Engine: Description and Operation

AJ16 Engine and Systems

General Description

The engine is available as a 3.2 and 4.0 liter unit. A 4.0 liter supercharged version is also available. Fuel is supplied to each cylinder via an injector fed from a regulated fuel rail. To comply with statutory regulations in some countries and to reduce emissions during the warm-up period, secondary air is delivered to the exhaust manifold by an electrically operated air injection pump. This improves oxidation until the catalytic converters are fully effective. All engine functions are controlled by an integrated engine management system, which incorporates the on-board diagnostic system (OBDII).

Engine Design and Construction




The 4.0 litre six-cylinder engine has been enhanced to improve performance, fuel economy and operational refinement.

Now designated AJ16, the revised engine has undergone an improvement program that has resulted in an approximate 10 % increase in power and torque, and improved fuel economy.

Construction

The skirted design crankcase is manufactured in cast aluminum alloy with shrink fit dry cast iron cylinder sleeves.

The crankshaft is manufactured from cast iron for the 3.2 liter engine, forged steel for the 4.0 liter engine and is nitro-carburize treated to give a very high quality finish on the bearing surfaces and increase the life of the journals.

The crankshaft is supported by seven iron bearing caps having bearings, which are lead bronze on split steel backed shells with a lead indium overlay.

Crankshaft end-float is controlled by half thrust washers fitted on each side of the center main bearing journal. The connecting rods are manufactured from carbon manganese steel, forged in an `H' section. The small end bushes are lead bronze with steel backing, machined to size after being pressed into the connecting rods. The connecting rod bearings are of a lead bronze alloy on split steel backed shells and with lead indium overlay.

The pistons are of monometal construction (aluminum) and have a spring assisted micro-land oil control ring situated below a barrel-faced internally tapered chrome plated compression ring and an externally stepped taper-faced secondary ring.

The pistons run on hardened steel piston pins offset from the center line of the piston towards the thrust face.

The cylinder head is cast from aluminum alloy with pent-roof shaped combustion chambers with cross-flow valve porting. Running directly in the cylinder head are two cast iron camshafts retained by machined aluminum caps. Each camshaft uses chilled cams to drive two valves per cylinder via chilled cast iron bucket tappets with shim adjustment. Control of each of the four valves per cylinder is maintained by single valve springs.

The camshafts are operated by a two stage `duplex' chain drive from the crankshaft. Each stage is controlled by a hydraulic tensioner operating through a pivoted rubber-faced curved tensioner blade. The first stage incorporates a three point drive via the crankshaft, intermediate shaft and auxiliary shaft. The intermediate shaft is live and provides a 0.75 x crank speed drive through the timing cover. This drive access is blanked off. The `live' auxiliary shaft is driven at crankshaft speed and is situated on the right hand side of the engine (looking from rear). In addition to driving the engine position sensor via a set of 2:1 reduction spiral gears, it provides an external drive for the power assisted steering pump at the rear. The second stage is a three point drive via the intermediate shaft and two camshafts. The 2:1 reduction ratio from crank speed is achieved by the combined ratio of the intermediate and camshaft sprockets sizes.

The oil pump is a rotor-type mounted on the underside of the front of the crankcase and driven by a `simplex' chain from the crankshaft nose. The pump incorporates a built-in pressure relief valve. Below the line of the crankcase, but above the oil pan oil level are two windage trays; these prevent oil being sucked up and thrown into the crankcase there- by alleviating windage and power losses through oil surge.

At the rear of the crankshaft is a new design of lip-type PTFE oil seal which provides a high degree of oil retention. It also allows the use of higher engine speeds and easier serviceability.

Cylinder Head Design

The four valves per cylinder are smaller in diameter than on a conventional two valve per cylinder engine and have a greater combined effective area. They are also lighter and apply less stress to the operating gear. The design increases the power at high engine speeds and allows an efficient combustion of the fuel. It also allows the spark plug to be situated in its ideal central position which creates efficient combustion and consequently enhances fuel economy.

Crankcase Breather

Blow-by gases are recycled via the air intake system to maintain a crankcase depression and so prevent their escape to the atmosphere. A baffled vent from the camshaft cover is used for both full and part load breathing. For full load breathing, a connection is made direct to the clean side of the air filter upstream of the throttle disc. Part load breathing is provided by a spur off the full load pipe to downstream of the throttle disc via the water heated restrictor. In this way, a crankcase depression is maintained at all throttle settings.

Lubrication System

Oil is drawn from the oil pan via a gauze filter. Pressurized oil, having been regulated by a relief valve, is then fed via internal galleries on the left hand side of the cylinder block. Pressurized and filtered oil is fed into the main oil gallery, the seven main bearings are fed and thence via crankshaft drillings to the connecting rod bearings. The intermediate shaft, auxiliary shaft and camshaft bearings are pressure lubricated by means of internal drillings directly fed from the front of the main oil gallery. For some markets an oil cooler is fitted to vehicles with 4.0 liter supercharged engines.

Cooling System

The engine is liquid cooled by a mixture of water and anti-freeze circulating around the coolant passages. The coolant pump is mounted on the left hand side of the cylinder block and is driven from the crankshaft nose by a three point belt drive (which includes the generator). The pump is a fully assembled bolt-on unit. The coolant is fed into the cylinder block at two places via an external delivery pipe. The coolant is drawn from the cylinder head via a self-contained thermostat housing back to the radiator or recirculated according to the thermostat position.

Supercharger

The supercharger optionally available on the 4.0 liter engine is a Roots blower type which gives better engine efficiency at part throttle conditions and responsive off-boost performance. The principal changes to the engine for the supercharged version are to the piston, valve timing, intake manifold and the additional drive to the supercharger. A lower compression ratio of 8.5:1 is used which provides the optimum balance between high speed performance and fuel economy.

The engine heat exchanger of the intercooler system is incorporated into the intake manifold. The blower incorporates an air by-pass system which improves efficiency under part throttle conditions. The by-pass valve is controlled by a vacuum actuator referenced to the intake manifold pressure.

Valve Timing




On 4.0 liter normally aspirated engines, valve overlap is reduced from 22 to 14 degrees of crankshaft movement, to improve engine idle quality. The change in performance due to the revised timing is minimal and within the tolerances of existing power curves.

The reduced valve overlap is achieved by the introduction of camshafts with re-positioned timing gauge slots. The new camshafts retard inlet valve opening and advance exhaust valve opening by 4 degrees each. Introduction of the new camshafts began in May 1995 and they are now build standard for all Sedan and XJS normally aspirated AJ16 engines.

If required, the valve overlap of earlier 4.0 liter normally aspirated AJ16 engines can be reduced to 14 degrees using their existing camshafts (for details see Technical Bulletin 03.1-04 for Sedan, or JD 38-95 for XJS).

Exhaust Manifold Heat Shield




The heat shield now has an aluminum finish, instead of chromed, to match the other engine covers. A profile change at the rear of the heat shield accommodates the exhaust gas recirculation valve on XJR Sedans.

Engine Specifications




Specification Chart




Engine External View