The surge tank

Monday, December 02, 2013

Tags :  induction-system

Innovation is the lifeblood of motorsport – or used to be. These days though, with the introduction of all manner of spec formulae with single supplier engines, the emphasis would seem to be more on the sport element than that of the motor, and anything even slightly outside the ‘spirit’ of the regulations (whatever that is) is deemed unacceptable. But it wasn’t always that way. Even as recently as 10 years ago, major factory teams would take a somewhat liberal interpretation of the regulations, and one example, if not well publicised even after it was eventually banned, is that of the surge tank.

It was all to do with the introduction of the turbocharger and the efforts to maintain or even increase the levels of intake manifold pressure when the engine was off-throttle. At the time, turbocharger anti-lag systems were in their infancy. Sometimes referred to as ‘bang-bang’ systems, these were designed to keep the turbocharger spinning and producing boost, even when the throttle was closed. However, being strangled by the regulation 34 mm air restrictor into the compressor under WRC rules, the performance of the engine was restricted even at wide-open throttle.

At the time, the Ford rally team realised that while it couldn’t flow more air when the engine was on full throttle, at part-throttle – which is a large part of the time in a rally car – an opportunity was being lost to get the air through the restrictor for use later. If this extra air could somehow be stored temporarily in a useful form and then released again when the engine was able to use it, a modest increase in engine power could result.

Ingenious or not but certainly within the rules, the 2003 Ford Focus rally car featured some interesting additions. Hidden behind the rear bumper was a carefully fabricated, 45 litre, 2 mm thick titanium tank, linked to the intake manifold by 4 m of 30 mm diameter titanium piping running the entire length of the car. Weighing in at somewhere near 20 kg, the system was connected to the rest of the engine system by something that was euphemistically called the ‘idle control valve’. This electronically controlled butterfly valve would flow large amounts of air when required yet still operate at the low flows of idle air demand when required, as specified in the homologation papers of the time.

With all air passing through the 34 mm restrictor in all its various aspects, the system was deemed fully legal. But using a highly aggressive anti-lag ‘bang-bang’ system as soon as the post-aftercooler air pressure was greater than the tank pressure (and the throttle closed), air would pass into the tank. When these pressures equalised, the valve would close again, only to be re-opened when full throttle was demanded to deliver a higher intake manifold boost pressure and an approximate 5% increase in power over the more usual arrangement.

Suitable only for asphalt rally stages where maximum grip and driver commitment were needed, the effectiveness of the arrangement saw increased engine performance which gradually fell on longer special stages. Although not the dominant effect the team had hoped it might be, to avoid all other teams developing similar – and, let’s be honest, rather fanciful – systems, after three rallies it is little wonder the system was banned from WRC.

It was an interesting and innovative approach to the use of the airbox, but even though a part of me understands the reason it was banned, the other half can’t help worrying that the decision to actually do so undermines one of the major strengths of the sport.

Fig. 1 - Titanium surge tank mounted across and behind the rear bumper

Written by John Coxon

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