Keith Seume examines the origins of PDK gearbox technology
If manufacturers had their way, double-clutch technology is the future, but certain Porsche Group C cars have used the system since the mid-1980s. However, PDK is nothing new – indeed, as Keith Seume discovered, the first patents were granted as far back as 1939…
The PDK transmission (that's short for 'Porsche Doppelkupplung', by the way – or, to put it in Queen's English, 'Porsche double clutch') was first introduced on a production model in 2008 on the second generation 997-series 911. This slick system replaced the ageing Tiptronic semi-automatic gearbox, Porsche claiming that not only was the PDK capable of shifting some 60 per cent faster than the Tiptronic, but it also helped to reduce fuel consumption thanks to its more efficient design.
To fully appreciate the benefits (or otherwise, as some might argue) of PDK, let's take a step back into history, for the DNA of this amazing design can be traced back far further than you might imagine. Forget for a moment the Tiptronics of the 1990s and Sportomatics of the 1970s, even though they were both important steps in the evolution of the modern 911. Instead, cast your mind back (or do a quick 'Google') to the days of pre-war cars like Armstrong-siddeleys and Daimlers. Or doubledecker buses and lumbering lorries… Seriously. They have more in common with the modern Porsche than you might at first realise.
The thread which connects the past with the present is the concept of the preselector gearbox. It's almost unheard of now outside the commercial vehicle and vintage car scenes but is a very clever idea. Vehicles with preselector gearboxes allowed the driver to select the next gear (either a higher or lower ratio) which was engaged only when the left-hand pedal was depressed. Note we don't call it the 'clutch pedal' as cars with preselector gearboxes use centrifugal clutches, which engage with rising engine speed.
In a similar fashion, with a Pdk-equipped Porsche the transmission is always in a state of readiness to almost instantly engage the next ratio, awaiting only for a signal from the driver (using the paddle shift) or the engine electronics. There is no clutch pedal. But we'll come back to that in a while.
Driving a car equipped with a preselector gearbox is a unique experience. To start the car, first you must make sure the 'gear lever' – usually nothing more than a spindly lever on the dashboard or steering column – is in the neutral position. Start the engine and then move the lever to 'first'. Nothing will happen until you depress the lefthand pedal and release it – that engages first gear. Now,
using the throttle, you bring the engine revs up to the point where the centrifugal clutch bites and the car moves off.
As soon as the car is underway, move the gear lever into second and, when you're ready, simply lift off the throttle and depress/release the left-hand pedal again. And that's it. Continue the process until you're in top gear, at which point you move the gear lever into the next lower ratio ready for when you need to change down a gear. If this sounds all very ponderous – for example, when struggling up a steep hill – never fear: you could effect the change of ratios without lifting off the throttle. Oh, and somewhat scarily, you can also move the lever into the reverse gear position when travelling forwards in anticipation of backing into a parking space…
Although never intended as a sporting option, the concept of 'having the next gear ready', so to speak, would clearly be of advantage to a driver who wished to press on without having to worry about grabbing a gear lever midway through a series of twist and turns.
In 1939, when preselector gearboxes were popular, a German engineer by the name of Adolphe Kégresse tested a new transmission in a Citroên 'Traction', a design which he felt would make driving easier, dispensing with the need for manual gear changes. Kegresse was born in 1879 in France but moved to Russia in 1905 to work for Tsar Nicholas II. There he developed the 'Kegresse track' a half-track conversion for conventional cars, allowing them to be driven in mud and snow.
On his return to France in 1919, Kégresse began work with Citroên but left after a brief few years to concentrate on developing his own gearbox: the twin-clutch Autoserve transmission. Pre-war manual gearboxes tended to be rather agricultural in operation, requiring drivers to carefully synchronise road- and engine speed to prevent clashes between gear teeth. Kégresse's patented design was ingenious, compact and efficient, and proved satisfactory in operation in his Citroên 'guinea pig'. However, the onset of hostilities brought a premature halt to his work, and Kégresse sadly passed away in 1943 at the age of 64.
He had already filed a patent in 1939 and further patents were submitted posthumously in 1946, and granted in 1951: 'The search for automatic operation of change-speed transmissions as applied to motor cars has led, in some systems, to connecting the engine to the gear trains by means of two independent clutches forming a unit and mounted on the same axis by means of two concentric shafts, as in French Patent No. 861,394 of 28th July 1939 in particular. An arrangement is thus obtained in which some of the shifts, the even numbered shifts for example, are taken on one of the clutches, and the odd numbered shifts on the other.'
The essence of Porsche's current PDK design, however, is contained in the following paragraph: 'The use of two clutches on concentric shafts enables a more compact transmission to be obtained with shorter shafts and having less parts than in the usual construction.'
Over the next decades, several related patents were applied for by other companies, many of which made direct reference to Kégresse's Autoserve design. Dodge in the USA (1950) and Panhard & Levassor in France (in 1957) both tipped their hats to the Frenchman, as did Zanhradfabrik Friedrichshafen AG, also in 1957. If that name is not immediately familiar, the initial letters will be: ZF. In fact, the list of patent applicants who made reference to Kégresse's design reads like a who's who of the motor industry – and right up until as recently as July