The Common rail high pressure direct injection diesel fuel system was designed and initially developed by the Fiat Group in the 1980s, with contributions from Magneti Marelli and the Fiat Research Centre. It was handed over to Bosch for the final stages of development and industrial production in the early 1990s, and the first car to use the new system was Fiat Group’s Alfa Romeo 156 JTD in 1997.

JTD revolutionised diesel engine design, allowing Fiat JTD units to set new dynamic standards and compete head-on with petrol engines across nearly all criteria, whilst still retaining the core diesel values of reliability, low maintenance, reduced CO2 emissions and exceptionally low fuel consumption figures.

In 2002, Fiat Group was awarded the prestigious Economist Innovation Award (Energy and Environment category) for its ground-breaking work in developing the ‘Common rail’ diesel technology that is now used by almost every major European car maker.

The sophisticated MultiJet technology of second generation JTDs differs from its UniJet predecessor in two essential areas; the injectors and the electronic control unit.

Within a UniJet engine, a pilot injection raises temperature and pressure inside the cylinder to improve combustion at the time of the main stroke. However, by dividing the main injection into a number of smaller injections, a MultiJet engine affords a fuller, more gradual combustion whilst the amount of diesel burnt at each stroke remains the same; thus abetting smoother, quieter combustion, reduced emissions and increased performance.

To facilitate multi-injection, the injectors can both reduce the time lag between injections from 1500 to just 150 microseconds, and reduce the minimum quantity of fuel injected from 2 to less than 1 cu mm. In tandem, the new control unit modulates injection strategy continually to adjust to changes within three parameters; engine rpm, torque required at any given time by the driver, and coolant temperatures.

Thus, when coolant temperature is less than 60 degrees and torque requirement low, two small and one large injection are performed. As torque demand increases, the number of injections drops to two; one small and one large. Under conditions of high rpm and high torque demand, only one injection is required, whilst with coolant temperature at over 60 degrees, emissions are minimised with one small, one large, then one small injection in the sequence.

Hence, whilst performance is substantially enhanced, start-up times and exhaust smoke are reduced, noise levels and vibration are lowered to ensure a quieter drive and improved passenger comfort, and both fuel consumption and emissions are significantly reduced. All Fiat MultiJet turbodiesel units easily fulfil Euro 4 emissions requirements.