Powertrain experts have been touting gasoline direct-injection engines for many years. As has been the case with automated cars and carbon-fiber construction, the passage of time hasn't brought this particular future any closer—until now. With its redesigned A6, Audi is introducing a 3.1-liter V-6 that will finally bring direct injection to the mainstream in America.
Diesel engines have always used direct fuel injection. That means squirting fuel, under high pressure, into an engine's cylinders rather than into the intake manifold, which is the approach used on virtually every current gasoline engine. In a diesel, the process of injecting fuel directly into the combustion chamber at the top of the compression stroke initiates and controls combustion. The modern mechanical injection pump for small diesels was perfected in 1927 by the Robert Bosch Company in Germany.
Gasoline-powered German fighter planes in World War II employed a variant of this system. Although there were some performance and fuel-economy benefits with direct injection (DI), the most important plus was the ability to provide uninterrupted power during violent maneuvers that often caused carbureted engines to sputter and hesitate.
The first mass-produced car to use fuel injection was the Mercedes 300SL Gullwing, currently celebrating its 50th anniversary. With DI, the roadgoing SL generated about 10 percent more power and consumed about 10 percent less fuel than the carbureted engine that won Le Mans in 1952.
Within a few years, fuel injection began appearing in numerous street and racing cars. However, virtually all these applications employed indirect, or what's called port, injection. This meant that fuel was sprayed into the intake manifold behind each intake valve instead of directly into the cylinders. Port injection was cheaper because the injection took place at much lower pressure, the injectors were not subject to the searing heat of the combustion chamber, and the injection-timing requirements were relaxed.
Since there was no major performance difference between those port-injection systems and DI, the port systems soon dominated and were universally in use by the late '80s because they were the only fuel-metering system that could satisfy the ever-stricter emissions regulations.
DI, however, was not totally forgotten. In the '70s, Ford and Texaco worked on a system called Proco, for "programmed combustion." The goal was to use direct injection to achieve lean combustion, which requires a much lower than normal proportion of fuel to air, which can in turn improve fuel efficiency. But there was a limited understanding of combustion in the '70s, and the electronics were primitive, too, so the work did not advance.
But by the mid-'90s, technology had progressed to a point where Mitsubishi introduced a number of direct-injection engines in Japan.
These operated in lean mode during light-throttle use and switched to stoichiometric—a balanced fuel and air mixture—at high power. The problem was that in lean mode, the NOx emissions ran a little high. Furthermore, the normal three-way catalysts that attenuate the NOx emissions don't work well when fed the exhaust from lean combustion. Therefore, this type of direct injection was only feasible on small engines in small cars.
Recently, however, a couple of direct-injection engines have made it to this country. One is the 6.0-liter V-12 in the BMW 760Li, and the other is the 3.5-liter V-6 in the Isuzu Axiom. Now comes the Audi V-6. I spoke with Axel Eiser, the man in charge of Audi's gasoline engines, to learn how it works.
Before getting into the DI details, you should know that the Audi engine, called FSI for "fuel stratified injection," is a showcase for modern engine technology. Its head and block are cast from aluminum. It has four valves per cylinder, operated by double overhead camshafts, each of which is adjustable over a 42-degree angle to optimize breathing and emissions at various rpm and throttle openings. Its valvetrain uses roller-finger followers for lower friction and greater valve lift. Its plastic intake manifold switches between runners that are short (15.6 inches) and long (27.2 inches) to enhance breathing across the rev range. The intake system even uses small retractable flaps in each intake port to increase turbulence during light loads.
The direct-injection system then takes this hardware up a notch. First, the evaporation of the tiny droplets of fuel injected directly into the cylinder (at between 450 and 1700 psi) cools the intake mixture, producing a denser charge, which means more power.
Furthermore, the cooler charge is less prone to detonation. Further detonation resistance comes from more rapid combustion, which occurs because although the overall charge mixture is stoichiometric, the charge is locally richer in the vicinity of the spark plug. This causes the mixture to light off more vigorously and progress more quickly. Faster combustion means less spark advance, which is inherently more efficient and further reduces detonation sensitivity.
The payoff is a lofty compression ratio—12.5 to 1. That's one to two points higher than that of typical engines, which often require premium fuel. Eiser promises that his engine will be happy on a diet of regular.
Higher compression extracts more energy from each droplet of fuel. Peak power for the 3123cc FSI V-6 is 255 horsepower at 6500 rpm; peak torque is 243 pound-feet at 3250 rpm. That represents 77.8 pound-feet for every liter of displacement. The comparable figure for BMW's 3.0-liter six is 71.8, 74.3 for the Infiniti G35 V-6, and 75.9 for the Porsche 911 flat-six.
Fuel-economy results for the new A6 are not yet available but are expected to be better than the previous model's, despite a slight increase in weight and substantially more power. Such efficiency makes DI attractive to automakers ranging from General Motors to Mercedes-Benz. Expect to see lots more of it in the coming years.