26.10.1997

Alfa 156: The engines


Introduction
The heart and the true strength of every Alfa Romeo is its engine. The Alfa 156 is no exception. That intriguing line with its forceful personality clothes a range of superlative engines, high-tech and big-hearted: four petrol and two turbodiesel.
Similar technological sophistication appears on the Alfa 156's petrol engines: the 1.6, 1.8 and 2.0 Twin Spark 16-valve and the 2.5 V6 24-valve. All the four-cylinder engines feature variable valve timing, and two of these, the 1.8 and 2.0 Twin Spark, feature a variable geometry intake system made of fibreglass-enriched nylon. This system delivers exactly the right quantity of air to the cylinders at all engine speeds. This makes for enhanced engine response in town or on the motorway by providing extra torque and power. The fuel line is returnless, in the interests of safety, combating the risk of fire and diminishing vapour emission.
The 2.5 V6 24-valve petrol engine features a new cylinder head, redesigned combustion chambers, intake and exhaust manifolds, and electronic rather than mechanical throttle control. A whole host of modifications have produced a car whose performance features make it a pleasure to drive. All the engines combine sparkling performance with low fuel consumption. The 1.6 T. Spark 16V delivers peak power of 120 bhp, peak torque of 14.7 kgm and gives the Alfa 156 a top speed of 200 km/h. Plus 0-100 km/h acceleration in 10.5 seconds and, in the combined cycle brought in by the most recent ECE regulations, a fuel consumption figure of 8.2 l/100 km.
Equipped with the 1.8 T. Spark 16V engine, with its 1747 cm3, 144 bhp maximum power output, and 17.2 kgm peak torque, the Alfa 156 is capable of a top speed of 210 km/h, does 0-100 km/h in 9.3 seconds and consumes 8.2 l/100 km. And at 82.4 bhp/litre this engine offers the highest specific power rating in its class.
The 2.0 T. Spark 16V vital statistics are 1970 cm3, 155 bhp, 19.1 kgm and 216 km/h. Equipped with this engine the Alfa 156 will accelerate to 100 km/h from a standing start in just 8.6 seconds and will cover 100 km on 8.5 litres. One incontrovertible indicator of engine performance is volumetric efficiency, a parameter that compares actual intake air volume at peak power to theoretically available cylinder capacity. The 2.0 T. Spark 16V scores 0.95 under this heading against an average 0.8 among its direct rivals.
And finally the 2.5 V6 24V 2492 cm3 6-cylinder: peak power 190 bhp, peak torque 22.6 kgm, top speed 230 km/h and a combined cycle figure of 11.4 l/100 km. It accelerates the Alfa 156 from 0-100 km/h in 7.3 seconds.
All these Alfa 156 engines require very little maintenance (they get their first service after 100,000 km, except of course for oil changes every 20,000 km). They are also extremely environment-friendly (noise levels below 74 dB and exhaust emissions within EU Phase 2 limits). Finally Alfa Romeo engineers have taken the utmost care to minimise noise and vibration in order to optimise cabin comfort.

Engineering of the petrol power units
Two of the petrol engines, the 1.8 and 2.0 Twin Spark, adopt a variable geometry intake system. What this means is that injection is handled electronically by the injection control unit which alters the length and position of the manifold ports to meet the needs of the moment, making them longer at low speeds (560 mm), shorter at high (380 mm).
Finally, the innovative technological choice of fibreglass-enriched nylon rather than aluminium for the intake module makes for:
- an improved overall engine layout that simplifies maintenance and servicing;
- a 4% reduction in engine and injection system weight (approximately 6 kg);
- enhanced quality and reliability when it came to assembling the engine components and installing the electrical contacts;
- minimised performance wastage thanks to improved intake manifold control.
The system exploits 'ram' effect and acoustic resonance phenomena to ensure that the cylinder is always appropriately filled, thereby guaranteeing optimum engine performance (specific powers of 82.43 bhp/l for the 1.8 and 78.68 bhp/l for the 2.0).
In practice, this system allows the motorist to exploit the full potential of the engine, both in town traffic, where these power units have proved stunningly responsive, and on the motorway. What really impresses about the 1.8 is its remarkably accessible torque and the way it supplies 90% of its power well before 3000 rpm. What you notice on the 2.0 is the 3% extra power provided by the variable geometry system.
