The Impreza 2.0D. Click to enlarge.

Peak torque is 350 N·m (258 lb-ft) between 1800 and 2400 rpm. The boxer diesel is matched to a six-speed manual gearbox.

Subaru has more than 40 years of experience with the horizontally-opposed ‘boxer’ engine layout. The design enables a light, compact unit which provides the strength necessary for the added stresses of diesel combustion, while reducing vibration, noise and internal friction compared to an in-line engine. There’s also the added advantage of a low center-of gravity. A liquid-filled engine mounting system ensures even less vibration and better handling performance.

The flat-four layout with its horizontally-opposed pistons effectively cancels out the second harmonic vibration (vibration at double the frequency of the engine speed) which means there is no need for balancer shafts, which in turn reduces inertia and friction. A compact crankshaft sandwiched by the highly rigid cylinder blocks helps minimize vibrational noise.

Compared to Subaru’s 2.0-liter gasoline engine, the Boxer Diesel has an 11 mm longer stroke and 6 mm smaller bore for a capacity of 1,998 cc versus 1,994 cc. Cylinder bore spacing is also reduced in comparison—98.4 mm, down from 113 mm—which means the engine block length is only 353.5 mm compared to the gasoline engine’s already compact 414.8 mm.

The good rigidity has allowed the blocks to be constructed from aluminium alloy for reduced weight. The block design uses the semi-closed deck type that has proven its durability in the turbocharged gasoline engine, while all five main bearings use metal composite supports for superior refinement due to their higher rigidity and thermal expansion properties similar to that of the crankshaft.

The 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.

The new boxer diesel uses an advanced common-rail fuel-injection system with the fuel being pressurized at 180 MPa (1800 bar). The variable nozzle turbocharger ensures the correct boost at all engine speeds; placed beneath the engine, it contributes to the Impreza’s low center of gravity, where it also connects directly to the catalytic converters.

The Impreza Boxer Diesel meets Euro 4 emission standards with an oxidation catalytic converter, a particulate filter and exhaust gas recirculation.