The Advent of Fuel Injection

by James Kraus

Messerschmitt Me 109 fighter powered by a Mercedes-Benz V12 fed with Bosch fuel injection

World War II, and the events preceding, did much to seed the development of automotive fuel injection. The concept of injecting precise amounts of fuel into the engine, as opposed to relying on vacuum to draw in approximately the right amount always held promise. The potential of overcoming the carburettor drawbacks of sensitivity to g-forces and altitude changes increased the allure. The war sped things along.

By 1940, Italy was suffering from widespread fuel shortages due largely to the vast amounts of gasoline Mussolini sent to Spain in support of Generalísimo Francisco Franco’s Nationalists during the Spanish Civil War. Shortages intensified when export of petroleum products to Italy was banned by the League of Nations.

Further putting the squeeze on supplies was government stockpiling for the upcoming war, all but inevitable after Italy signed the Pact of Steel with Germany on 22 May, 1939. Indeed, the extreme shortages of petrol led to the adoption of ethyl-alcohol blends, and other petrol substitutes that often played havoc with carburettors.

Alfa Romeo 6C 2500 SS Type 256 Ala Spessa with Iniezione at the Mille Miglia, 1940. Corrado Millanta photo

This situation inspired experimentation with electro-magnetic fuel injection on an Alfa Romeo 6C in preparation for the 1940 Mille Miglia. This ground-breaking system, designed by Ottavio Fuscaldo of Italian aircraft manufacturer Caproni, allowed the engine to run more efficiently on a wide variety of fuel substitutes. The system introduced three key features that would form the basis for later electronically controlled systems more than a decade hence; solenoid-operated injectors, low-pressure injection into the intake ports, and a continuous-circulation fuel loop.

Alfa Romeo 2.5 litre twin-cam six with Elettrico Iniezione. The six injectors can be seen on the left, outboard of the intake camshaft cover. Corrado Millanta photo

The injected 6C was driven by Antonio Chiodi, a test pilot working for Caproni. Fueled by a mixture of ethanol and palm oil, it finished behind the carburetted works Alfa 6C 2500 SS’s, but finished ahead of all independent 6C entrants.

Less than two months following the 1940 Mille Miglia, global hostilities leading to World War II intensified as Italy declared war on Britain and France.

Mercedes-Benz DB 601 A-1 1100 hp 34-litre Inverted V12 aero engine with the first-ever Bosch petrol fuel injection system

Throughout the war, thousands of Luftwaffe pilots took to the skies in fighters and bombers powered by engines fitted with fuel injection, with most of the systems supplied by Bosch. The first Bosch injection was installed on the Mercedes-Benz DB 601 A-1 V12 powering the Messerschmitt Me 109 interceptor. As in a diesel injection system, this introduced fuel at high pressure directly into the cylinders.

Gutbrod Superior 600

In 1952, as Germany began postwar recovery in earnest, Bosch introduced an automotive version of their mechanical direct injection that made its debut in the Gutbrod Superior 600 and Goliath GP 700, both powered by 2-stroke engines.

The cars enjoyed an extra benefit as the injection system also metered lubricant into the engine from a dedicated oil tank, obviating the need for owners to mix their own 2-stroke fuel blend. A small portion of oil was combined with fuel in the injection pump to lubricate upper cylinder walls and piston rings, the rest directed to the crankcase inlet port to lubricate bearings and lower cylinder walls.

When Goliath made fuel injection available on additional models, the presence of injection was designated by an “E” suffix added to the model name. This form of automotive nomenclature would prove to outlast the company by several decades.

Goliath GP 700 E Sportcoupé

Bosch direct injection really hit its stride in 1954 when installed in the now-legendary Mercedes-Benz 300 SL. The racing 300 SLR had used the system since 1952 with great success, resulting in its installation in the road-going 300 SL and the W196 Grand Prix car. Mercedes victories at Le Mans, Mile Miglia and in Formula One provided a boost of prestige for fuel injection and proof of the advantages it could provide in terms of increased power output.

Count Wolfgang von Trips with a works Mercedes-Benz 300 SL in Sweden following the Sports Car Grand Prix at Kristianstad, 1955. Denis Jenkinson photo

The direct injection system used by Mercedes was quite expensive and operated at very high pressures. It was also designed purely as a high performance system, lacking in such road car niceties as automatic cold-start enrichment.

1957 Chevrolet Corvette

In 1956, General Motors announced an optional Ramjet mechanical constant-flow indirect port-injection developed by Rochester for the 1957 Corvette, Chevrolet and Pontiac. Like the Alfa, this system injected fuel into the cylinder head intake ports rather than directly into the cylinders. While not as efficient as direct injection, this approach allowed for significantly lower system pressures and correspondingly reduced component costs.

Another concept that reduced complexity and expense was elimination of the timing function. Rather than timing the injection of fuel to occur as needed, all eight injectors continuously sprayed a mist of fuel into the inlet ports, whether the appropriate intake valve was open or closed.

