|Torque curve for the HDi 240. Click to enlarge.|
Citroën has introduced the new 3.0-liter diesel V6 DPFS HDi 240 engine for the C5 and C6. Improvements in the engine offer a reduction in fuel consumption and CO2 emissions by between 12% (in the C5) and 15% (in the C6) compared to the outgoing 2.7L version it replaces. The engine features a new third-generation direct common-rail injection system; new generation combustion chamber; dual variable geometry turbochargers; exhaust gas recirculation; and an alternator which recovers energy during deceleration and braking.
Power output is now 177 kW (237 hp)—an increase of 18%—with a maximum torque of 450 Nm (332 lb-ft) at 1,600 rpm. Fitted with an 6-speed automatic gearbox, the fuel consumption on combined cycle of the two cars is 7.4 L/100 km (32 mpg US) with CO2 emissions of 195 g/km. The engine complies with the future Euro V emission standards.
The maximum torque of 450 Nm is constant from 1,600 to 3 600 rpm—a range that corresponds with the primary use of the vehicle. Torque is greater than 400 Nm all the way from 1,400 and 4,100 rpm. Its value, at 1,500 rpm, is 422 Nm, 17% higher than the engine it is replacing.
One of the principal changes from the engine the new unit is replacing is an increase in capacity from 2721 to 2993 cm3 by increasing both the bore and stroke.
Fuel injection system. The third-generation fuel injection system operates at pressures up to 2000 bar compared to 1650 previously. Injectors have 8 holes as opposed to 6 allowing even better atomization in the cylinders. The system supports up to 5 injections per cycle: one or two pilot injections, two principal injections and one post-injection for the particulate filter system. These allow better fuel atomization for improved combustion, with reduced noise and lower engine-out emissions.
Piezoelectric injectors are utilized mounted at the center to reduce their emissions. In addition, the injection pump only delivers the fuel necessary for the operation of the engine, thus reducing the volume of fuel returned to the tank which in turn allows a reduction in the temperature of the fuel.
Combustion chamber. The ECCS technology (Extreme Conventional Combustion System) (earlier post) applied in the 2.2 DPFS 173 engine is again present in the new V6 Diesel. This technology comprises several elements:
- Optimal geometry for the desired level of power, compatible with the desired level of reduced emissions.
- Exceptionally durable alloy pistons, cooled by oil.
- A decreased compression ratio (16.1 instead of 17.3) which is beneficial for performance and allows a reduction in NOx emissions.
- A larger bore (84 mm instead of 81) reduces the volume of unburnt fuel due to less contact with the walls.
- A smoother cylinder head (of around 10%) for improved air circulation in the engine (improved consumption by reducing transfer losses and improved refilling).
- A reduction in swirl (air movement phenomenon in the combustion chamber) of around 10%, limiting thermic losses against the walls.
- Adoption of new pre-heating ceramic plugs at very high temperatures (1300° C) for an improved compatibility at low temperatures (used in post heating after start up) a reduction in cold engine emissions.
- Improved pre-heating time allowing a rapid start up at very low temperatures (-25° C, the preheating time is reduced from 12 to 2.5 seconds).
Turbocharging. The parallel bi-turbo design is continued with the 3 liter version. The two turbochargers have an identical function—each one feeds a line of three cylinders. The turbochargers offer improved output with low flow (reduction of inertia, optimization of internal aerodynamics). Optimum filling at high speed with a maximum speed capable of reaching 250,000 rpm instead of 230,000 contributes to increased performance compared to the 2.7 litre model. The maximum supercharge pressure is 1.4 bars (2.4 at the highest).
Actuation of the variable geometry is carried out via an electric motor which responds to the driver in less than 100 ms; this leads to possible variations in engine load from 0 to 100% in less than 500 ms.
EGR. The new engine offers a new Exhaust Gas Circulation (EGR) system in which the thermal capability of the fuel/water exchanger is improved by 40%. It comprises electric blades as well as a pneumatic control of by-pass valve both controlled by the engine control unit.
With a cold engine, the by-pass allows circulation through the exchanger, to limit the condensation of soot caused by the combustion and therefore the unburnt hydrocarbon emissions, and allows a more rapid increase in engine temperature and reduced combustion noise and overall improvement.
With a hot engine, the by-pass is closed and the exhaust gas crosses the exchanger with increased power, thus offering a net advantage in terms of reduced consumption and emission of NOx.