HDi diesels. The C4 range now offers the largest yet selection of Airdream engines, covering the 1.6-liter versions (HDi 92, HDi 110 DPFS with a manual gearbox and EGS6) and 2-liter versions with a manual gearbox. Airdream is Citroën’s  “eco-signature” label.

The 2-liter HDi unit has been upgraded to comply with the future Euro 5 standard. Fitted as standard with a particulate filter, the new HDi 140 DPFS develops 103 kW of power (140 bhp EEC). Fuel consumption is 5.3 L/100 km (44.4 mpg US) over a combined cycle, while CO₂ emissions are down to 140 g/km.

Fuel consumption of the other diesel units—the HDi 92 (with or without DPFS) and HDi 110 DPFS—with low-rolling resistance Michelin Energy Saver tires is 4.4 L/100km (53.5 mpg US). CO₂ emissions are 117 g/km, 119 g/km and 118 g/km, respectively.

Gasoline engines. The C4 is now available with two gasoline engines from the EP family co-developed with the BMW Group. It is the first Citroën to be fitted with engines from this family. The VTi 120 replaces the 1.6i 16V, and the THP 150 replaces the 2.0i 16V.

Mainly through better management of intake, the VTi (Variable valve lift and Timing Injection) 120 engine offers more power with low fuel consumption and lower CO₂ emissions. The C4’s VTi 120 engine has a capacity of 1,598 cm³ and develops 88 kW EEC (120 bhp EEC) at a speed of 6,000 rpm, for maximum torque of 160 Nm (118 lb-ft) at 4,250 rpm. Ninety percent or more of maximum torque is available between 2,500 rpm and 5,750 rpm.

Mated to a five-speed manual gearbox, the engine returns 6.7 L/100 km (35 mpg US) and 159 g/km of CO₂ over a combined cycle, down 6% on the old unit. The VTi 120 unit also ships with a four-speed automatic for fuel consumption of 7 L/100 km (33.6 mpg US) and CO₂ emissions of 165 g/km.

Fitted with an aluminum block and cylinder head, the 16 valves of the VTi 120 are driven by intake and exhaust camshafts with continuously variable valve timing. The intake valves also benefit from a variable raising system that allows the valves’ maximum travel to be adjusted gradually depending on the force applied to the accelerator pedal. These two features boost engine efficiency, increasing the responsiveness of the vehicle through higher available torque, especially in the most frequent driving conditions (part loads).

With direct gasoline injection and a turbocharger—both technologies a first on the C4 range—the new THP (Turbo High Pressure) 150 offers both high performance and low consumption. The new unit develops maximum power of 110 kW (150 bhp EEC) at 5,800 rpm, for maximum torque of 240 Nm (177 lb-ft) from 1,400 rpm to 4,000 rpm, with 156 Nm of torque available from as low as 1,000 rpm.

Mated to the new MCM six-speed gearbox (with the same architecture as the EGS6), fuel consumption over a combined cycle is 6.9 L/100 km (34 mpg US) for 164 g/km of CO₂, down 10% on the previous engine.

The C4 is also available with a four-speed automatic gearbox, for maximum power of 103 kW (140 bhp EEC). Maximum torque remains the same, at 240 Nm at 1,400 rpm. Combined-cycle fuel consumption is 7.7 L/100 km (30.5 mpg US), down 5% compared with the previous powerplant.

The THP 150 engine and the THP 140 version use the same architecture as the VTi 120. The THP engine (for Turbo High Pressure) features direct sequential injection fed by a high-pressure electronic pump mounted at the end of the intake camshaft. Maximum pressure of 120 bar allows the injectors to spray the mixture directly into the combustion chamber sideways, ensuring uniform distribution. The compression ratio is 10.5 to 1.

A twin-scroll turbocharger groups the gas ducts by pair (cylinders 1-4 and 2-3) in the exhaust manifold. The optimal gas flow combination this provides makes the turbine extremely fast-acting and considerably cuts down on lag time. The turbine can rotate at up to 200,000 rpm, developing pressure limited electronically by the discharge valve to 0.8 bars. Turbocharging is noticeable from 1,000 rpm.

The intake camshaft benefits from continuous variable timing, which provides more torque at low engine speeds while reducing fuel consumption and CO₂ emissions.

An oil pump with throughput regulation supplies the right amount of lubrication for optimal functioning and reduces fuel consumption by 1%. This system also features an oil intercooler, fitted in the filter and regulated by a thermostat, for faster heating and better temperature management, even during rough handling.

The new C4 will have its world premiere at the Moscow International Motor Show, which opens on 26 August 2008.