Even though the Motronic M 2.7, engine management system used in the CALIBRA TURBO 4x4, provides optimised ignition control, sequential fuel injection, air flow metering and knock control, as in the Motronic M 2.5 and M 2.8, used in 2 wheel drive Calibra models, it also controls the charge pressure produced by the turbocharger.

While this description describes the features of the Motronic M 2.7 engine management system, it will also prove useful as a general overview for both the M 2.5 and the M 2.8 versions.

MOTRONIC M 2.7 SYSTEM OVERVIEW

Illustration Key:

1.       Fuel tank

2.       Tank vent valve

3.       Active carbon canister ‑ tank vent valve

4.       Idle speed adjuster

5.       Intake air temperature sensor

6.       Inductive pulse pick‑up

7.       Inductive pulse pick‑up sensor gear

8.       Knock sensor

9.       High voltage distributor

10.     Coolant temperature sensor

11.     Oxygen sensor

12.     Turbocharger

13.     Bypass valve ‑ charge pressure control

14.     Hot‑wire mass air flow meter

15.     Ignition module

16.     Control unit with diaphragm, spring and actuating rod for charge pressure control valve

17.     Fuel filter

18.     Injection valves

19.     Fuel pressure regulator

20.     Hot start valve

21.     Charge cooler

22.     Throttle valve potentiometer

23.     Fuel pump

24.     Vibration damper

25.     Diagnostic plug

26.     Recognition – 1st gear

27.     Recognition ‑ Reverse gear

  COMPONENT DESCRIPTIONS

To provide a logical sequence to the various components that make up the Motronic M 2.7 engine management system, the following descriptions will proceed through various sub-systems of:

Fuel Flow.

Air Flow.

Electrical/Electronic

FUEL FLOW SUB‑SYSTEM

AIR FLOW SUB‑SYSTEM

HOT‑WIRE MASS AIR FLOW METER

Introduced with the Motronic M 2.5, the calibration of the air flow meter for the M 2.7 has been modified to suit the increased air flow rate. The optimal means of determining the load on a petrol engine is to measure the air mass taken in by the engine. The reading thus obtained, is independent of air pressure, height above sea level (important when traveling in mountains) and air temperature.

Construction of Hot Wire Air Mass Meter

Illustration Key:

m Intake air mass                                                                                 RK Temperature sensor

SE Control electronics                                                                           RH Hot wire

                                                                                                           RM Precision resistance

Operation

Measurement is made by guiding the air mass (m) into the engine, past a thin, electrically heated wire (hot wire, RH).

This hot wire is part of an electrical bridging circuit and is monitored by an electronic control unit (SE).

The electronic control unit (SE) regulates the flow of heating current (I) through the hot wire so that the hot wire always has a constant temperature.

If the mass of the intake air rises, this results in the hot wire cooling down proportionally.

Then the electronic control unit in the air mass meter (SE) increases the heating current so that the hot wire returns to constant temperature.

The heating current flows through the precision resistance (RM), causing a voltage drop that is always in the same proportion as the intake air mass.

This voltage drop is recorded at terminals 2 and 3 and conducted to the Motronic M 2.7 control unit as an air mass signal.

This heating current is therefore a measurement of the air mass flowing into the engine.

To avoid faulty measurement due to contamination, the hot wire is burnt free after each operation. A pre‑condition for this burning free period is that an engine speed of 1,000 rpm and an engine temperature in excess of 31 °C must have been reached. This means that the hot wire is not burnt free every time the ignition is switched ON or OFF.

Provided these pre‑conditions are fulfilled, the burning free begins approx. 4 seconds after the ignition is switched off and lasts for approximately 1.6 seconds when the process is visibly recognisable by the red glowing hot wire.

Illustration Key

I        Heat current                                                                                                    P 44              Hot wire air mass meter

m      Intake air mass                                                                                                P 44/Ter.1      Ground

SE     Control electronics                                                                                           P 44/Ter. 2     Ground to Motronic M 2.5 control unit

RK     Temperature sensor                                                                                         P 44/rer. 3      Signal lead hot wire air mass meter

RH     Hot wire                                                                                                          P 44/Ter. 4     Signal 'bum free' (pulsed ground)

Rl      Resistor in the measuring bridge of the control electronics                                   P 44/Ter. 5     Voltage supply +12 V

         (for better clarity, shown outside of the control electronics)                                  P 44/Ter. 6     Not used with Motronic M 2.7

RM Precision resistance measurement                                                                       K 61              Motronic M 2.7 control unit

A       Measuring leads                                                                                              K 68              Fuel pump relay

Should the hot wire air mass meter or the lead between the hot wire air mass meter and the Motronic M 2.7 control unit become defective, then the Engine Telltale Lamp will be lit and a diagnostic trouble code (DTC) will be stored in the Motronic M 2.7 control unit.

