Tech Briefs
Fuel injection: the inside story

The search for ever-greater automotive fuel-injection-system efficiency has brought enormous gains in performance, fuel consumption, and reliability. Over a relatively short period, the carburetor has been essentially relegated to the automotive museum. A central element of this achievement has been, and continues to be, the refinement of the design of fuel-injection nozzles. To achieve this, it is necessary to monitor droplet size in fuel sprays. Malvern Instruments has developed a system to achieve this and it is now being used by a research group at DaimlerChrysler that is working on injectors for direct-injection (DI) engines.

One of the most critical issues facing automotive engine designers is the resolution of two seemingly conflicting goals: the achievement of maximum power output from an engine while at the same time reducing fuel consumption and minimizing exhaust emissions. DI technology is now playing a central part in meeting those criteria. According to the company, there are considerable technical challenges to be addressed in developing such systems, and a detailed understanding of the fundamental technology is required to ensure continued progress.

Size distribution of droplets in a spray has a fundamental effect on both spark-ignition (gasoline) and compression-ignition (diesel) engines. Smaller fuel droplets vaporize much more quickly than large ones, but their penetration is shorter and therefore the size distribution must be optimized. In gasoline DI engines, fuel is injected directly into the cylinder, therefore the spray characteristics and mixture formation assume a high importance. Small droplet size, rapid evaporation, stable ignition, appropriate mixing, and minimal wall impingement are all usual requirements. DI engines, however, allow less time between the injection of fuel and sparking than conventional four-stroke engines, and a well-atomized spray is needed for optimized mixture formation prior to ignition.

Factors that may affect the droplet size distribution from an injector include the pressure and geometry of the injector and speed of operation of the needle valve. To study droplet formation from an injector effectively, very finely resolved time measurements are essential. It is also important to be able to study the opening and closing of the injector.

To simulate the workings of both gasoline and diesel engines, a test rig has been built at DaimlerChrysler's research center in Stuttgart, Germany. Simulation of the high pressures inside an engine is achieved via a model cylinder. Nitrogen gas is pressurized to between 1500 - 5000 kPa (218 - 725 psi) depending on the type of injector under investigation. Attached to the rig is a Malvern Spraytec system. The cylinder's glass walls enable laser light from the system to probe the spray, allowing measurement of the droplet size distribution produced by the injection event. The system uses established laser diffraction technology for the real-time measurement of particle or droplet size. It was developed specifically to overcome the fundamental problems associated with measuring continuous sprays, such as wetness, high concentration, and the short duration of many sprays. According to the company, droplet- or particle-size analysis using laser diffraction is based on the fact that when a particle passes through a laser beam it causes light to be scattered at an angle that is inversely proportional to its size. The scattered light is collected by a detector and analysis of the ensuing diffraction pattern enables the size distribution of particles in a given sample to be calculated.

To be sufficiently robust for continuous operation on an experimental test rig, the Spraytec system is designed to give wide clearances between the laser and detector modules. This allows large volume sampling. Separation of the control and electronics units means that these can be placed outside the testing area. In addition to the high time-resolution capabilities demanded for measuring these fuel sprays, fine spatial resolution is required to develop a complete understanding of the way in which droplet size distribution varies within the spray plume. To investigate this, the spray chamber is mounted on an X-Y-Z positioning device so that small sections of the spray can be accurately analyzed. To achieve the required level of spatial resolution, the Spraytec is fitted with a 3-mm (0.12-in) diameter laser beam.

Spraytec measures as many as 2500 times per second, or one measurement every 0.4 ms. A typical injection event occurs in 1 to 2 ms, so the system is able to provide the fine resolution needed to study these events. It also has multiple scattering correction and can make measurements at very short distances away from the injection nozzle, where the spray is very concentrated — capabilities particularly appropriate for this application. Synchronization of spray initiation and measurement events is achieved using an external trigger, ensuring that data is collected at all critical points in the development of the spray.

Once a nozzle has been fully characterized, it is tested in other research departments. A large number of techniques are used to produce a complete characterization of the fuel-injection nozzle. Among these are high-speed photography and Phase Doppler Anemometry, a technique that measures both droplet size and individual droplet velocity.

Stuart Birch