SIMK43 Airflow and Load Model

This document describes the airflow and engine-load model implemented in the Siemens SIMK43 ca663056 calibration, which should also apply on all other G4GC CVVT calibrations.

It documents how the ECU measures incoming air using the hot-film MAF sensor, independently calculates airflow using the intake-manifold model, determines cylinder air charge, supervises the measured and modeled airflow paths, and integrates the resulting load value into the torque, ignition and fueling systems.

1. General Architecture

The SIMK43 airflow system contains two parallel representations of engine airflow:

• Measured airflow derived from the physical hot-film MAF sensor.
• Modeled airflow calculated from engine speed, manifold conditions, valve overlap, temperature and ambient pressure.

These paths ultimately describe the same physical quantity: the mass of fresh air entering the engine.

The MAF sensor measures air travelling through the intake tract, while the combustion system ultimately requires an estimate of the amount of fresh air entering an individual cylinder.

The overall architecture is:

                     AIR ENTERING ENGINE
                            |
             +--------------+--------------+
             |                             |
             v                             v
       HFM / MAF SENSOR           INTAKE MANIFOLD MODEL
             |                             |
             v                             v
      Sensor linearization         RPM / pressure / VVT
             |                     temperature / ambient
             v                             |
      Measured airflow                     v
          [kg/h]                  Modeled engine airflow
             |                          [kg/h]
             |                             |
             v                             v
      Convert using RPM             Convert using RPM
             |                             |
             v                             v
     Measured cylinder             Modeled cylinder
          air charge                    air charge
          [mg/stk]                    [mg/stk]
             |                             |
             +--------------+--------------+
                            |
                            v
                  AIRFLOW SUPERVISION
                            |
                            v
                  FINAL CYLINDER LOAD
                         [mg/stk]
                            |
          +-----------------+-----------------+
          |                 |                 |
          v                 v                 v
       IGNITION           FUELING           TORQUE
                                             |
                                     actual load -> torque
                                             |
                                     torque request -> load

SIMK43 should therefore not be regarded simply as a MAF-based ECU.

It is more accurately described as a model-based cylinder-charge control system using the MAF sensor as its primary airflow measurement.

2. Airflow and Load Units

Siemens uses the term MAF for several related quantities. The engineering unit must therefore always be considered when interpreting a calibration or internal calculation.

MAF sensor signal

• Unit: V or internal ADC representation
• Meaning: electrical output of the MAF sensor.

Engine airflow

• Unit: kg/h
• Meaning: total fresh-air mass flowing through the complete engine intake.

Cylinder air charge

• Unit: mg/stk
• Meaning: fresh-air mass entering one cylinder during one intake event.

Torque

• Unit: Nm
• Meaning: indicated or effective engine torque.

The two principal airflow domains are:

MAF_KGH

and:

MAF_CYL

They describe the same airflow from two different perspectives.

MAF_KGH measured in kg/h represents the total airflow of the complete engine.

MAF_CYL measured in mg/stk represents the mass of fresh air entering one cylinder during one intake event.

Most combustion-related calibration maps operate in the cylinder-charge domain.

3. Measured Airflow Path

3.1 MAF Sensor Characteristic

The primary MAF characteristic in ca663056 is:

ID_MAF_TAB

Address:

0x11B4E

This table converts the electrical MAF sensor representation into total engine mass airflow.

Its output is expressed in kg/h.

Functionally:

MAF electrical signal
        |
        v
   ID_MAF_TAB
      0x11B4E
        |
        v
Engine airflow
     [kg/h]

The ca663056 disassembly directly references address 0x11B4E during initialization of the MAF conversion path, confirming that the table is actively used for MAF conversion.

The sensor characteristic can therefore be represented as:

MAF_RAW = ID_MAF_TAB(V_MAF)

This is the stage responsible for converting the physical MAF sensor signal into an engineering airflow quantity.

4. MAF Acquisition and Signal Conditioning

The ECU does not use a single instantaneous MAF sample directly as engine load.

The ca663056 disassembly shows that MAF measurements are accumulated over repeated acquisition periods and subsequently divided by the number of samples.

The measurement path therefore performs averaging before the airflow value is used elsewhere.

Conceptually:

MAF_AVG = Average(MAF_RAW)

The averaged airflow then passes through an operating-point-dependent correction.

The relevant calibration is:

IP_MAF_KGH_MES_FAC

Address:

0x193EC

SAM defines the function as:

IP_MAF_KGH_MES_FAC = f(N, TPS_SEG)

where:

• N = engine speed
• TPS_SEG = throttle operating region

The output of this map is a dimensionless correction factor.

