FACTfile - The new Alfa Romeo 156 JTS - Page 3

Setting a new performance standard

Alfa Romeo cars have always been distinguished by their up-to-date power units and scintillating performance, a motoring tradition that fills many pages of the international motoring history books and remains alive in the New Alfa 156. The heart under the bonnet of the new 156 is the revolutionary 2.0 JTS, forerunner of a new direct injection petrol power unit family that will equip brand models from now on. In addition the legendary Alfa Romeo V6 engine has been upgraded with more performance and lower emissions to translate heritage into a 21st century power unit.

Performance engines, therefore, that also offer moderate fuel consumption and respect for the environment. These engines come with efficient, precise gearboxes, such as the innovative Selespeed system that offers a revised operating strategy for greater driving satisfaction in complete safety.

JTS: Alfa Romeo's interpretation of the direct injection petrol engine
At its first outing in autumn 1997, the Alfa 156 immediately won the hearts of public and experts alike for its good-looking styling, thrilling performance and on-road behaviour and also for its sophisticated engineering, so perfectly consistent with the brand's great motorsport tradition. This was the first car in the world to be fitted with common rail direct injection turbodiesel power units. The engineering principle that was subsequently to win such success with other manufacturers.

The Alfa 156, aiming for more engineering excellence, now offers another world first: the first direct injection petrol engine with a specific power greater than 60 kW/l (82 bhp/l) and a specific torque of more than 100 Nm/l. An ultra-high performance power unit that takes the name of JTS (Jet Thrust Stoichiometric) from its specific combustion system, an acronym that is destined to identify an entire family of future Alfa Romeo engines.

As far as the customer is concerned, this means a two litre car that:

1. Already meets stringent Euro 4 emission limits
2. Does not need low sulphur petrol but is able to use the normal petrol already on sale in Australia, Europe and the United States

The New 156 is the first petrol engine from Alfa Romeo (and indeed Fiat Auto) with injectors that work directly in the combustion chamber. It achieves its end by interpreting the principles of stratified charge and the creation of motion in the mixture inside the cylinder in an entirely original way.

---

Engine

Lean burn, but not too lean

The possibility of injecting petrol directly into the combustion chamber instead of the intake duct has been known since Nikolaus Otto (who took out a patent in 1877) and has been applied for two different purposes over the years, on racing cars in the Fifties and Sixties to increase engine power and more recently from since 1996 to reduce fuel consumption.

Recently, manufacturers have devoted all their efforts to this latter direction and good results have been achieved with the stratified charge method. The principle is simple: instead of injecting all the petrol required to maintain the normal air-fuel ratio of 14.7:1 (stoichiometric) throughout the combustion chamber, only a small amount of fuel is injected that mixes with the air to form a core of almost stoichiometric composition about the spark plug. The resulting mixture is stratified or layered because it is richer where the ignition spark ignites and increasingly lean (more air and less fuel) as it approaches the outside of the chamber.

So far the benefits of this lean burn system, usually applied in the rpm band up to 3000 rpm, have amounted to a fuel saving of some 10 per cent. The disadvantages may be summarised as follows:

1. A drop in performance when the car is required to deliver full power (because the ducts and pistons are shaped in a certain way that is essential to reduce fuel consumption at low speeds)
2. The need to use sulphur-free fuel that is difficult to find in Europe and practically unknown Australia and in the US;
3. The requirement for exhaust gas treatment methods (DE-Nox) to reduce the higher nitrogen oxide emissions generated by the leaner burn.

It goes without saying that Alfa Romeo's approach to the new technology had to be quite different. Category-topping performance and irrepressible driving behaviour have always been essential requirements for all Alfa Romeo models. But what was to stop Alfa Romeo from using direct injection to increase engine power and torque in keeping with the sporty applications of this technology? Then, Alfa's engineers reasoned, the stratified charge system could be brought in to reduce fuel consumption within a restricted rpm band around idle speed. The result was an entirely original Alfa Romeo approach to applying direct injection in petrol engines. A solution that offered a compromise between the two methods pursued to date.

The New Alfa 156 2.0 JTS works using a lean burn approach up to around 1500 rpm and this saves fuel, although not as much as on other lean petrol direct injection engines. Above this rpm, the engine burns a stoichiometric air-fuel mixture, i.e. with a normal 14.7:1 ratio between both components. All this means outstanding performance. This is much better than would be obtained using a normal indirect injection petrol unit.

