1NZ-FXE Toyota engine

Introduction

Toyota’s 1NZ-FXE was a 1.5-litre four-cylinder petrol engine that was first introduced in the XW10 Prius. Toyota described the 1NZ-FXE engine as having an ‘Atkinson cycle’ since the compression stroke was shortened and the expansion stroke made longer.

A member of Toyota’s ‘NZ’ engine family, the 1NZ-FXE was closely related to the Otto cycle 1NZ-FE engine. Compared to the 1NZ-FE, however, the 1NZ-FXE engine had a maximum engine speed of 4500 rpm – this allowed the engine to have ‘mild’ valve timing, a smaller diameter crankshaft, lighter moving parts, less tension in the piston rings, reduced valve spring load and smaller bearings.

The 1NZ-FXE had a service weight of 87.1 kg.

1NZ-FXE block

The 1497 cc 1NZ-FXE engine had an aluminium alloy cylinder block with 75.0 mm bores and an 84.7 mm stroke. The 1NZ-FXE engine had thin-walled cast iron liners – which enabled a distance between the bores of 8 mm – with ‘spiny type’ construction in that their casting exteriors formed large, irregular surfaces for better adhesion with the cylinder block. This enhanced adhesion improved heat dissipation and reduced heat deformation of the cylinder bores.

The 1NZ-FXE block contained a water pump swirl chamber and an inlet passage, while the rear portion of the block had a conical shape to improve coupling rigidity with the transaxle. Since the rear oil seal was pressed into the 1NZ-FXE block, the rear oil seal retainer was omitted for compact packaging.

Crankshaft, connecting rods and pistons

The 1NZ-FXE engine had an offset crankshaft that shifted the bore centre 12 mm towards the intake (relative to the crankshaft centre). As a result, the side force when maximum pressure was applied was reduced and, according to Toyota, fuel economy was improved by between one and three per cent. Furthermore, the crankshaft had five journals, four balance weights and an integrated crankshaft position sensor.

The forged steel connecting rods for the 1NZ-FXE engine had caps that were held by plastic region tightening bolts. The aluminium alloy pistons had semi-floating type piston pins and low-tension rings.

Cylinder head

The 1NZ-FXE engine had an aluminium alloy cylinder head. To maintain uniform temperature for the combustion chamber wall, the cylinder head contained a water jacket that was positioned between the exhaust port and spark plug boss.

The 1NZ-FXE engine had double overhead camshafts (DOHC or Toyota’s ‘Twin Cam’) that were driven by a roller timing chain which had an 8.0 mm pitch; the timing chain was lubricated by engine oil from an oil jet. To reduce engine noise and frictional losses, the chain drive included a chain tensioner, chain tension arm and chain guide. The ratchet-type chain tensioner used a spring and oil pressure to maintain chain tension and suppress noise generated by the chain.

A single-piece, aluminium die-cast timing chain cover sealed the front portion of the cylinder block and the cylinder head. For servicing of the chain tensioner, the timing chain cover included an access hole.

Atkinson cycle

Toyota described the 1NZ-FXE engine as having an ‘Atkinson cycle’ since the compression stroke was shortened and the expansion stroke longer. 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 intake losses. Furthermore, closure of the intake valves was delayed until the end of the expansion stroke, thereby increasing the expansion ratio.

In a conventional (Otto) cycle engine, the compression stroke volume and expansion stroke volume were practically identical, such that the compression ratio and expansion ratio were also identical. Furthermore, any attempt to increase the expansion ratio also increased the compression ratio, and hence the likelihood of engine knock or pre-ignition.

Valves

The 1NZ-FXE engine had four valves per cylinder – two intake and two exhaust – that had a 33.5 degree included valve angle and were actuated directly by the camshafts. The valve lifters had shimless valve adjustment such that adjustment of valve clearance required the appropriate valve lifters to be replaced.

The dimensions of the 1NZ-FXE valves were as follows –

• Intake valve face diameter: 30.5 mm;
• Intake valve stem diameter: 5.0 mm;
• Exhaust valve face diameter: 25.5 mm; and,
• Exhaust valve stem diameter: 5.0 mm.

Variable Valve Timing – intelligent (VVT-i)

For the 1NZ-FXE engine, the ECU could calculate optimal valve timing from engine speed, vehicle speed, mass air flow, throttle position and water temperature inputs, and control the intake camshaft within a range of 43 degrees (of crankshaft angle).

