Pre-industrialisation of the Unijet, the system developed by Magneti Marelli, Centro Ricerche Fiat and Elasis on the basis of the Common Rail principle, began three years later, in 1990. This stage was concluded in 1994, when Fiat Auto decided to choose a partner with great experience in the field of injection systems for diesel engines. The project was therefore sold to Robert Bosch for the final part of the work, i.e. completion of the development process and industrialisation. In October 1997, eleven years after the Croma TDI, another record-setting car went on the market: the Alfa 156 JTD equipped with a revolutionary turbodiesel engine that gave hitherto unimaginable results. The cars equipped with this engine were incredible quiet, with a sparkling response on a par with that of a petrol engine and performance levels that were 12% higher than those of a similar pre-combustion engine, as well as 15% lower consumption. The success of the Alfa 156 with the JTD engine was immediate, and very soon not only was it being used on other Fiat Auto models, but numerous other car-makers were adopting similar engines. Now it is the turn of the second generation of JTD engines, the Multijet multivalve units.


The underlying principles of second generation turbodiesel engines remain those of the Common Rail, i.e. high injection pressure and electronic injector control. But one extra feature has been added: during each engine cycle, the number of injections increases over and above the current number of two. In this way, the same amount of diesel is burnt inside the cylinder but in several portions to achieve smoother combustion. The advantages include lower running noise, reduced emissions and a 6-7% increase in performance. All this comes with a level of engine efficiency that improves car handling still further.


These results are not to be underestimated, particularly because they are obtained with an engine that represents an incredible leap forward from pre-chamber diesels and even improves on first generation JTD engines.


The secret of the Multijet engine lies in the control unit that governs the electric injector opening and closure system (and also in the injectors themselves). The crucial element is the electronic control unit itself that can perform a set of injections that may be very closely spaced.


Fiat Auto's researchers developed the part (together with the injectors) especially for this application. It is designed to deliver the multiple injections that assure more accurate control of pressures and temperatures developed inside the combustion chamber and also more efficient use of air taken into the cylinders. This enables further goals to be achieved: quieter combustion, reduced emissions and increased performance.


The Multijet system is underpinned by long years of research. Our engineers began by resolving the problem of the limits imposed by the control units. Then they went on to map the benefits they could achieve by plotting different multiple injection sequences (two secondary injections very close to the main injection; one secondary injection not too close to the main injection plus two closely-spaced secondary injections; one secondary injection and then two main injections close together after a certain period, etc.) against different engine service conditions: in the idling region; with low loads and low rpm; with high rpm and moderate load; with low rpm and high load, etc..


The study revealed the potential of the system and showed that great benefits are achievable in all cases, though these tend to focus on one field or another according to the type of sequence chosen and the engine service area targeted. In some cases, for example, the priority is to reduce start-up times and fume levels, in other cases it is to increase torque and reduce noise while in others still it is to reduce emissions and ensure a quieter drive. And now this research strand has led to the creation of the Multijet engines, including the brand new 90 bhp 1.3 JTD Multijet 16v which debuts on Fiat Grande Punto.

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