OTC 65: Voltage CO potentiometer too low.

OTC 66: Voltage CO potentiometer too high.

OTC 73: Voltage hot wire air mass meter too low.

DTC74: Voltage hot wire air mass meter too high.

Once a trouble code is logged, the control unit sets a default value which allows the vehicle to still operate until the fault can be located and corrected. Refer to 5.

CHECKING PROCEDURES ‑ Motronic M 2.7 in this Section for further details of these and other diagnostic trouble codes.

BYPASS VALVE ‑ CHARGE PRESSURE CONTROL

Illustration Key:

1.   Charge pressure control bypass valve

2.   Charge pressure regulating valve control unit

3.   Air bypass valve

Intake air, charge pressure control is carried out via this pulsed, 3‑way valve ('1) which, depending on the actuation from the Motronic M 2.7 control unit, applies the charge pressure regulating valve control unit ('2'), to either the intake or pressure side of the turbocharger, thereby controlling the amount of boost provided.

Refer to Section 6A, ENGINE MECHANICAL, in this Volume for further information relating to the turbocharger operation.

The charge pressure control bypass valve is fastened by a retainer to the coolant return hose of the turbocharger, as shown in the above inset.

ELECTRICAL/ELECTRONIC SUB‑SYSTEM

With the complexity of the electrical/electronic interface with other vehicle components, the following block diagram shows the input signals required by the Motronic M 2.7 control unit to make decisions about the output circuits controlled by the control unit.

A brief description of the Motronic M 2.7 control unit follows, together with some of the input signal sources.

MOTRONIC M 2.7 BLOCK DIAGRAM

WIRING HARNESS

The separate wiring harness for the Motronic M 2.7 is in a self‑continued engine wiring harness that connects all sensors and actuators with the Motronic M 2.7 control unit.

Illustration Key

  1. Diagnostic plug (ALDL)                                                                   10. Trigger box

  2. Ignition coding plug                                                                         11. Engine wiring harness plug

3. Motronic M 2.7 control unit plug                                                       12. Fuel tank vent valve

4. Oxygen sensor                                                                              13. Hall sensor‑cylinder recognition

5. Hot wire air mass meter                                                                  14. Knock sensor

6. Throttle valve sensor                                                                       15. Idle speed adjuster

7. Fuel injectors                                                                                 16. Crankshaft inductive impulse sensor

8. Earth terminals                                                                              17. Engine coolant temperature sensor

9. Fuel pump relay

SIGNAL PROCESSING

BASIC CONTROL UNIT OPERATION

To establish a basic understanding of the way in which an electronic control unit functions, there are three different processes that perform separate tasks within the unit, which are:

a. Preparation of Input Signals:

§                 Interface

§                 A-D converter

b. Information processing:

§                 SEFI computer (Sequential Fuel Injection computer).

§                 CPU (Central Processing Unit).

§                 RAM (Random Access Memory, read/write memory with random access to individual data).

§                 ROM (Read Only Memory, non‑erasable program memory).

§                 Knock control computer.

c. Output Controls:

§                 Output stages (actuating signal boosters).

§                 Diagnostic plug (ALDL) for connecting to TECH 1

Operation

The CPU receives commands from the ROM and executes them.

This means that the CPU:

§                 Loads the measured values, which are edited by the interface, into the RAM.

§                 Recognises the various operating conditions on the basis of these values.

§                 Fetches the characteristic curves and diagrams, which belong to these operating conditions, from the ROM.

§                 Links the measured values with the characteristic curves/diagrams in accordance with the computing rules stored in the ROM

§                 Computes the actuating signals and passes these on to the output stages.

