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In Focus: Ti-VCT GDI Engine

New for the MkIII Focus is the Ti-VCT (Twin independent Variable Cam Timing) GDI (Gasoline Direct Injection) engine. While this isn't the first Focus with variable cam timing, it's the first one with such on both camshafts, independently modulated. Gasoline Direct Injection has been around for quite a while, especially in diesel engines, but this technology is also new to Ford's entry-level cars.

With traditional four-cycle engines, the camshafts are linked to the crankshaft by the timing belt or timing chain, and they usually operate at exactly half the speed of the crankshaft, because each four-stroke cycle takes two revolutions of the engine to complete. Ti-VCT allows both the intake and exhaust camshafts several degrees of advance or retardation away from the rigid timing of non-variable cams. This can allow the engine to run smoother at idle, achieve higher fuel economy under cruising conditions and higher torque under heavy acceleration.

As the name suggests, GDI injects gasoline directly into the combustion chamber as opposed to creating an air/fuel mixture ahead of the intake valves. This has some fascinating advantages. As the fuel is injected close to the spark plug, specially-shaped pistons can be used to create a small combustion chamber. Fuel can also be injected into the engine at any point in the combustion cycle and on a much more accurate basis. The engine can safely operate very lean under low loads, and richer as needed.

A side effect to GDI that I'll be keeping my eye out for is excessive carbon build-up in the intake port and on the intake valves. The PCV system pulls crankcase gases (such as exhaust that slips past the piston rings and fumes from oil) into the intake manifold. Similarly, EGR recirculates unburnt exhaust into the intake manifold as well. These fumes are usually brought into the engine in the company of the air/fuel mixture, which can act like a solvent against the faces of the intake valves. In GDI engines, this process doesn't occur, and the hot hydrocarbon fumes are thrown into the relatively cool intake ports on the head where they are prone to condense and build-up. This phenomenon has been seen on GDI engines from other manufacturers. We'll have to wait and see what transpires with my own Focus over the coming years.

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