GGHFM 57.60 (MAF Meter System Pulsations)

GGHFM57.60 (MAF Meter System Pulsations) Function Description

The MAF sensor output is sampled at 1 millisecond intervals. The sampled voltage value is first linearized using the 512 value characteristic curve MLHFM (which contains only positive values)​​ for further calculation of mass air flow. Therefore, when using a HFM5 sensor, an offset (defined by MLOFS) is required to take account of the reverse current region in the calculation of MLHFM values.

The calculated air mass values ​​are then summed in a memory segment. Once a segment is nearly full, the simple arithmetic average of the cumulative value over the last segment is calculated, i.e. it is divided by the number of samples of the last segment and then the offset MLOFS is subtracted.

During idle conditions, a selection is made between the measured air mass flow and the maximum possible air mass flow at this operating point, mldmx_w (taken at a height of -500 m and a temperature of -40°C) weighted by the multiplication factor FKMSHFM. By this measure, short circuiting of U_bat output to the engine can be prevented. [See module DHFM 63.130 Diagnosis: MAF sensor signal plausibility check: “With the HFM5 sensor, if the battery voltage is less than 11 V , no more information about the plausibility of the HFM signal is possible (basis: voltage levels of 0.5-2.0 V cause a short circuit between U_bat and U_ref)...”]

Then, the value is corrected via fpuk for pulsations and return flow (i.e. pressurized air dumped back to the intake tract on the overrun) and via fkhfm in areas with no pulsation and surging. When the turbo is on, the system constant SY_TURBO sets fpuk to 1.0 since there will not be any pulsations or return flow. The value mshfm_w is corrected in this case by the map KFKHFM.

Since different displacement elements of the engine hardware, such as the camshaft, intake manifold or charge movement flap can influence pulsation in the MAF sensor, the code words CWHFMPUKL1 and CWHFMPUKL2 determine which influencing factors are taken into account.

The air mass flow output is supplied as the 16-bit value mshfm_w. The RAM-cell mshfm_w is limited to zero. To take into account return flow (based on 1-segment) for turbo engines, the RAM-cell mshfms_w is provided, which is administered by the limiting value FW MLMIN.

The pulsation-correcting curve PUKANS corrects for the engine speed nmot so that intake air temperature-dependent displacements of actual pulsation areas are managed.

GGHFM 57.60 Application Notes

Pre-assignment of the Parameters

CWHFMPUKL1 = 1

CWHFMPUKL2 = 1

FLBKPUHFM = 0.5

FNWUEPUHFM = 0.5

KFKHFM = 1.0

KFPU = 1.0

KFPUKLP1 = 1.0

KFPUKLP12 = 1.0

KFPUKLP2 = 1.0

MLHFM = MAF sensor curve

MLMIN = -200 kg/h

MLOFS = 200 kg/h

PUKANS = 1.0

Application Procedure

1. Determine, input and review the MAF sensor linearization curve

2. Linearization curves depend on size and type (hybrid/sensor) of the MAF metering system deployed

3. For the HFM5 sensor, the curve with return flow, i.e., positive and negative air masses and use additional offset (MLOFS = 200 kg/h)

4. When using an alternative plug-in sensor, check the linearization curve is appropriate for the mounting position used.

Requirements for the Application of the Pulsation Map

Mixture pre-input path:

1. Normalise all enrichment (input factors and input-lambda), i.e. feed forward control to obtain lambda = 1;

2. In fuel systems where there is no constant differential pressure over the fuel injectors (e.g. returnless fuel systems, i.e. in which the pressure regulator is not working against the intake manifold pressure as a reference) this must especially be ensured for the application of pulsation maps (connection of a pressure regulator on the intake manifold).

3. If this is not technically possible, i.e. the differential pressure across the fuel injectors was previously considered in a correction curve (see note to returnless fuel systems), then carry out the following:

Pre-input charge detection:

1. Determine the MAF sensor characteristic curve

2. Normalise the pulsation corrections first (set KFPU, KFPUKLP1, KFPUKLP2, KFPUKLP12 to 1.0)

3. Set the MAF correction map values to 1.0

4. Limit rlmax by disabling or setting PSMXN to its maximum values

The pulsation correction depends on T_ans in the characteristic PUKANS stored as a factor and is addressed with T_ans/°C. This characteristic is used for engine speed correction to address the pulsation map KFPU.

PUKANS = (T₀/T_ANS)^0.5) where T₀ and T_ANS are absolute temperatures (i.e. in Kelvin)

The base temperature T₀ is 0°C = 273 K i.e. ftans (0°C) = 1.0

To apply the curve with 8 data points for pulsation corrections:

Application of the Pulse Maps KFPU, KFPUKLP1, KFPUKLP2, KFPUKLP12

The pulsation maps compensate for pulsation and reverse flow errors in the MAF meter system. There are four pulsation maps:

KFPU: the basic map

KFPUKLP1: pulsation-influencing adjustment element 1

KFPUKLP2: pulsation-influencing adjustment element 2

KFPUKLP12: pulsation-influencing adjustment elements 1 and 2

Parameterization of the code words CWHFMPUKL1 and CWHFMPUKL2:

Definition of adjustment element 1 for taking pulsation into account

CWHFMKLPU1:

1. 1 Intake manifold flap

2. Camshaft

3. Charge movement flap

Definition of adjustment element 2 for taking pulsation into account

CWHFMKLPU2:

1. 2 Intake manifold flap

2. Camshaft

3. Charge movement flap

Definition of the pulsation range:

MAF sensor voltage fluctuations with an amplitude of 0.5 V

Definition of the return-flow (i.e. pressurized air dumped back to the intake tract on the overrun) range:

MAF sensor voltage <1 V

Pulsation Map Adaptation:

Determining the pulsation or reverse flow region; possibly changing the sample-point resolution of pulsation maps to better cover the pulsation region.

The air mass in the intake manifold (ml_w) is compared with the calculated air mass in the exhaust gas via the characteristic curves KFPU, KFPUKLP1, KFPUKLP2 and KFPUKLP12. As an alternative to the calculated air mass in the exhaust, the air mass flow through a pulsation-damping volume to the air filter housing (e.g. a Helmholtz resonator device) can be measured instead.

Application of the MAF Correction Map KFKHFM:

In regions of no pulsation, the air mass comparison is carried out via the map KFKHFM. In this way, MAF-sensor errors caused, for example, by a problematic installation position can be corrected. For either, the balancing should maintain lambda of approximately 1.0, so the error in calculating the air mass in the exhaust gas is low. The residual errors (lambda deviation around 1.0) are interpreted as a mixture error and are compensated for by the characteristic curve FKKVS in the RKTI 11.40 module.

Abbreviations