The Output Stages trigger the actuators:

§                 Separate injection valves for each cylinder

§                 Idle speed adjuster

§                 Fuel pump relay

§                 Signal 'burn free' for hot wire air mass meter

§                 Tank vent valve

§                 Engine telltale (self‑diagnosis)

§                 Trigger box

BASIC IGNITION SYSTEM OPERATION

When computing the dwell angle, a difference is made between start, idling, partial load and full load. In addition, the dwell angle is also dependent on the anti‑ jerk function, knock control, the idle speed control and the deceleration fuel cutoff (these latter three are discussed in 2.3 BASIC OPERATION OF KNOCK CONTROL and 2.4 BASIC FUEL INJECTION SYSTEM OPERATION in this Section).

Start

When starting, the dwell angle is calculated according to a characteristic curve dependent on engine temperature and speed.

Idling

When idling, a characteristic curve dependent on engine speed becomes effective, which is corrected by the idle speed control program component of the CPU:

§                 If engine idle speed falls below the nominal value, the ignition is 'advanced' to raise the engine torque.

§                 If the engine speed rises above the nominal value, the ignition is 'retarded" to reduce engine torque.

Partial Load

In a partial load condition, the dwell angle is based on the dwell angle map, which is dependent on load and engine speed.

A dwell angle change limitation prevents the dwell angle from changing rapidly. The anti‑jerk function is an exception from this dwell angle change limitation. (See 'Anti‑Jerk Function' in this Section).

Full Load

In a fully loaded situation, a characteristic curve dependent on engine speed is valid, which is subject to an attitude correction. The control unit recognises the increased altitude when, with the throttle valve fully open (full load switch closed), a pre‑programmed air flow mass is not achieved. It then 'advances' the ignition to increase engine torque.

In this way a reduced performance, due to a reduced density in the intake air, resulting from low air pressure at high altitudes, is overcome.

Anti‑Jerk Function

The control unit recognises jerking by comparing the engine speed at two short consecutive intervals, filtering the values and computing the difference.

§                 If the engine speed is rising, the ignition is 'retarded' to reduce engine torque.

§                 If the engine speed is falling, the ignition is 'advanced' to increase engine torque.

This prevents jerking in the partial load range. Because jerking does not occur with greater loads or higher engine speed.

For this reason, the anti‑jerk function is disabled in this range.

BASIC KNOCK CONTROL OPERATION

Engines with high compression ratios cannot normally be operated with optimal spark advance as they would otherwise be damaged by detonation during combustion. As a result, the spark advance in a conventional ignition systems is set with a corresponding safety margin to the detonation limit. The use of knock control dispenses with the need for this safety margin and the need to have an octane number plug. This means that the engine is always operated with optimal spark advance and provides the following advantages:

§                 high performance

§                 good torque values

§                 low fuel consumption

§                 automatic adjustment to fuel quality

§                 no engine damage due to knocking combustion Function Diagram of Knock Control

FUNCTION DIAGRAM OF KNOCK CONTROL

The knock sensor supplies a structure‑bome signal in which all secondary noises are also contained (e.g. engine vibrations). Because the knocking frequency of the C 20 LET engine has been determined in trials in the region of 15 kHz, only this frequency is used for further evaluation.

This frequency is conveyed to the integrator only within the measuring window (10° ‑ 60° ATDC), where the integrator aligns the signal within the measuring window. The signal so formed, is allotted to the appropriate cylinder by the A‑D converter.

The actual value of this cylinder is now compared with is reference level (average value of the last 16 phases). If the actual value exceeds the reference level by a certain amount, the combustion is recognised as knocking.

If the actual value lies below a certain level related to engine speed, then the actual value is used as a new reference level for knocking recognition. Thus the knock control reacts to even minimal engine noise.

If the knock control has recognised knocking combustion for any one cylinder, the CPU will adjust the dwell angle by 3° in a retard direction for the next phase. The dwell angle of the other cylinders is not affected by this measure (cylinder‑selective control).  The dwell angle adjustment in a 'retard' direction is repeated for every combustion which is recognised as knocking and for each cylinder selectively (individually). If no more detonation is sensed, the ignition is adjusted by 0.75° in direction 'advance' after 20 to 120 knock‑free combustions (approx. 2 seconds). This is repeated until the pilot control value is reached again or until knocking combustion is registered. The knock control only affects the dwell angle in an engine speed dependent load range in which knocking combustion is to be expected.

As the knock limit varies from one cylinder to another in an engine and can change drastically within the operating range, every cylinder has its own ignition point for operation at the knock limit. This type of 'cylinder‑selective' knock recognition and control is an essential advantage in Motronic M 2.7 because it allows the optimisation of engine performance and fuel economy.

