Valve Covered Orifice Nozzle Research Articles

EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF FUEL INJECTION PARAMETERS ON DIESEL ENGINE PERFORMANCE AND EMISSIONS

The climate changes and increase in global temperature are the key factors that have mainly amplified the exploration studies of changing fuel injection and other constraints of compression ignition (CI) engines for the abatement of exhaust emissions. In current study, a direct injection (DI), CI engine was run on a test bench for the performance and emission analyses using different nozzles and injection timings. During the experiments, two types of nozzles known as sac and valve covered orifice (VCO) were used with hemispherical cavity and toroidal cavity pistons, respectively. Besides an already existing set of sac type nozzles, six distinct combinations of nozzles with varying cone angles and tip penetration (protrusion) lengths (designated as 135° × 3.5 mm, 140° × 3.5 mm, 145° × 3.5 mm, 150° × 3.5 mm, 150° × 2.5 mm, and 150° × 1.5 mm) were used at three different injection timings comprising 16° before top dead center (BTDC), 13°BTDC and 10°BTDC. Experimental results reveal that VCO nozzles in toroidal combustion chamber (CC) are better than sac nozzles with hemispherical CC, and that wider cone angle nozzles at 10° BTDC give the optimum results in terms of emissions and performance, relative to those of narrower cone angles. The 150° nozzles with 1.5 mm tip penetration give abated carbon monoxide (CO), hydrocarbon (HC) and smoke emissions along with better performance characteristics such as brake specific fuel consumption (BSFC) and brake power (BP), while exhibit slightly higher oxides of nitrogen (NOx) relative to other combinations. Moreover, the same combination also proves to be effective on emission control at 8 mode steady-state cycle.

On the Link between Diesel Spray Asymmetry and Off-Axis Needle Displacement

Cutting edge experiments and thorough investigations have pointed out that radial components affect the needle lift of diesel nozzles. So far, the effects of such needle “off-axis” have been investigated within the nozzle and immediately downstream from the hole outlet. Here, the focus has been extended to the spray ambient, far outside a multi-hole VCO (Valve Covered Orifice) nozzle. A reference off-axis configuration of the needle has been defined and used to investigate its effects on the spray, in terms of hole-to-hole differences. Indeed, the spray alterations due to the needle position have been addressed for those factors, such as the velocity-coefficient CV and the area-coefficient CA, able to describe the nozzle flow behavior under needle off-axis. The investigation has been based on 3D-CFD (three-dimensional computational fluid dynamics) campaigns. The modeling of diesel nozzle flow has been interfaced to the Eulerian–Eulerian near-nozzle spray simulation, initializing the break-up model on the basis of the transient flow conditions at each hole outlet section. The dense spray simulation has been on-line coupled to the Eulerian–Lagrangian modeling of the dilute spray region. Quantitative results on each fuel spray have been provided (in terms of penetration and Sauter Mean Diameter). The range of variability within the spray characteristics are expected to vary has been found and reported, providing reference information for lumped parameter models and other related investigations.

Comparative study of flow characteristics within asymmetric multi hole VCO and SAC nozzles

A three-dimension model of an asymmetric valve covered orifice (VCO) nozzle and an asymmetric blind hole nozzle (SAC) used in two valve engines were developed for investigating in-orifice flow characteristics. Two fluid model approach used for turbulence and cavitation modeling was used to replicate the turbulence and cavitation phenomenon within the nozzles. The formation and the subsequent development of cavitation phenomenon was comparatively more prevalent in the VCO nozzle orifices. Cavitation development intensity was observed to be more significantly influenced by orifice inclination angles in the VCO nozzle as compared to the SAC. Although in-orifice flow velocity of VCO are higher under cavitating condition, their corresponding injection rates are lower comparatively, due to the formation of smaller effective flow areas in the orifices. However under non-cavitating condition, the in-orifice flow velocities and effective injection velocities from the SAC orifices are higher, relative to that from the VCO. The cavitation phenomenal distribution within the SAC orifices were at the top areas while that of the VCO were mostly at the mid-sections of the orifices. Increasing orifice inclination angle increases the degree of cavitation phenomenon but decreases the uniformity of in-orifice flow velocity in the nozzles. The rate of non-uniformity in orifice flow velocity distribution is much higher within the VCO orifices as compared to the orifices in the SAC.With a comprehensive insight of in-orifice nozzle flow characteristics for asymmetric nozzles, the combustion efficiency and the rate of soot formation in a two valve engine can be managed and improve significantly by manufacturers.

