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From our experiences in converting diesel engine into diesel-dual-fuel engine with natural gas as primary fuel, accurate air/fuel ratio control is vital to the high engine performance, good vehicle drivability, and low emissions. Two components enter in calculating the air/fuel ratio, namely, the amount of fresh air and the amount of diesel and natural gas. Throttle and EGR valve are two actuators directly affect the amount of air, and the desired total fuel determines how much fuel should be injected at an instance. As opposed to inactive, fully opened throttle in typical diesel engine, the throttle in diesel-dual-fuel engine is regulated to cover wider range of desired air/fuel ratio. As a result, the problem of controlling the amount of air in diesel-dual-fuel engine becomes that of multi variables in which both throttle and EGR valve are involved. We present a novel algorithm that breaks the multi-variable control problem into two single-variable problems.

In this paper, we investigate a multivariable control of air path of a diesel-dual-fuel (DDF) engine. The engine is modified from a CI engine by injecting CNG in intake ports. The engine uses CNG as its primary fuel and diesel as its secondary fuel, mainly for initiation of combustion. The modification is economically attractive because CNG has lower price than diesel and the modification cost is minimal. However, for DDF engine, control of the air path becomes more difficult because the engine now has combined characteristics of the CI and the SI engines. The combined characteristics come from the fact that diesel is still directly injected into cylinders (CI engine) while CNG is injected at the intake ports (SI engine.) In pure CI engine, throttle is normally fully opened for maximum air intake, while EGR valve is actively actuated to obtain low emissions. In pure SI engine, however, throttle is an active actuator, driven by pedal.