The corrected measured airflow can therefore be represented as:

MAF_MES_COR = MAF_AVG x F_MAF(N, TPS)

The resulting quantity remains in:

kg/h

and represents the ECU's corrected physical measurement of total engine airflow.

5. Conversion from Engine Airflow to Cylinder Air Charge

The combustion model itself does not primarily operate in kg/h.

After corrected measured airflow has been obtained, the ECU converts total engine airflow into cylinder air charge using engine speed.

For a four-cylinder, four-stroke engine:

Intake events per hour = (RPM / 2) x 4 x 60

Therefore:

MAF_CYL [mg/stk] =
MAF_KGH [kg/h] x 1,000,000
--------------------------------
(RPM / 2) x 4 x 60

For a four-cylinder engine this simplifies to:

MAF_CYL [mg/stk] = 8333.333 x MAF_KGH [kg/h] / RPM

Example:

400 kg/h at 3000 rpm

MAF_CYL = 1111 mg/stk

At twice the engine speed:

400 kg/h at 6000 rpm

MAF_CYL = 556 mg/stk

The same total engine airflow therefore represents a different cylinder load depending on engine speed.

This distinction is fundamental to the SIMK43 load model.

6. Definition of Engine Load

For the remainder of the combustion-control system, the principal load quantity is cylinder air charge.

Conceptually:

LOAD = MAF_CYL

with units:

mg/stk

The basic conversion chain is:

MAF signal
    |
    v
Engine airflow
   [kg/h]
    |
    v
RPM conversion
    |
    v
Cylinder charge
   [mg/stk]
    |
    v
Engine load

This cylinder-charge quantity is used as the load coordinate by many ignition, fueling and torque-model functions.

7. Independent Intake-Manifold and Cylinder-Filling Model

In parallel with the physical MAF measurement, SIMK43 independently calculates airflow using a model of the intake system and the engine's cylinder-filling characteristics.

The core cylinder-flow model consists of two calibrations.

7.1 IP_EFF_VOL_OFS

Address:

0x1245E

Definition:

IP_EFF_VOL_OFS [kg/h] = f(N, VO)

where:

• N = engine speed
• VO = valve overlap

This calibration provides the base airflow offset of the cylinder-filling model.

7.2 IP_EFF_VOL_SLOP

Address:

0x1252A

Definition:

IP_EFF_VOL_SLOP [kg/(h*hPa)] = f(N, VO)

This calibration provides the pressure-dependent slope of the cylinder-filling model.

Together, the two maps describe the relationship between effective manifold pressure and cylinder airflow.

8. Offset and Slope Representation of Cylinder Filling

SIMK43 does not represent cylinder filling using one conventional percentage-VE table.

Instead, airflow is represented approximately as a linear relationship between an effective pressure quantity and airflow.

The basic relationship is:

MAF_MDL = O + S x P_EFF

where:

O = IP_EFF_VOL_OFS(N, VO)

and:

S = IP_EFF_VOL_SLOP(N, VO)

The units verify the relationship:

kg/h + [kg/(h*hPa)] x hPa = kg/h

The ca663056 disassembly loads the offset and slope calibrations as separate coefficients used in the same airflow calculation.

The effective pressure term should not automatically be interpreted as a raw MAP sensor-like value. It belongs to the larger manifold model and represents the pressure-domain state used by the cylinder-flow equation.

Functionally:

RPM
 +
Valve overlap
 +
Effective manifold pressure
        |
        v
Offset / slope cylinder model
        |
        v
Predicted engine airflow
       [kg/h]

9. Effect of Valve Overlap

Both base airflow coefficients depend on valve overlap.

O = f(N, VO)

S = f(N, VO)

The model is therefore explicitly aware of camshaft timing.

Changing valve overlap changes the predicted relationship between manifold pressure and cylinder airflow.

Conceptually:

Valve timing
     |
     v
Valve overlap
     |
     v
Cylinder filling characteristic
     |
     v
Predicted airflow

VVT therefore forms part of the physical air model rather than being treated only as an independent actuator.

10. Temperature Correction of Cylinder Filling

Cylinder filling changes with intake air temperature and engine thermal state.

The principal temperature-dependent corrections are:

IP_EFF_TIA_FAC

Address:

0x15539

Definition:

F_TIA = f(N, MAF)

This represents the effect of intake-air temperature on the cylinder-filling model.

