With the development of the Multijet system, the Fiat Group conquered an important first in the field of diesel engines. This achievement was made possible by the wealth of know-how built up since 1986, a year which saw the launch of the Croma TDI, the world’s first direct-injection diesel.

At that time, the car was a brilliant achievement, the first major step forward in terms of improved combustion efficiency for diesel engines. It was this technical advance, later to be used by other manufacturers, that allowed diesel cars to provide both enhanced performance and reduced consumption. There was just one problem - the loud engine noise at low revs and when speeding up or slowing down.

This is where the history of the Unijet begins – in other words the development of a more evolved direct-injection system that could drastically reduce the uncomfortably loud combustion. Several years later, research brought us the Unijet, while other important advantages were achieved in terms of performance and consumption. There were only two possible solutions: adopt a passive solution by insulating the engine to prevent the sound waves from travelling, or work towards eliminating the problem at source by developing an injection system that could reduce the combustion noise. Having chosen the second option, Fiat designers immediately began researching the principle of the “Common Rail”, having discarded other high-pressure injection systems after careful analysis, as they did not allow independent management of pressure compared to revs and engine load, nor pre-injection – both strengths of the Unijet.

The result of research carried out at the University of Zurich, and never used before on a car, the theory was both simple and ingenious. By continuing to drive the diesel into the tank, pressure is generated inside the tank itself, which becomes a hydraulic accumulator or "rail", i.e. a pressurised fuel reserve ready for use. Three years later, in 1990, pre-industrialisation began on the Unijet, the system developed by Magneti Marelli, the Fiat Research Centre and Elasis, based on the "Common Rail" principle. This phase ended in 1994 when Fiat Group Automobiles decided to form an alliance with a highly-skilled partner in the field of diesel injection systems.

The final stages of development and industrialisation were handed over to Robert Bosch.
Eleven years after the launch of the Croma TDI, another record-breaking car arrived on the market in October 1997: the Alfa 156 JTD, fitted with a revolutionary turbo diesel which gave a performance that was previously unimaginable. Cars fitted with this engine are incredibly silent and give a lively response similar to petrol engines. Compared to similar ante-chamber diesels, they give a performance which is improved by 12% on average, and also reduce fuel consumption by 15%. The Alfa 156 JTD was an instant hit, and the engine was not only used on other Fiat models, but also by many other car manufacturers who used similar engines.

In 2002, Fiat Group Automobiles presented the second generation of JTD engines with multiple injection and 16 valves. Autumn 2002 saw the launch of the progenitor, the 1.9 Multijet 16v, but the series was soon extended to include the revolutionary 1.3 Multijet 16v (2003) and later the powerful 2.4 Multijet 20v (2004). Now it is the turn of the new 1.6 Multijet 16v to take over the baton of the undisputed leader in this sector.

The secret lies in the Multijet system

Second-generation turbo diesels are again based on the principles of Common Rail, i.e. high-pressure electronically-controlled injection. Yet they have something extra: the number of injections during each engine cycle is increased, compared to the two of the Unijet. The same quantity of fuel is burned inside the cylinder, but it is divided into more parts, so that combustion is more gradual. The advantages include reduced noise and emissions, and performance is enhanced by 6-7%, accompanied by an engine yield that makes the car even more enjoyable to drive.

These results are not insignificant, especially because they come from an engine that not only represents an incredible technological leap forward compared to a precamera diesel, but is also a huge improvement on the first-generation JTDs.

The secret of the Multijet lies in the control panel which opens and closes the injectors (and also inside them). The key component is the electronic control panel itself, which can perform a series of very rapid injections.

Developed by Fiat Group researchers for this very purpose, the component and injectors perform multiple injections which allow for greater precision of pressure and temperature in the combustion chamber, and also use the air entering the cylinders more efficiently. This allows further objectives to be achieved in terms of controlling combustion noise, reducing emissions and boosting performance.

The Multijet system is the result of a lengthy research process. First, the technicians solved the problem of the limits imposed by the control unit. They then mapped the benefits obtained by cross-referencing a certain sequence of multiple injections (two secondary ones shortly after the main one, a secondary one not too close to the main one followed by two more secondary ones; a secondary injection and then, at a given distance, two main injections close together, etc.) against various fields in the engine’s operating range: in the minimum zone; with low loads and low revolutions; at high revs with medium load, at low revs with a high load and so on.

The study revealed the system’s potential, which always yields great benefits, accentuated in one field or another depending on the type of sequence chosen, and the field of engine operation in which it is applied. In some cases, it reduces start-up times and smokiness, in others it increases torque and reduces noise, and in others it reduces both emissions and noise. Today, that research has given us the Multijet engine.