Schwarz, Stumpp, Knapp: "K-Jetronic", original German article, ATZ No. 11 (November 1973)

In this document

This is the original article in German published in the journal Automobiltechnische Zeitschrift (ATZ), 75th year, No. 11, November 1973 pp. 93-98, signed by Bosch engineers Reinhard Schwarz, Gerhard Stumpp and Heinrich Knapp, titled "K-Jetronic — kontinuierliche Benzineinspritzung von Bosch" p. 93. It is "Teil 1" (part 1) of an article announced in several installments ("wird fortgesetzt", p. 98).

Introduction and genesis of the system

The text indicates that the development of the system began in 1967, with the objective of finding a simple mechanical successor to the in-line pump, while retaining the advantage of nascent electronic injection: ease of installation without mechanical drive p. 93. The authors recall Bosch's previous experience with the quasi-continuous injection of the Mercedes-Benz 220 SE p. 93 and explain the choice of a continuous air measurement rather than the Venturi principle, which they deemed incapable of handling the required 1:30 flow range p. 93. The initial code name for the project was "KA-System" ("kontinuierlich" and "antriebslos"), p. 93.

General principle

The system combines an air flow meter with a flap (Luftmesser) and a fuel distributor (Mengenteiler), connected by a single lever forming the "mixture regulator" p. 94. The general diagram Bild 1 p. 94 numbers 31 components, including the vane pump (15), the filter (17), the pressure regulator (19), the injectors (20), the warm-up regulator (22) and the auxiliary air valve (27).

Air flow meter (Luftmesser)

Based on the suspended body principle (Schwebekörperprinzip): a shutter flap moves in a cone, with the aerodynamic force balancing a constant hydraulic counter-force pp. 94-95. The fundamental equation (1)-(5) is developed on p. 95, leading to a linear relationship between the air flow Qₗ and the flap travel y. The air force is indicated at approximately 3 N, the pressure drop at 10³ N/m², corresponding to an engine filling loss of approximately 1% p. 95.

Fuel distributor (Mengenteiler)

Based on rectangular control slots (8) each associated with a differential pressure valve (10) with a metal diaphragm (11), maintaining a constant pressure difference regardless of the flow rate pp. 95-96. Equations (6)-(11) p. 96 show that the fuel flow Qₖ is linear with respect to the control piston travel x. The text specifies a pressure difference Δphy of approximately 0.1 bar, regulated to within ± 1.0%, with slot widths of 0.1 to 0.2 mm obtained by electrical discharge machining p. 96.

Fuel circuit

The circuit distinguishes between a supply system at approximately 5.7 bar and a control system with variable pressure between 1.5 and 4.7 bar p. 97. The roller pump (15) feeds via an accumulator (26) — which dampens noise, delays pressure build-up at start-up and maintains pressure for hot starting — and a filter (17). The injectors (20) have an opening pressure of 4.3 bar p. 97.

Cold start and warm-up correction

Two corrections are described: compensation for increased friction at idle via an auxiliary air valve (27) controlled by an electrically heated bimetallic strip (25), and mixture enrichment via a pressure control valve (22) adjusted by a spring (23) and bimetal (24) pp. 97-98. Cold starting uses an electromagnetic valve (29) and a thermal switch (31) p. 98.

Electrical diagram

Bild 10 p. 98 shows that the fuel pump (15) only operates when the engine is running, the criterion being the air flow detected by an electrical contact (30) on the flap.

The article closes with a bibliography of eight Bosch and MTZ/ATZ references p. 98, and the page continues with a separate article on trends in racing car construction, unrelated to the K-Jetronic.

Context

The K-Jetronic, developed by Bosch and described here from its complete scientific presentation, remained famous as a continuous mechanical injection system equipping many European models of the 1970s-1980s, including several Citroën and Mercedes vehicles.

Historically, it is part of the transition between carburetors, the first electronic injections (D-Jetronic, L-Jetronic) and future systems regulated by lambda sensors — mentioned in the article as a perspective for evolution towards the definitive name "K-Jetronic" p. 94.

Paul Magès, at Citroën, was closely interested in developments in carburetion and injection for his own work on engine supply, in a regulatory context of tightening anti-pollution standards in the early 1970s.

Archivist's view

This document is a first-rate technical reference piece: it is the founding article, in the original language, where the Bosch engineers themselves set out the equations and design choices of the K-Jetronic. Its presence in the Magès archives likely testifies to attentive technological monitoring of competing or complementary innovations in fuel injection.

It seems that this German text was considered important enough to be subsequently translated in full, as evidenced by the French translation of this same ATZ article kept in the same collection — a sign of studious rather than anecdotal interest.

It can also be compared to the Revue Automobile clippings on the K-Jetronic, which provided a more general public popularization of the same system, thus completing Magès' documentary file on carburetion and injection.