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April-June 1997 Issue

Improving Piston Engines Through Better Ring Design

As engineers try to improve today's internal combustion engine, an important place they look is inside the cylinder. Some 8% of the fuel used in automotive engines is consumed by friction related to the moving pistons. Altering the piston's design and behavior to reduce friction could improve fuel efficiency and reduce the rate at which engine components wear. In-cylinder refinements could also reduce oil consumption, making possible the use of less viscous oils--a change that would itself reduce friction throughout the engine. However, making such improvements is difficult because it requires changing the "microgeometry" of the engine, that is, the details of how the components inside the cylinder are designed and the precision with which they fit together. Given the subtlety of the changes to be made and the complexity of today's engines, engine designers must rely on computer models to test the impact of proposed changes.

In past work, Dr. Victor W. Wong, principal research scientist in the Energy Laboratory and lecturer in the Department of Mechanical Engineering, and collaborators from MIT's Sloan Automotive Laboratory and Nissan Motor Company developed a model that is now helping engine designers reduce one counterproductive type of piston behavior: "piston slap." As a piston moves up and down inside its cylinder, it also shifts from side to side, sometimes hitting one side of the cylinder wall with a hard slap. Such slapping of the piston on the cylinder wall generates noise, increases friction, and reduces fuel economy; and it can also damage the engine. For a specified engine design and operating conditions, the MIT model determines the piston's behavior within the cylinder, including where and how hard the piston slaps and the amount of friction that results (see e-lab, January-March 1996).

Cross Section of Engine Cylinder and PistonNow Tian Tian, a recent PhD graduate in the Department of Mechanical Engineering, Remi Rabute of Dana Corporation, Dr. Wong, and Professor John B. Heywood, Sun Jae Professor of Mechanical Engineering, have developed a model that focuses on an even more detailed engine component: the piston rings. In a typical engine, three metal rings are mounted on each piston (see the figure to the left). Each ring extends to the cylinder wall and slides along a film of lubricating oil on the wall as the piston moves up and down. The top ring seals shut the chamber above the top of the piston where combustion occurs, creating the high pressures needed to push the piston down. The second and third rings primarily control the flow of lubricating oil along the cylinder wall.

However, as the piston goes up and down, the rings move around in their grooves. If the rings no longer seal the combustion chamber, gases can flow around the rings, and lubricating oil can leak into the combustion chamber and subsequently burn. The movement of the rings also affects friction and wear: despite the layer of lubricating oil, the rings inevitably come in direct contact with metal surfaces, particularly the surfaces of their grooves in the piston. The wear that results can significantly alter the shapes of both the rings and the grooves. Engine designers control ring behavior by making the rings different sizes and shapes or by cutting tiny notches out of them. But a design that provides a tight seal when new may lose its effectiveness over time as the rings and grooves wear.

The new MIT model predicts not only how the rings move within their grooves for a given piston and ring design but also how that motion affects oil consumption, friction, and wear. To determine the motion of the rings, the model calculates all the forces acting on each ring. It determines the forces due to the piston's motion within the cylinder, including the effects of slap (as described by the piston-slap model) and inertial forces that arise when the piston reverses direction and the rings tend to keep going (much as a passenger jerks forward when a train stops suddenly). The model determines how the rings bend and twist and calculates the forces that tend to bring them back to their original positions (like coil springs). It also calculates the behavior of the oil films on the cylinder wall and in the ring-grooves--films measured in thousandths of millimeters-and the effects on the rings of interacting with those films. It calculates when the rings come into direct contact with other metal surfaces and the forces that result. And it tracks how gas pressures change in the spaces above, below, and between the rings and the net forces on the rings resulting from those pressures.

Based on all the forces, the model calculates precisely how each ring moves as the piston travels up and down inside the cylinder. The model predicts both how the ring moves up and down within its groove and how it twists--a feature that makes the simulation realistic and the model valuable for practical engine studies.