Figure 6C‑22 shows the individual cylinder knock control for a 4 cylinder engine, such as the C 20 LET fitted to the Calibra 04 Turbo.

Automatic Octane Number Adjustment

The knock control makes automatic octane number adjustment possible.

Two ignition characteristic maps are programmed in the control unit.

The knock control computer selects the appropriate ignition characteristic map for the fuel quantity according to the following criteria:

§                 After 50 knocking combustions the control unit switches to the map with the more retarded dwell angle (low octane number).

§                 After approx. 8.5 minutes of knock‑free operation, the control unit switches back to the map with the more advanced dwell angle (higher octane number).

BASIC FUEL INJECTION SYSTEM OPERATION

Motronic M 2.7 continues with sequential fuel injection, as introduced with the earlier M 2.5.

What follows is an illustration of the difference between simultaneous and sequential fuel injection:

Figure 6C‑23 With simultaneous injection, all injection valves inject once per crankshaft revolution regardless of which phase each cylinder is in.

With sequential injection, only the cylinder in the induction phase is supplied with fuel.

The advantages of sequential injection are:

§                 exact amount of fuel required for each cylinder

§                 spontaneous reaction to load change

§                 high performance

§                 high torque

§                 low fuel consumption

§                 uniform mixture distribution

§                 improved exhaust emissions (no injection onto open intake valve) A separate microprocessor and one output stage for each injection valve are provided in the control unit to provide exact computation and triggering of the injection.

INJECTION TIMING COMPUTATION

The injection timing is dependent on the load signal.

The load signal is computed from the voltage reduction in the hot wire air mass meter, the engine speed and an injection valve constant.

In order to counteract vibrations (jerking), this signal is put through an electronic filter which collects these vibrations.

The injection timing is computed from the product of these processed load signals and all correction factors in the current operating condition.

The mixture is enriched in the following dynamic operating conditions:

§                 after‑Start

§                 warming‑up

§                 acceleration

§                 re‑engagement after deceleration fuel cutoff

The mixture is also corrected in the following stationary operating conditions:

§                 when idling above an engine speed dependent characteristic curve (idling, deceleration fuel cutoff)

§                 at partial load via a characteristic curve dependent on engine speed and load

§                 at full load via an engine speed dependent characteristic curve

§                 Start

Starting is divided into two phases.

In phase 1 the load signal is not yet useable and is therefore replaced by a fixed value of 2.5 ms. Depending on engine temperature, it is determined whether it is a:

Cold start (engine temperature below 0 °C)

Normal start (engine temperature from 0°C‑125 °C) Hot start (engine temperature above 125 °C)

Phase 1 is valid as long as the engine speed has not yet exceeded an engine speed threshold, dependent on engine temperature or that 12 ignitions after starting have not been exceeded.

Intake air mass and engine speed are not considered until phase 2.

§                 After‑Start

When starting is finished, the so‑called after‑start phase begins.

Now the load signal is used together with an after‑start correction to compute the injection timing.

After a cold start, a cold start correction follows, after a hot start, a hot start correction.

With a hot start, an injection time reduction occurs for a pre‑set period of time.

§                  Warm‑up

When idling, the mixture is enriched In accordance with a characteristic curve dependent on engine temperature and engine speed.

When not idling, a characteristic map dependent on load and engine speed is called upon.

§                 Acceleration Enrichment

The acceleration enrichment is triggered when the intake air mass increase per second exceeds a certain value. In order to attain a better dynamic ratio during the acceleration phase, auxiliary injectors are actuated to supplement the injection extension. The extent of acceleration enrichment is determined by the degree of acceleration, the engine temperature and a characteristic map dependent on load and engine speed.

DECELERATION FUEL CUT-OFF

Conditions for deceleration fuel cutoff are:

§                 idle contact closed or load signal below a certain threshold

§                 engine speed below a threshold dependent on engine temperature

Once these conditions are met, the deceleration fuel cutoff immediately begins. That is:

§                 the ignition is retarded to the idling dwell angle.

§                 then fuel injection is switched off.

Before re‑engagement occurs, either the engine speed must fall below a certain engine temperature dependent threshold, the load signal must exceed a certain threshold or the idling switch must open.

Injection is resumed and the dwell angle is slowly adjusted to the characteristic map value (soft re­engagement).

