This article emphasizes on the use of thermoplastics in engines. One case in point is a reinforced nylon multifunctional module involving three components—air intake manifold, cam cover, and design cover-on the Alfa Romeo 156 engine. The project was a collaboration of Rhodia Engineering Plastics of France, Robert Bosch GmbH of Germany, and Magneti Marelli S.p.A. of Italy, system supplier for Fiat, which manufactures the Alfa Romeo. The article also shows an example where an under-the-hood reinforced nylon module on the Alfa Romeo 156 engine saved weight, allowed part integration, and incorporated a snap-fit assembly technique. According to an expert, the nylon also made possible more efficient assembly techniques. The air intake manifold, for example, is assembled using snap-fits, eliminating the need for welding or screw. The part provides high dimensional stability, good heat distortion temperature, and resistance to oils.
Car manufacturers are faced with never-ending pressure to wring every excess pound from their models. What’s more, manufacturers of just about everything are constantly looking for ways to make assembly faster and more efficient.
These concerns have opened the hood to the use of thermoplastics in engines.
One case in point is a reinforced nylon multifunctional module involving three components—air intake manifold, cam cover, and design cover—on the Alfa Romeo 156 engine.
The project was a collaboration of Rhodia Engineering Plastics of France, Robert Bosch GmbH of Germany, and Magneti Marelli S.p.A. of Italy, system supplier for Fiat, which manufactures the Alfa Romeo.
Air intake manifolds are usually manufactured of cast aluminum or nylon fabricated by lost-core or vibration welding; cam covers may be of stamped steel sheet, aluminum, or magnesium. (In lost-core, a process for molding hollow precision thermoplastic parts with complex geometry, plastic is molded over a tin-bismuth alloy “core,” which is subsequently melted and evacuated, leaving the plastic part.) According to Siegfried Bossecker-Königs, automotive technical manager of Rhodia’s Farmington Hills, Mich., plant, who was involved with the Alfa Romeo project, using his company’s nylon, the module came in at half the weight of an aluminum assembly.
Weight saving, although important, is not the only pitch Rhodia makes for thermoplastics. According to Bossecker-Königs, the nylon also made possible more efficient assembly techniques. The air intake manifold, for example, is assembled using snap-fits, eliminating the need for welding or screws.
Other advantages include longer tool life and fewer postmolding operations, such as removing flash, compared tc die-cast or thermoset parts, he said. Injection molded nylon also allows the design of thinner ribs in the part and offers more opportunities for part integration.
The assembly had to perform reliably in the harsh under-the-hood environment, standing up to heat, oil, and vibration. In specifying thermoplastics for its integrated module. Rhodia paid special attention to material formulation, design analysis, and part testing, according to the company.
Parts testing included a backfire test that the company designed in-house, which simulates an explosion without igniting a fuel-air mixture.
Nylon for Metals
To meet performance requirements of tasks usually reserved for metals, Rhodia developed reinforced nylon grades that were dimensionally stable and resistant to heat and oil. (Moisture absorption, which is often a concern with nylon, a hygroscopic material, is minimal in un-der-the-hood environments where temperatures range around 80°C, said BosseckerKönigs.) Three different reinforced grades of Technyl nylon 6 and 66 were specified for specific components in the module. Thermoplastics are often reinforced with glass or minerals to add stiffness, reduce shrinkage, or minimize warpage.
The shells of the air intake manifold were molded of 30 percent glass fiber-reinforced nylon 6. The properties of this material, in particular, are high impact strength and elongation at break— favoring clip assembly for the intake manifold housing and certain components. Glass fiber, reinforcement gave the part the required rigidity.
The cam cover, which seals off the oil-bearing section at the top of the engine and carries the additional mass of the ignition coils, was molded of nylon 66 reinforced with 25 percent glass and 15 percent mineral. Properties of the part include high dimensional stability, stiffness and strength, good heat distortion temperature, excellent thermal aging, and high resistance to oil, according to Rhodia.
The design cover was molded of 30 percent mineral-filled nylon 66/6, for good surface finish, dimensional stability, and stiffness. The design cover is fixed to the cam cover with four bolts, hiding the cam cover and part of the air intake manifold, and providing additional sound insulation.
Rhodia provided CAE support to Bosch, which supplied the original CAD files. Rhodia performed mechanical analysis using I-DEAS Master series meshing software from Structural Dynamics Research Corp. of Milford, Ohio. Static and dynamic calculations used software supplied by ANSYS Inc. of Canonsburg, Pa., and Insight flow analysis and warpage prediction software supplied by Moldflow Corp. of Lexington, Mass.
Rhodia performed mechanical analysis on the cam cover, which seals off the oil bearing section of the engine. Because the integrity of the seal is critical, it’s essential that the compression of the rubber gasket is evenly distributed and not too high, to avoid deforming or breaking the part.
Bossecker-Königs noted that the cam cover is attached to the cylinder heads with bolts 200 mm apart—a large spacing that required added part stiffness. To avoid deformations that result in leakage, Rhodia suggested a number of features to keep deflections within the allowable range.
Ignition coils, bolted directly to the cam cover, also presented a design challenge. Four ignition coils—each weighing 0.6 pound—increase the weight at the center of the cam cover. A plate, located atop the cam cover, holds the four separate coils for the eight spark plugs of the Twin-Spark in-line engine. To avoid leakage, the cam cover had to be stiff enough to accommodate the added weight of the coils. The cam cover is bolted to the cylinder head so that it is acoustically insulated to prevent transmission of excess vibrations and related noises.
