Some info on the 136bhp JTD i've just dug out with a bit of help from Google
Alfa Romeo 156
156 2.4 JTD - FIRST DIESEL ALFA IN UK
When Italian sporting car manufacturer, Alfa Romeo, introduced the award-winning 156 in 1997, the company also unveiled a highly advanced diesel engine - the first in the world to incorporate a Unijet system - in the 156 JTD.
Today, some 30% of Alfa Romeo's European business involves diesel powered cars, and its diesel standard bearer, the 156 2.4 JTD, has just become available here in right hand drive, further expanding the highly successful 156 sports saloon range.
The upper medium saloon car segment of the UK market has grown in recent years from just over 536,000 units in 1995 to over 596,000 in 1998. Within this segment, diesel share has decreased over the same period from 25% in 1995 to 21% in 1998. However, the 'performance diesel' sub-segment (over 110bhp) is actually growing rapidly: from 2355 units in 1995 to 30,243 in 1998.
With 136 bhp and a remarkable 224 lb.ft of torque available, the Alfa 156 2.4 JTD can truly be regarded as a performance diesel, putting many petrol engined competitors to shame in the process. Alfa GB is thus confident that far from compromising the brand's sporting ethos, the JTD's arrival actually enhances it.
The 126 mph Alfa 156 2.4 JTD has a 2.4 litre, five-cylinder turbodiesel engine, featuring an innovative high pressure direct injection system called Unijet. It is a 'common rail' system designed and initially developed by the Fiat Group, with contributions from Magneti Marelli and the Fiat Research Centre, before it was handed over to Bosch for the final stages of development and industrial production.
Compared to conventional injection systems, Unijet injection offers not only a general improvement in performance, 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 emissions characteristics.
By contrast, with 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 with 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 customers. 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 to take place. Preheating the combustion chamber drastically reduces the pressure gradient which is the source of uneven combustion, and hence the clatter typical of direct injection engines.
The system, as it appears on the Alfa Romeo 2.4 JTD engine, consists of a small pump immersed in the fuel tank, which delivers diesel oil to the primary (high pressure) pump (driven by the timing belt), which continuously 'pushes' diesel oil into an 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 returns excess diesel oil into circulation in order to reduce pressure inside the accumulator to the level required 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's power curve. All of which makes for improved combustion.
Efficient pressure management across the entire power curve optimises combustion efficiency, thereby producing enhanced performance and fuel economy. That is because the higher the pressure of diesel oil entering the injector, the finer the fuel spray produced, which makes for an enhanced air-fuel mix and more 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 is required is an ability to select the best possible balance between combustion efficiency and low noise levels at each point on the engine power curve, which is what the Unijet system delivers.
To reduce noise, JTD injection demonstrates another effective solution: pilot injection. This takes about 200 micro-seconds, thanks to an electro-hydraulic injector system governed by an electronic control unit that delivers just 1.5-2 mm≥ of diesel oil.
This pre-injection raises combustion chamber temperature and pressure just as each piston reaches TDC and prepares the chamber for combustion proper. The advantages are obvious when the combustion curve in this unit is compared with that achieved by a conventional diesel.
Thanks to pilot injection, the heat release curve at the moment of main injection is very much less steep, and the temperature and pressure peak is also at a lower level. Hence the same amount of energy is produced, but it is delivered in a more graduated manner, 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.
Thus, 'common rail' enhances combustion efficiency and provides better performance, while pre-injection promotes quieter combustion, easier cold starts and reduced exhaust emissions.
But this is not the end of the story. In order to optimise combustion inside each cylinder, Alfa Romeo adopted multi-jet injectors with the smallest possible apertures to atomise the diesel oil, and used a spiral shape for the intake port inside the cylinder head to optimise air swirl. A combination of an atomised fuel spray and swirling air produces an air-fuel mixture that burns ultra-efficiently. Compared with a pre-combustion chamber type of engine, a Unijet unit of the same size delivers an average 12% improvement in performance together with an average 15% reduction in fuel consumption.
The 15% fuel economy bonus is an average figure, because in practice, the saving ranges from 6% at idle to 20% at full power.
In particular, fuel economy is influenced by three parameters: enhanced combustion efficiency, reduced heat dispersion to the cylinder walls, and a reduction in energy expended on pumping fuel (in the port that links the pre-combustion chamber and the piston crown, found on indirect injection engines).
The resulting five-cylinder diesel engine (2.4 JTD) is ultra-reliable and makes this version of the Alfa 156 more refined, quieter and more vibration-free than diesel cars could have been hitherto.
The five-cylinder 2.4 JTD is a 2387 cc unit that develops 136 bhp at 4200 rpm, together with remarkable peak torque of no less than 224 lb. ft, (considerably more than Alfa's 3.0 V6 24v petrol engine!), delivered at just 2000 rpm. The Alfa 156 JTD achieves a top speed of 126 mph, and 0-62 mph acceleration in 9.5 seconds.
It is also notably economical. Official fuel consumption figures are - ECE: 31.7 mpg: EUDC: 52.3 mpg: Combined: 42.1 mpg.
The 2.4 JTD engine features a multi-layer cylinder head gasket that provides a perfect seal. It also boasts a closed deck spheroidal cast iron engine block with built-in cylinder liners and a light alloy cylinder head. Pistons and omega-type combustion chambers are housed inside the engine block. Inside the cylinder head itself are coiled intake manifolds that create the swirl effect to facilitate air-fuel mixing. Two parallel vertical valves per cylinder are driven directly by a single overhead camshaft. And there are no pre-combustion chambers since the entire combustion process takes place inside the chamber, which is carved out of the piston.
Auxiliaries are driven by a double-acting pulley that reduces crankshaft torsional vibration and stresses on both the belt and the driven auxiliary components.
The engine uses a variable geometry Garrett VNT 25 turbocharger from which it derives its outstanding peak power and excellent torque, at remarkably low engine speeds: The term 'variable geometry' reflects this turbocharger's ability to vary its blade angle to speed up or slow down gas flow and hence the turbine, which is coupled to an intercooler.
It is also equipped with a two-mass engine flywheel (one half is built into the crankshaft, the other into the main shaft). This reduces vibration and makes the whole transmission system quieter and more refined. The 2.4 JTD also features a counter-rotating balancer shaft which further reduces noise and vibration.
A story of supremacy
In creating the Unijet system, Fiat Group has made another major step forward in diesel engine technology. Back in 1986 Fiat Auto's Croma TDI became the world's first direct injection diesel-powered car. It was a spectacular achievement for the era and marked the first 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 very noisy at low RPM and in the mid-range.
Further research was carried out into a more advanced direct injection system to drastically reduce noise. This research led, a few years later, to Unijet, having picked up a number of other major advantages in engine efficiency/fuel economy along the way.
Essentially, there were two ways to solve the noise problem. Engineers could have settled for a passive approach, which would have meant sound-insulating the engine itself. Or they could be 'proactive' and work to eliminate the problem at source, which would mean developing an injection system that possessed an ability to minimise combustion noise.
Having opted for this latter approach, Fiat Group engineers focused their attention on the 'common rail' principle, having carefully analysed and rejected other high pressure injection systems. Even the best of them offered no way of regulating pressure independent of RPM 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 exploiting that pressure.
Three years later, in 1990, the system known as Unijet, jointly developed by Magneti Marelli and the Fiat Research Centre 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.