Engine design for the 1990's has to meet substantially tougher goals than in the past. With E.P.A. and C.A.F.E. edicts from Washington, a more sophisticated customer, and world competition, the big three automotive companies and truck manufacturers are looking at new designs and new materials to meet this challenge. Specifically, some of these goals are higher horsepower and torque per liter, reduced noise, lower weight, improved durability, better aesthetics, and lower cost to meet world competition.
Material selection can play a large role in meeting the challenges for the 1990's. Composites are now being designed in many engine applications which have been traditionally metal. Proper design of these composites can provide many advantages over other materials. One promising thermoset composite is vinyl ester sheet molding compound, which is a vinyl ester resin matrix material using 30% glass fiber for reinforcement and is loaded into the mold in sheet form.
Valve covers, oil pans, intake manifolds, etc., are being designed with composite materials. The traditional materials such as steel stampings, aluminum die castings, and in rare applications, magnesium die castings and thermoplastics, have disadvantages when compared to SMC composite components. The composite valve cover is lighter and dampens noise better than steel, is less expensive and dampens better than aluminum and magnesium. The thermoset composite material generally has higher heat distortion properties than thermoplastics.
With the help of finite element analysis modeling, extensive testing programs and actual production experience, compoILLIGEBLE molders are beginning to make their presence known to engine designers.
When developing design criteria for valve covers on a passenger car engine, one must determine seal loads and bolt locations. This is a formidable task in any material, and a composite cover is no exception. Working closely with the seal manufacturer and the product engineers make the task considerably less difficult.
One advantage sought by the engine designer is improved sound damping. New isolated systems using composite valve covers with silicone seals combined with isolated bolts will help meet Motor Vehicle Noise (MVN) Emission Limits.
The silicone gasket can be designed to fit into a molded groove in the cover. This will help to retain the seal in position with less load and provides for easy assembly. Some applications will require an adhesive to help hold the gasket in place.
The silicone gasket is friendly to a composite cover because sealing can be achieved with relatively low loads. Previous gasket designs using cork or paper required very high loads and were not as effective in isolating noise.
The hold-down bolts can be isolated with silicone and metal sleeves. Although the isolator is a little more expensive than a standard shoulder bolt with washer, it is an essential link to isolating the cover (Figure 1).
The captured fastener isolater design is probably the most desirable because of its easy assembly. The bolt and isolator are sub-assembled and the silicone is shot around the sleeve. This subassembly is easily pushed through the bolt hole and held in place with a molded bead on the isolator.
The basic seal load requirements on an isolated system must also be determined. This information can be obtained from a seal manufacturer. When this load is established, the number of bolts and bolt loads can be predicted. Location of the bolts on the head are dependent on clearances with the head layout and general packaging of the engine. Most product engineers will want to minimize the number of bolts because of the cost of the isolators, increased machining, and additional assembly.
The designer should understand that the composite material is less stiff than aluminum. Suggested locations for bolts are in corners, severe transition areas of the part section, or in areas of multiple seal joints. An example of severe section change would be the transition area between the normal section of an oil pan and the sump area or the clearance housing for the chain (overhead cam) on a valve cover (Figure 2).
Corner bolting should be considered on any design especially when section depth is reduced for clearance. Many times the corners are included in the longest bolt span on the valve cover. This is an undesirable situation especially on a V-8, V-6, etc., engine. The outboard side of the valve cover in a V-type engine is where the oil will tend to accumulate and most likely to leak. Additionally, the proximity of the exhaust manifold can increase the operating temperature locally to over 197°C (350°F), reducing the composite cover and silicone gasket properties. These design conditions must be taken into account when determining bolt location (Figure 3).
T-joints are quite common in the newer engine designs. Sealing can be especially difficult because additional seal area may be required to contain the intersecting seal. An example of this would be the intersection between the head, front cover and valve cover (Figure 4). The composite valve cover can be locally widened at the seal to contain the excess seal coming from the head.
Once the seal load and initial locations of the bolts have been established, a finite element analysis should be performed to determine stress points and predict deflection. A shell model should be developed using known physical properties of the material. This model may be run at different wall thicknesses to optimize the design for minimum weight.
Selecting the optimum section or sections on the part to develop the model is very important. Also, setting effective boundaries between the bolt and seal area will require some discussion between the product and structural engineers. At Budd, the structural engineer, material research and development engineer, and finite element analysis engineer worked closely to develop an effective model.
Initially, the model did not produce strain numbers that we felt were accurate, however, the result gave us the high stress locations which proved to be very accurate in future bench testing.
While we were continuing the prototype on the valve covers, Budd's Research & Development Center performed a number of bench tests to prove the durability of the cover. One test was to load the pockets to failure on an Instron test machine. Our Research & Development engineers used sound equipment to detect internal delamination while loading up the pockets.
Other tests included thermal cycling of the covers with seals. This test was done on a flat plate representing the head. Holes were drilled and tapped in the plate to accept the bolts. The covers were assembled and bolted to the plate. The covers were then filled with oil and air pressure was applied until oil leakage was detected. After the pressure test, the air was vented. The plate and assembled covers were placed in an oven at 149°C (300°F). The parts were removed after a pre-determined time and pressurized. This was repeated at 163°C (325°F) and 177°C (350°F) to prove the robustness of the design.
To increase seal load we conducted bench testing and dimensional scanning was conducted to determine which wall thickness would provide maximum stiffness and seal load without significantly increasing weight. The technique involved scanning the cover as molded from the prototype mold at critical sections, then scanning a part from dyno testing, and comparing both of those to a panel that was exposed to thermal cycling. The scanning of all parts was done at The Budd Plastic Division Design Center. The results were plotted on a ten times size plot. We were able to detect deflection areas indicating where we should add stiffness. The Research and Development Center provided our structural engineers with pocket strength data at elevated temperatures. This was analyzed and factored into our FEA data. We were able to predict more accurately the stress and deflection on the bolt pockets.
The stiffness was increased by adding wall thickness. The walls were basically doubled and blended out to specific points. Subsequent testing showed a .14 to .21 kg/cm2 improvement over previous pressure testing on the valve covers.