Firstly, because petrol injected directly into the combustion chamber instead of the duct cools intake air to increase the engine's volumetric efficiency. As temperature drops, the gases increase in density and their volume therefore decreases: this means that more air can be introduced into the combustion chamber. Power unit susceptibility to knock is also reduced by chamber cooling. It is therefore possible to increase the compression ratio - in this case from 10:1 for the 2.0 T. Spark to 11.3:1 for the 2.0 JTS.

This means more power for the new Alfa Romeo engine that is, moreover, able to deliver its power unhindered because the exhaust gas treatment system used on the 156 does not generate the strong back-pressure typical of the Nox catalysts used by lean petrol DI engines. Direct introduction of petrol into the chamber improves power unit response speed to the accelerator control and it is faster overall than a conventional petrol engine.

Benefits: higher performance and lower fuel consumption
Compared to the current 2.0 Twin Spark unit and other currently-available direct injection petrol engines, the 2.0 JTS unit fitted to the New Alfa 156 offers slightly lower fuel consumption and a generous increase in power and - above all - torque. The new JTS engine has exactly the same capacity as the Twin Spark engine it replaces, yet power is up from 114 kW to 121 kW; torque rises from 187 Nm to a remarkable 206 Nm. And all this is achieved using petrol currently on sale and current catalytic converters.

A new combustion chamber principle
The new JTS combustion system displays two distinctive features:

1. The principle followed to generate the movement that propels the air and fuel mixture toward the spark plug inside the cylinder;
2. The range of rotation speeds within which the engine works using a lean burn system.

In other Gasoline Direct Injection (GDI) engines, the air's force drags the fuel spray into the area where the ignition spark ignites. This option is determined by a desire to achieve a very lean mixture (up to 60:1) and thus consistent fuel savings. But it brings a need to change the air's motion within the combustion chamber - the charge motion - according to rpm level and this complicates the air input mechanisms, such as throttles, duct closure systems etc.

On the 2.0 JTS, however, the force of the fuel spray (Jet Thrust) propels the fuel toward the spark plug as it mixes with the air. In this way, it achieves a charge that is less lean overall (the ratio remains constant at all speeds and is 25:1) and less fuel is consequently saved. But the engine's internal mechanism is far less complicated because it lacks systems for altering the air's motion.

The same process of simplification also guarantees the limitation of lean burn technology to rpm levels around idle speed (up to 1500 rpm). GDI engines that use stratified charge within a broader speed band (up to 3000 rpm) must employ modified piston and duct profiles. The resulting shape does not allow power to be optimised at high speeds. The use of stratified charge only up to 1500 rpm, however, means that the pistons and ducts on the 2.0 JTS Alfa Romeo are hardly altered. Because their shape is more similar to those of current indirect injection engines, they are able to exploit all available power at high speeds. The addition of an exhaust gas treatment system (Nox catalyst) to remote nitrogen oxides is also only required when the lean burn range is extended up to 3000 rpm. This also dictates the use of sulphur-free fuel, i.e. the only type that will not damage the catalytic converter.

The use of stratified charge only at speeds around idle speed, however, allows the 2.0 JTS unit fitted to the New Alfa 156 to use a conventional catalytic converter system. This result is also made possible by a more extensive use of exhaust gas recirculation, which reduces the production of nitrogen oxides (Nox). Because Alfa Romeo engines are fitted with variable valve timing, exhaust gas is recirculated to the intake on the 2.0 JTS directly between the intake and exhaust valves (internal EGR).

Engineering: what changes
The main engineering changes on the 2.0 JTS compared to the corresponding Twin Spark engine affect the cylinder head (with Bosch injectors fitted in the chamber), pistons, camshafts and exhaust system. All these components are completely new.

The intake ports are high performance; the fuel manifold is high pressure (common rail type); piston compression ratio is higher - and the exhaust - built to Euro 4 standards - is cascade type.

The exhaust gas treatment system works conventionally despite an unconventional layout: the system no longer consists of a preconverter and a main converter located under the body. Instead it comprises two main catalytic converters built into the manifold (each connected to a double branch that leads to two cylinders). This frees up the space under the body for a silencer that is more permeable and thus more able to reduce back pressure for fuller engine power delivery.

previous page

next page