The injected 4.7 litre V8 was lauded by U.S. enthusiasts for attaining the elusive “one horsepower per cubic inch” criteria. Other productions cars of the time that met or exceeded that specific output figure were indeed an elite group: the Alfa Romeo Giulietta Veloce, Porsche Carrera, Jaguar XK-140 SE, Maserati A6GC, Ferrari 250, Chrysler 300B and the aforementioned Mercedes-Benz 300 SL.

Although the Ramjet system provided good performance and high engine output, sales were limited and production ceased in 1965. The continuous injection principle would be revived in the 1970’s in the form of Bosch K-Jetronic, but it would prove to be the final curtain bow for mechanical injection.

Concurrent with the development of Ramjet and spurred by the recent introduction of the transistor, Bendix conceived their electronic Electrojector fuel injection system that was offered to the public on the 1958 Chrysler 300D, Desoto Adventurer and Dodge Custom Royal Lancer D-500. This was a timed port-injection system that triggered injection through timing pulses generated by a signal from the distributor.

Chrysler 6.0 V8 with Bendix Electrojector system. The four injectors for the right-hand cylinder bank can be seen in the foreground

The Electrojector system took into account ambient temperature, engine temperature and altitude. Alas, electronics were still in their infancy. Transistors, having just been made available in quantity in 1952, were still very expensive, and to in an effort to rein in costs, Bendix used low quality capacitors that were prone to failure. In addition, the systems electrical connections were insufficiently sealed against moisture.

Customer complaints piled in and Electrojection was phased out in late 1958, with little more than 50 cars built with the system. To placate unhappy owners, Chrysler arranged to replace the fuel injection with twin four-throat carburettors at no charge.

Mercedes-Benz W111 220 SE

In September of 1958, Bosch introduced an indirect mechanical system for the new Mercedes-Benz 220 SE, using timed low-pressure port injection. The new system provided fuel to the engine based on RPM, temperature and atmospheric pressure. The injection allowed for 18% more power and 8% better fuel economy as compared to the twin Solex carburettors of the 220 S. Mercedes quickly adopted this new system across their top-line six-cylinder range and on the big M-100 6.3 V8 throughout the 1960’s.

Nearly a decade after its introduction, other manufacturers began emulating the Bosch indirect mechanical system. Lucas developed a version for the Triumph TR5 PI and 2500 PI. Italy’s SPICA built one for U.S.-bound Alfa Romeo 1750’s, and Kugelfischer supplied injection for the BMW 2002tii, Capri RS 2600, Lancia Flavia, Triumph 2000 and various Peugeot models. In the fall of 1968, Porsche adopted the Bosch system for the 911E and 911S.

BMW 2002 tii – the second “i” signifying the presence of Kugelfischer mechanical injection

A number of these installations were undertaken to enable engines to comply with new U.S environmental legislation that set limits on automotive exhaust emissions beginning in 1968. The more precise fuel metering offered by fuel injection often allowed engines to meet the regulations without resorting to various add-on subsystems such as engine-driven air pumps to supply oxygen to the exhaust manifold for afterburning.

While superior in most ways to carburetion, the timed mechanical systems were expensive; in particular the pump, the internals of which had to be precision manufactured within extremely close tolerances. This was necessary due to the fact the beyond providing the requisite fuel pressure, the pump determined injection timing and quantity. The real breakthrough that allowed for widespread adoption of automotive fuel injection was the dawn of the more affordable and precise electronic Bosch Jetronic.

In 1965, Robert Bosch licensed the patents for the ill-fated Bendix Electrojector system. The burgeoning electronics industry had made great strides since the late 1950’s, offering components of both greater reliability and lower cost. Bosch took full advantage of the improving technology. Their new Jetronic system debuted in the autumn of 1967 on the Volkswagen 1600 LE and TLE. In the next few years, it was offered by Citroën, Mercedes-Benz (on their new 3.5 V8), Saab, Volvo and others. VW further expanded its use, incorporating it into the new 411 LE and VW-Porsche 914.

Added to the models that had recently introduced mechanical injection, the widespread adoption of Jetronic meant that most of the world’s automakers had at least a few models in their range sporting the increasingly ubiquitous “E” (Einspritzer) or “I” (Injection) suffix.

Volkswagen 1600 TLE with Einspritzer and Fräulein

The Jetronic system housed all its discrete analogue components on a single circuit board inside a metal o-ring-sealed Electronic Control Unit (ECU), slightly smaller than a tissue box. Like the original Alfa-Caproni and later Electrojector systems, it utilized solenoid-operated injectors, low-pressure injection into the intake ports, and a continuous-circulation fuel loop.

