Direct Fuel Injection (DFI)
Standard on the Cayenne V-engines since 2007, Porsche is now moving over to Direct Fuel Injection (DFI) for the flat-six engines in the 911 series. Forming exactly the right mixture of fuel and air directly in the combustion chamber, this sophisticated technology serves to develop more horsepower and torque from less fuel..
As well as offering lower running costs, the direct fuel injection engines fitted in the 911 are also more powerful than the previous generation power units. Maximum output of the 911 Carrera 3.6-litre is up by 20 bhp to 345 bhp (254 kW). Output of the 911 Carrera S 3.8-litre power unit is equally impressive, up by 30 bhp to 385 bhp (283 kW). With this extra power, the Carrera S Coup now offers a top speed of slightly over 300 km/h or 188 mph.
The driver benefits from a further advantage of direct injection every time he moves the throttle pedal: with fuel being injected fractions of a second prior to combustion, the engines respond even more directly and spontaneously to even the slightest movement of the driver’s right foot. This is not only the case when accelerating, but also when lifting off the throttle, for engine speed drops more quickly and smoothly since there is no residual fuel left in the intake manifold which might otherwise prolong the combustion process.
Direct fuel injection also serves to ensure ‘homogeneous’ operation of the power unit with a consistent balance of the fuel/air mixture in the combustion chamber at all times and under all running conditions. Such homogeneous operation guarantees optimum combustion, maintaining consistently low emissions, without making any particular requirements in terms of fuel quality. Depending on engine load and speed, fuel is injected into the combustion chamber at a pressure of 120 bar. The big advantage is that unlike conventional intake manifold injection, direct fuel injection serves to form the fuel/air mixture directly in the combustion chamber. This better mixes the air and fuel in the cylinder, establishing an important prerequisite for clean and complete combustion.
With the evaporation of fuel reducing the temperature in the combustion chamber, the amount of air drawn in may be increased accordingly, helping to improve the cylinder charge and the level of engine power. At the same time, the cooler fuel/air mixture allows a higher compression ratio in the interests of greater efficiency and extra power from less fuel. Thus, the combustion chambers in both the 3.6-litre and 3.8-litre engines now have a compression ratio of 12.5:1, as opposed to 11.3:1 on the 3.6-litre and 11.8:1 on the 3.8-litre to date.