IP_EFF_TCO_FAC

Address:

0x154F9

Definition:

F_TCO = f(N, MAF)

This represents the effect of coolant temperature on the cylinder-filling model.

The ECU combines the temperature influences into a common correction quantity.

The resulting correction is filtered before it is applied to the cylinder-flow model.

The relevant filtering calibration is:

IP_CRLC_EFF_VOL_COR

Address:

0x154ED

Definition:

CRLC_VE = f(N)

The resulting thermal correction can therefore be represented conceptually as a low pass filter:

F_THERM = LPF[f(TIA, TCO, N, LOAD)]

where:

LPF represents the filtering applied by the ECU.

11. Ambient-Pressure Compensation

The base cylinder-flow characteristic is also corrected according to ambient pressure.

The relevant calibrations are:

IP_EFF_VOL_OFS_AMP

Address:

0x1251E

Definition:

F_OFS_AMP = f(AMP_AD)

and:

IP_EFF_VOL_SLOP_AMP

Address:

0x125EA

Definition:

F_SLOP_AMP = f(AMP_AD)

Both outputs are dimensionless correction factors.

AMP_AD represents the ECU's adapted ambient-pressure state.

The ambient-pressure corrections modify both the offset and slope of the cylinder-flow characteristic.

12. Corrected Cylinder-Flow Coefficients

The complete cylinder-flow coefficients can be represented as follows.

The base offset is:

O_0 = IP_EFF_VOL_OFS(N, VO)

The base slope is:

S_0 = IP_EFF_VOL_SLOP(N, VO)

The effective offset is:

O_EFF = O_0 x F_OFS_AMP

The effective slope is:

S_EFF = S_0 x F_THERM x F_SLOP_AMP

The disassembly shows the slope being corrected by the filtered thermal correction and then by the ambient-pressure correction.

The offset is separately corrected by its own ambient-pressure factor.

The final modeled airflow equation is:

MAF_MDL_KGH = O_EFF + S_EFF x P_EFF

This is the central cylinder-filling relationship of the SIMK43 airflow model.

13. Manifold Filling and Pressure-Ratio Model

The effective pressure term used by the cylinder-flow equation comes from the larger intake-manifold model.

SAM identifies several associated functions relating to:

• throttle effective area
• pressure ratio across the throttle
• upstream pressure
• intake-system pressure loss
• intake-air temperature
• manifold filling dynamics
• predicted manifold state

One relevant calibration is:

IP_FAC_AR_RED_COR

Definition:

IP_FAC_AR_RED_COR = f(PQ)

where:

PQ represents a pressure-ratio quantity.

Additional functions include:

ID_MAF_FAC_OFS

ID_MAF_FAC_OFS = f(PQ_ESTIM)

and:

ID_MAF_FAC_SLOP

ID_MAF_FAC_SLOP = f(PQ_ESTIM)

These functions describe nonlinear airflow behaviour associated with the pressure ratio across the intake restriction.

Another manifold-model coefficient is:

IP_MAF_MDL_CON_1

Definition:

C_MAN = f(TIA)

Its engineering unit is approximately:

s/m

This coefficient participates in the physical manifold-filling model.

The intake-manifold model can therefore be represented as:

Throttle effective area
        |
        +---- Upstream pressure
        |
        +---- Pressure ratio
        |
        +---- Intake temperature
        |
        +---- Manifold filling dynamics
        |
        v
Effective manifold pressure state
        |
        v
Cylinder offset / slope model
        |
        v
Predicted engine airflow

The modeled airflow is therefore derived from pressure and filling behaviour rather than from a simple throttle-angle-to-load lookup.

14. Intake-System Pressure Loss

The intake model also accounts for pressure loss between the outside atmosphere and the pressure available upstream of the throttle.

SAM defines:

IP_AMP_DEC

as:

DELTA_P_INTAKE = f(MAF_THR)

As airflow increases, the pressure loss through the intake tract also increases.

Conceptually:

P_UPSTREAM = AMP_AD - DELTA_P_INTAKE

This upstream pressure participates in the throttle pressure-ratio and manifold-filling calculations.

The model therefore does not assume that atmospheric pressure is always fully available at the throttle inlet.

15. Modeled Airflow to Modeled Cylinder Charge

Once the model has calculated total engine airflow in kg/h, that airflow is converted into cylinder charge using the same physical relationship as the measured-airflow path.

For the four-cylinder engine:

MAF_CYL_MDL [mg/stk] = 8333.333 x MAF_MDL_KGH [kg/h] / RPM

The measured and modeled paths therefore converge into the same engineering domain:

mg/stk

This allows the two airflow estimates to be compared, supervised and substituted.

