FB20 Subaru Engine
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Subaru FB20 Engine

Introduction

Subaru's FB20 was a 2.0-litre horizontally-opposed (or 'boxer') four-cylinder petrol engine. Effectively replacing the EJ204 engine, the FB20 engine was a member of Subaru's third generation 'FB' boxer engine family which also included the FB25, FA20D, FA20E and FA20F engines. The FB20 engine first offered in Australia in 2012 Subaru GP/GJ Impreza.

Key features of the FB20 engine included its:
  • Aluminium alloy cylinder block with open-deck design;
  • Aluminium alloy cylinder head with double overhead camshafts;
  • Four valves per cylinder actuated by roller rocker arms;
  • Subaru’s dual ‘Active Valve Control System’ for variable intake and exhaust valve timing;
  • Auto Start Stop function for engine shutdown when stationary in traffic;
  • Compression ratio of 10.5:1; and,
  • Cooled exhaust gas recirculation.

The FB engines were manufactured at Subaru's Gunma Oizumi factory which was developed exclusively for the production of the FB engine. Please note that this article considers the FB20 engine as it was supplied in Australian-delivered vehicles; specifications for other markets may vary.
Subaru FB20 engine
Model Engine Trans. Power Torque Years
Subaru GP/GJ Impreza 2.0-litre petrol F4 6sp man.,
6sp CVT
110kW at 6200rpm 196Nm at 4200rpm 2012-on
Subaru GP XV 2.0-litre petrol F4 6sp man.,
6sp CVT
110kW at 6200rpm 196Nm at 4200rpm 2012-on
Subaru SJ Forester 2.0-litre petrol F4 6sp CVT 110kW at 6200rpm 198Nm at 4200rpm 2013-on

FB20 block

The FB20 engine had an open-deck, aluminium alloy block with 84.0 mm bores and a 90.0 mm stroke for a capacity of 1995 cc; within the cylinder bores, the FB20 engine had cast iron liners. Due to its revised connecting rods and valvetrain components, the FB20 block was the same size as its EJ204 predecessor, despite its smaller bore and longer stroke. According to Subaru, the longer stroke improved fuel efficiency by enabling faster air induction and reduced unburnt fuel during cold starts.

The FB20 engine had separate cooling circuits for the cylinder block and head to improve coolant distribution. Around the block, the flow rate was limited to maintain a high temperature for the cylinder liner oil, thereby reducing friction from the motion of the pistons.

Connecting rods and pistons

To reduce engine width and enable its longer stroke, the FB20 engine had asymmetrical, diagonally-split connecting rods. Compared to its EJ predecessor, the FB20 engine achieved an approximate 30 per cent reduction in frictional losses due to its lighter connecting rods, lighter pistons and wrist pins, and lower piston-ring tension.

Intake

The FB20 engine had a plastic intake manifold with reshaped branches to reduce pressure losses and thereby increase power; the valves were also revised to reduce drops in pressure when open and to increase tumbling when closed. Upstream of each intake port, a metal partition acted as a tumble generator valve (TGV) to increase air tumble and create vortices within the combustion chamber. Furthermore, the size and shape of the resonators in the intake manifold were streamlined.

Cylinder head

The FB20 engine had an aluminium alloy cylinder head with separately cast camshaft carriers so that cores in the cylinder head could be omitted for a reduction in metal thickness. As noted above, cylinder head cooling was enhanced by using separate circuits for the cylinder block and head.

Camshafts and dual AVCS

The FB20 engine had double overhead camshafts that were driven by a maintenance-free chain. For the FB20 engine, a chain drive was adopted because it enabled a narrower included valve angle and a reduction in the sprocket diameters of the crank and camshaft for reduced width. The four valves per cylinder were actuated by roller rocker arms (previously valve lifters for the EJ253).

The FB20 engine had Subaru’s dual Active Valve Control System (AVCS) which provided variable intake and exhaust valve timing.

Injection and ignition

Whereas the fuel injectors for the EJ204 engine were in the intake manifold, the fuel injectors for the FB20 engine were moved to the cylinder head. According to Subaru, positioning the injectors in the cylinder head enhanced the flow of atomised fuel, thereby improving fuel efficiency and reducing exhaust gas emissions.

The FB20 engine had coil-on-plug ignition with an integrated igniter for each cylinder. The spark plug caps, which provided contact to the spark plugs, were integrated with the ignition coil assembly. For the FB20 engine, cooling around the spark plugs was improved for a higher knock limit, increased power and improved fuel efficiency.

Exhaust

For the FB20 engine, the diameters and lengths of the exhaust manifold were modified to improve catalytic converter warm-up and increase power output. Furthermore, the more free-flowing exhaust system contributed to higher power output.

The FB20’s exhaust gas recirculation (EGR) system included a cooling circuit that enabled greater exhaust gas volumes to be recirculated than occurred in the EJ engines. By using EGR, combustion temperatures were reduced such that the engine was less susceptible to knock and injection timing could be advanced.
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