D-Jetronic History and Fundamentals


The D-Jetronic system developed by Bosch in the early 1960's was the first mass-production electronic fuel injection system. It was primarily based on patents that Bosch licensed from the Bendix corporation. Bendix developed the basic idea of using an inductive element coupled to manifold vacuum as a component in a loop circuit ("multivibrator") to develop the basic injection pulse width. The system was first used on the 1967 VW Type 3 motors. Bosch continued development of the system, and it was last used in the D-Jetronic form in about 1976. Variants of D-Jetronic were used by other manufacturers (Ford, Toyota, etc.) for many years, and various forms of "speed-density" injection systems similar to D-Jetronic are still in use today. Bosch developed many more types of electronic fuel injection after D-Jetronic (L-Jetronic, K-Jetronic, etc.) that had improved characteristics, and still is a dominant force in fuel injection systems today.

D-Jetronic References

Two papers were published in the Bosch Technical Journal that gave overall descriptions of the D-Jetronic system, components, and operation. I have recently secured these documents from the Bosch archivist and there are links below to the PDF files. Both articles are in German. I have done a rough translation of the Scholl article that is in text file format. The system as described in the Scholl article is very similar to the implementation of D-Jetronic on the Porsche 914 1.7 and 2.0L motors.

"An Electronic Fuel Injection System for Automobiles", Von Gunther Baumann, Bosch Technical Journal, 1967.

"Electronic Fuel Injection - Jetronic", Von Hermann Scholl, Bosch Technical Journal, 1969 (part 1)
"Electronic Fuel Injection - Jetronic", Von Hermann Scholl, Bosch Technical Journal, 1969 (part 2)
English Translation

D-Jetronic Patents

Thanks to Dirk Wright, 914 owner and USPTO employee, I have a good list of fundamental D-Jetronic related patents, and I've found quite a few using those as a starting point. Here are links to the patents at the USPTO web site, along with a short description of the gist of each patent. NOTE - you may have to load the TIFF file applet to be able to view and print the patents, click on any of the patents below and follow the "Help" link for more information. If anyone can dig up patents on other D-Jetronic components, please let me know and I'll include the numbers here.

UPDATE: The USPTO has changed its URL format since I first wrote this document. To avoid issues in the future with changing URL's, I'm only going to list the base URL for the USPTO search page below:


Copy the patent number from below that you want to see, then click the link above. Enter the number, then click on the "Images" button at the top of the page to see the patent....

"Speed-Density" Fuel Injection

NOTE: The discussion here is limited to constant fuel pressure, multi-port, pulsed injection systems.

The primary function of any injection system is to control the mass air/fuel (A/F) ratio of a specific engine over all expected operating conditions. The desired A/F value for a specific operating condition depends on the optimization of fuel economy, performance, emissions, and other factors. The parameter that is to be controlled is the mass of fuel to be injected, so the problem is to determine the mass of the air in the cylinder. A "speed-density" system does this by measuring the density of air in the intake manifold (via a pressure or vacuum sensor), correcting for the pumping efficiency of the engine (which is a function of engine speed). The result gives the density of air in the cylinder. If the cylinder volume is known, then the mass of the air in the cylinder can be determined. Since the fuel pressure and flow rate of the injector is also known and constant, the exact pulse duration to deliver a fuel mass needed to produce the desired A/F value can be determined.

In the D-Jetronic system, the manifold pressure sensor senses the intake manifold vacuum, and engine speed is sensed by the trigger contact points in the base of the distributor. The contact trigger points also determine the timing of the injection pulses. D-Jetronic is a grouped injection system, where half of the injectors are in a group that are pulsed simultaneously. In four-cylinder implementations, there are two cylinders per group. There is one injection pulse per power cycle. With grouped injection, one cylinder gets the injection pulse right before the intake valve opens, the other cylinder gets the injection pulse about 180 degrees before the intake valve opens.

Below is a plot of data taken on a 2.0L Porsche 914 D-Jetronic system under simulated operating temperature, engine load, and engine speed conditions using an EFI Model 1401 Electronic Analyzer, a D-Jetronic tester:

The speed-density nature of the system is obvious from this plot. As engine load increases (decreasing manifold vacuum), the injection pulse width increases (the "density" part). At a constant load, injection pulse width varies with the pumping efficiency of the motor (the "speed" part).

