ABSTRACT:
Today, the major problem which troubles the engineers is controlling the emissions from an automobile with out sacrificing its efficiency. The one way to solve the problem is utilizing the alternate fuels which will be an eco friendly. But the main disadvantage is the power output from such alternate fuels is less compared to the conventional fuels such as petrol and diesel. So through this paper we present the various advanced technologies of fuel injection which will be an eco friendly as well as an efficient one.
This paper reports a new application of a thermal micro injector for fuel injection. The paper estimates the ejected velocity of the diesel fuel droplet, and the minimum temperature for bubble formation. The effective fuel spray penetration using the thermal micro injector is explained elaborately. Analysis of the temperature profile evolution and activation curve between water and diesel fuel is helpful for optimization of the micro injector design. The micro injector is fabricated using combined surface and bulk micromachining. Also we have discussed some advantages of the micro injector over the conventional fuel injector
INTRODUCTION:
The fuel injection system is the heart of the internal combustion engine. The function of the injector is to provide fuel to the engine chamber. The fuel will be atomized into small droplets, and begin to evaporate as it moves away from the nozzle and mixes with hot air. The fuel-air ratio is not uniform across the engine chamber because of the complexity of the mixing process. The fuel injection system mainly deals with the atomization step for a typical combustion engine sequence that consists of
1. Atomization,
2. Vaporization,
3. Air entrainment,
4. Ignition, and
5. Combustion.
The typical nozzle diameter for a diesel fuel injector lies between 200 µm and 1000 µm, while the ratio of the length to diameter of the nozzle ranges from 2 to 8. The fuel injection pressure is usually very high (20-170 MPa). Also, the temperature in the cylinder at the time of injection is about 1000 K. The diameter of a typical diesel fuel droplet with a back pressure of 39.3 MPa (5700 psi) ranges from 10 µm to 100 µm, with the average about 51 µm (0.002 in) as shown in Fig. 1.
Fuel droplet size distribution of micro injector for different input pressure
To make the sizes of fuel droplets smaller than those of nozzles fabricated with traditional machining methods, a high pressure is needed to operate the nozzle in atomization regime. The typical speed of the injection at the exit of the nozzle is about 100 m/s and the spray penetration depth is about 50-100 mm.
The most important characteristics of the diesel fuel injector system are
1. Droplet size distribution,
2. Spray penetration,
3. Spray evaporation,
4. Dribbling, and
5. Leakage.
The typical frequency bandwidth of the diesel fuel injector is about 100 Hz. However, it could be higher at a faster engine speed if the engine is made smaller while the desired power output is kept the same.
Since the four-stroke, direct-injection diesel engine has a higher thermal efficiency, about 40 %, than most of the conventional gasoline engines, it has been chosen by the United States Council For Automotive Research as the candidate for the Partnership for a New Generation of Vehicles program (PNGV). To increase the fuel combustion efficiency to 80 miles per gallon, a novel fuel injection system is desired due to the conventional system’s inability to provide smaller and precisely volume-controlled droplets.
OPERATION PRINCIPLE:
The operating principle of micro injector is similar to that of commercial thermal inkjet print heads. The thermal jet technique uses thermal energy to grow bubbles inside a chamber, functioning as a pump to eject droplets. Fig. 2 illustrates the droplet ejection sequence of a typical thermal jet. The jet uses an electric current pulse to boil liquid inside the micro chamber. The expanding bubble pressurizes the chamber and ejects a column of liquid through a nozzle. The liquid column further breaks into a sequence of droplets through the interaction between surface tension and inertial force.
A typical thermal bubble jet a) growth of bubble and ejection of liquid column
b) Collapse of bubble and droplet formation
After the liquid column is separated from the nozzle, the chamber is refilled by liquid from the manifold via the capillary force. The size of a fuel droplet highly depends on the shape and size of the micro nozzle. A monolithic silicon micro injector has been fabricated using MEMS technologies and successfully demonstrated the ability of single-droplet ejection with water and ink as working fluids.
FABRICATION AND PACKAGING OF MICROINJECTOR:
The micro injectors were made on a 4-inch silicon wafer in the UCLA Nanolab by surface and bulk micromachining with a process similar to one reported by Tseng et al. (1998b). On the front side of the chip, there are 12 micro injectors positioned along the manifold, shown as a horizontal centerline. Each vertical line on the top portion of the chip is connected to one of the micro injectors. The test chip is glued onto a printed-circuit board with patterned electrodes and two through holes, and wire-bonded. Two plastic tubes are connected at the back of the board to supply liquid to the manifold.
