Open-loop control

For a warmed-up engine, the controller will operate in an open loop if the closed-loop mode is not available for any reason. For example, the engine may be warmed sufficiently but the EGO sensor may not provide a usable signal. In any event, it is important to have a stoichiometric mixture to minimize exhaust emissions as soon as possible. The base pulse width T_b is computed as described above, except that the desired air/fuel ratio (A/F )_d is 14.7 (stoichiometry):

Corrections of the base pulse width occur whenever anything affects the accuracy of the fuel delivery. For example, low battery voltage might affect the pressure in the fuel rail that delivers fuel to the fuel injectors. Corrections to the base pulse width are then made using the actual battery voltage.

An alternate method of computing MAF rate is the speed–density method. Although this method has essentially been replaced by direct MAF measurements, there will continue to be a number of cars employing this method for years to come, so it is arguably worthwhile to include a brief discussion in this chapter. This method, which is illustrated in Fig. below.

Engine control system using the speed–density method

is based on measurements of MAP, RPM, and intake air temperature T_i. The air density d_a is computed from MAP and T_i, and the volume flow rate R_v of combined air and EGR is computed from RPM and volumetric efficiency, the latter being a function of MAP and RPM. The volume rate for air is found by subtracting the EGR volume flow rate from the combined air and EGR. Finally, the MAF rate is computed as the product of the volume flow rate for air and the intake air density. Given the complexity of the speed–density method it is easy to see why automobile manufacturers would choose the direct MAF measurement once a cost-effective MAF sensor became available.

The speed–density method can be implemented either by computation in the engine control computer or via lookup tables. Fig. below is an illustration of the lookup table implementation. In this figure, three variables need to be determined: volumetric efficiency (n_v), intake density (d_a), and EGR volume flow rate (R_E). The volumetric efficiency is read from ROM with an address determined from RPM and MAP measurements. The intake air density is read from another section of ROM with an address determined from MAP and T_i measurements. The EGR volume flow rate is read from still another section of ROM with an address determined from DP and EGR valve position. These variables are combined to yield the MAF rate:

where D is the engine displacement.

Lookup table determination of da, R_E, and n_v