When Ford engineers embarked on the EcoBoost program six years ago, they plucked the 3.5L Duratec V-6 out of the corporate engine lineup to use as a starting point. The naturally aspirated Duratec is plenty capable in its own right, putting out 265 hp and 250 lb-ft. Despite the extra 100 hp that the EcoBoost was projected to produce, only minor revisions were necessary. The EcoBoost retains the Duratec's 3.64x3.41-inch bore and stroke dimensions, but the block has been fortified with additional reinforcement ribs. Likewise, Ford beefed up the crank and rods by using an alloy with greater nickel and chromium content. The pistons, while cast, feature a low-friction coating on their skirts, and are cooled by oil squirters positioned in the crankcase. Furthermore, the cylinder heads were fitted with 1.456/1.220-inch Inconel valves, and modified to facilitate fitment of the high-pressure fuel injectors. In addition to a slight drop in compression ratio, multi-layer steel head gaskets have been added to help seal the chambers under boost.
Of course, the EcoBoost's impressive output wouldn't be possible without the twin Garret 48mm turbos mounted on each bank of cylinders. Their wastegates are controlled electronically, allowing precisely regulating boost between 8-12 psi for an extremely flat torque curve and a linear power band. Additionally, to reduce mass Ford opted for stamped-steel exhaust manifolds instead of cast iron. The insulating effect of the twin-wall stamped manifolds help retain heat in the turbos for improved thermal efficiency and quicker spool up.
When running down the 3.5L EcoBoost's spec sheet, some of the most surprising revelations are its 10.0:1 static compression ratio and ability to run on 87-octane fuel. Most hot rodders wouldn't dare to run that much compression in a naturally aspirated carb motor with iron heads, let alone a forced-induction application pushing 12 psi of boost. Likewise, the absence of forged rods and pistons is sure to raise some eyebrows. What makes it all possible is gasoline direct injection, and despite what that vague description may imply, it has far more to do with the entire fuel delivery and management process than the placement of the injectors themselves.
Gasoline direct injection draws heavily from advancements in modern common-rail diesel technology. As with a diesel, fuel is drawn from the tank at conventional pressure until it reaches a cam-driven injection pump positioned just before the fuel rails. On the EcoBoost, the injection pump is mounted on the driver side cylinder head, and boosts fuel pressure up to 2,200 psi. Depending on the needs of the engine, an electronically operated piston on the pump can reduce rail pressure down to 200 psi. While the operating pressure of a gasoline direct-injection system is substantially less than the 26,000 psi of a diesel, it's still quite a bit more than the 40-60 psi at which most late-model EFI systems function. When combined with injectors positioned in the combustion chamber, right below the intake valves, the result is a very fine spray pattern exactly where you need it. "By bringing the fuel injector right into the combustion chamber, the air is cooled right where you're going to burn it," says EcoBoost Design Manager Brett Hinds. "This action both improves the breathing of the engine and minimizes knocking. Since the fuel is directly introduced into the combustion chamber, you don't get fuel wetting of the combustion wall like with port fuel injection, you don't saturate the ports, and you don't get droplets that might recombine and add to saturation."
In a direct-injection engine,...
In a direct-injection engine, precisely directing a stratified fuel charge near the spark plug is critical. This enables running a locally stratified air/fuel mixture near the plug under light load, while leaning the air/fuel mixture surrounding the plug. The contours of the EcoBoost's pistons help accomplish just that.
As engine load and rpm increases,...
As engine load and rpm increases, a direct-injection engine transitions from a stratified fuel charge in the chamber to an evenly distributed homogeneous mixture. This precise fuel control is why the EcoBoost can run just fine on 87 octane, despite 12 psi of boost pressure and a 10.0:1 compression ratio.
A direct-injection EFI system...
A direct-injection EFI system is very similar in principle to common-rail diesel technology. Fuel is first pumped to the motor at conventional pressure levels, then bumped up to nearly 2,200 psi by an injection pump mounted on the driver side cylinder head. The pump is driven off of a separate lobe integrated into the intake camshaft, and uses an electronically operated piston to vary pressure between 200 and 2,200 psi, depending on engine load.