16. MAF Pulsation Supervision

The physical MAF signal can become unreliable when intake-flow pulsation or reverse-flow effects become sufficiently large.

SIMK43 therefore contains a dedicated MAF-pulsation supervision system.

One of the principal calibrations is:

IP_MAF_PULS_MAX

Address:

0x12F50

Definition:

MAF_PULS_MAX = f(N)

The system also contains maximum-difference and hysteresis parameters.

The ECU evaluates the spread of the sampled MAF signal.

That spread is converted into the cylinder-charge domain using engine speed.

The calculated pulsation is then compared with the permissible pulsation limit.

Conceptually:

MAF samples
    |
    v
Maximum / minimum spread
    |
    v
Convert spread to mg/stk
    |
    v
Compare with IP_MAF_PULS_MAX
    |
    v
MAF pulsation state

The pulsation state participates directly in airflow-source supervision.

It is therefore part of the functional airflow model and not merely a diagnostic function.

17. Measured-Airflow and Modeled-Airflow Arbitration

SIMK43 contains source-selection logic between the measured and modeled airflow paths.

Functionally:

If measured airflow is accepted:
    MAF_WORK_KGH = MAF_MES_KGH

If measured airflow is rejected:
    MAF_WORK_KGH = MAF_MDL_KGH

During normal operation, the physical MAF measurement is the primary airflow source.

When the measured signal is considered unsuitable because of pulsation, faults or other fallback conditions, the independently calculated manifold-model airflow can replace it.

The airflow architecture therefore contains two complete sources for engine airflow:

Measured MAF airflow      Modeled manifold airflow
        |                             |
        +--------------+--------------+
                       |
                       v
                 Source selector
                       |
                       v
                 Working airflow

18. Final Cylinder-Load Determination

Following airflow supervision, the ECU maintains several representations of cylinder charge.

The principal sources include:

• measured-air-derived cylinder charge
• manifold-model-derived cylinder charge
• filtered or alternate cylinder-charge representations
• fallback cylinder charge

These ultimately produce one effective load quantity:

MAF_CYL_WORK

with units:

mg/stk

The rest of the combustion-control system uses this working cylinder-charge state without needing to know whether its source originated from the MAF measurement or the manifold model.

19. Reference Torque Model

The principal forward relationship between cylinder charge and torque is:

IP_TQI_REF

Address:

0x14D68

Definition:

TQI_REF = f(N, MAF_CYL)

where:

• MAF_CYL is cylinder fresh-air charge in mg/stk.
• TQI_REF is reference indicated torque in Nm.

The function therefore represents:

Cylinder air charge
      [mg/stk]
          |
          v
     IP_TQI_REF
          |
          v
Reference indicated torque
          [Nm]

This map describes the torque the engine is expected to produce from a given amount of cylinder fresh air.

20. Torque Loss Model

Indicated torque is not identical to torque available at the crankshaft.

SIMK43 separately models friction and pumping losses.

The principal calibration is:

IP_TQFR

Address:

0x14AE0

Definition:

TQFR = f(N, MAF_CYL)

This represents base friction and pumping losses as a function of engine speed and cylinder load.

Conceptually:

TQ_EFFECTIVE =
TQI
- TQ_FRICTION
- TQ_PUMPING
- TQ_ACCESSORY
- other losses

The torque system therefore combines the calculated indicated combustion torque with independently calculated engine losses.

21. Inverse Torque-to-Air Model

SIMK43 also contains the inverse relationship between torque and cylinder air charge.

The relevant calibration is:

IP_MAF_SP

Address:

0x12F5C

Definition:

MAF_SP = f(N, TQI_SP)

The output is:

mg/stk

This calibration converts desired indicated torque into the cylinder air charge required to produce that torque.

The two principal torque/air relationships are therefore:

Forward model

MAF_CYL -> TQI

through:

IP_TQI_REF

and:

Inverse model

TQI_SP -> MAF_CYL_SP

through:

IP_MAF_SP

Together, these functions form the central air-to-torque and torque-to-air relationship of SIMK43.

22. Air-Charge Control Objective

Once the torque system has calculated a desired cylinder charge:

MAF_CYL_SP

it can be compared with the estimated actual cylinder charge:

MAF_CYL_ACT

The conceptual air-control error is:

E_AIR = MAF_CYL_SP - MAF_CYL_ACT

The air-path controller can then act on the available air-control actuators (mainly the IAC Valve, as the throttle is driven by a steel cable) in order to move actual cylinder charge toward requested cylinder charge.

