By Jim Kerr
Every year, automotive journalists are presented with the latest in technologies at the AJAC (Automobile Journalists Association of Canada) annual “testfest” gathering at Shannonville raceway in Ontario. In Part 2 of my report on the contenders for the “Best New Technology” award, I’ll be examining Audi’s FSI fuel system. (See also)
FSI (fuel straight injection), more commonly known as direct fuel injection, has entered the Canadian market. Companies such as Mitsubishi and Nissan have been using direct fuel injection overseas for years, but there have always been difficulties in meeting North American emissions standards. The latest designs have overcome these difficulties and now Audi is making it available on their A6, A4 and A3 models.
Direct fuel injection is distinctly different from the port fuel injection systems we currently find on almost all gasoline-powered vehicles. Port injection sprays the fuel into the engine’s intake manifold near the back-side of the intake valve a few degrees of crankshaft rotation before the intake valve opens. As the valve opens during the intake stroke, the air/fuel mixture is drawn into the cylinder as a homogenous mixture. This mixture is then compressed when the piston rises on the compression stroke and ignited by the spark plug.
Direct injection is currently used on many diesel engines, and the gasoline direct injection uses similar principles. The injector sprays fuel directly into the cylinder. Only air enters past the intake valve. On diesel engines, the fuel starts burning as soon as it is injected due to the high heat of compressed air in the cylinder. On direct injection gasoline engines, the fuel is still ignited by the spark plug, because compression ratios in gas engines don’t compress the air high enough to generate as much heat.
3.2-litre V6 FSI. Click image to enlarge
Introduced first in the 3.2 litre V6 in Audi’s A6 sedan, this gasoline direct injection offers both increased power and fuel economy. A high (for gas engines) compression ratio of 13.2:1 is used and the fuel is injected late in the compression stroke so that it doesn’t have time to pre-ignite.
One of the problems with direct injection is there is a limited amount of time for the fuel to evaporate before it is ignited. Raw or liquid fuel doesn’t ignite, so excessive hydrocarbons can be emitted out the tailpipe. Audi has corrected this by combining the position of the
injector with the shape of the top of the piston. This creates a swirl effect that aids complete combustion. Another design feature is the use of an air valve in the 30/70 split intake manifold. This valve directs air through the 30% area at programmed operating points to provide a tumble effect for air entering the combustion chamber. Combined with
special piston head shape, the engine can operate at 16:1 air fuel ratio.
Two fuel pumps are used for the Audi FSI. A primary pump operates at 6 bar (87 PSI) and is activated when the driver’s door is opened. A second pump is operated on demand and uses a single piston pump to push fuel to a common rail that feeds all injectors. This pump can develop between 30 and 110 bar (435 to 1600 PSI) depending on engine requirements.
2.0 T FSI. Click image to enlarge
Audi is also using FSI or the A4 and A3 models. A 2.0 litre turbo engine is used (a first turbo application for FSI), with the turbo providing the intake air swirl necessary for complete combustion.
Direct injection can be used for power or economy gains. Most manufacturers strive for a combination of these. Higher compression ratios produce more power. FSI produces torque over a large rpm range, so the engine performance is improved under all driving conditions. More power output allows the manufacturer to decrease engine size while retaining the same performance – and smaller engines typically use less fuel. The direct injection fuel spray can be directed precisely at the spark plug so leaner fuel ratios can be used. This improves fuel economy too.
I didn’t drive an Audi with FSI direct injection far enough to verify any fuel economy gains, but I can tell you this new fuel system starts quickly, runs smoothly and produces lots of power. Audi’s Le Mans-winning R8 used FSI direct injection. Now you can drive the latest