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by Thomas J. Dunn, Ph.D.

The automotive industry has long been driven by demands for better fuel economy and lower exhaust emissions. In Europe these forces have led to a resurgence of the once-lowly diesel engine. Early diesels were often perceived as dirty and noisy but cheap and easy to operate and manufacture. In contrast, the latest generation of passenger car diesels is quiet, clean, efficient and powerful. These improvements result largely from the development of high-pressure fuel injectors that provide thorough mixing of fuel and air as well as precise control of the injection profile.

"If you can't measure it, you can't make it." Never was this adage more true than during the development of these new injectors. With operating pressures approaching 30,000 psi, they require sealing and bearing surfaces with form tolerances as tight as 1 micron. To further complicate matters, the critical surfaces sit at the blind end of a long, narrow bore, making them difficult to access. Conventional measurement technologies offered neither the necessary accuracy nor the speed to control such a high-volume production process.

To meet this need, Corning Tropel Inc. developed a laser interferometric metrology system that combines submicron accuracy with measurement and setup times on the order of one minute per measurement. Although developed specifically for fuel injectors, the technique will likely apply to other high-value manufacturing processes that incorporate deeply recessed, rotationally symmetric surfaces.

Europe's diesel renaissance

Diesel engines possess a fundamental fuel efficiency advantage over spark-ignition gasoline engines. In addition, diesel fuel is both less expensive to produce and, because it's less volatile, safer to use than gasoline. These advantages have long secured the diesel engine's position in industrial applications, in which purely economic considerations outweigh those of aesthetics. But who hasn't changed lanes to avoid following a smelly, smoky diesel on a long, slow climb, or cast uncharitable thoughts toward the smug driver who saves his fuel dollars at the cost of our fresh air?

All of this is about to change. The diesel has been reborn, and from the soot have risen smooth, powerful, clean and quiet engines that maintain fuel economy. This renaissance began during the early 1990s in Europe, where high fuel prices underscore the benefit of the diesel's economy and strict regulations limit exhaust emissions. Luxury car manufacturers Alfa Romeo and DaimlerChrysler were the first to introduce the new diesel technology, and they were quickly followed by others. Between 1996 and 2001, the percentage of newly registered European cars with diesel engines had more than doubled. Volkswagen currently holds the fuel economy record of 100 km on less than a liter of fuel. At the performance end of the spectrum, Volkswagen will soon introduce a five-liter, 10-cylinder engine that produces more than 300 horsepower, and a diesel BMW recently won the 24-hour endurance race at Nürburgring race track in Nurburg, Germany. In terms of cleanliness, the emission of noxious engine substances has dramatically dropped during the last 10 years. Particulate emissions are down 80 percent, nitrous oxide by 90 percent and carbon monoxide by 97 percent.

High-pressure fuel injection systems lay at the heart of the diesel renaissance, led by pioneering manufacturer Robert Bosch GmbH. High-injection pressure promotes finer fuel atomization and better mixing with air to ensure complete combustion. When combined with electronic actuation, the high pressure also permits precise control of the injection timing and volume. Bosch recently announced the delivery of its 10 millionth high-pressure injector and delivered more than 4 million during 2002. This compares to only 200,000 delivered as recently as 1999.

A fuel injector is fundamentally a highly sophisticated needle valve (See below). Injector ports at the bottom of the valve direct the fuel into the combustion chamber in a precisely controlled pattern, and a solenoid or piezoelectric actuator opens and closes the valve. Precision mechanical surfaces include the guide bearings, cylindrical surfaces on the shaft of the needle and body that control the needle's alignment; and the valve surfaces, mating conical surfaces that control the flow of fuel. Thirty-thousand psi operating pressures require form tolerances on these surfaces as tight as one micrometer. The critical valve seat is typically located at the bottom of a blind hole a few millimeters in diameter and tens of millimeters deep. Created by grinding, these surfaces can be relatively rough.

In any high-volume manufacturing process, control is essential to high yield and maximum profitability. As operating pressures and manufacturing volume climbed, engineers soon realized that they had no adequate means of measuring their manufactured surfaces at an adequate speed. Contact methods lost much of their precision in the inaccessible blind hole and weren't fast enough to provide a high-density surface map in a reasonable period of time. Optical methods also ran into problems with accessibility and the roughness of the measured surfaces. Developing a viable measurement technique became essential to continuing progress in fuel injector manufacturing.

The ThetaForm metrology system

Corning Tropel's ThetaForm is a miniaturized, dual-wavelength interferometer configured to measure difficult-to-access, rotationally symmetric surfaces.

In distance-measuring applications, interferometers work by comparing the distance traveled by one beam of light, the test beam--which reflects back to the interferometer from the measured surface--with the distance traveled by another beam, the reference beam--which reflects back from a reference surface fixed at some arbitrary distance, as illustrated to the left. The interferometer makes the comparison by looking at the way in which the two beams interfere when recombined. If the crests and troughs of the test and reference beams coincide when they return to the interferometer, they're said to interfere constructively. The difference between the distances traveled by the test and reference beams must be an even multiple of half the light's wavelength. However, if the crests of one beam coincide with the troughs of the other (i.e., 180 degrees out of phase), they interfere destructively, and the distance difference must be one-quarter of the wavelength greater or less than an even multiple of the half-wavelength. Adjacent points spread over a surface will compose a pattern of dark and light bands, with the dark bands corresponding to destructive interference and the light bands corresponding to constructive interference. The overall pattern represents the deviation of the shape of the test surface from the shape of the reference surface.

