The Subaru Forester | Part Two (2008)

Subaru Forester

Subaru Forester

Dedicated to the horizontally-opposed 'boxer' engine layout for over 40 years, Subaru engineers were determined to use this format for their first diesel engine.

The reasons were to create a light, compact, strong unit with reduced vibration, noise and internal friction compared to an in-line engine and for an ultra-low centre-of gravity.

This enhances handling agility and road holding - especially when combined with Subaru's renowned symmetrical all-wheel drive system. The entire engine and transmission are not only mounted very low down but also within the wheelbase which further boosts chassis dynamics. And with a minimum 45kg weight increase over the 2.0 litre petrol engine, the boxer diesel actually lowers the centre-of-gravity.

The 2.0 litre Boxer Diesel has a class-leading light weight, with the horizontally-opposed pistons effectively cancelling out the secondary harmonic vibration (vibration with twice the frequencies of the engine speed).

The crankshaft is short and highly rigid, minimising the vibrational noise of many diesel engines. And the superior balance of the horizontally-opposed engine means internal rotational inertia and friction are drastically reduced compared with an in-line engine.

Reduced internal friction means throttle response is exceptionally lively and in keeping with Subaru's sporty heritage.

Thanks to a bore pitch shortened from 113mm to 98.4mm, the overall engine block length is only 353.5mm compared to the petrol's already compact 414.8mm.

The block is of a rigid semi-closed deck design, while all five main bearings in the alloy cylinder block use metal composite supports for added strength and durability. It also enhances refinement, providing a similar thermal expansion to that of the crankshaft.

Extra water-cooling slits have been added between the cylinder bores, and the high-strength pistons are squirted with oil to enhance cooling. The large big ends of the connecting rods feature an asymmetrical profile for assembly precision and reduced friction.

The high-strength crankshaft has a special surface treatment to withstand the diesel engine's high combustion pressures while the uprated cylinder heads have roller rocker arms. In addition, the cam-drive is via a chain system, that better handles the variations in torque of a diesel engine.

Published 19 November 2008 Staff

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