The total pulse duration described above determines the "basic injection quantity", or Tb. Other factors must be accounted for to determine the actual injection pulse, Tinj. These factors include:

  1. Air Temperature: The density of air is a function of temperature and the basic injection quantity must be corrected for the effect. In the D-Jetronic system, the TS1 sensor is a negative-temperature-coefficient resistor (resistance goes down as temperature goes up), mounted on the intake plenum. The ECU uses the value of the TS1 resistance to correct the injection quantity for the effect of temperature.
  2. Acceleration: Due to the finite response time of the pressure sensor and the ECU, and the inertia of the air mass in the intake manifold, there is a delay between the opening of the throttle and the response of the system to the need for added fuel. To reduce this response time, a separate acceleration system is required for good engine response. D-Jetronic uses a set of contacts in the throttle position switch to provide both immediate injection pulses and a temporary acceleration enrichment effect when the throttle is opened.
  3. Idle: For good transition when accelerating from a stop, low emissions, and for smooth and stable idle while powering accessories (e.g. lights, cooling fan, a/c, etc.), control over the idle mixture is required. In D-Jetronic, an idle switch on the throttle position switch sends a signal to the ECU when the throttle is closed. A separate idle mixture control circuit in the ECU sets the idle mixture, which is adjustable via an external potentiometer.
  4. Cold Starting: When the engine is cold, additional fuel is required for starting due to poor mixing and condensation of the A/F mixture on cold intake, cylinder, and piston surfaces. The ECU senses the cold start with the engine temperature sensor, TS2, and provides additional fuel when cranking. At temperatures below 32 deg. F ("Cold-Cold Start") , a separate thermo-switch and cold-start valve (CSV) in the intake manifold are activated. This injector sprays a fine mist of fuel into the intake system that mixes better with the cold air than the injector spray, and helps make cold starting easier. Some later D-Jetronic systems used a thermo-time switch which limited the duration the CSV would remain open, to prevent flooding and spark plug fouling.
  5. Warm-Up Transition: When the engine is below its normal operating temperature, more fuel is needed to account for condensation and incomplete mixing until the engine is fully warmed up. Engine temperature is sensed by the TS2 sensor. The ECU has a warm-up enrichment circuit that senses the resistance of the TS2 sensor and corrects the mixture for the engine temperature. When the value of TS2 drops below a threshold value (typically 300 ohms), the warm-up circuit in the ECU has a cut-off characteristic, so that no additional decrease in TS2's resistance will affect the mixture (i.e. the engine is fully warmed-up).
  6. Over-Run: When the throttle is closed while the car is moving and in gear, there is a high vacuum in the intake manifold while the engine is at a fairly high speed. For lower emissions, early D-Jet system shut off the fuel supply until the engine had dropped below some target speed (1300 rpm or so). Over time, this was found to increase HC emissions when the fuel supply was restarted, due to cooling of the cylinder walls, and the over-run circuit was deleted from the ECU. Surprisingly, later cars (1975-1976 2.0L) have ECU's where this circuit was restored, possibly to reduce unburned fuel from being passed through the exhaust to the catalytic converter, which can cause the temperature of the converter to rise to dangerous levels. On some cars, an air bypass valve (deceleration valve) was incorporated that passes additional air past the throttle plate when closed, to provide better combustion during over-run.
  7. Full-Load: When the engine is under full-load (wide open throttle) conditions, maximum power is required and an enriched mixture is needed. Additionally, air cooled engines under full-load conditions need the additional fuel for cooling. A full-load diaphragm is incorporated into the manifold pressure sensor to enrich the mixture under full-load conditions. This diaphragm is activated when the pressure differential between the intake manifold and the atmospheric pressure drops below 100 mTorr. The diaphragm moves more per unit pressure difference than the part-load aneroid cells, causing an abrupt increase in the injection duration.

For a detailed description of how the ECU operates and accommodates the various operation conditions, see my ECU web page. For a detailed description of how the manifold pressure sensor responds to part-load and full-load conditions, see my manifold pressure sensor web page.

D-Jet System Engineering - My Conjectures...

Bosch used D-Jetronic for many different applications. Here are a few conjectures of mine as to how they made the system manufacturable by using standard components and customization:


Both of these issues are specific to air-cooled D-Jetronic applications (VW and Porsche)

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