MECHANICAL ANIMAL
Porsche’s adoption of fuel injection.
As clever as the carburettor is, fuel injection offers more precise control and more efficient use of fuel. In the second part of our series looking at fuel injection systems, we take a look at Porsche’s use of mechanical fuel injection in air-cooled cars…
Carburettors, as we detailed in the previous issue of Classic Porsche, were an ingenious invention to atomise fuel and mix it in the correct ratio with air rushing into a petrol engine. They’re still in use in various applications around the globe, though not in new cars. This is chiefly because, as brilliant as carburettors are, they don’t allow for precise metering of fuel, something that became more important as economy and emissions came under the spotlight in the 1970s. Fuel injection soon took over, but it wasn’t an overnight transition — the next step on from the carburettor was mechanical fuel injection.
In fact, for performance reasons as much as efficiency, Porsche had been investigating the use of fuel injection since the 1950s. In partnership with Bosch, direct fuel injection was looked at first, primarily with the 356’s flat-four engine the focus of attention, but the technology proved heinously expensive and difficult to implement successfully, even when applied to Porsche’s racing engines. And racing is where Porsche’s mechanical fuel injection system was first used. For its first Formula One car, the Type 787 of 1961 (a major development of the better-known 718/2 racer), however, it turned not to Bosch, but to Kugelfischer. The central component, the fuel pump, is impossible to miss in the 787 engine bay, being a big chunk of metal ‘hanging’ off the front of the engine, offset to one side. There are four plastic fuel supply pipes exiting the top of the pump leading to injectors in each of the inlet ports, while the pump itself is driven by a toothed timing belt from the right-hand camshaft and operating at the same speed. Internally, there are four plungers in the pump, one for each cylinder, pressurising the fuel for their respective cylinders at, theoretically, precisely the right time.
The fuel was fed into the intake ports below the inlet trumpets and a slide throttle, while the control of the latter was mechanically connected to the pump operation — when the driver pressed the throttle pedal, it opened the throttle for air and operated the fuel pump at the same time. Fuel pressure, according to Classic Porsche contributor, Karl Ludvigsen (in his book, Excellence Was Expected), was 31bar in order to atomise the fuel. While the fuel injection system allowed the engine to idle well and reduced plug fouling at lower engine speeds because of better control over the amount of fuel injected, Porsche didn’t find any significant performance advantage. Indeed, the system was blamed for reliability issues in the manufacturer’s Grand Prix cars and cited as the cause of vapour locks in fuel lines. Moreover, the system didn’t respond favourably to changes in ambient temperature, meaning the host engine was rarely running optimally.
AROUND THE BENZ
Porsche eventually gave up on the 787 and its injection system, though a special mention should be given to Michael May, who joined Porsche from Daimler-benz, having established direct fuel injection there. Using his prior experience as the foundation for further Porsche product development, he designed a working direct injection system for the more-or-less abandoned 787’s flat-four engine by using a similar set of components to that of the port-injection installation. A four-plunger Bosch pump was installed, driven by a toothed rubber timing belt, though its operation was said to be more sophisticated than the Kugelfischer system. The fuel was fed to holes in the cylinders below the inlet valves during the compression stroke, holes protected by the piston as the power stroke began.
Along with other significant changes to the 787’s engine, May’s direct injection system contributed to a much more powerful and reliable unit. Alas, it never proved itself in competition, not least because Porsche was focused on the development of its new eight-cylinder racing engine. Tantalisingly, May’s directinjection engine was installed in the then new Type 804
Formula One car as a back-up when Porsche realised the flat-eight’s development wasn’t going to plan, but at the last moment, Porsche managed to overcome its frustration and the direct-injected engine was removed. That, so it was thought, was the end of the story — the flat-eight reverted to carburettors and, though port injection was briefly investigated for this engine, the unit had a short shelf life due to the brand’s decision to pull out of Formula One as a manufacturer after only a couple of seasons. The 1966 906 (also known as the Carrera 6) took up the mantle.
Although the 906 was initially homologated with carburettors, nine cars were fitted with prototype versions of the flat-six engine featuring mechanical fuel injection, the result of a research project, once again in collaboration with Bosch. The latter’s fuel pump featured six plungers (again, one for each cylinder) feeding pressurised fuel through plastic pipes to the six individual intake runners, located below the intake trumpets and slide throttles as before. This pump was driven at half engine speed by a toothed belt from the right-side camshaft and fuel from the tank was supplied by an electric pump, with a return system for unused fuel. A linkage between the throttle and pump turned a cam inside the pump to alter the amount of fuel to be injected, controlled by a speed-sensing governor system. This was further adjusted by a mechanism sensitive to air pressure.
Drivers confirmed throttle response was notably improved, but fuel consumption measured in the two prototype Carrera 6s entered into the 1966 Targa Florio road race was higher than if the two-litre flat-six engines had been making use of carburettors. For unrelated reasons, these cars didn’t finish the race, but Porsche persevered with the injection system and, alongside other changes, it came good for the 906s making up the works entry into the year’s 24 Hours of Le Mans. Fuel economy was usefully better, power was up and throttle response was consistent. With the podium locked out by Ford’s all-conquering, seven-litre, V8-powered GT40, fourth, fifth and sixth-place finishes confirmed the system’s reliability. It was enough to convince Porsche that mechanical fuel injection was ready for sale to customer teams running the 906, and for it to be a feature of series assembly lines.