Piston Ring TwistThe figure to the right shows two examples of how a ring can twist and the implications for engine operation. The drawing on the top shows a ring twisted so that it forms a seal at the bottom surface toward the inside of the groove. When the pressure of the gases above the ring is high, the force downward on the ring is strong and the seal is tight. In the drawing on the bottom, the ring tilts so that the seal is at the outer edge of the groove. The high-pressure gases can now flow below the ring and press upward. The net downward pressure on the ring is not so high, increasing the likelihood that the ring will lift off the surface of the groove as the piston moves.

Having calculated each ring's detailed movements, the model then determines what those movements mean for engine operation. For example, it calculates how much of the high-pressure combustion gases or unburned fuel/air mixture escapes from the combustion chamber when a ring lifts off its groove. It also predicts the flow of gases through the tiny gap between the two ends of the ring. (A ring is not a continuous circle but rather has an opening that allows it to be mounted on the piston.) It then determines the effect of such gas flows on the layer of lubricating oil. As the gases jet through a small opening, they pick up lubricating oil from adjacent surfaces. If the gases escape downward, the oil simply returns to the crankcase to be recirculated. However, during some parts of the piston's travels, the pressure is higher below the rings than above, and gases escape upward. In that case, the entrained lubricating oil blows into the combustion chamber, where it vaporizes and burns up. The model calculates the oil losses that result.

The model also predicts how the ring's position will affect friction and wear. If the ring-groove contact occurs at a single point (as in both drawings), the layer of lubricating oil tends to be pushed aside. Metal-to-metal contact occurs, and friction and wear are high. If the ring lies flat on the bottom of the groove, the lubricant is better able to keep the ring off the surface; and friction and wear are lower. The model predicts where contact occurs, what the contact pressure is, and how much wear results. Finally, the model calculates how the friction and the gas and oil flows contribute to the forces that determine the movement of the rings.

The researchers have verified their model using three experimental approaches. They have measured the thickness of the oil film using the laser fluorescence technique they developed a decade ago (see e-lab, April-June 1987 and October 1989-March 1990). The experimental measurements confirm the model's ability to predict the behavior of the oil film, which is a critical input to other calculations. In other experiments the researchers mounted probes on the cylinder wall in an operating engine and measured the pressure between the wall and the piston, in particular, in the spaces above and below the top piston ring. Pressures predicted by the model match the measured pressures well-an indirect indication that the model correctly simulates the behavior of the ring and the resulting flows of high-pressure gases from one space to another. Finally, the researchers have verified the model's ability to predict wear by measuring the shapes of the rings and the grooves when they are new and after hours of operation. Tests show that the regions of the grooves where the contact pressure is highest according to the model correspond to the regions where the actual erosion of material is greatest in the experimental engine.

The researchers have now used the model in several practical studies. One focus has been reducing wear. Model simulations show that the contact pressure, hence wear, peaks at one area on the groove's bottom surface when combustion is occurring. Using the model, Dr. Tian and Mr. Rabute showed that cutting the groove at a specific angle produces a lower, more uniform contact pressure all along the bottom of the groove. Not surprisingly, the shape of that angled groove corresponds closely to the profiles of worn grooves measured in long-term engine tests.

The model has also enabled the MIT researchers to clarify and quantify a potentially important source of oil consumption. Under certain conditions, when the piston moves upward, the top ring scrapes oil off the cylinder wall and pushes it along. When the piston moves downward, a ring of oil is left behind, some of which vaporizes and burns. According to the model, such ring "up-scraping" can deposit significant amounts of oil. Precisely how much depends on how the piston is tilted and how the ring is twisted within its groove. Further analysis should identify ring designs that will reduce this oil-consuming behavior.

Finally, the researchers are using the model to examine a troublesome phenomenon known as "ring flutter." Under certain operating conditions, a ring moves rapidly up and down in its groove, never staying at the top or the bottom, sometimes for an extended period of time. The seal between the ring and the groove is never tight, so the leakage of gas and entrained oil is extremely high. Using the model, the researchers are defining the exact conditions that lead to flutter and are exploring changes in piston and ring design that may prevent it, thereby reducing oil consumption.

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