Rover's Vikingship The Airflow Meter Rover's Vikingship

The air flow meter is a very important part in the L-jetronic system because it measures the amount of air the engine is taking in. From this measurement the ECU can calculate how much fuel must be injected to get the right fuel/air ratio.

If we look at the picture below we see that the housing contains a double flap pivoting on a vertical spindle. The measuring flap is closed on a stop when the engine is not running by a light return spring.When air is entering the engine the flap opens. The damping flap moves into the damper chamber it compresses the air a bit so damping the rate of movement. If throttle is held the air pressure in the chamber will fall until it is equal to the pressure at point B. Both flaps are slightly twisted to the pivot axle in opposite directions to allow smooth movement of the flaps. This to prevent oscillation of the flaps and thus a rough running engine.

Airflow meter  (15,1 kB)

Whenever the measuring flap is closed on its stop the electrical contacts (no's 36 and 39) of the fuel pump are open. These contacts operate the fuel pump. Thus the pump is not running. Whenever the flap is opened (i.e. the engine is turning and sucks in air) the contacts are closed and the pump is running.

Coupled to the flap spindle is a potentiometer which gives a signal to the ECU. The potentiometer is fed from pin 6 from the ECU. Depending on the posistion of the flap a variable signal is send back to the ECU at pin 7 and this signal tells the ECU how much air is entering the engine.

Airflow meter  (9,0 kB)

The potentiometer is of a very special construction. Whenever the car would hit a bump or have vibrations which could send the pick-up arm in resonance the signal to the ECU would be far off the mark. To prevent this a double pick-up construction is used with every pick up point mounted on an arm with a different length en thus its own resonance frequency. This way the correct signal is always been given to the ECU regardless of the road conditions.

An interesting thing is the way the potentiometer resistance circuit is built up. The signal at pin 7 is an indication for the amount of air. However it is very difficult to get the right resistance characteristic of the potentiometer in relation to the measured amount of air. Therefore the path of the potentiometer is divided in eight segments with the same build up. A resistor is placed parallel to each segment. This way the wanted characteristic can be achieved and also leaves room for some fine tuning by changing the values of the resistors depending on the car application.

Also incorporated in the Airflow meter body is a temperature sensor which measures the temperature of the incoming air. It is basically a NTC resistor. The resistance drops with increasing temperature. The temperature signal is picked up at pin 27 of the ECU.
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Airflow meter  (38,0 kB)
 
Rover's Vikingship The ECU Rover's Vikingship

The ECU, the heart of the matter. Well this unit gets all the engine data and from this data it calculates when and how much should be injected. The ECU receives the following information:

  • Engine speed (from distributor)
  • Cranking (from ignition lock)
  • Engine temperature (from coolant sensor)
  • Air temperature (from the sensor in the Air flow meter)
  • Amount of air going into the engine (from the Air flow meter)
  • Position and movement of the throttle (from throttle potentiometer)
  • Engine mixture (from lambda sonde, if fitted, not on the system as used for the SD1)

From this data the ECU can calculate the correct inject time and fuel amount. The ECU then sends its output to:

  • The injectors (two signals with the same duration but different timing,....two banks of four injectors, remember)
  • The Extra Air Valve

There is a difference between the standard Bosch ECU and the Lucas ECU as used in the SD1. The Rover SD1 had all L-jetronic parts EXCEPT the electronic control box which was a digital, mapped system permitting stereo lambda feedback from the outset. Euro models were not fitted with lambda sensors or catalysts. This digital system did not have a microprocessor but an integrated hard wired logic custom IC made at Ferranti for Lucas. The silicon dies were too big to get a good yield as one chip so the design was split into two.......a disintegrated chip! The fuel map was reduced from 16 x 16 to 8 x 16 also to reduce die size. The look-up parameters were air-flow per stroke and RPM.

Below you will find how the L-Jetronic ECU operates (not the Lucas one!). However input and output signals of both ECU systems are equal. In other words the injectors of the Rover V8 are also controlled by varying the length of the pulse depending on the input signals.

The operation of the Bosch ECU
(No not the Lucas one)
ECU diagram  (10,5 kB)

A volume of air (QL) is flowing through the air sensor.This would need amount Qk of fuel, point D. The flap is deflected at angle a, point A.The potentiometer connected to the flap gives a voltage signal Us to the ECU, point B. The ECU calculates the amount of fuel to be injected, point C. In the diagram it can be seen that points C and D line up so the right amount of fuel is injected despite the fact that various parameters are non-linear.

The signal from the distributor is converted to a digital pulse in the Shaper circuit. Basically a Schmitt-trigger circuit (For the Electronic Whizz kids). Then the pulse coming out of this circuit is divided by two. (Also see the info at the distributor).

The signal goes on into the Division Control Multivibrator (What's in a name...!). The measured air quantity signal(Us in the diagram) is converted into rectangular control pulses with a duration Tp. A higher voltage means more air thus a longer Tp. This pulse Tp is then generated everytime a signal is given by the shaper/divider. The signal is then fed into the Multiplying stage.

enrichment faktor (4,5 kB)

The multiplying stage determines a multiplying faktor (K)dependend on load, engine- and air temperature. Then a new pulsetime is calculated with Tm=Tp*K. Then this time is added to the basic time with T=Tm+Tp.

Finally a correction time for the battery voltage is added to keep the injected amount of fuel constant even if the battery is low.

The resulting signal is then send to the Final stage where the signal is split over two banks and amplified.

ECU diagram  (24,9 kB)

As you can see the L-jetronic system isn't a mapped injection system where all the data is programmed into memory. The system continuously calculates the needed amounts of fuel. To change the influence of for instance air temperature or engine temperature it isn't a case of just swapping chips like on the newer systems. If you want to change the characteristics of the L-jetronic then you have to take your solder and replace resistors to achieve the wanted characteristic. This is why the L-jetronic isn't too popular these days with engine tuners.

If you tune your engine the EFI is able to deliver about 20% extra power before the Air flow meter and the injectors limit the power increase. A first thing to change when tuning is to replace the fuel pressure regulator for a rising rate pressure regulator. This gives an adjustable pressure difference instead of the fixed difference. This way more fuel can be injected over the entire speed range. If this isn't enough then the Air flow meter can be exchanged with the unit from the Jaguar XJ-6 series along with bigger injectors. But then we are beginning to talk about very expensive changes better left to the specialists.

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© rwp Mach. 2001