2AR-FXE Toyota engine

Introduction

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

• Atkinson cycle operation; and,
• A unique cylinder head, camshafts, pistons and fuel supply system.

To reduce fuel consumption, the 2AR-FXE used electric motors to power ancillaries such as the water pump, variable air-conditioner compressor and electric power-assisted steering, thereby removing the need for an accessories belt. The key difference between the 2AR-FXE and 2AR-FSE engines was that the latter had direct fuel injection in addition to conventional port injection.

For Australia, the 2AR-FXE engine was used in the Toyota XV50 Camry Hybrid, Lexus XV60 ES 300h and Lexus NX 300h.

2AR-FXE block

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

Cylinder head

The 2AR-FXE 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-FXE 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-FXE 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-FXE 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-FXE 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-FXE 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.

Toyota described the 2AR-FXE 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.

Toyota quoted the 2AR-FXE engine as having a compression ratio of 12.5:1.

Intake

Like the 2AR-FE, the 2AR-FXE engine had a long-port plastic intake manifold to reduce induction noise and increase performance. The 2AR-FXE engine had 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-FXE engine had L-type sequential multiport fuel injection (SFI) system with a hot-wire type mass air flow meter to measure intake air density. The 2AR-FXE engine used long nozzle-type fuel injectors, each of which had twelve-holes. To reduce evaporative emissions, the 2AR-FXE engine had a returnless fuel system used to reduce evaporative emissions.

The 2AR-FXE engine had a ‘Direct Ignition System’ (DIS) in which there was one ignition coil (with igniter) for each cylinder. Furthermore, the 2AR-FXE 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-FXE engine had a long branch, stainless steel exhaust manifold and a three-way catalytic converter.

The 2AR-FXE 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-FXE engine, the EGR system included a step-motor and a water cooler to reduce gas temperatures.