The Mulsanne straight at Le Mans, where the leading competitors reach speeds of over 200mph each lap during 24 hours of non-stop racing, may seem an unlikely proving ground for a new breed of highly economical petrol engines. But, true to Audi's famous 'Vorsprung durch Technik' philosophy, Audi FSI technology played its part in three consecutive Le Mans victories for the all-conquering Audi Infineon R8 racers before making its UK debut in the more familiar surroundings of the public roads.
Le Mans is, of course, hardly an economy run. But for the teams who enter the annual contest of speed and endurance it still pays dividends to have an efficient engine, one that can make the most effective use of every drop of petrol, thus reducing the number of pit stops a car has to make. And the same goal - fuel efficiency - makes FSI technology highly attractive to the company car driver, too.
Today, Audi FSI engines combine good performance, low emissions and excellent economy. The 2-litre, 4-cylinder A4 2.0 FSI, for example, has a top speed of 136mph and can cover the 0-62mph dash in just 9.6 seconds. And yet CO2 emissions of just 180 g/km put it into the 19% BIK band. Most impressive of all are the economy figures: 28.5mpg on the urban cycle, 52.3mpg on the extra-urban and a combined figure of 39.7 mpg are the kind of figures a diesel would be proud to claim.
FSI? In essence it's a form of direct petrol injection, and it has been hailed as the most significant step forward for the petrol engine since fuel injection itself replaced the carburettor. But it is also a piece of intelligent thinking, about the time-hallowed principles of creating and igniting the fuel-air mixture inside the combustion chamber, in particular when running under part-load or with the throttle closed.
Most engines work on the principle that an even – or 'homogenous' – mixture of fuel and air, at the correct proportion, is the ideal for combustion. And so it is, especially if full power is required, for example when accelerating. But how much of the time are we accelerating? What happens under a trailing throttle, for example when slowing for a junction or holding station in traffic? Audi's engineers discovered that under those conditions having a homogenous fuel-air mixture could be wasting energy, and thus fuel.
At such times the FSI engine switches to something called a 'stratified' charge, where the injector releases a precisely-timed and directed smaller charge of fuel, creating a variable mixture of fuel and air that is at its richest around the spark plug, for instantaneous combustion, but that becomes leaner further away from the spark. Result? Enough power for the situation and a clean, highly efficient burn. In simple terms, this means that the driver has all the power he or she needs, whenever it is needed, but that the engine smoothly and seamlessly switches to a leaner burn mode whenever the car is in a steady-state cruise or is decelerating.
At the heart of the FSI engine is an Audi-developed fuel injection system, which is able to control, precisely, the timing and duration of each charge. This calls for common rail injection with a range of operating pressures from 40 to 110bar so that the amount of fuel and timing of each injection can be accurately delivered. A tumble flap in the intake manifold, to create a healthy vortex of swirling air as it enters the combustion chamber, and a specially-shaped piston crown together create the ideal combustion conditions for both homogenous and stratified mode.
Unlike other methods of combining 'small engine' economy with 'big engine' performance, the switch between homogenous and stratified FSI modes is all-but-undetectable by the driver, with each press of the throttle pedal being met with crisp, instantaneous response. Currently, Audi's FSI engines range from the 1.6 FSI offered for the A2 through to an all-new 3.2 litre FSI engine that may be specified for the new A6. In each case the Audi FSI engine has all the refinement and performance that the most demanding driver could wish for, whilst offering frugal economy and low Co2 emission figures. Although FSI is designed to run efficiently on ultra-low sulphur unleaded petrol, which is available throughout the UK, use of the new environmentally-friendly sulphur-free fuel will provide further bonuses in both fuel economy and 'lean burn' performance.
There is, of course, one further Audi engine that features FSI, but it is not available to the company car buyer, and nor would it impress around the M25 during rush hour. The Audi UK R8, fresh from its triumph at the 2004 Sebring 12 hours, will be attempting to claim the top step of the Le Mans podium for a 4th time this June, powered as before by a twin-turbocharged Audi V8 FSI engine producing around 610PS. Thanks to Vorsprung durch Technik, their victory could be your gain.