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A heavy-duty, naturally aspirated diesel engine was converted into a turbocharged, aftercooled, compressed natural gas engine. Engine test results show that excess air ratio and ignition timing strongly affect NOx and THC emissions. Leaning the air-fuel mixture reduces NOx emission, but it increases THC emission and combustion becomes unstable above a certain excess air ratio. Retarding the ignition timing reduces both the NOx and THC emissions. Dual-plug ignition improves brake thermal efficiency. The NOx emission level can be reduced to meet the Japanese long-term emission regulation limit for heavy-duty gasoline engines with a sufficient safety margin by appropriately selecting the air-fuel ratio and ignition timing so as to keep the THC emission level below the regulation limit without using any after-treatment. The engine full torque characteristics were almost the same as the base engine throughout the engine speed range, while the maximum exhaust gas temperature was lower.

To improve the low-speed followability of semi-trailers, a control method has been developed for trailer-wheel steering. The basic concept of the control method is to steer the wheels so that the trailer rear end follows the tractor front end. Computational simulation results show that the control method minimizes the swept path of the tractor and trailer combination without causing any rear end overhang swing problems. To evaluate the effectiveness of the control method, the approximately one twelfth scale model vehicles of the tractor and semitrailer were produced. Tests on the articulated model vehicles show that better low speed follwabilty improvement is expected with path-following control compared with other trailer wheel steering control methods.