New Alfa Romeo Diesels In The UK | Part TwoPublished 26 March 2003
This same engine will also be available in Turismo and Veloce versions of the Alfa 156, priced at £15,590 and £16,400 respectively; while the comprehensively equipped 156 Sportwagon 1.9 JTD Turismo goes on sale at £16,710.
Alfa Romeo’s tradition of combining the latest technology with legendary roadholding and handling prowess is set to continue later this year with the introduction of a 140 bhp 1.9 JTD 16-valve unit, followed by an uprated version of the 2.4 JTD engine which is already available in the Alfa 156.
The JTD Engine: A Breakthrough In Diesel Technology
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, dependent on engine speed).
In conventional diesel injection systems, fuel delivery to the injectors is driven by a mechanical pump. Injection pressure rises as engine speed increases, imposing 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, since the injection pump generates pressure on a cumulative basis. Moreover, since pump and fuel injectors are electronically controlled, both the injection pressure and fuel quantities injected can be optimised at every point on the power curve.
The system 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 the accumulator. Thus the accumulator tank, or ‘rail’ always contains pressurised oil whatever the engine speed and ‘load’ (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, varying fuel pressure continuously. Such precise pressure management across the entire power curve optimises combustion efficiency, thereby producing enhanced performance and fuel economy. That is because the higher the diesel oil pressure entering the injector, the finer the fuel spray produced, making for an enhanced air-fuel mix, more complete combustion and a reduction of exhaust fumes.
However, the higher the pressure inside the combustion chamber, the louder the noise. So the Unijet system combines very high injection pressures with the electronically controlled delivery of minute quantities of diesel fuel to create pre-ignition. Known as pilot ignition, this pre-ignition system takes 200 micro-seconds to deliver just 1.5-2 mm3 of diesel oil, raising combustion chamber temperature and pressure at the moment the piston reaches Top Dead Centre, thus optimising conditions for combustion proper.
Thanks to pilot injection, the heat release curve at the moment of main combustion is very much less steep, and the temperature and pressure peak is also at a lower level. The same amount of energy is produced, but in a more gradual manner. And it is this progressive delivery that drastically reduces engine noise and eradicates the lumpy, clattering noise normally associated with diesel engines.
In order to further optimise combustion, the cylinder intake port is formed in a spiral shape to optimise air swirl. The combination of an atomised fuel spray and swirling air produces an air-fuel mixture that burns ultra-efficiently.
So, common rail technology guarantees enhanced combustion efficiency and better performance; pre-injection offering quieter combustion, easier cold starts and reduced exhaust emissions.
Compared with a traditional, pre-combustion chamber type of unit, a Unijet engine of the same size delivers an average 12% improvement in performance, plus an average 15% reduction in fuel consumption; the saving ranging from 6% at idle to 20% at full power.