During mold flow simulation, Rhodia paid particular attention to minimizing part warpage and shrinkage, Bossecker-Königs said. He added that one way to avoid warpage is to “overcorrect” for it in the tool. Minimal warpage is critical to ensure a good seal against leakage.
The main body of the air intake manifold consists of two separate injection-molded shells that snap-fit together after the insertion of a gasket. Plastic air intake manifold housings are usually vibration welded or produced with the lost-core technique. Plastic snap-fit assembly is simpler and more efficient than other methods, according to Rhodia.
According to Bossecker-Königs, snap-fit assembly will not damage sensitive internal components of the intake manifold, which could be affected by vibration welding.
The air intake manifold is attached to the cylinder head by means of a rubber sleeve and a flange. It also carries components supplied by Bosch, such as a throttle valve with microhybrid control unit, wire harness, vacuum actuator, fuel tank vent valve, and various connecting hoses.
The variable air intake manifold of the 1.8- and 2.0-liter engines provides the optimum amount of air for combustion to the engine running at low and high speeds, increasing the engine’s flexibility. A baffle inside the intake manifold channels the air to either a short or a long tube. The long tube improves torque at lower rpm, and the short tube increases horsepower, BosseckerKönigs said. Smooth surfaces of the airflow channels are essential to provide good airflow.
The thermoplastic allows components to be integrated fairly easily. For example, the oil separator, which removes oil vapor and dust from the air that is vented from the crankshaft, is clipped into the cam cover. In the future, another component, the positive crankshaft ventilation valve, will be integrated directly into the cam cover, further eliminating assembly steps and parts, he added.
Rhodia put the components of the assembly through a series of leakage, heat aging, thermal cycling, and vibration tests to make sure that the parts would stand up to real-world conditions.
In the heat-aging test, the cam cover was attached to an aluminum plate with an acrylic-rubber gasket and placed in an oven, where it was exposed to 120°C for 700 hours.
The cam cover received a quantity of oil, and was placed on top of the acrylic-rubber gasket. Testers bolted it to the aluminum plate with the required torque and in the same pattern as it would be affixed to the engine block. The cover was put through a leakage test to make sure it was sealed, then exposed to heat over time, followed by a second leak test.
In a thermal cycling test, the cam cover, bolted to the aluminum plate, was connected to the leak-testing device, and placed in a temperature chamber. A leak test was conducted at startup. Beginning at ambient, the temperature was increased stepwise until failure. The part was tested between -40°C and 150°C. “We want to find out where the limits are and where there could be difficulties,” Bossecker-Königs explained.
A thermal shock test was conducted on the cam cover to simulate cold starts and observe how the part behaves as the engine heats up from extremely cold temperatures. As the engine heats up, expansion behavior of the aluminum cylinder block is different from the expansion behavior of the gasket and thermoplastic cam cover, Bossecker-Königs said.
The thermal shock test device is equipped with two chambers. The lower half is a freezer that can be cooled down to -70°C; the upper chamber is an oven that can be increased to temperatures approaching 200°C. The assembly is exposed to the low temperature for about an hour, and then raised to the high-temperature chamber on a sort of elevator platform. By exposing the part to sudden changes of temperature extremes, the test determined whether the part was susceptible to cracking or seal failure.
The cam cover was also put through vibration testing to make sure it could support the added weight of the ignition coils. The cover was placed on a vibration table and given a workout simulating actual service to test the part for signs of fatigue. During vibration testing, the part may be subjected to changes in temperature. At higher temperatures, damping behavior of the thermoplastic improves, but mechanical properties drop.
The ignition coils add significant weight to the part, he said. Vibrating the cam cover along its outside lines creates a different excitation at the center of the part that supports the coils. This relative movement creates flexural behavior, resulting in fatigue.
Vibration testing was also conducted to ensure that the resonance frequencies of the plastic part are different from those of the engine. If the resonance frequencies of the part and the engine are too closely matched, the vibration can result in acoustical problems, as well as part fatigue, Bossecker-Königs explained.
The air intake manifold was put through static and dynamic burst pressure tests to determine if the snap-fits would be able to meet minimum backfire requirements.
Rhodia developed its backfire test in-house, simulating the explosion without actually igniting a fuel-air mixture. During the test, a pressure increase of up to 10,157 psi per second is realized in the part. One advantage of this approach is that the test can be conducted repeatedly in a short time, said Bossecker-Königs. The test can be used to determine the pressure at which failure occurs, and can also simulate a series of backfires in rapid succession, to determine how well the part stands up to repeated backfiring.
The air intake manifold was also put through a pressure cycling test, in which air pressure was dynamically increased and decreased through the part, to analyze fatigue behavior. In addition, the switching mechanism to channel the airflow inside the air intake manifold was tested for hundreds of hours in a heat-aging chamber, to make sure it would work reliably.
Both the cam cover and the intake manifold are assembled with numerous brass inserts. BosseckerKönigs said they were checked, too. Following heat aging, thermal cycling, temperature shock, and fatigue tests, the inserts underwent over-torque testing and pullout force measurements, to make sure they were embedded securely.