Subaru EE20 Diesel Engine

Subaru's EE20 engine was a 2.0-litre horizontally-opposed (or 'boxer') four-cylinder turbo-diesel engine. For Australia, the EE20 diesel engine was first offered in the Subaru BR Outback in 2009 and subsequently powered the Subaru SH Forester, SJ Forester and BS Outback. The EE20 diesel engine underwent substantial changes in 2014 to comply with Euro 6 emissions standards – these changes are discussed below and summarised under ‘Euro 6 changes’.

Please note that this article considers the EE20 engine as it was supplied in Australian-delivered vehicles. As such, it does not consider the Euro 4 emissions compliant EE20 engines that were available in Europe. Furthermore, specifications for other markets may differ from those in Australia.

The EE20 engine was manufactured on the same assembly line as Subaru's six-cylinder horizontally opposed petrol engines at its Oizumi factory.

The EE20 engine had an aluminium alloy block with 86.0 mm bores and an 86.0 mm stroke for a capacity of 1998 cc. For its Euro 4 and Euro 5 versions, the EE20 engine had a semi-closed block (i.e. the cylinders bores were attached to the outer case at the 12, 3, 6 and 9 o’clock positions) for greater rigidity around the head gasket. For the Euro 6 EE20 engine, however, an open deck design was adopted which eliminated the 12 and 6 o’clock supports.

For the EE20 engine, all five main bearings in the cylinder block had metal matrix composite journals (inserted during the cast process) for rigidity and due to their similar thermal expansion to the crankshaft. Furthermore, cooling slits between the cylinder bores provided water cooling channels.

For comparative purposes, dimensions of Subaru’s EE20, EJ20 and EZ30 engines are given in the table below.

To withstand the high combustion pressures of a diesel engine, the crankshaft for the EE20 engine was subjected to a surface treatment for increased strength. Furthermore, the crankshaft journals were made from aluminium and cast iron due to the high pressure applied on both side of the cylinder block.

The forged connecting rods had fracture split bearings for the crank end and an asymmetrical profile which increased precision during assembly. The pistons had internal cooling channels, while oil jets in the crankcase sprayed the underside of the pistons. The EE20 engine had an aluminium alloy cylinder head that was 17 mm thinner than the EJ20 engine. Furthermore, the intake ports and the diameter of the intake valves were designed to create a swirling effect for the air as it entered the combustion chamber.

The EE20 engine had double overhead camshafts (DOHC) per cylinder bank that were driven by a chain and gear with a speed-reducing gear. The four valves per cylinder (two intake and two exhaust) were actuated by pivot-type roller rocker arms. The EE20 engines have IHI turbochargers with variable nozzle turbines (VNTs). Generally, VNTs use movable vanes in the turbine housing to adjust the air-flow to the turbine to realise comparable exhaust gas velocity and back pressure throughout the engine’s rev range. To enhance torque at engine speeds below 1800 rpm, the nozzle vanes would close to narrow the air path and increase the speed of the air flow. At higher engine speeds, however, the vanes would open to reduce airflow resistance and improve fuel consumption.

Initially, the turbocharger was positioned under the engine. For the Euro 6 EE20 engine, it is understood that the turbocharger was relocated to the bottom right of the engine. It is understood that the maximum turbine speed for the IHI turbochargers used in the EE20 engine is 190,000 rpm. The Euro 4 and Euro 5 EE20 diesel engines had a Denso common-rail injection system with eight-hole, solenoid-type injectors that achieved an injection pressure of 180 MPa. For the Euro 6 EE20 engine, however, injection pressure was increased to 200 MPa. For the EE20 engine, the injectors were positioned at an almost 90 degree angle to the cylinder and were 40-50 mm shorter than those used in inline four-cylinder diesel engines.

The Euro 5 and Euro 6 EE20 engines are understood to have ceramic-type glow plugs. The EE20 diesel engine had a water-cooled exhaust gas recirculation (EGR) system which recirculated exhaust gases to the intake to lower combustion temperatures and reduce NOx emissions.

The Euro 5 and Euro 6 EE20 engines had a closed-loop diesel particulate filter (DPF); both the oxidation catalyst and DPF were positioned next to the turbocharger to utilise the heat of the exhaust air. The alternator for the EE20 diesel engine had a voltage charging control system which, to reduce the alternator’s load on the engine, reduced the charging voltage when the vehicle was idling or being driven at a constant speed and increased voltage at low speeds. The Euro 6 emissions compliant EE20 diesel engine was introduced in the Subaru BS Outback in 2014 and the Subaru SJ.II Forester in 2015. Relative to the Euro 5 version, changes for the Euro 6 EE20 engine included:

• An open deck cylinder block;
• An increase in piston crown capacity;
• A new piston skirt coating was introduced to reduce friction;
• A reduction in the compression ratio to 15.2:1 to lower combustion temperature and reduce NOx emissions;
• A fourth generation common rail injection system was introduced for higher injection pressure (200 MPa, previously 180 MPa) and a finer fuel spray;
• Each diesel injector had an integrated driver unit to reduce fuel leak volume, fuel pump load and improve fuel economy;
• A low-friction timing chain was introduced to drive the fuel pump (previously gear-driven) for quieter operation;
• The glow plugs were revised to improve pre-heating temperature at start-up and increase after-glow time;
• Oil jets were added to the timing chain drive;
• A low-pressure EGR circuit was introduced to increase the EGR rate, while the high-pressure EGR circuit was ‘optimised’;
• The turbocharger repositioned at the bottom right of the engine (previously under the engine) and improved vane control was achieved;
• The diesel particulate filter (DPF) substrate specifications were revised and regeneration performance enhanced. The type and amount of precious metals in the oxidation catalyser and DPF catalyst were also revised;
• The number of idlers used in the auxiliary belt system was reduced;
• A more precise sensor measured battery current, voltage and temperature; and,
• The rear flange and bracket material, exhaust pipe and end plate material were changed for rust prevention.