Improved atomization is just part of the equation. High operating pressure combined with powerful engine management electronics allow precisely manipulating the shape and location of the fuel charge in the chamber. This is accomplished by making multiple injections in the combustion chamber every intake cycle, and varying fuel pressure anywhere between 200-2,200 psi. In other words, direct-injection fuel systems have the ability to administer fuel in multiple spurts at any point between TDC on the intake stroke and TDC on the compression stroke. A standard port fuel injection system, on the other hand, can only introduce fuel into the chamber when the intake valve is open, which is dependent on the profile of the cam lobes. "By injecting fuel directly into the combustion chamber and under high pressure, the fuel can be directed to exactly where we want it to be for a given combustion cycle," Hinds adds.
So how exactly does this translate into improved fuel economy and power? It's all in direct-injection's ability to separate the combustion chamber into separate zones. At idle and under part-throttle, the fuel charge is stratified, which means that it's concentrated near the spark plug. Conversely, the layer of air surrounding this zone is extremely lean. The result is an air/fuel mixture that's lean overall, yet stoichiometrically ideal near the spark plug. Under part-throttle operation and at idle, this reduces fuel consumption by up to 40 percent. As throttle input and load increases, a direct-injection system transitions to a homogeneous fuel charge, in which fuel is dispersed evenly throughout the entire combustion chamber. In addition to reducing emissions during cold starts by warming up the catalytic converters more quickly, the leaner overall air/fuel mixture boosts torque by about five percent. "The EcoBoost engine is constantly controlling the rail pressure to a target level based on what the customer is demanding from the engine," explains Todd Rumpsa, EcoBoost calibration supervisor. "As the driver increases demands on the engine, higher rail pressures maintain optimized combustion performance by delivering the best atomization of fuel and precise injection duration." For extra peace of mind, the Bosch engine management system used in the EcoBoost can retard timing up to 20 degrees on the fly, which negates the need for costlier forged rods and pistons.