All of a piece and smoothly progressive, the 1.8 and 2.0 T. Spark 16V units also feature gearboxes whose ratios were selected to combine good performance with fuel economy, thereby benefiting the environment on the one hand and the motorist's pocket on the other.
Other strengths of the 1.8 and 2.0 T. Spark 16V include safety and respect for the environment. In order to minimise the risk of injuries in the event of a crash, for example, both engines adopt a 'returnless' fuel system that makes do with just one tank-to-engine line (instead of two), thereby minimising the risk of fire. The same system also reduces the release of petrol vapours into the air, adding an extra 'cleanliness' dimension to an engine that already features an electronic emission control system which confines exhaust emissions to within the limits established by the EU's Phase 2 regulations.
The entry level Alfa 156 engine is the 1.6 T. Spark 16V, which already powers other Alfa models. Like the other two Twin Spark units, this ultra-reliable and much-loved engine features twin spark ignition, variable valve timing and one of the most advanced electronic engine management systems around.
At the other end of the Alfa 156's petrol engine range we find the 2.5 V6 24V supplying the perfect balance between performance and flexibility, thanks to a six-speed gearbox (6th is not an overdrive) which has its gear ratios distributed in such a way as to exploit the engine fully on all types of route.
The 2.5 V6 24V adopts a new cylinder head with a covered combustion chamber, a central spark plug, and a 37º angle between the valves. The electronic throttle control unit eliminates the need for a mechanical linkage connecting the accelerator pedal to the throttle. Engine management is also electronic and supplied by the Bosch Motronic ME 2.1 system.
The electronic throttle envisages that the system's memory can be programmed to set throttle apertures which can provide both a sporty reaction at high speeds, enhanced flexibility at slower speeds, smooth acceleration/deceleration and prompt response to cold starts.
The electronic throttle also speeds up catalyst activation with the engine cold (by supplying a wider throttle aperture than the accelerator pedal demands) for enhanced control over exhaust emissions.
Several other engineering features on the 2.5 V6 24V are worth mentioning. They include graphite pistons to reduce the loss of efficiency that comes from friction between cylinder liner and piston, intercooler, a 'returnless' fuel system, platinum twin-spark plugs, hydraulic tappets and automatic tensioners on the timer belt and on the accessory V-poles.
All these features reduce the V6's maintenance requirements and indeed it is not scheduled for servicing until it has covered 100,000 km.
Finally, on the environmental protection front, the engine boasts a three-way catalyst with heated Lambda probe, low inertia stainless steel manifolds and the electronic throttle that effectively controls exhaust emissions in the catalyst-heating phase.

Unijet system and engineering of the diesel power units
The two Alfa 156 turbodiesels feature - a world first - an innovative high pressure direct injection system, never seen before on any car worldwide. It is called Unijet and it is a 'common rail' system designed and initially developed inside the Fiat Group with contributions from Magneti Marelli, the Fiat Research Centre and Elasis, before it was handed over to Bosch for the final stage of development and industrial production.
Compared to conventional injection devices, Unijet injection offers not only a general improvement in performance compared to traditional injection systems but also makes for a far quieter engine (up to 8 dBA quieter, depending on engine speed).
In today's (precombustion or direct injection) systems, fuel delivery to the injectors is driven by a mechanical pump (often controlled electronically) and injection pressure rises as engine speed increases. It is a system that imposes limits on the optimisation of combustion and the engine's performance, not to mention its noise and emission characteristics.
By contrast, on the Unijet system, injection pressure is independent of engine speed and load (i.e. accelerator pedal position), since the injection pump generates pressure on a cumulative basis. Moreover, since both pump and injectors are electronically controlled, the injection pressure and fuel quantities injected can both be optimised at every point on the power curve.
This makes it possible to combine very high injection pressures and the electronically controlled delivery of minute quantities of diesel fuel to achieve pre-injection, known as pilot injection. These two characteristics offer enormous advantages to the driver. The first 'pressure pile-up' concept makes for more efficient combustion, hence improved performance, while pilot injection reduces combustion noise.
High pressure injection significantly reduces both fuel consumption and exhaust fumes, while pilot injection creates the ideal temperature and pressure conditions inside the combustion chamber for combustion proper. Preheating the combustion chamber drastically reduces the pressure gradient which is the source of uneven combustion and hence the typical clatter of standard direct injection engines.
