Air-Fuel Ratio Adaptive Control for Two-Stroke S.I. Engines with Direct Injection

In two-stroke S.I. engines, direct fuel injection prevents fuel short-circuiting from the exhaust port. Charge stratification helps solving combustion problems at light loads due to excessive ratio of residual-to-fresh gas, however it requires precise determination of the air-fuel ratio. Unfortunately, this ratio undergoes unpredictable variations during engine life, on account of variations of trapping efficiency, mainly caused by carbon deposits. A customary closed-loop control strategy cannot be adopted, since exhaust gas contains short-circuit air. The purpose of the paper is to show that the problem can be solved with the adoption of an innovative diagnostic and control method. A special test strategy, which is activated both periodically and/or when suitable data are not congruent, allows to acquire and refresh information on short-circuit oxygen. This makes it possible to operate an adaptive control of air-fuel ratio, based on the use of a proportional oxygen sensor.