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The third generation four valve lean combustion engine controlled by newly designed combustion pressure sensor has been developed. This combustion sensor composed of a metal diaphragm and a thin silicone layer formed on devitron piece detects the combustion pressure in the No.1 cylinder. Comparing with the lean mixture sensor equipped in the first and second generation lean combustion engine, the lean misfire limit was detected directly with this sensor, and the lean operation range was expanded, which realized lower fuel consumption and NOx emission. The output torque fluctuation was minimized by precisely compensating the fuel supplied to individual cylinder based on the crank angle sensor signal. Separated dual intake ports, one with the swirl control valve and the other with helical port shape was designed and a twin spray injection nozzle was equipped between those ports. The swirl ratio was lowered from 2.2 to 1.7.

The lean combustion of an SI engine has been recognized as one of the most promising methods for further improvement of fuel economy. There has been, however, difficulty in extending the lean misfire limit enough to realize NOx emission levels below the mandatory level and still keep satisfactory driveability. A simulation study has been carried out to search for the possibility of getting better fuel economy under the constrainsts of NOx emission and driveability. To realize the optimum calibration, the lean misfire limit has been extended by the introduction of (1) high swirl and high combustion chamber turbulence through the use of a helical port with an unique swirl control valve, (2) a newly developed ZrO2 lean mixture sensor and (3) the multi-point fuel injection with sophisticated control. A very good fuel economy level of 17.0 km/1 (Japanese 10 mode) has been accomplished while still meeting the NOx emission cycle regulation of 0.25 g/km.