Depending on engine configuration, the system supplied fuel in groups of two or three cylinders at a time, triggered by a second set of points in the ignition distributor. One cylinder would receive fuel as the intake valve opened, the fuel for the other grouped cylinder(s) being sprayed atop their closed intake valves. Fuel injection quantity was varied by modulating the opening time of the injectors. This was determined by the ECU based on manifold vacuum, inlet air temperature, engine temperature, engine speed, atmospheric pressure (thus offering automatic altitude correction) and throttle position.

Drivability of the Jetronic was very well received; power output was comparable to multiple carburetion, exhaust emissions were decreased and fuel consumption was drastically reduced in city driving due to complete fuel-feed cut-off on deceleration above 1200 rpm.

Citroën DS 21 Pallas with Bosch Injection Électronique

In 1969, a much improved on-demand enrichment circuit was added (analogous to a carburettor’s accelerator pump) that significantly quickened response when accelerating from low engine speeds. While seemingly a simple matter, this necessitated a second circuit board half again as large as the original, although still fitting into the original ECU housing.

Electronic control over fuel mixture would prove to be the wave of the future. While mechanical injection would make a brief comeback in the form of the elegantly simple Bosch K-Jetronic; vast strides in digital electronics and component miniaturization ultimately doomed such designs. The relatively simple Jetronic injection ECU of the 1960’s would later expand its function to encompass control over ignition timing and other parameters, eventually evolving into total electronic engine management.

11 thoughts on “The Advent of Fuel Injection

  1. JK:

    I have always wondered where the idea for fuel injection arose. And, here it is.

    I am never ceased to be amazed by the amount of information you provide in your articles. Here is another example with the same attention to detail and depth. Keep the good stuff coming! I only hope that you continue to do so–and more often.



  2. Great topic. Do you have more images or larger of the 1958 Chrysler fuel injected?

  3. Not only was mechanical fuel injection more expensive, it was also less reliable.

    In 1952, as Germany began postwar recovery in earnest, Bosch introduced an automotive version of their mechanical direct injection that made its debut in the Gutbrod Superior 600 and Goliath GP 700, both powered by 2-stroke engines. The cars enjoyed an extra benefit as the injection system also metered lubricant into the engine from a dedicated oil tank, obviating the need for owners to mix their own 2-stroke fuel blend. A portion of the oil was combined with fuel in the injection pump to lubricate the cylinders and piston rings, the rest was ported to the air intake to lubricate the crankcase.

    The main benefit of direct fuel injection is that the air is mixed with fuel after the exhaust port was closed, so none leaked out the exhaust. But at the time, fuel was much cheaper, and the smoke from a carbutretted two stroke was much less than the smog coming from factories and steam trains at the time. It was the oil injection that metered lubricant into the crankcase, and this was neccesary because the fuel did not pass through the crankcase.

    But apparently service stations used to have pre-mixed petroil available straight from the pumps when carburetted two stroke engines were more common, so even owners of carburetted two stroke engined vehicles didn’t need to mix their own blend.

    Because of this, it seems that neither the Gutbrod Superior 600 nor the Goliath GP 700 could stop at the same pump as other vehicles also powered by two stroke engines. So direct injected two stroke engines may have been less well off at that time.

    • Despite the likelihood of minor bugs accompanying the adoption of any groundbreaking new technology, I was not aware of any reliability problems with the Bosch mechanical injection. If there were any significant issues, I am sure Mercedes-Benz would have had second thoughts as to utilizing the system to power their Formula One and Le Mans victories in the early 1950s.

      Filling the cars with pre-mixed fuel, while fine in a pinch, would be at best a compromise. Most two-stroke engines were designed to run on a specific blend of petrol to oil. Some scooters ran a mix as rich as 20:1, Saab recommended 32:1, and the DKW 3=6 was best fueled at a relatively lean 40:1.

      As to pulling up to the pumps, I would think it would have been just as easy to queue up at the more numerous straight-petrol pumps used by drivers of VWs, Opels and other traditional four-stroke automobiles.

  4. I have over the years owned a number of 60’s era Mercdes (250SE, 280SE) with Bosch mechanical injection and had no issues at all, very reliable. Cold starts were much better than any carburettor alternative, power was increased and smoothly delivered. All in all very impressed with this system.

    • But I’ve heard the 300SL has to be driven fast to keep the engine clean. If the engine runs too slowly, it destroys lubrication by mixing with oil.

      • The oil dilution problem in the 300 SL was due to two issues. First, drivers in the 1950s (particularly in the U.S) had a tendency to rev the engine before turning off the ignition in the belief that the attendantly high closed-throttle vacuum would draw oil up the cylinder walls and coat the rings in oil prior to the next start-up. In such a case, the SL’s mechanically driven injection pump would keep pumping fuel directly into the cylinders until the crankshaft ceased turning. This fuel would seep down into the crankcase and land in the sump.

        Secondly, many drivers bought the car for its glamour and prestige, never driving aggressively enough (or on trips of adequate length) to warm the engine oil sufficiently for the petrol to evaporate out.

Comments are closed.