Comparison of microsac and VCO diesel injector nozzles in terms of internal nozzle flow characteristics

A computational study focused on the inner nozzle flow and cavitation phenomena has been reported in this paper in order to investigate the two most common types of diesel injector nozzles at the present: microsac and valve covered orifice (VCO). The geometrical differences among both types of nozzles are mainly located at the needle seat, upstream of the discharge orifices. In the case of microsac nozzles there is a small volume upstream of the discharge orifices which is not present in VCO nozzles. Due to these geometrical differences among both type of nozzles, differences in the inner flow and the cavitation development have been found and analysed in this research. For the study, two cylindrical nozzles with six orifices and the same outlet diameter have been experimentally characterized in terms of mass flow rate. These measurements have been used to validate the CFD results obtained with the code OpenFOAM used for the analysis of the internal nozzle flow. For the simulations, two meshes that reproduce the microsac and VCO nozzles seat geometry while keeping the same geometry at the orifices have been built. The simulations have been carried out with a code previously validated and able to simulate cavitation phenomena using a homogeneous equilibrium model (HEM) and with RANS approach for the turbulence modelling (RNG k–ε).For the computational study, three injection pressures and different geometries simulating different needle lifts have been used. The comparison among nozzles has been made in terms of mass flow, momentum flux and effective velocity and in terms of other non-dimensional parameters which are useful for describing the inner nozzle flow: discharge coefficient (Cd), area coefficient (Ca) and velocity coefficient (Cv). The analysis performed by studying and comparing the particularities of the flow in each nozzle has been useful in order to explain the experimental differences found in terms of mass flow rate and critical cavitation conditions.One of the main conclusions of this study is the higher influence of the needle on the mass flow, momentum and injection velocity results for the VCO nozzle as compared to the microsac one. Hence, whereas in the first one these variables scale with the needle lift value, in the second one there is an intermediate needle lift from which they stop being influenced by the presence of the needle.Furthermore, the study has also revealed important differences in the proneness to produce cavitation and its morphology. For the VCO nozzle, cavitation phenomenon occurs only in the upper part of the orifice inlet. However, for the microsac nozzle cavitation appears both at the upper and the lower part of the nozzle orifice entrance.

Numerical analysis of needle eccentricity effects on internal flow of enlarged VCO diesel injector

The VCO (Valve Covered Orifice) nozzle is known to reduce unburned hydrocarbon emission from direct-injection diesel engine. However, high-pressure fuel injection causes needle eccentricity which results in non-uniformity of sprays and deterioration of emission characteristics. Present study numerically investigates the needle eccentricity effects on the internal flow of the large-scaled VCO nozzle by using STAR-CCM+ (ver. 8.02.011), and obtained results are compared with those of a corresponding experiment. Swirling flow is formed in the nozzle hole due to the needle eccentricity. As increasing the swirl strength, air entrainment from the nozzle hole outlet is generated, and spray cone angle is decreased. However, in the range of large needle eccentricity, growth of the swirling flow decays, and the spray cone angle is saturated at certain value. These tendencies are also observed in the experiment. Thus, it is confirmed that the present numerical simulation reproduce the actual phenomena qualitatively. Furthermore, effects of edge curvature of the nozzle hole inlet is also numerically investigated as a key factor acting on the internal flow characteristics. As the edge curvature increases, the swirling flow in the nozzle hole considerably grows, and the air entrainment into the nozzle hole is greatly enlarged. Also, spray characteristics is extremely changed.

