The evolution of Subaru design is represented by the FB SUBARU BOXER engine. A 2.5-liter version powers the Forester and the new 2.0-liter unit is in the 2012 Impreza. They provide improved fuel economy with reduced emissions.


Part one referred to the lightweight engine components that contribute to improving fuel economy. These are complemented by component and system designs within the engine that help reduce friction – the greatest obstacle to good fuel economy.


Here are some items that help improve fuel economy in Subaru engines.


Cylinders, Pistons, and Crankshafts



Subaru uses low-viscosity oil in its engines, which offers less resistance to engine components and requires less engine power. The low-tension piston rings throughout the engine line also require less engine power to overcome the friction between the cylinder walls and rings.


FB and 3.6-liter six-cylinder engines have smaller bores in combination with longer strokes, which help reduce friction and the amount of engine power required to function.


Cylinder Head, Valvetrain


Overall, Subaru valvetrains function with low reciprocating masses. As a result, less engine power is needed to actuate them, thereby saving fuel. The engines also feature variable valve control, which optimizes valve timing for the ideal volume of air to be drawn into the combustion chambers for increased fuel efficiency.


All Subaru engines have multi-spray fuel injectors, which distribute fuel more evenly throughout the combustion chamber for cleaner, more efficient burning. In the FB engines, the injectors are mounted in the cylinder heads rather than the intake manifolds. This reduces the amount of vaporized fuel that adheres to cylinder-head surfaces, helping to improve fuel efficiency.


FB engines differ, too, in that they have roller rockers rather than lifters, which require less engine power to actuate. Tumble generator valves create vortices within the combustion chambers of FB engines, which helps improve fuel efficiency. Plus, their chain drives have a narrower profile to help reduce friction and the amount of fuel required to operate them.


Controls and Other Engine Systems


All Subaru engines feature electronic throttle control, which saves fuel through precise measurement, and efficient intake and exhaust system designs. In addition, Exhaust Gas Recirculation (EGR) helps reduce the volume of fuel sent to the fuel injectors. (In the 2012 Impreza, the EGR system is cooled, allowing an even larger amount of gas to be redirected.) The cooling systems are designed to lower engine temperatures efficiently, resulting in less engine power required to drive the fan belt.


The new 2012 Impreza has a battery charging control system that reduces generator voltage when the engine is at idle or at a fixed speed and raises voltage when decelerating – this reduces engine load overall, helping to boost fuel economy.


While not conclusive, these features demonstrate how attention to detail makes a difference. When the designs in the new FB engine are taken as a whole, they help make it approximately 10 percent more efficient.



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