics in a spray guided direct injection spark ignition (DISI) engine under lean stratified operation. In-cylinder pressure analysis, exhaust emissions measurement, and visualization of combustion and spray were applied. Combustion in a stratified DISI engine was found to have both lean premixed and mixing controlled flame characteristics. The stratified mixture characteristic for the injection timing was verified as a dominant factor of the flame characteristics. For the early injection timing, non-luminous flame and low combustion efficiency were observed due to the over-mixed mixture formation. On the contrary, luminous sooting flame was shown at the late injection timing because of under-mixed mixture formation. In addition, smoke emission and incomplete combustion products were increased at the late injection timing due to increased locally rich area of the mixture. On the other hand, nitrogen oxides (NOx) emissions were decreased while indicated mean effective pressure (IMEP) was increased as the injection timing was retarded. The retarded combustion phasing was verified as the reason in this observation.

Effects of a Catalytic Volatile Particle Remover (VPR) on the Particulate Matter Emissions from a Direct Injection Spark Ignition Engine

Emissions of fine particles have been shown to have a large impact on the atmospheric environment and human health. Researchers have shown that gasoline engines, especially direct injection spark ignition (DISI) engines, tend to emit large amounts of small size particles compared to diesel engines fitted with diesel particulate filters (DPFs). As a result, the particle number emissions of DISI engines will be restricted by the forthcoming EU6 legislation. The particulate emission level of DISI engines means that they could face some challenges in meeting the EU6 requirement. This paper is an experimental study on the size-resolved particle number emissions from a spray guided DISI engine and the performance of a catalytic volatile particle remover (VPR), as the EU legislation seeks to exclude volatile particles. The performance of the catalytic VPR was evaluated by varying its temperature and the exhaust residence time. The effect of the catalytic VPR acting as an oxidation catalyst on particle emissions was also tested. The results show that the catalytic VPR led to a marked reduction in the number of particles, especially the smaller size (nucleation mode) particles. The catalytic VPR is essentially an oxidation catalyst, and when post three-way catalyst (TWC) exhaust was introduced to the catalytic VPR, the performance of the catalytic VPR was not affected much by the use of additional air, i.e., no significant oxidation of the PM was observed.

Effect of Fuel Properties on Spray Breakup and Evaporation Studied for a Multihole Direct Injection Spark Ignition Injector

Mixture formation in spray-guided direct injection spark ignition engines (SG-DISI) with late injection timing is mainly controlled by spray atomization and evaporation and strongly depends on fuel properties. The influence of fuel composition on the liquid spray structure was determined for a 12-hole solenoid injector with integral and light sheet Mie scattering as well as phase Doppler anemometry (PDA). Late injection timing in a high pressure atmosphere was simulated in an injection chamber (1.5 MPa, 293 and 673 K) to characterize the spray propagation and evaporation of alkanes with high and low volatility (n-hexane, n-heptane, n-decane) and a 3-component mixture of these alkanes with similar fuel properties like gasoline fuel. Under high chamber pressure and low ambient temperature, the spray propagation and the resulting droplet sizes are similar for all fuels. However, for n-decane, the droplet size distribution is shifted to smaller droplets and the spray appeared to be less dense because of fuel-dependent internal nozzle flow which results in a reduced injected fuel mass. In contrast, under high ambient temperature conditions for more volatile fuel components, the liquid spray length is reduced and droplet size as well as droplet momenta are decreased. Small amounts of high boiling fractions delay the evaporation and support the overall spray stability also for multicomponent mixtures which is indicated by increased spray length as well as larger droplet sizes and momenta. Moreover, the droplet size distributions and the small liquid Peclet numbers (PeL ≈ 1) of the 3-component fuel indicate a demixing of light and heavy boiling components in the 3-component fuel under conditions which are typical for DISI strategies with late injection.

In situ fuel concentration measurement near a spark plug in a spray-guided direct-injection spark-ignition engine using infrared absorption method

Vaporized fuel concentration in a spray-guided direct-injection spark-ignition (SG-DISI) engine was measured using an optical sensor installed in a spark plug. A laser infrared absorption method was applied to quantify the instantaneous gasoline concentration near the spark plug. This paper discusses the feasibility of obtaining in situ air–fuel ratio measurements with this sensor installed inside an SG-DISI engine cylinder. First, the effects of the spray plume from a multi-hole injector on the vaporized fuel concentration measurements near the spark-plug sensor were examined using a visible laser. We determined the best position for the sensor in the engine, which was critical due to the spray and vapor plume formation. Then, a 3.392-μm He–Ne laser that coincided with the absorption line of the hydrocarbons was used as a light source to examine the stratified mixture found during ultra-lean engine operation. A combustible mixture existed around the spark plug during the injection period when a preset air–fuel ratio of 45.0 was used with different fuel injection timings and net mean effect pressure conditions. The effects of the orientation of the spark plug on the measured results and ignitability of the SG-DISI engine were examined. Orienting the spark plug vertically to one of the spray plumes provided more accurate results and better engine reliability. The study demonstrated that it was possible to qualify the air–fuel ratio near the spark plug during the injection period using the developed spark-plug sensor in an SG-DISI engine.

Investigation of spray characteristics in a spray-guided DISI engine using PLIF and LDV

The spray characteristics of a prototype piezo pintle-type injector are investigated in a single-cylinder optical direct injection spark ignition (DISI) engine based on the spray-guided concept where mixture preparation is dominated by the spray dynamics. The high-pressure piezo electric injector has an annular nozzle providing a hollow cone spray. Planar Laser-induced Fluorescence (PLIF) and 2-dimensional Mie scattering were used to study the spray structure and characteristics under different in-cylinder back pressures and to obtain cycle resolved measurements of the planar fuel distribution inside the cylinder. The obtained results give information about the extent of charge stratification, spray stability and fuel/air mixture formation process under operating conditions. PLIF results revealed that there is a rich mixture at the spark plug position for all these conditions, ensuring stable combustion over the entire engine operating range. In addition, LDV measurements were carried out in order to provide information on the in-cylinder flow field near the spark plug, and to assess its influence on the mixture formation process. LDV results showed that the influence of the flow field strength during the late compression injection on the liquid fuel spray structure is low. The tumble generated in the cylinder is transformed into turbulence at the end of the compression stroke, which aids fuel evaporation and vapour dispersion just before ignition.

Stratified-charge combustion: modeling and imaging of a spray-guided direct-injection spark-ignition engine

Spray-guided spark-ignited direct-injection (SG-SIDI) engines are likely to be the next generation of gasoline engines. Compared to current wall-guided (WG-SIDI) engines, they offer improved fuel economy and substantially reduced hydrocarbon and soot emissions. Two stages of stratified combustion (partially premixed flame propagation and mixing-controlled combustion of rich products left from partial combustion of initially fuel-rich mixtures) are investigated here in an experimental SG-SIDI engine using detailed CFD modeling, high-speed spectrally resolved combustion luminosity imaging, and cylinder pressure analysis. The CFD model reproduces the spatial and temporal distribution of both combustion stages. Correlation of CFD calculations and experimental combustion measurements demonstrates that optimum ignition timing occurs for somewhat rich equivalence ratios (0.9–1.6) and relatively low flow velocities (<15 m/s) at the spark location. Both model and experiment show: (1) for fixed injection time, increased rich burning occurs for earlier ignition times and (2) higher in-cylinder swirl leads to asymmetric flame propagation and increased rich burning. Calculated and measured overall heat-release rates agree quite well over a wide range of engine operating conditions. These experiment-model comparisons provide a detailed picture of combustion processes in spray-guided SIDI engines and a sound scientific basis for engine design optimization.