The first production Porsche road car to be fitted with fuel injection came with the introduction of the longerwheelbase B-series
911 in 1969. Increased manufacturing costs meant not all models in the line-up received the system to start with — the rangetopping 911 S and the new 911 E (E standing for the German word, einspritzung, which translates as injection in English) were the first. At a glance, the new mechanical fuel injection system was little different to that used in the 906. It included a six-plunger Bosch fuel pump driven by a toothed belt to send pressurised fuel to the inlet runners at the correct time, but it was refined substantially, partially because it had to cope with a wide range of ambient conditions without a team of factory mechanics on hand to tweak settings. To aid packaging, the six plungers were arranged in a vee formation, while more durable metal pipes replaced the plastic tubes used by the race cars. In the interests of weightloss, the pump housing was cast in magnesium.
Other control systems were added, enhancing sophistication: enrichment of the fuel-air mixture
THE BRIDGE BETWEEN FULLY MECHANICAL FUEL INJECTION AND LATER ELECTRONIC SYSTEMS
was altered for starting, with a different setting for firing up in cold conditions. Meanwhile, when the driver took their foot off the throttle pedal, fuel flow to the engine was stopped to prevent both backfiring and unnecessary expenditure of fuel, not to mention emissions of unburned fuel through the exhaust. You see, the introduction of mechanical fuel injection to Porsche’s road cars was as much to do with lower emissions as it was performance. This focus was led by US road safety legislation, which had already ruled the higher performing 911s out of the North American sales market in 1968 because of their prohibitively high emissions. Cars that were sold in the USA had to make use of a secondary air injection system, where fresh air is injected into the exhaust in a bid to cause more complete combustion of any excess fuel emitted. Though widely adopted by mass volume manufacturers, it was an expensive installation, requiring extra pipework and a power-sapping air pump. Thanks to Porsche’s use of mechanical fuel injection, this apparatus was no longer needed.
From 1970, the evaporation of fuel from a new car sold in the USA had to be actively regulated, causing issues for carburetted cars, which usually have fuel remaining in the carburettors, effectively open to atmosphere. Fuel injected cars did not meet the same obstacle. Improved emissions and fuel economy were not the only benefits, though — interestingly, while the injection system didn’t directly increase power, it allowed for an increase in output. The 911 E’s engine, for instance, didn’t need the modest camshaft profile of ‘lesser’ models, which was designed to enable smooth idle at the expense of top-end power. The injection system resulted in a more stable idle, allowing a ‘racier’ cam to be used, resulting in more power. In the 911 S, it was possible to raise the compression ratio, too, which, in conjunction with redesigned inlet ports, also released extra power.
FIRST OF MANY
Unsurprisingly, and despite its cost, the system slowly made its way across the 911 range in most markets, until the well-known Bosch Continuous Injection System — or K-jetronic (K for kontinuierlich in German, meaning continuously in English) — took over, launching in the 1973 911 T. Porsche was, in fact, the first car maker to use this advanced new system, which served as the bridge between fully mechanical fuel injection and later electronic systems.
As the name suggests, there is continuous injection happening here, not the pulsed high-pressure injection as before. In fact, once the system is up to operational pressure, the injectors don’t open and close at all — they stay open and allow fuel into the inlet ports, right at
the backs of the inlet valves. The injectors are designed to atomise the fuel as it passes through them. Notably, K-jetronic requires much lower fuel pressure than its predecessors, operating around the five-bar mark. This is provided by the same electrical pump bringing fuel from the tank to the engine, meaning the cam-driven (and heavy) plunger pump of before is no longer necessary. Nonetheless, there’s a lot more to the K-jetronic system, as anyone who has grappled with troubleshooting it will attest. After the air is filtered, it goes through the all-important airflow sensor (a mechanical precursor to the modern-day hot film airflow meter), which is conically shaped, wider at its outlet, where it feeds the engine with air through the single throttle valve. Within the airflow sensor is a thin plate lifting higher the faster air flows through the cone shape. The position of this plate is mechanically translated to what is known as the control plunger in the attached fuel distributor (a key component in the Continuous Injection System) which then meters fuel to the injectors. The distributor also regulates system pressure.
The design of the control plunger is quite complex. Its position opens and closes metering slits to the injectors depending on a variety of conditions, and this position can also be influenced by outside factors. For example, the warm-up regulator monitors engine and ambient temperatures and adjusts the control plunger position to suit. For cold starts, there’s actually a fifth ‘injector’ that squirts extra fuel into the intake system in order to enrich the mixture. This is controlled electrically. There’s an accumulator in the system, too, which essentially holds pressure in the fuel lines when the engine isn’t running. This eases hot starts and reduces vapour lock in the lines.
There’s no doubt Porsche (and other manufacturers) needed the precise fuel metering K-jetronic allowed, enabling adherence to emissions legislation of the day, though it was a challenge to extract performance from an engine equipped with such a system. Additionally, pressure waves in the inlet could affect the movement of the airflow sensor plate, reducing its accuracy.
Moving the sensor further from the intake manifold wasn’t a great solution due to increased response delay. Instead, Porsche altered cam timing, much to the detriment of performance. Seemingly, the workaround was an increase in engine displacement to 2.7 litres, commencing in anticipation of the 1974 model year.
Porsche and Bosch gently improved the system, bringing it closer to full electronic control with each stage of development. It was in use for two decades, up to the end of 964 production, but by then, electronic fuel injection was well-established in other Porsche models. We’ll cover this in the next issue of Classic Porsche. Subscribe at bit.ly/subscp and we’ll send each magazine direct to your door with heavily discounted cover price and free UK delivery or low-cost overseas shipping.