If the engine speed falls very quickly, the injection re­engages earlier in order to prevent the engine from dying (fast re‑engagement).

If the load signal increases sharply, the ignition is immediately related to the characteristic map value so that the engine torque is increased (fast re‑engagement).

OXYGEN REGULATION SYSTEM

When catalytic converters are used for exhaust gas conversion, then unleaded fuel must only be used and the air‑fuel ratio may only deviate very slightly (t 0.15%) from the stoichiometric ratio (Lambda = 1, which corresponds to approximately 14 kg air to 1 kg fuel).

Only under these conditions can the exhaust constituents CO, HC, NO. be reduced by 90%.

Such accuracy in mixture formation is not possible without regulation. Therefore the computation of injection timing described above is supervised by the oxygen regulator.

Two factors are responsible for oxygen regulation:

1.     The integrator regulates without delay.

2.     The block learn function adapts the regulator to long term changes, as for example those which occur due to running in and aging of the engine, density and changes in quality of the fuel, air leaks etc.

Block learn function 1 is effective during idling.

Block learn function 2 is effective in the partial and full load phases.

IDLE SPEED CONTROL

The idle speed is controlled by means of the idle speed adjuster and the dwell angle adjustment. The dwell angle adjustment is a fast but limited measure that operates until the idle speed adjuster takes over regulation with the slower volume control. The dwell angle control is described in Section 2.2 BASIC IGNITION SYSTEM OPERATION, in this Section.

The idle speed adjuster is actuated in all operating conditions.

The following additional functions are fulfilled above and beyond the true idling zone:

§                 Auxiliary air valve: for mixture enrichment at low engine temperatures

§                 Vacuum limitation: If the throttle valve is closed, the control unit opens the idle speed adjuster in order to limit the vacuum in the intake system.

§                 Soft deceleration fuel cut‑off and re engagement: before switching off the injection, the idle speed adjuster is closed and does not return to the open position until after re‑engagement. In this way, together with the dwell angle control (see Section 2.2 BASIC IGNITION SYSTEM OPERATION, in this Section), a smooth deceleration fuel cutoff and re­engagement is achieved.

§                 Idle speed modification when the air conditioning system is activated.

COMPUTING THE NOMINAL ENGINE SPEED

In order to compute the nominal idle engine speed, a characteristic curve dependent on engine temperature and engine speed is drawn on during starting. After starting, a corresponding characteristic map serves this purpose.

Apart from this, the nominal engine speed is dependent on the battery voltage. If the battery voltage falls below a pre‑set value, the nominal engine speed is increased. This increase is not reversed until the voltage rises again.

At low temperatures the nominal engine speed is also increased to guarantee smooth engine running.

Computing the Nominal Air Requirement

The nominal air requirement (the air which should flow through the idle speed adjuster) is computed in accordance with the PI regulator principle (Proportional Integral Regulator). Proportional means that the idle air adjuster is opened by an amount corresponding to engine speed deficit when compared to the nominal value. The integral part results from the average of all previous engine speed deviations. This results in the equalising and compensating function of the integrator.

Idle Speed Adjuster Triggering

The nominal air requirement which has been computed is converted to a frequency with which the idle speed adjuster is triggered.

When the engine idles for quite a long period, the computed nominal air requirement is compared to the actual intake air mass and the computer adjusts its calculation by adapting to the actual situation.

In this way slowly changing conditions are taken into account. For example:

§                 An increased idle air requirement in brand new engines (due to greater friction).

§                 Air leaks in older engines.

§                 Diversity of engines due to manufacturing tolerances.

TANK VENTILATION

If the fuel in the tank becomes warmer due to outside influence or in passing through the fuel supply system (fuel pump, fuel line, distributor pipe), then vapours form which cannot be released into the atmosphere in a vehicle with catalytic converter.

The vapours which form in the tank are released into the atmosphere via the active carbon filter when the engine is not running The petrol vapours are retained by the active carbon and temporarily stored until the next time the engine is operated.

In the partial and full load ranges the tank ventilation valve is opened by the control unit. Due to the vacuum in the intake manifold, fresh air induction takes place via the active carbon filter when the engine is running.

The temporarily stored fuel vapours are thus expelled.

In order to prevent this flushing of the active carbon canister from interfering with the engine running, the tank ventilation valve is only triggered during active oxygen regulation, when the engine temperature is greater than 49.8 °C and when the idling switch is open.