Analyzing injection science: characterizing the high-speed output from fuel injectors is critical to improving automotive engine efficiencies.
Once fuel is released from an injection nozzle, combustion efficiency is determined by the droplet size. Smaller droplets vaporize more quickly than larger ones, so they generally enable more rapid and efficient combustion. Larger droplets, however, facilitate better penetration of the spray into the engine's cylinder head.
Fuel injection, which has now virtually replaced the carburetor in all vehicle engines, helps to reduce engine emissions with a variety of efficiency improvements.
A fuel injection system essentially comprises a fuel-dispensing nozzle and a solenoid or piezoelectric valve. An external pump forces fuel through the nozzle under high pressure, atomizing it as it is injected into an engine's combustion chamber. The Engine Control Unit (ECU) monitors several engine parameters to determine the volume of fuel to be injected, triggering the opening of the solenoid for the required duration.
Fuel injection systems provide the greatest combustion efficiency if the injected fuel is vaporized rapidly. Because evaporation rate increases as fuel droplet size decreases--smaller droplets have a higher ratio of surface area to volume--this implies that droplet size should be as small as possible.
Controlling droplet size is therefore essential, affecting both the fuel mixing process and the evaporation rate, as well as influencing vehicle emissions, fuel efficiency and power output. This control demands an understanding of atomization behavior, including recognition of the impact on the performance of fuel properties.
Mode of operation
Direct injectors, standard across the automotive industry, offer improved droplet penetration into the combustion chamber. The kinetic energy of the fuel as it passes through the nozzle is used to achieve atomization. Increasing the pressure of the fluid increases the available energy, reducing droplet size.
However, there are practical limits to the amount of pressure that can be applied to maintain the production of small fuel droplets. First, too high a pressure can result in cavitation--where turbulent flow causes gas bubbles to form. These bubbles affect the liquid atomization process, reducing the volume of fuel that passes through the nozzle. Second, there's a limit to the amount of energy that can be used to pressurize the fluid in the pumping system.
The ECU in a direct injector determines valve opening time, which in turn controls the amount of fuel released per cycle. Typical injection times range from 0.2 to 10 msec, depending on engine load, intake airflow, ambient temperature and pressure. This short duration of the injection cycle has presented one of the main challenges to characterizing droplet size in fuel injection systems. Data must be acquired at high speeds to study the profile of an individual injection event in detail.
Laser diffraction is a well-established particle sizing technique. Fuel droplet size information is obtained by measuring the angular dependence of light scattered by the droplets as they pass through a laser beam.
In the experiment described below, the droplet size distribution produced by a fuel injection system was measured at a range of fuel pressures, from 5 to 40 bar using a Malvern Spraytec. At each pressure the droplet size was measured over 25 injector cycles.
Traditional measurements of fuel injection systems require a certain amount of temporal averaging. While temporal averaging of size distribution may be sufficient for initial testing and for assessing repeatability, the ability to capture time-resolved information is valuable in understanding nozzle performance. A particle size history for a single injection is shown in Figure 1.
[FIGURE 1 OMITTED]
In this example, data were acquired at 10 kHz, producing a particle size distribution every 100 [micro]sec. The size history shows the variation in three key distribution statistics (Dv10--particle size below which 10% of the sample volume is detected; Dv50--volume median particle size; Dv90--particle size below which 90% of the sample volume is detected) together with the volume concentration (Cv). Analyzing nozzle performance over time-scales under a millisecond shows that large droplets are produced at the beginning of the injection cycle during a phase of atomization when the droplet concentration increases. This phase is followed by a rapid decrease in particle size as the flow stabilizes.
This phenomenon occurs in many types of fuel injection systems. The larger droplets observed in the initial stages of the injection cycle result from lower levels of turbulence within flow emerging from nozzle. Retention of liquid in the nozzle inhibits acceleration of liquid at the beginning of the cycle and promotes liquid deceleration towards the end of injection.
The large amount of data obtained can make interpreting particle size information challenging. In this experiment, each injection cycle lasted between 20 and 30 msec, generating between 200 and 300 particle size distributions for each pulse. Twenty-five injection events were characterized for each set of injection conditions, yielding 7,000 records/experiment.
To maximize value from the data, the relevant parameters relating to injector performance must be easily accessible. The first step is to confirm how the droplet size produced by the injector changes as a function of injection pressure. This can be achieved by averaging the data from 25 injection cycles to produce a single particle size distribution for each pressure (Figure 2). From this it can be seen that as the pressure of the fuel increases, the particle size decreases--the result of increased flow rate through the nozzle at higher pressures.
[FIGURE 2 OMITTED]
This analysis demonstrates that variability is greater at lower pressures since the nozzle is operating below the optimum flow rate. Control of the applied pressure is less precise at low pressures, leading to variability in the achieved droplet size. Increasing pressure leads to more reproducible atomization.
The next logical step is to try to understand the source of any injection variability as this knowledge can help determine how to adjust the injection cycle to improve performance. One way of accessing valuable information is to calculate the average and standard deviation of the size distribution recorded for every time point across each of the injection cycles.
The average size history profiles calculated for the Dv50 are shown in Figure 3. At lower pressures the initial droplet size is much larger and more variable, and it takes longer for a stable particle size to be achieved. As the pressure is increased, the initial size and variability decrease, with stabilization being achieved more rapidly. In each case, the point of maximum variability correlates with the time when the concentration is highest (Figure 1).
[FIGURE 3 OMITTED]
Laser diffraction particle size analyzers permit highly detailed analysis of the droplet size distribution of a fuel injection system at a range of injection pressures. Here data were analyzed using a number of methods. Ensemble averaging reveals that variation of droplet size within all of the pulses changes with pressure. The variability of the particle size at each pressure was also investigated by averaging the data from each pulse, showing a general decrease in variability as the pressure was increased.
For more information, contact Paul Kippax, Ph.D., Product Manager, Diffraction Products, Malvern Instruments Ltd., at Paul.Kippax@malvern.com.
At a glance
* Droplet size is critical to injection system efficiency.
* Controlling droplet size effects vehicle emissions and output.
* Pressure, droplet size, and flow rate affect injection variability.
* Laser diffraction is used to characterize droplet size distribution.
* Measuring droplet size requires analysis of large data sets.
Malvern's Spraytec incorporates features that make it suited for automotive applications, including: high-speed data acquisition up to 10 kHz, ensuring that rapidly changing events can be captured in detail; a range of triggering options to coordinate size measurement with the spray event; and patented multiple scattering analysis for accurate measurement of high concentration sprays.
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|Title Annotation:||Fuel Technologies|
|Author:||Yonyingsakthavorn, P.; Dumouchel, C.; Cousin, J.; Virden, A.; Kippax, P.|
|Date:||Jan 1, 2010|
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