₦airaland Forum

Introduction


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 2AZ-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. The 2AZ-FXE engine had a relatively high mechanical compression ratio of 12.5:1, but the effective ratio was slightly lower.

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.
Injection and ignition
The 2AZ-FXE engine had an L-type sequential fuel injection (SFI) system which used twelve-hole injectors. The 2AZ-FXE engine had a coil-on-plug ignition system, Toyota’s ‘Direct Ignition System’ (DIS), in which the spark plug cap was integrated with the ignition coil. Furthermore, Toyota’s ‘Electronic Spark Advance’ (ESA) which selected optimal ignition timing in accordance with inputs from sensors.



2AZ-FXE block
The aluminium alloy cylinder block of the 2AZ-FXE engine had 88.5 mm bores and a 96.0 mm stroke for a capacity of 2362 cc. The 2AZ-FXE engine had cast iron liners which were manufactured so that their casting exteriors formed large, irregular surfaces (‘spiny type’) for better adhesion between the liners and the cylinder block.

Other features of the 2AZ-FXE cylinder block included:
Air passage holes in the crankshaft bearing area of the cylinder block for better air flow and reduced back pressure at the bottom of the pistons;
Water jacket spacers which suppressed water flow in the centre of the jackets and guided the coolant around the cylinder bores;
Oil filter and air conditioning compressor brackets that were integrated into the crankcase; and,
The water pump swirl chamber and thermostat housing were also integrated into the cylinder block.
Balance shafts
The 2AZ-FXE engine had twin counter rotating balance shafts that rotated twice for each rotation of the crankshaft to cancel secondary inertia forces. To cancel the inertial forces generated by the balance shaft itself, a second balance shaft rotated in the opposite direction. The drive for the no.1 balance shaft was driven via a gear on the number three crankshaft counter weight. To suppress noise and vibrations, the balance shafts were driven by resin gears.
Crankshaft, connecting rods and pistons
The 2AZ-FXE engine had a forged steel crankshaft with five journals, eight balance weights and roll-finished pin and journal fillets. The 2AZ-FXE’s crankshaft was offset by 10 mm to the thrust side of the cylinder bore centre line to reduce friction.

The connecting rods and caps were made of high-strength steel. To reduce mass, nut-less type plastic region tightening bolts and narrow connecting rod bearings were used.

The 2AZ-FXE engine had aluminium alloy pistons. While the piston head had a taper squish shape, the piston skirt was coated with resin to reduce frictional losses.
Cylinder head
The 2AZ-FXE engine had an aluminium alloy cylinder head which was mounted upon a steel-laminate type head gasket; to increase the sealing surface, a shim was used around the cylinder bore. To reduce mass, the 2AZ-FXE engine had a magnesium alloy die-cast cylinder head cover.
Camshafts
The 2AZ-FXE engine had double overhead camshafts (DOHC) that were driven by a roller chain which had an 8 mm pitch and was lubricated by an oil jet. The chain tensioner used a spring and oil pressure to maintain chain tension; it also used a ratchet type non-return mechanism.
Valves and VVT-i
The 2AZ-FXE engine had four valves per cylinder (two intake and two exhaust) that were positioned such that the included valve angle was 27.5 degrees. The intake valves each had a diameter of 34.0 mm, while exhaust valve diameter was 29.5 mm. Furthermore, the 2AZ-FXE engine used shimless type valve lifters that provided a large cam contact surface.

The 2AZ-FXE engine had valve overlap that ranged from 3 degrees to 33 degrees (a range of 30 degrees), intake duration of 280 degrees and exhaust duration of 228 degrees.