As per the table below, valve overlap for the 1NZ-FXE engine ranged from 20 degrees to -23 degrees (a range of 43 degrees). Furthermore, the 1NZ-FXE engine had intake duration of 270 degrees and exhaust duration of 216 degrees.

The VVT-i controller was installed on the front of the intake camshaft and consisted of:

• The housing driven from the timing chain; and,
• The vane coupled with the intake camshaft.

The camshaft timing oil control valve controlled the position of the spool valve according to signals from the ECU – this enabled hydraulic pressure to be applied to the VVT-i controller advance or retard side and cause rotation in the VVT-i controller vane circumferential direction. The ECU used signals from the camshaft position sensor and crankshaft position sensor to detect actual valve timing, thus providing feedback control to achieve the intended timing.

When the engine was stopped, the intake camshaft would be in its most retarded pressure for easy starting. When hydraulic pressure was not applied to the VVT-i controller immediately after the engine was started, the lock pin prevented movement of the VVT-i controller.

Intake and throttle

The 1NZ-FXE had an aluminium intake manifold – unlike the 1NZ-FE which had a plastic intake manifold – and vertical intake ports. The 1NZ-FXE engine had a linkless-type throttle body and electronic throttle control (Toyota’s ‘Electronic Throttle Control System – intelligent’, ETCS-i) whereby the throttle valve opening was controlled according to accelerator pedal effort and engine conditions. The ETCS-i also controlled the Idle Air Control (IAC) system and cruise control system.

Injection and ignition

The 1NZ-FXE engine had sequential fuel injection via 12-hole injectors that were fitted in the intake port in the cylinder head. The 1NZ-FXE engine had pentroof-type combustion chambers with a slanted (oblique) squish design to improve thermal efficiency and reduce the chance of engine knock (pre-ignition). The squish angle was shaped obliquely along the wall surface of the combustion chamber, to improve airflow, promote swirl and speed flame travel.

The 1NZ-FXE operated at a compression ratio of 13.0:1 for the Prius and 13.4:1 for the Prius c.

The 1NZ-FXE engine featured Toyota’s distributorless ‘Direct Ignition System’ in which there was one ignition coil for each cylinder and the spark plug caps were integrated with an ignition coil. The 1NZ-FXE engine used long-reach iridium-tipped spark plugs to increase the thickness of the surrounding cylinder head and so that the water jacket could be extended near the combustion chamber. The iridium-tipped spark plugs had 100,000 kilometre replacement intervals.

With Toyota’s ‘Electronic Spark Advance’ (ESA), ignition timing was determined by the ECU based on inputs from sensors.

Exhaust and emissions

The 1NZ-FXE engine had a long branch-type stainless steel exhaust manifold to improve torque at low to medium engine speeds. Furthermore, the 1NZ-FXE engine had a rearward exhaust layout and double-wall construction for the exhaust pipe which connected to the three-way catalytic converter (TWC) – this contributed to faster warm-up of the TWC.

Ball joints were used for coupling the exhaust manifold to the front pipe and the exhaust pipe to the main muffler. The muffler included a spring control valve that could vary the length and resistance of exhaust gas flow. The valve opened steplessly when exhaust gas pressure overcame spring pressure. As a result, the valve would be closed at lower engine speeds to reduce exhaust noise and open at higher engine speeds to reduce back pressure and increase power output.

To reduce emissions, the 1NZ-FXE engine had a returnless fuel system to reduce evaporative emissions. To reduce emissions, the 1NZ-FXE engine had:

• A returnless fuel system to reduce evaporative emissions; and,
• An On-Board Refueling Vapor Recovery (ORVR) system that used a charcoal canister to recover fuel vapour that was generated during refuelling and reduce the discharge of fuel vapour into the atmosphere.

Prius c updates

For the Prius c, the following changes were made to the 1NZ-FXE engine:

• A higher 13.4:1 compression ratio;
• Introduction of a one-piece ‘maniverter’ exhaust system combined the manifold and catalytic converter for faster warm-up of the catalytic converter, mass reduction and more compact packaging;
• Belt-less operation of the water pump and air conditioning compressor;
• Introduction of water-cooled exhaust gas recirculation;
• Fitment of a low-friction cam chain; and,
• Improved cooling efficiency which included an air guide to direct air from the grille through the radiator.