The system, as it appears on the Alfa Romeo 156's 1.9 and 2.4 JTD engines consists of a small pump immersed in the fuel tank which delivers diesel oil to the primary pump. That is the high pressure pump (driven by the timing belt) which continuously 'pushes' the diesel oil into the accumulator. This means that the accumulator tank or 'rail' always contains pressurised oil whatever the engine speed and 'load' (i.e. the accelerator pedal's demand for power).
A pressure sensor on the rail and a pressure regulator on the pump respectively monitor oil pressure inside the accumulator, and adjust it as instructed by the electronic control unit. The pressure sensor 'reads' the bar level inside the accumulator and if that level is too high, passes the information on electronically to the pressure regulator (an hydraulic valve) which sends the excess diesel oil back into circulation in order to reduce the pressure inside the accumulator to the bar level demanded by the control unit.
In this way, fuel pressure is continuously variable and the ideal pressure can be selected for any individual point on the engine power curve. And that makes for improved combustion.
Evidently, efficient pressure management along the entire power curve is going to optimise combustion efficiency, thereby producing enhanced performance and fuel economy.
That is because the higher the pressure of the diesel oil entering the injector, the finer the fuel spray produced, which makes for an enhanced air-fuel mix and complete combustion (with less diesel oil remaining in liquid form on the cylinder walls or being converted into exhaust fumes). However, the higher the pressure, the louder the noise. Ideally then, what we want is the ability to select the best possible balance between combustion efficiency and quietness at each point on the engine power curve, which is what we get with the Unijet system.
To solve the noise problem, the injection system on the Alfa 156 diesel engines demonstrates another highly effective device: pilot injection. This takes just 200 micro-seconds or thereabouts, thanks to the electro-hydraulic injector system governed by the electronic control unit that delivers a bare 1.5/2 mm3 diesel oil.
This pre-injection raises combustion chamber temperature and pressure just as the piston reaches TDC and prepares the combustion chamber for combustion proper. The advantages are obvious when we compare the combustion curve in this unit with that achieved by a traditional diesel.
Thanks to pilot injection, the heat release curve at the moment of the main injection is very much less steep and the temperature and pressure peak at a lower level. Hence the same amount of energy is produced but it is delivered more gradually and it is precisely this progressive delivery that drastically reduces noise. In fact, the steeper the heat release curve the more violent and therefore the noisier the combustion.
So, 'common rail' for enhanced combustion efficiency and better performance; pre-injection for much quieter combustion, easier cold starts and reduced exhaust emissions. But it didn't stop there. In order to optimise combustion inside the cylinder, Alfa Romeo adopted multi-jet injectors with the tiniest possible apertures to atomise the diesel oil, and used a spiral shape for the intake port inside the cylinder head to optimise air swirl. Combine an atomised fuel spray and swirling air and you get an air-fuel mixture that burns ultra-efficiently. Compare a pre-combustion chamber type of engine with a Unijet model of the same size and you find that Alfa's new Unijet engines deliver an average 12% improvement in performance plus an average 15% reduction in fuel consumption.See TABLES. The 15% extra fuel economy is an average figure because in practice, the saving ranges from 6% at idle to 20% at full power. The graphs also show the increases in performance and peak torque delivered by the two engines.See TABLES.
In particular, fuel economy is influenced by three parameters: enhanced combustion efficiency, reduced heat dispersion to the walls and a reduction in energy expended on pumping the fuel (in the port that links the pre-combustion chamber and the piston crown found on indirect injection engines).
The resulting four- and five-cylinder diesel engines (the 1.9 JTD and 2.4 JTD) are ultra-reliable and make the diesel versions of the Alfa 156 superlatively comfortable, quieter and more vibration-free than any diesel car could ever have been hitherto.
The 1.9 JTD is a 1910 cm3 four-cylinder engine that delivers 105 bhp and torque of 26 kgm. This Alfa 156 version produces a top speed of 188 km/h, 0-100 km/h acceleration in just 10.5 seconds and a pick-up of 60-100 km/h in 7.2 seconds in fourth gear (80-120 km/h in 10.2 seconds in fifth).
