In the feedforward part of an SI (Spark Ignition) engine Air/Fuel control system, the in-cylinder mass air flow rate has to be accurately estimated to determine the fuel amount to be injected. Generally, this evaluation is performed either with a dedicated sensor (MAF sensor) or with an indirect evaluation based on the speed-density method.
In order to meet the stricter and stricter emission regulations, automobile gasoline engines are equipped with a three-way catalytic converter (TWC). A precise control of air-fuel ratio (A/F) to the stoichiometric value is necessary to achieve a high efficiency of the TWC converter in the conversion of the toxic exhaust gases (CO, NOx, HC) into less harmful products (CO2, H2O, N2). Typically, this control is performed in a spark-ignition engine through a so-called lambda sensor. The lambda sensor generates a signal representative of the value of the ratio
  λ  =            Air      /      Fuel              Air      /              Fuel        stoichiometric            from the amount of oxygen detected in the exhaust gas mixture. If λ<1 the mixture is rich of fuel, while if λ>1 the mixture is lean of fuel.
In order to keep the air/fuel ratio (AFR) as close as possible to unity, the lambda sensor is inserted in the outlet of exhaust gases for monitoring the amount of oxygen in the exhaust gases. The signal generated by the lambda sensor is input to the controller of the engine that adjusts the injection times and thus the fuel injected during each cycle for reaching the condition λ=1.
Traditional Air/Fuel control strategies include a feed-forward part in which the amount of fuel to be injected is calculated on the basis of the in-cylinder mass air flow and a feedback part which uses the signal of an oxygen sensor (lambda sensor), located in the exhaust system, to ensure that the Air/Fuel will remain in the neighborhood of the stoichiometric value [1].
FIG. 1 shows a block diagram of a traditional Air/Fuel control system. Generally, the feedback part of the Air/Fuel control system is fully active only in steady-state conditions; moreover the lambda sensor signal is available only after this sensor has reached a fixed operating temperature. In transient and cold start conditions the feedback control is disabled, thus the feedforward part of Air/Fuel control is particularly important.
As mentioned before, the air flow estimation is the basis for calculating the injected fuel quantity in the feedforward part of Air/Fuel control system.
A conventional technique [1] for estimating the cylinder air flow into a SI (Spark Ignition) engine involves the so-called “speed-density” equation:
            m      .        ap    =            η      ⁡              (                              p            m                    ,          N                )              ·                            V          d                ·        N        ·                  p          a                            120        ·        R        ·                  T          m                    where {dot over (m)}ap is the inlet mass air flow rate, Vd is the engine displacement and N is the engine speed; Tm and pm are the mean manifold temperature and pressure and η. is the volumetric efficiency of the engine. This is a nonlinear function of engine speed (N) and manifold pressure (pm), that may be experimentally mapped in correspondence with different engine working points. A standard method is to map the volumetric efficiency and compensate it for density variations in the intake manifold.
One of the drawbacks in using the “speed-density” equation for the in-cylinder air flow estimation is the uncertainty in the volumetric efficiency. Generally, the volumetric efficiency is calculated in the calibration phase with the engine under steady state conditions. However variations in the volumetric efficiency due, for example, to engine aging and wear, combustion chamber deposit buildup etc., may induce errors in the air flow estimation.
Moreover, the low-pass characteristic of commercial sensors (Manifold Absolute Pressure or MAP sensors) used for the determination of the manifold pressure pm, makes the signal affected by a delay which, during fast transients, introduces a relevant error in the air flow estimation.
This problem is not solved by using a faster sensor: in this case the sensor also captures pressure fluctuations due to the valve and piston motion [2]. In engines equipped with an EGR (Exhaust Gas Recirculation) valve, the MAP (Manifold Absolute Pressure) sensor cannot distinguish between fresh air and inert exhaust gas in the intake manifold. Therefore, in this case the speed-density equation (1) cannot be used and the air charge estimation algorithm must provide a method for separating the contribution of recalculated exhaust gas to the total pressure in the intake manifold [4].
An alternative method for the air charge determination is to use a Mass Air Flow (MAF) sensor located upstream from the throttle body, which measures directly the inlet air flow. The main advantages of a direct air flow measurement are [1]: automatic compensation for engine aging and for all factors which modify engine volumetric efficiency; improved idling stability; and lack of sensibility to the system to EGR (Exhaust Gas Recirculation) since only fresh air flow is measured.
Anyway, air flow measurement by means of a MAF sensor (which is generally a hot wire anemometer) accurately estimates the flow in the cylinder only in steady state, while in transients the intake manifold filling/emptying dynamics play a significant role [3], [5]. Moreover, a MAF sensor for commercial automotive applications has a relatively high cost compared to the cost of MAP (Manifold Absolute Pressure) sensor used in the “speed density” approach.