DIFFERENCE BETWEEN WATER AND DIESEL FUEL USED
IN MICROINJECTOR:
The major difference between water and diesel fuel as injecting medium is that diesel fuel is a mixture of different chemicals, including
1. n-paraffin,
2. Isoparaffin,
3. Olefins,
4. Naphthene,
5. Aromatics,
6. Non-hydrocarbon compounds while water is a pure substance.
For e.g.: the diesel fuel from Philips Chemical Company consists of paraffinic hydrocarbons (>80%), aromatic hydrocarbons (<10%),>
PROPERTIES | NO.2-D | WATER @100 (DEGREE)C |
CETANE NO | 52 | - |
GRAVITY ATP | 36.3 | - |
DENSITY | 843.3 | 958.3 |
KINEMATIC VISCOSITY | 3.0*10-7 | 1.02*1 0-6 |
DYNAMIC VISCOSITY | 2.53*10-3 | 9.8*10-4 |
DISTILLATION TEMP | - | - |
AMT RECOVERED | - | - |
50% | 274.4 | - |
90% | 318.3 | - |
End Point, °C | 346.7 | 100(boiling point) |
Flash Point, °C | 71.1 | |
Auto ignition temp °C | ~317 | |
Sulfur, Mass % | .017 | |
Surface Tension,MN/m | 30 | 59 |
Heat of evaporation KJ/Kg | 230-~270 | 2257 |
Specific Heat | ~1.8 | 4.211 |
Thermal conductivity w/mk | .1768 | .68 |
Thermal diffusivity m2/s | 1.165*10^-7 | 1.627*10^-7 |
DIESEL DROPLET DYNAMICS:
The investigation of the speed of the droplet of diesel fuel due to the bubble growth is not trivial. Although in-depth study would be helpful for the understanding of the mechanism behind the diesel fuel micro injector, it is not practical to fully model the phenomena for the preliminary design. Therefore, a simplified model for the bubble dynamics to generate the droplet is introduced here.
The fuel droplet velocity at the exit plane of the nozzle will essentially determine its subsequent trajectory. The estimation of the droplet velocity can be used to predict its flow regime, either laminar, transition or turbulent. The driving mechanism for the micro injector is the pressure build-up due to the bubble formation.
MINIMUM TEMPERATURE FOR BUBBLE FORMATION
The minimum temperature required for water bubble formation is well documented but not for diesel fuel. In order to find the optimal design for a diesel fuel micro injector, a one-dimensional heat conduction model is used for the temperature evolution from heater-fluid interface before the bubble formation. There exists a minimum temperature to generate enough bubble pressure to push the diesel fuel out of the nozzle. The activation curve is the minimum temperature for a bubble with a radius R to generate sufficient vapor pressure to overcome the diesel surface tension. Based on the Clausius-Clapeyron equation, the activation curve for the bubble can be approximated by the following formula
T=Tsat + (2sPsat)/(Rrvhfg)
where Tsat is the saturation temperature at 1 atmosphere, R (= 2x) is the radius of the bubble, x is the distance calculated from the heater-fluid interface, hfg is the latent heat of evaporation, and s is the surface tension of the fluid. Since the properties of diesel fuel, a mixture, are not exact numbers, but lies between two values, it is better to describe the activation region to be lie between the lower and upper activation curves for the diesel fuel .
For the micro injector with a 30 mm nozzle, the temperature profile evolution and activation curve for water and diesel fuel are calculated. the temperature profile at 10 ms will reach the activation curve. It indicates that if the pulse width of the electric signal applied to the heater is equal or larger than 10 ms, then the bubble can be activated and grow. On the other hand, the temperature profile for the diesel at 5 ms will reach the lower activation curve, but not reach the upper curve. It is suggested that the bubble from some portion, but not all, of the diesel fuel may gain enough energy to grow. By comparing the temperature gradient for the diesel fuel is higher than that of the water. Therefore, the temperature near the heater-diesel interface will increase faster than that of the water. There is another difference between water and diesel fuel. No. 2-D diesel fuel has a flash point at around 71 °C and auto-ignition temperature at 317 °C. Depending on the heat transfer rate, before the temperature reaches the activation temperature, it may generate combustible vapor mist or the diesel fuel might burn. For water, it is not easy to visualize the water vapor mist because the latent heat of evaporation is much higher.
