Analyzing Injection Science
Characterizing the high-speed output from fuel injectors is critical to improving automotive engine efficiencies.by P. Yonyingsakthavorn, C. Dumouchel and J. Cousin, UMR 6614 CORIA Research Institute, University of Rouen, France, and A. Virden and P. Kippax, Malvern Instruments Ltd, UK
Spraytec from Malvern Instruments
| The performance of a fuel injection system is a fundamental determinant of a reciprocating engine’s efficiency and power output. It also has a key influence on the production of pollutants.
The current climate of relatively high oil prices and increasingly stringent legislation governing vehicle emissions, therefore, places emphasis on the design, optimization and maintenance of fuel injection systems—a challenge complicated by the growing use of biofuels. Biodiesel has a lower calorific value than petrodiesel, intensifying the need for efficient design and different physical properties, most notably viscosity, which alter atomization and combustion behavior.
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.
Combustion efficiency
Over the last 30 years, increasing legislation has required a reduction in tailpipe exhaust emissions of more than 90%[1]. The automotive industry employed a number of technologies to meet these progressively tougher limits, of which the development of advanced fuel injection systems has played a major role. Fuel injection, which has now virtually replaced the carburetor in all vehicle engines, helps to reduce engine emissions with a variety of efficiency improvements.
Figure 1. Particle size history obtained for a single injection cycle using a pressure of 5 bar. Click to enlarge. | 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. However, there are many applications, such as manifold port injection systems, where this requirement must be balanced against the need for the fuel spray to penetrate into the cylinder head. Often, spray penetration is improved by increasing the droplet size, which may improve mixing.
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.
Biodiesel, compared to petrodiesel, tends to be more viscous, requiring greater shear to atomize to an equivalent droplet size. While such fuels may be advantageous in terms of lubricity and cetane number (relative to low sulfur diesels), accurately characterizing atomization behavior is vital to avoid potential problems with incomplete combustion when modifying or developing engines for their use.
Mode of operation
Figure 2. Average particle size distributions obtained for injection pressures between 5 and 40 bar. Click to enlarge. | Direct injectors, standard across the automotive industry, offer improved droplet penetration into the combustion chamber. They release fuel either as a conical liquid sheet or multiple cylindrical jets, with flow being controlled by either a piezoelectric or solenoid valve. 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. Firstly, too high a pressure can result in cavitation—where a turbulent flow causes gas bubbles to form in a pressurized fuel. These bubbles affect the liquid atomization process, reducing the volume of fuel that passes through the nozzle. Secondly, there’ is a limit to the amount of energy (and its associated cost) 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 ms, depending on a number of measurements, including 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 for users to be able to study the profile of an individual injection event in detail.
Laser diffraction
Figure 3. Average Dv50 calculated for each injection cycle for the different injection pressures. Click to enlarge. | 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. The international standard ISO 13320-1 covers the application of laser diffraction for this purpose, and states the minimum requirements for instrument performance[2].
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.
Time-resolved data
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 highly valuable in understanding nozzle performance. A particle size history for a single injection is shown in Figure 1.
In this example, data were acquired at 10 kHz, producing a particle size distribution every 100 μsec. 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 timescales of well 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 dramatically. This phase is followed by a rapid decrease in particle size as the flow through the nozzle 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. For this reason, these droplets are characteristically produced either at the beginning of the pulse, before the main part of the spray plume, or at the end of the injection. Improvements to the nozzle design, i.e. the removal of areas that retain liquid, can greatly reduce their occurrence [3].
Data analysis
The large amount of data obtained can make interpreting particle size information challenging. In this experiment for example, each injection cycle lasted between 20 and 30 msec, generating between 200 and 300 particle size distributions (or records) for each pulse. Twenty-five injection events were characterized for each set of injection conditions, yielding 7,000 records/experiment.
Figure 4. Relative standard deviations for the Dv10, Dv50 and Dv90 calculated for each of the different injection pressures. Click to enlarge. | To extract 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. (see 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.
Although averaged size distributions are useful for understanding the general trends in particle size produced as a function of injector conditions, they do not enable assessment of injector reproducibility. Isolating the data for each injection cycle within the experiment data set enables calculation of the average size distribution produced for each injection. Figure 3 shows the output from such an analysis, with the averaged Dv50 presented as a function of start time of each injection at each pressure. Similar data can be obtained for Dv10 and Dv90, allowing easy calculation of the variability of injector output, as seen in Figure 4.
Thise above 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, although there is a small increase in Dv50 variability at higher pressures. This may be caused by degradation of the spray profile, resulting from the onset of cavitation within the nozzle.
Injection variability source
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
Figure 5. Ensemble average Dv50 profiles calculated for each of the five injection pressures. The error bars represent +/-1 standard deviation from the mean. For clarity, the error bars are only shown for every 10th data point. Click to enlarge. | 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. This is achieved by analyzing variability as a function of time using the single average size histories produced from the 25 repeat injections at each pressure.
The average size history profiles calculated for the Dv50 are shown in Figure 5. 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).
Conclusion
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.
High-speed data acquisition overcomes the traditional challenges associated with the analyzing droplet size in fuel injection systems, allowing the detailed observation of variations in concentration and particle size within individual injection cycles. Systems such as the Spraytec, which have this capability, offer major time and cost savings in the research and development of increasingly efficient fuel injection systems.
References
1. 2000. CONCAWE, Motor Vehicle Emission Regulations and Fuel Specifications—Part 2. Detailed information and historic review (1970–1999).
2. 1999. ISO 13320-1 Particle Size Analysis—Laser Diffraction Methods Part 1: General Principles.
3. Robart, D., R. Kneer, and M. C. Lai. 2001. Application of a High Temporal Resolution Laser Diffraction Technique to the Characterization of Gasoline Direct Injection Sprays. Direkteinspritzung im Ottomotor III, Expert-Verlag. 169-187.
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