Overview: Infiniti’s V6 diesel

For the benefit of those blessed with youth, Infiniti appeared in the late 1980s as a luxury offshoot of Nissan. Unlike Toyota’s luxury brand Lexus, which was born around the same time, Infinity stayed away from Europe. Now that has changed.

There are three model series. The ‘G’ comprises a saloon, a coupé and a convertible on a 2850mm wheelbase. The saloon, with an overall length of 4775mm, is squarely in the premium executive class: the Mercedes E-class and BMW 5-series saloons are a little longer overall (4868mm and 4841mm respectively) and in the wheelbase (2874mm and 2888mm). The coupé (4655mm) and convertible (4660mm) also compare closely with their Mercedes E-class equivalents (both 4698mm). The ‘G’ range uses a single 3.7-litre V6 petrol engine, delivering 320PS at a rather lively 7000rpm.

With a 2900mm wheelbase and overall length of 4945mm, Infiniti’s ‘M’ saloon was designed for European tastes in ride and handling. Deliveries to customers are scheduled to kick off in September. The M is rear-wheel drive with its engine and gearbox mounted largely inside the wheelbase. Diesel and petrol versions are available; a hybrid variant will be along next year. Interestingly, the M uses anti-noise — played over the sound system — to counteract low-frequency cabin noise.

Infiniti EX 30d.

Infiniti’s EX and FX are ‘crossover’ cars. They have four-wheel drive, but their structures and running-gear are set up for road driving.

The EX is the smaller of the two models, with a wheelbase of 2800mm and an overall length of 4645mm for the diesel model. The FX is longer by 85mm in the wheelbase and 220mm overall. The same three-litre diesel and 3.7-litre petrol V6s are available in both models; the FX can also be had with a five-litre V8.

The new diesel engine

Designated V9X, Infiniti’s new V6 diesel has been developed in Europe by an engineering team from Infiniti, Renault and Nissan. It is built in Cleon in France. The unit will also be used in various models from Renault and Nissan as part of a variety of different drivetrains, and it complies with EU5 emission levels.

Development work on the V9X began in 2005. The possibility of a V8 diesel was investigated early on, but the format was soon abandoned, as it would limit the engine’s possible range of applications across the three Alliance brands.

The EX and FX models were designed initially to use only the 3.7l V6 petrol engine, and their narrow engine bay architecture called for a compact power unit. An unusual 65° V angle was adopted, allowing a more rigid block casting than a conventional 60° V6 — of which more anon — and more space inside the V for mounting the single turbocharger.

The need to achieve good block rigidity as an important key to operating refinement led not only to the unusual V angle but also to the decision to use compacted graphite iron (CGI) for the block casting in preference to aluminium. There are good reasons for this.

All V-format engines share a design weakness. There is a lot of flexing in the acute angle between the cylinders when the power-unit is under load. The use of CGI allows this area to be built stronger than would be the case with conventional cast iron or aluminium allow, without significant weight penalties. Audi was an early user of CGI in casting V engines: all of the Company’s V6 and V8 diesel blocks are made from CGI.

Although CGI is not quite as strong as ductile iron, it is up to 75 per cent. stiffer than grey iron and twice as resistant to metal fatigue. CGI is also considerably more fatigue resistant than aluminum at higher temperatures. The nodularity* of CGI — the number of graphite spheres in the metal structure — and tensile strength increase as wall-section decreases, which is very helpful in designing a block with a good strength-to-mass ratio.

* The number of graphite nodules in an iron alloy is important in determining the characteristics of the casting. The more graphite nodules, the higher the strength of the casting and the greater its ability to elongate before fracture — in other words, to resist cracking.

Improving the stiffness of a cylinder block is not just a question of refinement. Block stiffness defines the limits of the loads that can be placed on the engine: in other words, the maximum torque output that can be achieved before flexing begins to affect cylinder tolerances and engine life. Daf Trucks have been able to improve the output of a 12.6l engine from 480PS to 530PS by switching from cast iron to CGI for the block casting: and this engine is a straight-six, not a V design. The switch also saw a significant decrease in block weight — typically, weight savings are around 20 per cent.

Infiniti FX 30d.

Other measures taken to improve the running refinement of the Infiniti diesel include direct bolting of ancillaries on the crankcase, a structural oil pan, a high-stiffness torque-converter housing and an axial driveline bearing on the gearbox side. Infiniti claims that the V9X engine has the lowest 250Hz and 500Hz vibrations of all its benchmarked competitor engines. The unit’s cylinder heads, which are not a serious source of noise, are cast from aluminium alloy.

Inside the engine, a mini-sac seven-hole injector nozzle design has been adopted. Internal engine friction is managed by the use of ultra smooth components such as the micro-finished forged steel used for the crankshaft. The Bosch common-rail fuel injection system uses piezo injectors and operates at 180MPa. Idle speed has been set at an exceptionally low 650rpm.

An overcooled Exhaust Gas Recirculation (EGR) system is used to limit NOx emissions. The system incorporates an exhaust gas bypass in the EGR cooler which helps the power-unit achieve operating temperature as quickly as possible after a cold start.

The exhaust system also uses a metallic oxidation catalytic converter. These generate lower back-pressures than ceramic systems. An exothermic catalyst and a catalytic diesel particulate filter (DPF) are incorporated in the same housing. The DPF incorporates a seventh fuel injector which fires periodically to burn off the solid build-up in the particulate trap.

The new engine is coupled to a seven-speed epicyclic automatic transmission; each production engine is subjected to a ‘hot bench test’ cycle before it is delivered to the vehicle assembly plant.

Differences between petrol and diesel versions of the EX and FX models include a new front subframe, different front bumper designs to enhance airflow into the engine bay, and some differences in the sheet metal inside the engine bay.

Infiniti EX
Body type 5-door hatch 5-door hatch
Cylinders 6V 6V
Valves 4 4
Bore/stroke 84.0/90.0 84.0/90.0
Swept volume 2993cc 2993cc
16.0:1 16.0:1
PS/rpm 238/3750 238/3750
Nm/rpm 550/1750 550/1750
Maximum speed 137 132
0-100km/h 7.9s 8.3s
Combined MPG (l/100km) 33.2 (8.5) 31.4 (9.0)
CO2 g/km 224 238
Transmission A7 A7
Driven wheels All All
Length 4645 4865
Width 1800 1925
Height 1600 1680
Wheelbase 2800 2885
Track: front
Track: rear
Kerb mass 1985 2150
Wheels 18x8 (a) 20x8 (a)
Tyres 225/55 265/50
CdxA 0.33x2.44 0.35x2.71
Fuel tank 80l 90l
(a) Alloy
Infiniti three-litre V6 diesel engine..
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