MICROINJECTOR TESTING:
To characterize the droplet ejection process of the fabricated Micro injector, a visualization system is used. A light emitting diode was placed under the micro injector to back illuminate the droplet stream. Two signals, one for driving the micro injector and the other one for the LED, were synchronized with an adjustable time delay. Ink droplets were ejected from a micro injector by a current pulse train with a 10 ms duty cycle at 1 kHz. The ink droplet images were captured by the flashing light from the LED at specified time delays. In the fuel droplet ejection testing, No. 2-D diesel fuel from Chevron was placed into the injectors, and a periodic input signal with a 10 ms pulse width and 1 kHz frequency was supplied to energize liquid fuel in the micro chamber. Both commercial inkjet print heads (HP 51626A) and our micro injector were tested. The fuel droplets were intermittently illuminated by a light emitting diode synchronized with the firing frequency, visualized by a microscope with a CCD camera and recorded by a VCR. The droplet ejection results are shown in Fig. 3(a) for the commercial inkjet and in Figure 3(b) for the Micro injector. Ejection velocity increased with higher power input as shown in Fig. 3(a). The inkjet heater was burnt out at around 130 mJ/pulse. However, it was observed that the operation lifetime of the commercial inkjet is less than 30 min (5 cartridges were tested), and the droplet ejection distance decreased with time quickly. It is suggested that the chamber structure of the commercial inkjet be damaged by diesel fuel. To examine this suggestion, a new inkjet Cartridge was taken apart and the chip was diced into halves and inspected under the microscope as shown in Fig.(b). After dipping the diesel fuel on the cross section of the chip, the photoimageable barrier (polyimide), is observed to be dissolved in less than 30 seconds as shown in Fig. (c).
Cross section of a commercial inkjet print head a)schematic view b)picture of cross section showing the barrier c)picture of cross section showing the barrier damaged by diesel
In contrast, the problem of short lifetime was not observed in our micro injector because of the compatibility of the diesel with the micro injector’s materials. In the testing of our micro injector, the energy supplied was gradually increased from 0 to 36 mJ/pulse until the fuel flowed outward to form a liquid puddle. When the energy was increased to 57 mJ/pulse, diesel mist of which the droplet size is smaller than 1 mm was observed. When the energy was further increased to 120 mJ, fuel droplets were clearly seen. The amount of the energy needed to eject a diesel droplet is higher than the energy needed for water, as expected. The diesel mist shows unstable movement which depends on the surrounding airflow .Apparently, the fuel droplet from the micro injector is much smaller (~30 mm) and more uniform than the droplets generated from the commercial fuel injector, about 200 mm which can contribute greatly in decreasing the droplet evaporation time and increasing the fuel-air mixing efficiency. In addition, the frequency response of the micro injector is on the order of 10 kHz, which may open the possibility for the diesel engine to compete with gasoline engines even in terms of acceleration.
SUMMARY AND CONCLUSION:
The conventional diesel fuel injector is operated by a fuel pump to provide high pressure so that the fuel jet coming out of the nozzle will break into small droplets which are much smaller than the nozzle diameter in the atomization flow regime. The cyclic high pressure will introduce high stress near the nozzle which is undesirable for mechanical design. The size of the generated fuel droplet typically ranges from 10-100 mm. The moving parts in the conventional fuel injector and the fuel pump will cause wear problems. In contrast, diesel fuel injection by the reported thermal micro injector is not operated at such high pressure and there is no need of the fuel pump and moving parts. The ejected droplet size is primarily determined by the nozzle diameter and is more uniform. The operating bandwidth of the micro injector, on the order of 10 kHz, is much higher than the conventional one. It can be advantageous for more sophisticated engine control systems. The cost of monolithically-manufactured micro injector can be competitive by batch processing.
However, the spray penetration of the micro injector is not deep enough, owing to the high viscosity-to-inertia ratio for small droplets. This issue can be further addressed by either airflow assisted fuel intake or a premix procedure in the fuel-air mixing process. Finally, the dribbling problem, commonly seen in traditional fuel injectors, is expected to be solved by a micro injector with a special micro channel design.
No comments:
Post a Comment