17.02.2004

The Common Rail Multijet: the diesel of the future


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 Multijet 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°C 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°C, things change again and to minimise emissions the injection arrangement becomes: one small, one large, one small.
On display in the showcase at Geneva, visitors will have a chance to take a close-up view of the 70 bhp 1.3 16v Multijet and the 140 bhp 1.9 16v Multijet.

1.3 16v Multijet
The 1.3 16v, the smallest, most advanced second generation Common Rail direct injection diesel on the market, belongs to the Multijet family. To create it, its designers borrowed the technology from the very recent 140 bhp 1.9 Multijet engine: a high pressure Common Rail device, multiple injections, 16 valves, intercooler and a combustion chamber with a particularly efficient shape. Then they added an intake system with a plastic manifold and directional ports. Their last task was to reduce engine weight and size without losing anything in terms of technical sophistication and quality.
The end result was the 1.3 16v Multijet, 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.
To sum up, the 1.3 16v Multijet is a true masterpiece of technology in miniature: it weighs just 130 kg with all its components fitted. It is small in size at less than 50 centimetres in length and 65 cm in height and its components are laid out to take up the least possible space. It assures the same advantages as bigger engines because it has not been reduced but miniaturised.
The power unit, designed for maximum rationalism, efficiency and reliability, is now the smallest four cylinder Common Rail diesel engine on the market. It is the only unit able to enclose no fewer than six normal-sized components in a cylinder with a bore of under 70 millimetres. In other words, four valves, one injector and a glow plug. This record-breaking miniaturisation means the unit can be fitted to segment B superminis and also segment A city cars. And the new car can now claim another record: it is the most powerful. Despite a minuscule cylinder capacity of 1251 cc, the small Multijet engine can stand comparison with any other small fixed geometry turbine diesel engine currently on the market. Even the most hyped. Suffice it to say that it offers better specific performance figures than any other diesel engine between 800 and 1500 cc: 41 kW/l of power and 144 Nm/l of torque. This compact, technologically sophisticated new engine is also capable of outstanding performance and is effectively maintenance-free for life. Suffice it to say that the 1.3 16v Multijet engine is designed to last for 250,000 km instead of the normal 150,000. During this long life, the engine requires no maintenance to its mechanical parts (not even the obligatory fan-belt change at 80,000 km). The oil change intervals are also extended from 20 to 30,000 km. The oil is naturally low viscosity (i.e. designed for fuel economy) and environment-friendly: when it is changed, only the paper cartridge need be discarded, not the entire filter.
And more. The 1.3 16v Multijet is also environment-friendly because it already meets Euro 4 emission limits not due to come into force until 2006. It is also one of the very few engines in the world to achieve this result without adding a sophisticated exhaust post-treatment device such as a particulate trap. All in all, an intrinsically clean car: the particulate emission level (responsible for dust and fine dust) - for example - is actually lower than the level set by upcoming Euro 4 legislation.
The 1.3 16v Multijet represents a true technological leap that will allow Fiat to tap into the growing interest for diesels, particularly smaller diesels. Due to lower capacities and weights, low emissions and an advantageous ratio of performance to fuel consumption, the diesel engine's share of the overall market is rising quickly even in lower segments: the figure was 5% in 1997, nowadays it is almost 20% and over the next three years it should rise to 30-40%.
These results mean that the market for this type of engine should, if the estimates are correct, soon rise to 1,500,000 units. Small Fiats equipped with the sophisticated 1.3 16v Multijet engine could carve out a significant niche within this market. The history of diesel suggests that the market is product-driven. If this is true, Fiat is again in the vanguard of change and ready to introduce new models to make the most of the revolution it helped to bring about.

Benefits to the customer
As far as customers are concerned, the technology packed into the pint-sized 1.3 16v Multijet ultimately means a reduction in fuel consumption. The figure is 10% lower than the current 1.9 JTD engine while emissions are 50% lower. Other benefits include:

  • lower noise levels (imagine cylinder knock as drum beats: three beats on a small drum are not as noisy as a single beat on a big drum);
  • improved comfort: fewer reciprocating masses means fewer vibrations;
  • a smooth, satisfying drive due to very gradual torque delivery (in turn assured by improved control of combustion, moment by moment);
  • all the flexibility and prompt responses of a diesel that thinks it's a petrol engine due to its wide rpm range (for example, the fuel cut-off at just over 4000 rpm is imperceptible);
  • the green features that allow this engine to improve on the main environmental advantage of diesel engines - i.e. fuel economy - by reducing their main drawback, i.e. particulate emissions.

140 bhp 1.9 16v Multijet
This engine is derived from the tried and tested 1.9 JTD 8 valve Common Rail unit and takes the form of a 4 cylinder in line unit with bore of 82 millimetres and stroke of 90.4 millimetres. The four valves per cylinder are driven directly by a twin overhead camshaft. The new turbodiesel has undergone several engineering changes to increase performance and engine torque at low speeds and to reduce noise levels and vibration.
For example, the Common Rail system used on the 1.9 Multijet 16v unit includes two new strategies for automatically calibrating and balancing the diesel injected to lower noise and reduce vibration. Certain engine components are brand new: a cylinder head with hydraulic tappets, steel connecting rods and crankshaft, a piston with an internal channel to carry cooling oil to the main and connecting rod bearings that are made out of different material to the previous unit. The exhaust and intake manifolds are also new: the former is made out of a special high-strength material while the latter is made out of pressure cast aluminium. The electronically-controlled EGR system is cooled by exhaust gas. The lubrication circuit has a new oil pump and an external heat exchanger (air/oil) for cooling the oil. The cooling system is fitted with a different water pump. This long series of improvements and changes have created a reliable, powerful engine with low fuel consumption. The new 1.9 JTD 16v power unit thus promises great power (103 kW) and generous torque (31 kgm). These results have been achieved by adopting a new engine setting, increasing the direct injection pressure from 1300 to 1400 bars and introducing a new turbocharger setting. The power units are turbocharged via a Garrett turbocharger with variable geometry turbine that helps improve power delivery by allowing very high torque delivery even at low rpms. Suffice it to say that 90% of maximum torque is available between 1750 and 3250 rpm. These data translate into great driving satisfaction and truly inspiring performance.
The Fiat Stilo 1.9 Multijet 140 bhp (3 door), for example, touches a top speed of 203 km/h (200 km/h for the 5 door and Multi Wagon), accelerates from 0 to 100 km/h in 9.7 seconds and covers a kilometre from a standing start in 30.9 seconds. Despite these searing performance figures, the fuel consumption is low: the saloon returns 7.6 l/100 km over an urban cycle, 4.2 l/100 km over an extraurban cycle 5.4 l/100 km for a mixed cycle, while the figures for the 5 door and Multi Wagon versions are 7.8 l/100 km - 4.4 l/100 km - 5.6 l/100 km respectively.

Direct injection diesel engine: a Fiat story
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 set-up, 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 transients.
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, nine 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.


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