Oct
8
Machining a Better Piston Engine
October 8, 2012 | 1 Comment
The cylinder block of your engine isn’t so stable as one might imagine. Two factors change the shape so carefully engineered and machined into each piece.
Fraunhofer’s Adjustable Cylinder Honing Tool in Operation. Click image for the largest view. Image Credit: Fraunhofer.
The first problem is bolting an engine together. The assembly and compression of fasteners distorts the shape of the parts, which is known as static distortion. The second occurs when the engine is running and higher temperatures warp the parts with thermal distortion.
Piston in cylinder internal combustion engines generate friction as the piston traverses the cylinder – four trips per power cycle. The piston rings need plenty of lubrication in order to be able to move in the cylindrical sleeves in the engine block.
Most modern engine manufacturers compensate for static distortion. During the final machining stage, honing, technicians mount a honing liner to the engine, which simulates the cylinder head that will be mounted later. Then the work on the borehole for the cylinder sleeve is completed.
The second distortion, a thermal change from the difference in shape from ambient at machining to operating temperature, depends on prevailing engine temperatures and the specific engine model. In factual reality, the piston does not follow a perfectly smooth up and down motion, but instead rubs at points within the borehole.
The price for the rings rubbing the cylinder wall is the engine requires more oil as well as more gas. Fraunhofer a figures 2 to 3% improvement, and that’s in Germany where machining is a high form of art.
Fraunhofer believes the problem has now been solved. Working in collaboration with a car manufacturer and a machine tool producer, IWU head of department André Bucht says, “Our technology makes it possible to compensate for both static and thermal distortion. This can lead to a fuel saving of two to three percent in combustion engines, and remove one step in their production.”
The new technology is based on a tool that can adapt its own shape. Researchers started by working out how an engine block is likely to become distorted. First they determine the level of static distortion by unscrewing the cylinder head and measuring the extent to which the borehole has been warped. Second, they simulate thermal distortion that occurs in each engine series, using an operating temperature of 90º C as their operating temperature reference point.
Based on these calculations the honing tool adjusts its shape altering the profile of the borehole so that motions of the piston are perfectly smooth later on when the engine is running, preventing excessive friction.
The researchers get the fine adjustments by integrating small Piezo actuators into the tool, which alter its shape and expand the diameter as required. “This is how we can incorporate any ‘imperfections’ in the otherwise perfectly round shape of the finished bore hole,” says Bucht.
A prototype of the tool is already at work. The researchers have put it to use to prove that they can achieve the surface accuracy required without slowing down production – adding no more than 20 to 30 seconds for the assembly of each engine.
Research is currently being performed on the test rig in collaboration with more auto manufacturers so an engine produced by using the tool can be put through its paces. The tests allow researchers to examine to what extent piston friction and fuel consumption are reduced, and how the lifespan of the engine might be affected in comparison with engines manufactured using conventional tools.
The tests are scheduled to be complete by the end of the year. Researchers then plan to design the tool and the production process so that manufacturers can adopt them.
Two to three percent doesn’t sound like a lot until the realization comes that the numbers will apply across the whole operating range, all of the time.
Pistons travel immense distances, and a multi cylinder engine produces huge numbers compared to the miles traveled. A six-cylinder engine turning 2000 rpm would produce over 13 miles of piston travel per mile. A little bit of friction found and taken out is a very good thing.
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