Until recently, power units with unijet Common Rail technology represented the final frontier in diesel engineering. Despite the name 'unijet', the engines make not one but two diesel injections into the combustion chamber, a small initial injection and a bigger main injection. But all this has changed now, because the Fiat-GM Powertrain engineers have developed a second-generation of Common Rail power units that are multijet, i.e. capable of multiple injections (3 to 5 in fact). 

The engineering principle behind both systems is the same. Even in the unijet version, the pilot injection raises temperature and pressure inside the cylinder to improve combustion at the time of the main stroke. Because the main injection can now be divided into many smaller injections, the amount of diesel burnt inside the cylinder remains the same but combustion is fuller and more gradual. This allows further progress towards the aim of quieter combustion, reduced emissions and increased performance. 

Multijet Common Rail engines differ from unijet Common Rail engines in two essential parts: the injectors and the electronic control unit. Injections that can reduce the time between one injection and the next were required to increase the number of injections. And the intervening time thus drops by one order of magnitude: from 1500 to 150 microseconds. Then the engineers had to reduce the minimum injected quantity: from two to less than 1 cubic millimetre. We therefore needed a smarter control unit, i.e. a unit able to modulate injection strategy continually to adjust to changes in three parameters: engine rpm, torque required at any given moment by the driver and coolant temperature. 

While the new 1.3 Multijet 16v engine is in operation, the control unit continually adjusts injection arrangement and number (as well as the amount of diesel injected). When coolant temperature is lower than 60° and torque requirement is low, two small and one large injection are performed, very close together. As torque increases, the number of injections drops to two: a small one and a large one. Under conditions of high rpm and high torque demand, only one injection is performed. With coolant temperature over 60°, things change again and to minimise emissions the injection arrangement becomes: one small, one large, one small. 

The 1.3 Multijet 16v is a 1251 cc 4 cylinder in line power unit with a bore of just 69.6 mm and a long 82 mm stroke. The four valves per cylinder are driven directly by a twin overhead camshaft. Maximum power output is 51 kW at 4000 rpm (70 bhp) and the torque delivered is 180 Nm at just 1750 rpm. 

Amongst other things, the new power unit features a chain-driven timing system with rocker control (a maintenance-free solution), hydraulic tappets, a cast iron crankcase with an aluminium base, an aluminium cylinder head and steel crankshaft and connecting rods. 

The cylinder head is also designed with highly efficient directional intake ports to ensure an optimum fuel mix despite the small bore. 

The fuel and combustion systems are designed to operate at very high pressures that range from 1400 bars of diesel in the rail to 160 bars of gas in the cylinder. The 1.3 Multijet 16v is turbocharged by a turbocharger with wastegate and intercooler. A variable geometry version will soon also be available that is capable of developing even higher power and torque levels. 

Last but not least, the 1.3 Multijet 16v also features an electronically-controlled EGR system and an exhaust gas cooling device. 

The new Multijet system has allowed the Fiat Group to achieve an important new record in the diesel engine field. All this was possible because we have been building up know-how in this field since 1988, the date that marked the arrival of the Croma TDI, the first direct injection diesel vehicle in the world. The Croma engine was an outstanding result for the time and the first major step toward automotive diesel engines with more efficient combustion. 

The engineering, subsequently copied by other manufacturers, meant that diesel cars were able to ensure better performance coupled with lower fuel consumption. One problem remained: excessive engine noise at low rpms and during speed transitions. 

 This was the cue for the start of the Unijet story, i.e. the quest for a more advanced direct injection system that could drastically reduce the problem of excessive combustion noise. Some years later, this research effort brought us the Unijet itself and significant benefits in terms of efficiency and fuel consumption. 

We reasoned that the problem could be solved in one of two ways: we could be content with a passive system and simply insulate the engine to prevent sound waves from reaching the passenger compartment - or we could work actively to eliminate the problem at its source by developing an injection system that actually reduced combustion noise. 

Fiat Group engineers chose the second option and immediately went for the Common Rail principle after considering and then rejecting other high-pressure injection systems. The other systems do not allow pressure to be managed independently of rpm and engine load and neither do they include a pre-injection, which are essential attributes of the Unijet system. 

The theory that lay behind our research was originally developed by researchers at Zurich University but had never previously been applied to a vehicle. This simple yet elegant theory is based on the assumption that if you continue to push diesel into a tank, the pressure inside will rise and the tank itself will become a hydraulic accumulator (or rail), i.e. a reserve of pressurised fuel ready for use. 

Three years later, in 1990, the Unijet system developed by Magneti Marelli, Fiat Research Centre and Elasis on the Common Rail principle entered the pre-production stage. This stage was completed in 1994, when Fiat Auto started to look for a partner with leading-edge knowledge of diesel engine injection systems. The final stage of the project, i.e. completion of development and industrial production, was eventually entrusted to Robert Bosch. 

In October 1997, eleven years after the Croma TDI, the market welcomed another record-breaking car: the Alfa 156 JTD. The new model was equipped with a revolutionary turbodiesel engine that was to deliver previously unimaginable results. 

Vehicles equipped with this engine are incredibly noiseless and have all the alacrity of a petrol power unit. They improve on the performance of a similar prechamber engine by an average of 12 percent and reduce fuel consumption by 15 percent. 

The Alfa 156 equipped with a JTD engine won immediate success and similar power units soon appeared on other Fiat Auto models and were adopted by many other motor manufacturers. 

Now the turn has come for a second generation of JTD engines, in other words the multiple injection, 16 valve engines that made their debut last autumn in Paris in the shape of the 1.9 JTD engine fitted to the Alfa 156 and Alfa 147. 

And that family has now been extended by the pocket-sized 1.3 Multijet 16v.