The five-cylinder 2.4 JTD is a 2387 cm3 unit that develops 136 bhp and a peak torque of 31 kgm. Powered by this engine, the Alfa 156 achieves a top speed of 203 km/h, 0-100 km/h acceleration in 9.5 seconds and even more outstanding pick-up: 60-100 km/h in 7.1 seconds in fourth; 80-120 km/h in 9.8 seconds in fifth.
It also does well under the new combined ECE cycle (urban route plus EUDC out-of-town route) fuel consumption ratings: 5.8 l/100 km for the 1.9 JTD; 6.7 l/100 km for the 2.4 JTD.
Distinctive constructive features of the 1.9 and 2.4 JTD engines created for the Alfa 156 include: the multi-layer cylinder head gasket that offers a perfect seal; a closed deck spheroidal cast iron engine block with built-in cylinder linings; a light alloy cylinder head. The pistons and the omega-type combustion chamber are housed inside the engine block. Inside the head we find the coiled intake manifolds that create a swirl to facilitate air-fuel mixing. The two parallel vertical valves per cylinder are driven directly by a single overhead cam. And there are no pre-combustion chambers since the entire combustion process takes place inside the chamber which is carved out of the piston.
On both engines, auxiliary parts are driven by a double acting pulley that reduces both the torsional vibration of the crankshaft and stresses on both the belt and the driven auxiliary components.
The 1.9 JTD engine is turbocharged by a Garrett T15 blower with wastegate valve (to eliminate part of the exhaust gas energy loss at high speeds) and intercooler.
The more powerful 2.4 JTD adopts a variable geometry turbocharger from which it derives its outstanding peak power and excellent torque, at low speeds. The term 'variable geometry' reflects this turbocharger's ability to vary the blade angle to speed up or slow down the gas flow and hence the turbine. The Garrett T15 is also coupled to an intercooler.
Finally both engines are equipped with a two-mass engine flywheel (one half built into the crankshaft, the other into the main shaft). This reduces vibration in low gear and makes the whole transmission system much quieter, for enhanced ride comfort.
And the 2.4 JTD also features a counter rotating balancer shaft which further reduces noise and vibration, enhancing comfort.

A story of supremacy
In creating the Unijet system, the Fiat Group has racked up another major first in diesel engine technology. Back in 1986 Fiat Auto's Croma TDI had already become the world's very first direct injection diesel-powered car. It was a spectacular achievement for the era and the first great step towards enhanced combustion efficiency in automotive diesel engines. Soon to be copied by other manufacturers, the engine layout on the Croma TDI allowed diesels to combine enhanced performance and fuel economy for the very first time. Just one problem remained: these engines were still excessively noisy at low revs and in the transients.
Enter the Unijet, or rather research into a more advanced direct injection system to drastically reduce the irritating inconvenience of excessively noisy combustion. This research would indeed lead, a few years later, to the Unijet, having picked up a number of other major advantages in engine efficiency/fuel economy level along the way.
There were only two ways to solve the noise problem. The engineers could have settled for a passive approach, which would have meant insulating the engine itself to prevent sound waves seeping out. Or they could be 'proactive' and work to eliminate the problem at source, which would mean developing an injection system that possessed the ability to minimise combustion noise.
Having opted for this second approach, Fiat Group engineers soon focused their attention on the 'common rail' principle, having carefully analysed and rejected other high pressure injection systems. Even the best of them, in fact, offered no way of regulating pressure independent of revs or engine load; nor did they allow for a pre-injection function and these are precisely the strengths of the Unijet system.
Developed by a Zurich University research team, the common rail concept on which Fiat Group engineers began to work was both simple and brilliant. As diesel oil is continuously injected into a tank, pressure is built up inside the tank itself, transforming it into a kind of hydraulic accumulator (or 'rail') i.e. a reserve supply of pressurised fuel ready for use. It was up to Alfa Romeo engineers to design a system capable of using that pressure.
Three years later, in 1990, the system known as Unijet, jointly developed by Magneti Marelli, the Fiat Research Centre and Elasis on common rail lines, went into pre-industrial production. That phase ended in 1994 when Fiat Auto decided to choose a manufacturing partner with outstanding competence in diesel engine injection systems. At that point the final development phase was handed over to Robert Bosch for completion followed by industrial production.
Alfa Romeo is now launching the world's first diesel engines to incorporate a Unijet system.


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