2AR-FSE Lexus engine

Introduction

The Lexus 2AR-FSE was a 2.5-litre inline four-cylinder petrol engine. Based on Toyota’s 2AR-FE engine, the 2AR-FSE differed in that it had:

• Atkinson cycle operation;
• Direct fuel injection (in addition to conventional port injection); and,
• A unique cylinder head, camshafts, pistons and fuel supply system.

According to Lexus, the 2AR-FSE engine achieved thermal efficiency of 38.5 per cent.

The 2AR-FSE was used to power the Lexus RC 300h, Lexus L10 GS 300h and Lexus XE30 IS 300h.

2AR-FSE block

The 2AR-FSE had an aluminium alloy cylinder block with 90.0 mm bores and a 98.0 mm stroke for a capacity of 2494 cc. With the cylinder bores, the 2AR-FSE had ‘spiny-type’ liners which were manufactured so that the casting exteriors of the liners had irregular surfaces for better adhesion between the liners and the cylinder block.

The 2AR-FSE block also contained:

• Oil jets for cooling and lubricating the pistons and bores;
• Water passages between the cylinder bores so that coolant could flow and keep the temperature of the cylinder walls uniform; and,
• A shallow bottom water jacket used to reduce the volume of engine coolant for faster warm-up. The water jacket included a water jacket spacer to suppress water flow in the bottom of the water jackets and guide the coolant in the upper area of the water jacket for uniform temperature distribution. As a result, the viscosity of the engine oil that acted as a lubricant between the bore walls and the pistons was lowered.

Within the crankcase, the 2AR-FSE contained:

• Two balance shafts to counteract the secondary inertial forces that were generated twice for each rotation of the crankshaft. The crankshaft had a balance shaft drive gear that was used to drive the no.1 balance shaft, while the no.2 balance shaft was gear-driven from the no. 1 balance shaft. To cancel secondary inertial forces, the balance shafts rotated twice for each rotation of the crankshaft to generate inertial force in the opposite direction. To cancel the inertial force generated by the balance shaft itself, the balance shaft consisted of two shafts rotating in opposite directions;
• Blowby gas passages with an oil separator that would separate oil from the blowby gas to reduce oil degradation and consumption; and,
• Oil drain passages to prevent the crankshaft from mixing the engine oil (reducing rotational resistance).

Crankshaft, connecting rods and pistons

The crankshaft for the 2AR-FSE engine was made of micro-alloyed steel, had five journals and eight balance weights. To reduce lateral forces to the cylinder wall, the crankshaft was offset to move the bore centre 10 mm towards the exhaust side (relative to the crankshaft centre). Other attributes of the crankshaft included:

• A micro-grooved lining surface for an optimal amount of oil clearance; and,
• Narrow crankshaft bearings with eccentric oil grooves to reduce the amount of oil leakage from the bearing.

The connecting rods and caps were made of micro-alloyed steel; to reduce mass, the connecting rods used plastic region tightening bolts. Like the crankshaft, the connecting rod bearings had micro-grooved lining surfaces and a narrow width to reduce friction.

The 2AR-FSE engine had aluminium alloy pistons with resin coated skirts and low tension piston rings.

Cylinder head

The 2AR-FSE engine had an aluminium cylinder head in which the camshaft housing (cam journal portion) was separated from the cylinder head. The cylinder head was affixed to the block with a triple-layer metal type cylinder head gasket the surface of which was coated with fluoro rubber.

The 2AR-FSE engine had a magnesium alloy die-cast cylinder head cover which contained an oil delivery pipe for lubrication of the sliding parts of the roller rocker arm.

Camshafts and roller rockers

The intake and exhaust camshafts for the 2AR-FSE engine were driven by a 9.525 mm roller chain. The timing chain was lubricated by an oil jet, while the chain tensioner – a ratchet type with a non-return mechanism – used a spring and oil pressure to maintain chain tension. The 2AR-FSE engine had roller rocker arms with built-in needle bearings that reduced friction between the cams and the roller rocker arms (which actuated the valves down).

Dual VVT-i

For the 2AR-FSE engine, the ‘dual variable valve timing with intelligence’ (Toyota’s ‘Dual VVT-i) system controlled the intake and exhaust camshafts to vary valve timing according to operating conditions.

Each VVT-i controller consisted of a housing that was driven by the timing chain and a vane that was coupled with the intake or exhaust camshaft. Both the intake and exhaust sides had four-blade vane-type actuators. The camshaft timing oil control valve controlled the spool valve using duty cycle control from the engine control module (ECM) – this allowed hydraulic pressure to be applied to the advanced or retarded side of the VVT-i controller, causing rotation in the VVT-i controller vane circumferential direction to vary intake and exhaust valve timing. Once target timing was attained, valve timing was held by keeping the camshaft timing oil control valve in its neutral position.

The 2AR-FSE engine had 36.5 mm diameter intake valves and 31 mm diameter exhaust valves.

Atkinson cycle

In a conventional (Otto) cycle engine, compression stroke volume and expansion stroke volume are practically identical, such that the compression ratio and expansion ratio are also identical. Hence, any attempt to increase the expansion ratio also increases the compression ratio, and hence the likelihood of engine knock or pre-ignition. Please note that:

• Expansion ratio = (expansion stroke volume + combustion chamber volume)/combustion chamber volume; and,
• Compression ratio = (compression stroke volume + combustion chamber volume)/combustion chamber volume.

Lexus described the 2AR-FSE engine as having an ‘Atkinson cycle’ since the compression stroke was shortened and the expansion stroke extended. This was achieved by keeping the intake valves open during the initial stage of the compression stroke (when the piston was ascending) to allow a reverse flow of intake air into the intake manifold – this allowed for an increase in throttle valve opening in part load conditions, thereby reducing intake manifold vacuum and pumping losses. Furthermore, closure of the intake valves was delayed until the end of the expansion stroke, thereby increasing the expansion ratio.

Since this ‘Atkinson’ operation used a smaller portion of the compression stroke to compress the intake air, it did not take in as much air as a comparable Otto cycle engine and had lower power density, yet higher thermal efficiency.

Lexus quoted the 2AR-FSE engine as having a compression ratio of 13.0:1.

Intake

The 2AR-FSE engine had a plastic intake manifold, a linkless-type throttle body and Toyota’s ‘Electronic Throttle Control System – intelligent’ (ETCS-i) which controlled throttle valve opening according to with the amount of accelerator pedal effort and the condition of the engine.

Injection and ignition

The 2AR-FSE engine had electronic fuel injection (EFI) via port injectors or direct injectors. The port injectors operated when the engine was idling and on low driving loads. The direct injectors operated at higher engine speeds and loads to increase power output; for the direct injectors, maximum injection pressure was 208 MPa.

The 2AR-FSE engine had a ‘Direct Ignition System’ (DIS) in which there was one ignition coil (with igniter) for each cylinder. Furthermore, the 2AR-FSE engine had ‘Electronic Spark Advance’ (ESA) which determined ignition timing according to inputs from sensors, including a wave-form analysis type knock sensor.

Exhaust and emissions

The 2AR-FSE engine had a stainless steel exhaust manifold and an integrated three-way catalytic converter.

The 2AR-FSE engine had an exhaust gas recirculation (EGR) system which recirculated a portion of the exhaust gas back into the intake air to slow down combustion in the cylinder and lower the combustion temperature – this, in turn, reduced the amount of NOx emissions. For the 2AR-FSE engine, the EGR system included a step-motor and a water cooler to reduce gas temperatures.