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Atomizing Fuel: Continuous Injection

January 30, 2012 by Matt

Audi 4000 4K CIS K-Jetronic K-Jet CIS-E Bosch

Bosch’s proprietary continuous injection system (CIS), also known as K-Jetronic or K-Jet, is an interesting hybrid of mechanical and modern fuel injection.

First fitted to the early ’70s Porsche 911, CIS was eventually adopted by a whole host of European automakers, from Audi and VW to Volvo, Ferrari and Lotus. It functions exactly as its name would suggest: Fuel is pressurized by the pump and metered continuously to injectors near the engine’s intake ports, the flow rate controlled by a movable circular plate mounted in the intake stream, attached to the fuel distribution unit. CIS resembles mechanical fuel injection in that there’s a direct relationship between the position of the plate and the flow of fuel, but does allow for some electronic control and closed-loop O2 sensor feedback. It straddles the two methodologies, developed before mass production of fully digital fuel injection was realistic, but band-aided in its later years as a less expensive stopgap system while Bosch’s much more advanced digital Motronic system came to market.

Advantages? It’s cheap, and once properly dialed-in, very reliable. CIS’s rudimentary nature (read: lack of sensors) eliminates many potential failure points, and the basic components used to deliver fuel—the injectors and fuel distributor / air flow plate assembly—are quite robust. Compared to the carbureted systems it replaced, CIS offers the ability to meet a broader envelope of engine fueling requirements, was considerably more efficient while still being emissions-compliant, and isn’t nearly as affected by weather vagaries or other environmental factors.

CIS K-Jetronic K-Jet CIS-E Bosch Diagram Schematic Drawing Operation

Downsides? Fuel metering, while more precise than most carburetors, still isn’t as accurate as sequential common-rail port injection. Additionally, the presence of the air flow plate and fuel distribution constrain the intake path considerably. Air has to flow up through the plate and then embark on whatever twists and turns it must make in order to reach the cylinders. By contrast, the metering unit of fuel injection with a flapper-door AFM or MAF sensor can be positioned wherever it needs to be to optimize the intake path. And finally, CIS is constrained by its semi-mechanical nature, lacking flexibility in the face of ever-changing emissions and efficiency requirements.

I drove a CIS-equipped car for the better part of 3 years. After an initial pig-rich condition was sorted out by a local specialist, the car’s fuel injection system ran perfectly and required absolutely nothing of me for the remainder of our time together. CIS is unique and certainly doesn’t readily surrender its secrets, but I grew to respect and appreciate the durable nature of the system Bosch developed.

Editor’s note: This post is part of an ongoing series highlighting various obsolescent methods of fuel delivery. Read the other installments here:

Filed under: Atomizing Fuel, Technical

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