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Chapter 2 of D-Jetronic article series of Dr-DJet gives a functional overview of D-Jetronic fuel injection system from fuel supply and ignition to injectors


2.1 Components overview

components of an 8-cylinder
components of an 8-cylinder

D-Jetronic system is composed of following components: Damper (1, not on all cars), fuel pump with pressure relief valve (2), fuel filter (3), fuel line (4), electrically operated injectors (5), pressure adjuster (6), damper (7, not an all cars), trigger contacts (8, in distributor), motor cabling, engine control unit (9, ECU), manifold pressure sensor (10, MAP sensor), thottle switch (11), cold starrt valve (12), air temperature sensor (13, not on all cars), engine temperature sensor (14, water or block temperature), thermo time switch (15, not on all cars), auxiliary-air device (16), injection relais (17), fuel pump relais (18), cold start relais (not on all cars), absolute pressure switch (only very early cars).

2.2 Fuel-air ratio determines performance, fuel consumptions and emissions

Every engine needs a suitable and ignitable fuel-air mix for all engine revolutions and with all possible loads. Additionally torque, fuel consumption and emissions need to be watched. Adjusting this from exhaust gases via closed looped lambda control is a concept that was not yet known in the 60s. It was introduced in late 70s in L-Jetronic. That is why injection systems in the 60s had to find out how much air was curently in ach cylinder to adjust injected fuel accordingly. In principle an air to fuel atio of 14:1 is desirable, measured as air number λ. If  λ is above 1, mixture is too lean, and below it is too rich, emissions and fuel consumption increase.

2.3 How does D-Jetronic control air-fuel mixture?

D-Jetronic was the first commercially successful open loop control system that could control air-fuel ratio according to the air mass sucked in by the engine. It therefore measures intake manifold pressure, temperature of air and engine revolutions in order to determine load and evolutions. Being an open loop control it always adjusts a slightly rich mixture. Only in idle-run one can adjust the mixture via a potenitometer on most ECUs. This was introduced to fulfil US emission rules. Calculated values are adjusted by factors like cylinder filling ratio, engine temperature, acceleration detection, full load recognition and a possible altitude compensation. It was a first try to automatically adjust fuel-air mixture to engine state instead of just defining it. Today's technologies use a heat wire driven air mass sensor and a cheap silicon MAP sensor for such a task.

Each cylinder has an electrically operated injector valve that injects fuel in a spray into the manifold just before intake valve. Those injectors have a constant fuel pressure supply, so that opening times determine the amount of injected fuel. Opening times are 2 to 10 milliseconds, while it takes 0.15 milliseconds to fully open. Opening times are calculated by ECU with the input of all available sensors. ECU always combines 2 to 3 injectors into one injector group and injects fuel on all associated cylinders at the same time. Injected fuel then "waits" in manifold until the intake valve is opened and fuel and air is sucked into the cylinder. Timing of injection is determined by trigger contacts (2 for 4 and 6 cylinder models, 4 for 8 cylinders) in lower body of distributor. ECU also learns engine revolutions from them. Ignitions distributors and temperature sensors were not always supplied by Bosch. Citroen and Jaguar decided to use their local suppliers instead.

2.4 Adjusting fuel pressure

The fuel pump sucks fuel from tank through a damper and pushes it though fuel filter to a fuel line that is connected to all injectors and cold start valve. At the end of this fuel line is an adjustable pressure relief diaphragm and a damper (not on all cars). This pressure adjuster is used to exactly adjust fuel pressure in the fuel line to 29 +2  psi (meaning 29 to 31 psi or 2.0 to 2.1 Bar) and keep it there, no matter how much fuel is consumed. Nowadays such a system is called common rail. Unconsumed fuel is returned via a return line from pressure adjuster to tank. It is important that you use a suitable fuel pump with a pressure relief valve that limits fuel pessure peaks to 40 psi (3 Bar), otherwise the diaphhagm in pressure adjuster will suffer. Misaligning fuel pressure will affect injected fuel in each state of the engine, therefore fuel pressure always has to be within reference limits. Saab and Volvo during some years used different pressures. of 31 to 32 psi ( 2.1 or 2.2 Bar). Fuel pump is switched off 2 seconds after engine dies or does not start. K-Jetronic pumps should only be used with a pressure limiting valve  or you isk damaging your pressure adjuster. It will run well first, but over time pressure peaks will destroy the diaphragm.