Flow in valve covered orifice nozzles with cylindrical and tapered holes and link to cavitation erosion and engine exhaust emissions

Results from a research programme addressing the development, testing, and production of valve covered orifice (VCO) nozzles operating with current production Tier 3 off-highway diesel engines are reviewed. The common rail injectors operate at pressures exceeding 1300 bar and include pilot and main injection events. Although acceptable engine exhaust emissions can be obtained with conventional VCO nozzles, cavitation erosion may lead to mechanical failure of the nozzle. Redesigning the injector in terms of its durability against surface erosion has been obtained through use of a computational fluid dynamics (CFD) flow solver incorporating a two-phase cavitation model and flow visualization in enlarged transparent nozzle replicas. The model has provided evidence of the flow distribution under realistic pressure and needle lift opening scenarios while at the same time it has been calibrated to indicate the locations where the possibility of cavitation erosion may become significant. The experiments performed in enlarged transparent nozzle replicas have provided evidence of the string cavitation structures formed inside the different nozzle designs. Cross-correlation with engine emission tests indicates that string cavitation may be associated with increased engine exhaust emissions. Proposed injector designs with geometric modification easily implemented in the production series have been proved to be erosion-free while at the same time have improved the engine exhaust emissions.

Study of the influence of nozzle seat type on injection rate and spray behaviour

A deep analysis of the injection rate characteristics and spray behaviour of the most used nozzle types in diesel engines [microSAC and valve covered orifice (VCO)] has been carried out. In order to compare the injection characteristics and the spray behaviour of both nozzle types, several experimental installations were used, such as the steady flow test rig, injection rate test rig, spray momentum test rig, and nitrogen test rig, to obtain a full hydrodynamic and spray characterization. The study of the flow in both nozzles was analysed under steady flow conditions in the steady flow test rig and in real unsteady flow conditions in the injection rate test rig and the spray momentum test rig. The macroscopic properties of the spray (tip penetration and spray cone angle) were characterized using a high-pressure test rig. From the point of view of the internal flow behaviour, the results showed interesting differences in the permeability of both nozzle geometries, with a higher discharge coefficient in the microSAC nozzle. However, from the point of view of air entrainment, the results showed a better quality of fuel-air mixing in the VCO nozzle. Besides the evidence from the experimental results, a theoretical analysis was carried out in order to identify the most important parameters that determine the spray behaviour and thus justify the different macroscopic behaviour of both nozzles.

Ultrasonics in the steel foundy industry

A three-dimension model of an asymmetric valve covered orifice (VCO) nozzle and an asymmetric blind hole nozzle (SAC) used in two valve engines were developed for investigating in-orifice flow characteristics. Two fluid model approach used for turbulence and cavitation modeling was used to replicate the turbulence and cavitation phenomenon within the nozzles. The formation and the subsequent development of cavitation phenomenon was comparatively more prevalent in the VCO nozzle orifices. Cavitation development intensity was observed to be more significantly influenced by orifice inclination angles in the VCO nozzle as compared to the SAC. Although in-orifice flow velocity of VCO are higher under cavitating condition, their corresponding injection rates are lower comparatively, due to the formation of smaller effective flow areas in the orifices. However under non-cavitating condition, the in-orifice flow velocities and effective injection velocities from the SAC orifices are higher, relative to that from the VCO. The cavitation phenomenal distribution within the SAC orifices were at the top areas while that of the VCO were mostly at the mid-sections of the orifices. Increasing orifice inclination angle increases the degree of cavitation phenomenon but decreases the uniformity of in-orifice flow velocity in the nozzles. The rate of non-uniformity in orifice flow velocity distribution is much higher within the VCO orifices as compared to the orifices in the SAC.With a comprehensive insight of in-orifice nozzle flow characteristics for asymmetric nozzles, the combustion efficiency and the rate of soot formation in a two valve engine can be managed and improve significantly by manufacturers.