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Toyota Motor Corporation has developed a dedicated compressed natural gas (CNG) powered vehicle “Camry” sedan to achieve near zero emissions and to meet rising demand for alternative fuel. A new engine that uses CNG as fuel has been developed by modifying the base 2.2 liter gasoline engine. In the unmodified base engine, torque and power for CNG decrease compared to gasoline. The new engine has adopted a high compression ratio, intake valves with early closed timing, intake and exhaust valves with increased lift and a low back pressure muffler, which thereby restored the loss of engine power. In order to greatly reduce exhaust emissions, a multi port injection system was chosen, and the injectors and pressure regulator have been newly developed. At the same time, precise A/F ratio control and special catalysts for CNG exhaust gas have been utilized.

In order to meet requirements for lower fuel consumption, we have developed a technique for significantly decreasing valve train friction for a direct acting OHC engine. Droplets of pure titanium generated by the titanium nitride coating process of the shims improves the surface roughness of the cams, which eliminates the need to polish the cams. In an engine with these shims, the surface roughness of the cams is considerably improved within a few minutes of initial operation by the polishing action of the droplets. Valve train friction is greatly reduced by improving the surface roughness of the cams and shims, which results in better fuel economy.

An investigation was made into the effects of valve lifter material on fuel consumption and engine noise. It was found that the use of aluminum not only improves fuel economy but also reduces valve-train chatter because it is lighter in weight and less hard than steel. The stresses to which the valve lifters are subjected and their surface temperatures were measured in bench tests, and durability tests were conducted to ascertain the problems which might be expected. Based on the results of these tests, the shape was modified, a new aluminum alloy was developed and a coating was applied to the surface. The aluminum valve lifters thus developed were found to be as durable as conventional steel lifters and have been used in the new Toyota V8 engine (IUZ type).