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“People say the engine is the heart of a car, but to me the piston is the very heart of the engine,” remarks power-train development specialist Masamichi Suzuki.  Most
drivers understand that the pistons transmit power from the combustion chamber
to the crankshaft through connecting rods, thereby rotating the wheels. Perhaps
it’s less well known that pistons, constantly reciprocating in the cylinder
at high speeds and subjected to extreme heat and pressure generated in the combustion
chamber, must be extremely durable. On the other hand, light weight is also an
important requirement, definitely a delicate balancing act for developers.“The piston’s weight determines that of the counterweights on the crankshaft, as well as the power loss in motion,” explains Masamichi Suzuki, Deputy Staff Manager of Design Group II, Automobile Power Train Design Dept. “It influences the whole engine configuration.” The first task for Mr. Suzuki’s team in development of the new-generation 2.7-litre V6 engine for the Grand Vitara XL-7, for example, was to improve piston durability in the face of higher combustion temperature caused by the leaner air/fuel ratio it takes to suppress fuel consumption and emission of hydrocarbons (HC). Yet without increasing the piston’s weight. “Heat concentrates at the thinnest area in the centre of the piston head, causing excessive thermal stress in the rib of piston-pin boss just beneath there,” said Mr. Suzuki. “So, based on extensive computer-aided-engineering analyses, we thickened the piston head centre by only 0.8mm for increased thermal capacity and modified the rib shape for better heat dispersion – for a combined weight increase of just 9g per piston – creating the ideal figure to withstand the thermal stress.” The team’s second task was to reduce unburned gas HC deposited above the top piston ring between the pistons and cylinder walls. “Reducing that top space cuts HC, but if you shorten the piston-head height above the top ring groove, closer proximity to the combustion chamber heats up the groove. That causes groove surface wear or aluminium adhesion to the ring surface, which both deteriorate the seal of the top ring and increase the blow-by gas (unburned gas leaking from combustion chamber to cylinder interior), degrading lubricant performance and damaging the engine.”  Generally,
this problem is solved by electrolytic creation of a hard oxide film on the groove
surface, but a film that is both even enough and thin enough is extraordinarily
difficult to create. To do that, Mr. Suzuki and his team thoroughly refined the
electrolytic treatment procedure to achieve a film-surface evenness of Rz7µm,
meaning an average surface-height difference of only 7µm as measured in
10 places. Compare that to the 70µm thickness of an average human hair for
an idea of the technical precision required. |
