2008 Saab 9-3 Turbo Diesel

Super efficient and powerful, the 9-3 Aero TTiD boasts a unique two stage turbocharging system, which was a world 'first' in the premium car segment. Matched to either a six-speed manual or six-speed automatic transmission, the 1.9-litre, 16-valve two-stage turbo engine puts the performance and efficiency of the Aero TTiD among the best in its class. The responsible performer delivers a sporty drive with 132kW and 400Nm of maximum torque, in gear performance similar to the 9-3 petrol Aero and a frugal combined 5.7 litres per 100km for the manual Sport Sedan model.

Two stage turbocharging
The two stage turbocharger housing is integrated with the exhaust manifold and contains two turbochargers of different sizes. The pair of turbine/compressor wheels are sequentially mounted and each is able to by-pass the input and output feeds of the other, while also being able to operate in tandem. This arrangement provides a driver with the best of both worlds - good, instant torque at low engine speeds - via the low-inertia small turbo when the exhaust pressure is light - and strong 'top end' power at higher revolutions, when the larger turbo is engaged. Saab's two stage turbocharging uses a system of by-pass valves that directs the exhaust gas stream between the two turbine wheels and also separates the air intake charge from the compressors.

From tick-over to 1,500 rpm the exhaust feeds only the small turbo. Between 1,500 and 3,000 rpm the flow is split between both turbines, giving stronger boost and ensuring a smooth transition phase to full power operation, above 3,000 rpm, when only the larger turbo is engaged. The compact design provides more efficient packaging than systems using two separate turbochargers linked together externally in series. It ensures that the complex control and regulation of gas flows on both the exhaust turbine and intake compressor sides is achieved with a minimal number of seals and connections.

TTiD engine
The TTiD engine is a substantial development of the current 16-valve 1.9TiD unit, using common rail, direct and multiple fuel injection. It operates with a maximum boost pressure of 1.8 bar (1.4 bar for the current engine) together with a slightly lower compression ratio of 16.5: 1 (17.5:1). The cast-iron block, alloy cylinder head and all internal components are strengthened as necessary to withstand the higher thermal pressures. The exhaust gas re-circulation (EGR) system is characterised by extremely efficient cooling and now includes an electronically controlled by-pass, and a low-maintenance exhaust particulate filter for improved emissions. Other technical enhancements include the adoption of a larger air-to-air intercooler, for better cooling of the intake charge; the introduction of more efficient pre-heating elements inside the cylinder head, for faster starting in cold conditions; and the use of a weight saving plastic intake manifold.