Higher combustion pressures produce more power with less fuel, a magic combination for commercial truck producers looking to get the most out of mammoth diesel engines. Those higher pressures put a lot of stress on engine components, including gaskets, requiring new designs to seal in gases and fluids.
“Typically, when you move combustion pressure from 200 bar to 250 bar, you’d have to increase the available bolt load in the joint to ensure proper combustion sealing,” explains Mike Gerulski, director, global research and development, sealing, Federal-Mogul Powertrain. Engineers at the supplier of gaskets, piston rings, pistons, and other powertrain parts believe they can seal in higher-pressure combustion without that commensurate increase in bolt load.
Combining multi-layer steel (MLS) gasket designs, born from the company’s work with mass-market passenger cars, with high-tech elastomers, Gerulski says Federal-Mogul can support a 25% boost in engine pressures without forcing truck and engine companies to redesign their powertrains.
Avoiding tradeoffs
Boosting combustion pressures in big diesel powertrains can be a challenge. Because engines already run at fairly high pressures, going to 250 bar only boosts efficiency by a few percentage points, probably less than 5%, Gerulski explains. If an engine company redesigns the head or engine block to handle the higher pressures, increased structural weight could eliminate efficiency improvements gained from higher pressures.
“If the customer doesn’t have a technology for sealing the joint, he may have to make the block or head heavier,” Gerulksi adds. “Now there is a weight penalty for the engine that may offset any potential savings. You need a technology solution in that gap that allows you to increase the combustion pressure without increasing the weight.”
Conversely, if the customer is willing to redesign an engine, reducing weight with similar combustion pressure, the reverse can be true. Engine designers could remove some of the supporting structures in their engines, counting on the hybrid steel-elastomer gaskets to seal combustion. Such engines would not be able to operate at elevated engine pressures, but lower engine weights could improve fuel efficiency.
Whether the goal is higher pressures or less mass, the key, Gerulski says, is to provide more reliable sealing of the combustion opening, instead of spreading sealing pressure evenly across the entire gasket.
“What we can do is allow for the bolt pattern around the combustion opening to be maximized to actually seal combustion instead of being used to seal the fluids on the perimeter of the gasket,” Gerulski explains. Doing that requires different materials and different gasket design approaches.
Hybrid gasket desgins
The high-pressure, hybrid head gasket design has three main parts:
Elastomer seal – A fluid- and temperature-resistant polymer gets dropped into a steel plate or molded to the plate. The elastomer seal covers the coolant- and oil-port openings.
“By adding elastomer sealing for the fluid passages, which require much lower loads to seal, we can concentrate the sealing forces where they are most useful,” Gerulski explains.
Plate – A cold-rolled steel plate serves as the solid core of the gasket. Designed to the specific geometry of each engine, the plate provides gap control between the engine block and cylinder head, giving designers control over the combustion height.
MLS rings – Two to four stacked embossments made of hardened stainless steel drop into the plate around the combustion opening, providing precision sealing of the combustion event. Automakers have used MLS technology in car engines since the 1980s, and some global truck manufacturers use it on their large diesels. But Gerulski says it hasn’t been popular in North American commercial trucks.
“There are different ways of packing these components together, based on how much space we have and what the customer wants to see,” Gerulksi says. “Then it becomes a packaging issue to see how to put it all together so it fits the head block plan view layout.”
Complete system design
Various groups in Federal-Mogul’s powertrain division collaborated to find new ways for customers to deal with higher combustion pressures. Sealing engineers, piston designers, piston ring engineers, and computer simulation experts discussed some of the challenges, including higher combustion pressures, facing their customers. The goal was to see which technologies could support engine improvements across multiple systems.
“As you bolt the head to the block, the gasket, through the combustion area, will direct some of that load into the combustion opening. Depending on the configuration, you can actually put enough load into the cylinder walls that you can distort the cylinders,” Gerulski states. By addressing much of that load at the gasket, he adds that engine designers can minimize bore distortion, and “our brothers in our ring group can use a lower-tension ring pack, which could drive another reduction in the friction of the rotating mass of a piston inside the cylinder liner.”
Eliminating friction between the moving parts in the cylinder reduces the parasitic drag on the engine, further improving power and fuel economy. Ideally, head gasket improvements can lead to a waterfall of engine improvements – higher combustion pressure, lower engine mass, less engine friction. As each of those benefits boosts fuel economy by a percentage point or two, the overall savings can build quickly.
“We call our steel-elastomer head gaskets and other technologies enabling technologies. They provide engine designers options to improve efficiency without having to compromise on mass,” Gerulksi concludes.
Lead-free engine bearings
Lead-free bearings offer environmental benefits for engine producers and improve bearing fatigue life, increasing long-term durability. With Federal-Mogul’s latest lead-free bearing systems, the upper and lower halves of a bearing shell pair are individually optimized. Heavy combustion loads occur only on the upper half of the connected rod and the lower half of a main bearing. The new bearing system provides a coated bearing for the high-loaded side combined with an aluminum substrate for the counter (less-loaded) shell.
Company engineers tested the new systems using energy dispersive X-ray (EDX) spectroscopy and scanning electron microscopy (SEM). Additionally, elastohydrodynamic (EHD) studies or finite element methods (FEM) were used to optimize the bearing and its environment. Along with the development of lead-free bearing materials, necessary new manufacturing processes and test procedures, such as wear and embeddability tests, were also introduced. One heavy-duty engine producer has begun using the bearings, and another is considering the technology. Meat products must be refrigerated or frozen after processing and before shipment to inhibit spoilage and growth of pathogens. The challenge is to maintain proper temperature in the meat delivery trucks during transportation.
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