Fuel pump is controlled by ECU through a relais. If engine evolutions are below 200 to 300/min, the fuel pump is deactivated. After switching on ignition, it will run for 2 seconds, to build up pessure in fuel line. During start the fuel pump is definitely running. If the engine does not start or dies while running, the fuel pump will be switched off to avoid wear and the risk of fire with fuel continuously leaking.

2.5 Where does the ECU get the infomation from?

The ECU is a surprisingly stabile analogue and totally discrete electronics, The MAP sensor coils are an integral part of this electronics. ECU reads the sensors and calculates injector opening times from that information. Main sensor is the MAP sensor that reads the vacuum pressure in intake manifold. Air mass is corrected by an air temperature sensor (exception: early Opel ECUs). Engine speed and when to inject is read from trigger points in lower body of ignition distributor. Acceleration is detected when gas pedal is pushed down. A full load situation is detected by various ways: Very early ECUs read that from an absolute pressure sensor in intake manifold. After that a full load diaphragm was introduced in MAP sensor and latest versions have a full load contact in throttle switch. All these methods are used to further enrich the mixture in full load. Full load switch via absolute pressure sensor has a big disadvantage: In highr altitudes the absolute pressure sensor will never be able to detect a full load situation, thus decreasing engine performance. D-Jetronic is not as sensitive to bypass air as e.g. K-Jetronic, but massive bypass air will make it fail like any other injection system.

2.6 Warm-up run

By reading the engine tempeature the ECU can heavily enrich fuel-air mixture up to a factor of 3 to compensate for fuel condensation on intake manifold. On some cars it also controls the cold start valve. Mostly this is done via a thermo time switch (in my picture I show a water temperature sensor as I did not have a thermo time switch at hand), that will enable cold start valve during starting the engine only for a time of up to 20 seconds at -4°F (-20°C). Above 95°F (35°C) it is normally deactivated.. Additionally engine idle speed is increased by a auxiliary-air device by typically 300 to 500 rpm. This is like opning the thottle just a little more and does not affect the mixture. ECU recognizes increased speed and adjusts the mixture all by itself.

2.7 Idle-run CO

In ilde-run CO emission can be adjusted by a potentiometer on ECU. Very early ECUs do not have such a potentiometer (even early MB ECUs for 250CE 0 280 001 002 do not have it). This is no control, just an adjustment. It only works, when idle contact in throttle switch is closed. One can adjust values of somewhat 1% to 5%. During bi-annual emission test I always surprise my engineer by quickly adjusting CO from co-pilots seat.

2.8 Over-run shut-off

Many ECUs also have an over-run shut-off. Exception: Early Volkswagen and late Opel. If engine speed is above a certain limit while idle contact is closed, fuel injectors are simply switched off until speed sinks below a second lower limit. Exact values are engine specific and also varies with engine temperature. In wamr-up both limits are higher. This shut-off causes well known saw-tooth behaviour if idle contact is closed and bypass air (e.g. from a auxiliary-air device staying open) increases engine speed. First rpm increase to upper limit, then injectos are switched off. Then engine speed decreases immediately to lower limit when injectors are switched back on. This happens in a typical 1 to 2 second interval calledsaw-tooth behaviour. Some vendors like Opel had problems with transition from shut-off to restarting injection or feared that cylinder walls would not be properly cooled without fuel and have therefore decided to omit over-run shut-off.

2.9 Engine wiring harness

Engine cablingwiring connects sensors, injectors and ECU. For EMC reasons those cables shall not be too close to ignition cables. Motor cabling is always specific for both engine and car model and sometimes even year of make. Depending on car model, sensors and ECU even for same engine can be differently located. And year of make can influence whether a 4 or 5 pin throttle switch is used. Cable isolation can start to break after 40 or more years and create terrible intermittant short cuts, false currents or high contact resistance. Those problems are more common than defetive ECUs or MAP sensors.

2.10 Diagnosis

Few D-Jetronic cars (e.g. MB as of 74) had a diagnosis connector. And if it was only to monitor ignition. This however does not help today's "mechanics". D-Jetronic does neither have self-diagnosis nor does it recognize non-operating or wrongly connected components. Whatever sensors deliver is used for calculation, be it infinite resistance of a boken temperature sensor, a broken cable or be it e.g. VW sensors installed on a Renault. Diagnosis only works by using your head and systematically testing all components, that's why it was also called "Denker-Jetronic" (Thinker-Jetronic).


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