With the ThetaForm system, the interference map comprises sequential measurements acquired point-by-point as the test surface rotates on its axis in close proximity to the interferometer probe. The probe is mounted to a programmable stage that can move left, right, up and down, thus allowing the probe to follow the surface in a spiral pattern over any rotationally symmetric form. The test beam passes through the probe, reflects from the surface and returns through the probe to the interferometer. The interferometer's reference arm travels with the probe but not through it; therefore the interference map represents the deviations of the test surface from an ideal, virtual surface symmetric about the rotational axis of the measured part.

The wavelength of the light provided by the laser sources used in interferometry is precisely known and extremely stable; thus, the measurements offer reliable precision and accuracy. Using specialized analytical techniques, it's possible to resolve point-to-point distance changes of one-hundredth of the wavelength or better. The ThetaForm uses two lasers with wavelengths of 1,310 and 1,550 nm that offer point-to-point resolution of a few nanometers.

Although they're highly precise, interferometers have limited dynamic range. Because each light wave is exactly like every other wave, an interferometer is like a ruler without numbers (i.e., it can accurately measure fractions of an inch, but it has a hard time distinguishing one inch from another). As a rule of thumb, interferometric measurements require a surface roughness much less than the wavelength of the light. The ground surfaces found in fuel injectors can easily exceed this criterion. However, the ThetaForm's dual wavelength interferometer offers a solution. By combining the interferometric patterns at the two different fundamental wavelengths, it generates a composite pattern with a synthetic wavelength of 8.46 mm. Both interferometers are independently capable of measuring smooth parts. Together they can measure ground parts with surface roughness (Rz) up to 2 mm.

Probe design

One of the most difficult surfaces to access on a fuel injector is the valve seat, which is located at the bottom of a deep, narrow hole. The ThetaForm's probe design allows the test arms of both interferometers to be delivered to the bottom of a 3.5 mm diameter by 45 mm deep blind hole, as illustrated to the left. At the end of the probe, miniature optics split the test arms into two different beams. The beams each contain both wavelengths but leave the probe at different angles, permitting measurements of two different surface types with the same probe. Because neither the part nor the probe is disturbed between measurements, the system can make relational measurements such as runout, perpendicularity and coaxiality among different regions of the part. For example, the fuel injector requires precise alignment of the guide bearing and the valve surfaces.

Stage, spindle and environmental control

The interferometers and probe ride on an X-Z stage system that allows the probe to follow the part surface. Although it's a high-precision, crossed-roller bearing stage, its motion isn't perfect. A separate, three-axis displacement measuring interferometer constantly monitors the straightness, yaw and displacement errors of the stage motion. Stage position data is recorded for each measured point, permitting error correction during subsequent processing.

The part is mounted to a precision air-bearing spindle by a hydraulic expansion chuck. Software analysis after the measurement removes any residual tilt or decenter--typically less than a few microns.

The stages, interferometers and spindle are mounted on a granite base, which is suspended by a pneumatic isolation system. They're enclosed in an actively controlled environmental chamber that maintains the entire measurement area at a constant temperature within +_ 0.25° C.

Results with the new system

At 600 rpm, data acquisition time for typical measurements is short. Even allowing time for sample mounting, probe positioning and data analysis, total measurement throughput equals a minute or two per measurement. Moreover, the high sampling density provides thorough coverage of the full surface and minimizes the probability of significant sampling error.

The repeatability of these measurements has proven to be quite good. We can typically achieve a standard deviation of 10 to 30 nm, depending on the parameter being measured. For example, a roundness test of the inside cone for 25 different parts was measured 20 times during a period of five days, as illustrated below. For this set of data, the average standard deviation was 11 nm.

The measurements' accuracy is verified by NIST/PTB-certified artifacts. Corning Tropel has manufactured artifacts for each of the measured parameters: roundness, straightness, parallelism, angle, diameter and runout. Gage studies comparing the measured values to the certified values achieved an accuracy of 40 nm or better for each of these parameters.

Fuel injectors' future

High-pressure fuel injectors have permitted dramatic improvements in automotive diesel engines. The new generation of engines will ultimately affect the lives of millions, not only by improving the quality of the driving experience, but also by reducing the negative environmental effects of driving we all share. The new injectors' performance, however, is heavily dependent on maintaining extremely tight manufacturing tolerances that, until now, have been difficult or impossible to measure. Although the ThetaForm was developed to satisfy the specific requirements of injector manufacturers, it will certainly apply to other precision manufacturing operations.

About the author

Thomas J. Dunn, Ph.D., is manager of the engineering group working within Corning Tropel's Metrology Products Division. Dunn is responsible for the development of the ThetaForm Metrology System. Letters to the editor regarding this article can be sent to letters@qualitydigest.com.