This illustrates why the SIMK43 torque-control structure is fundamentally based around cylinder air charge rather than throttle angle alone.

23. IVVT Airflow Prediction

SIMK43 contains an additional airflow estimator associated with the IVVT system.

One relevant calibration is:

IP_MAF_IVVT_1

Address:

0x193A4

Definition:

MAF_IVVT = f(N, TPS)

Its output represents calculated airflow for the VVT-related airflow-prediction path.

SAM also identifies:

• IP_MAF_IVVT_2 - alternate airflow calculation used when a TPS error exists.
• IP_MAF_TPS_FAC - throttle-position weighting.
• IP_MAF_ADD_ISAPWM - estimated additional airflow through the idle-air actuator.
• IP_MAF_FAC_TIA - intake-air-temperature correction.

This auxiliary IVVT airflow estimator should be distinguished from the main manifold and cylinder-filling model.

The primary physical cylinder model is based on:

• IP_EFF_VOL_OFS
• IP_EFF_VOL_SLOP

The IVVT airflow calculation represents an additional prediction path associated with VVT, throttle and idle-air operating conditions.

24. End-to-End Functional Sequence

24.1 Measured-Air Path

MAF electrical signal
        |
        v
ID_MAF_TAB
0x11B4E
        |
        v
Total engine airflow
kg/h
        |
        v
Sampling and averaging
        |
        v
Operating-point MAF correction
IP_MAF_KGH_MES_FAC
        |
        v
Corrected measured airflow
kg/h
        |
        v
RPM conversion
        |
        v
Measured cylinder air charge
mg/stk
        |
        v
Airflow / load source supervision
        |
        v
Final working cylinder load
mg/stk

24.2 Modeled-Air Path

RPM
Valve overlap
Ambient pressure
IAT
Coolant temperature
Throttle / manifold state
        |
        v
IP_EFF_VOL_OFS
IP_EFF_VOL_SLOP
        |
        v
Temperature correction
        |
        v
Ambient correction
        |
        v
Corrected offset and slope
        |
        v
Effective manifold pressure
        |
        v
Predicted engine airflow
kg/h
        |
        v
RPM conversion
        |
        v
Predicted cylinder charge
mg/stk
        |
        v
Airflow / load source supervision

Both paths therefore converge before the main combustion-control functions.

25. Complete Mathematical Model

25.1 Measured Airflow

MAF_RAW = ID_MAF_TAB(V_MAF)

MAF_AVG = Filter(MAF_RAW)

MAF_MES_COR = MAF_AVG x F_MAF(N, TPS)

25.2 Measured Cylinder Charge

MAF_CYL_MES =
8333.333 x MAF_MES_COR / RPM

25.3 Base Modeled Cylinder-Flow Coefficients

O_0 = IP_EFF_VOL_OFS(N, VO)

S_0 = IP_EFF_VOL_SLOP(N, VO)

25.4 Corrected Cylinder-Flow Coefficients

O_EFF = O_0 x F_OFS_AMP

S_EFF = S_0 x F_THERM x F_SLOP_AMP

25.5 Modeled Airflow

MAF_MDL_KGH = O_EFF + S_EFF x P_EFF

25.6 Modeled Cylinder Charge

MAF_CYL_MDL = 8333.333 x MAF_MDL_KGH / RPM

25.7 Working Engine Load

MAF_CYL_WORK =
Select(
    MAF_CYL_MES,
    MAF_CYL_MDL,
    MAF_CYL_FALLBACK
)

25.8 Forward Torque Model

TQI_REF =
IP_TQI_REF(
    RPM,
    MAF_CYL_WORK
)

25.9 Inverse Torque Model

MAF_CYL_SP =
IP_MAF_SP(
    RPM,
    TQI_SP
)

26. Functional Interpretation

The SIMK43 airflow architecture can be divided into four principal functional layers.

1. Measurement

The physical MAF signal is converted into corrected total engine airflow in kg/h.

2. Physical Model

An independent intake-manifold model predicts airflow using manifold conditions, engine speed, valve overlap, temperature and ambient pressure.

3. Load Estimation

Measured and modeled airflow are converted into cylinder charge in mg/stk and supervised to produce the ECU's effective working load.

4. Torque Coordination

The ECU converts actual cylinder air charge into estimated torque and requested torque back into required cylinder air charge.

Physical MAF measurement and a parallel intake-manifold model are used together to determine the amount of fresh air available for combustion and to provide a consistent load quantity for the torque, ignition and fueling systems.