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Citroën Adds New Electronic Transmission to Diesel C4, Boosting Fuel Economy to 52 MPG

The C4.

Citroën is introducing a new six-speed electronic transmission system on its 1.6 HDi 110 hp diesel models of the C4 in Europe.

The automated gear changes and clutch control of the new system reduce fuel consumption by up to 6% compared to current models, resulting in a promised 4.5 liters/100 km (52 mpg US) and CO2 emissions of just 120g/km. These new 1.6 HDi 110hp models are now also fitted with a Diesel Particulate Filter System (DPFS).

Click to enlarge.

The new transmission eliminates the clutch pedal, and gives the driver the flexibility to choose between a manual mode, with paddleshift or gear lever (which has no mechanical link with the gearbox), or an automatic mode. A Sport function is also available, while a hill-start assist feature contributes to improved safety and ease-of-use.

Using new electro-hydraulic actuators to ensure gentle and smooth gear shifts, the system can also recognize and automatically adapt to individual driving styles. In manual mode, the driver maintains full control of the gear change by using either the paddles that are set behind the steering wheel, or the sequential gear lever, which can be flicked forwards or backwards to change up or down.

The Sport function is available in both manual and automatic modes. Engaged by pressing the S button next to the gear lever, the Sport function speeds up the gear changes, from 0.8 to 0.4 seconds in manual mode and from 1.2 to 0.5 seconds in automatic mode.

The hill-start assist makes it easier to start on slopes by preventing the car from rolling and giving the driver time to accelerate away safely and confidently. Activated on both uphill and downhill slopes with a gradient of 3% or higher, the control holds the car steady for two seconds when the brake is released.

A control unit controls the two actuators of the gearbox system. The first—the gearbox actuator—ensures the selection and engagement of the gears, and the second—the clutch actuator—controls the clutch.

When a gear change is requested, either by the driver directly issuing a gear change command or indirectly by the driver depressing the accelerator or the brake or as a function of the gradient of the road, engine speed, vehicle speed and grip conditions, the gearbox control takes control of the engine controller and the clutch actuator.

Engine torque is reduced and the clutch is opened progressively to avoid jolting. Once the clutch is open, the gearbox control unit activates the gearbox actuator. The selector fork is disengaged, the new gear is selected and synchronized, the selector fork is reengaged.

The gearbox control unit then progressively closes the clutch and controls torque increase to complete the gear change without jolts or rebounds. When upshifting, the entire process takes between 0.4 and 1.2 seconds.

A 1.6 HDi C4 was also one of the platforms for PSA Peugeot Citroën’s introduction of two diesel-electric parallel hybrid applications earlier this year. The C4 Hybride HDi prototype delivers average combined city and highway fuel consumption of 3.4 liters per 100 kilometers (69 mpg US), with 90 grams of CO2 emitted per kilometer. (Earlier post.)


Rafael Seidl

Note that this is a single clutch robotized step-by-step transmission. These systems have been around for a while. However, the inevitable loss of torque during the gear change impacts driver comfort, since the shift points are defined by the electronics, not the driver. Perhaps Citroen PSA has succeeded in minimizing this problem by executing the shift faster and integrating the drivetrain controls such that the engine is already very nearly at the right speed when the clutch is re-engaged.

At 4.5L/100km diesel (120 g/km CO2) the variant described here comes close to the numbers for the similarly-sized and -priced Toyota Prius (4.3L/10km gasoline, 104g/km CO2), especially if you consider that these are NEDC rather than real-world numbers. In many European countries, diesel is cheaper by volume than gasoline due to tax differences due to tax differentials that ought to be questioned as diesel contains 12% more energy by volume.

Btw, a dual-clutch transmission completely avoids the transient torque loss but is more expensive. In the high-torque wet cltuch version used by VW, the fuel economy gains are lower, only about 4% (in L/100km).

Bud Johns

Still,you gotta be impressed to match the Prius in mileage, although you won't have the perfect smoothness of no shifts at all, and I dare say the Prius will burn it in any acceleration contest.......


Bud, you might lose that acceleration bet. The Prius albeit smooth does 0 to 60 in 10.2. Stock TDI's range from 10.3 to 10.8 depending on model. A $400 mod gets TDI's in the 8 second range.

Rafael Seidl

Joseph -

this particular diesel vehicle will do 0-60kph in 11.2 secs. The European version of the Prius is advertised as doing it in 10.9 secs.

It's kind of a moot point, though. As soon as you floor it on any car, your mileage will go down the toilet. If you're going to drive that aggressively, why bother buying a fuel miser?

Sid Hoffman

Rafael, have you ever gotten on a freeway in a dense area like L.A. before? Every time I go to southern California I'm amazed at how short the onramps are relative to the 65-70mph freeway traffic speed. You absolutely need a vehicle that can do 0-70mph in 11-12 seconds or so if you want to have any hope of merging on many of the short, and especially uphill onramps. Vehicles like this are at the edge of being barely quick enough to get away with anything less than redlining the engine in every gear just to get on a freeway with short onramps.


Thats pretty amazing that the fuel economy can be boosted so much by only changing the transmission.

On the Prius I read that Toyota gimped the acceleration.. to make it smoother and seem more like a combustion engine. Not everyone can handle rapid acceleration, for example an elderly person without great coordination hitting the pedal too hard.

For someone like Sid needing to merge rapidly however cars that sacrifice acceleration for mileage would be a bad idea. People have to find a vehicle that fits their needs.

fyi CO2

I rent a Prius every time I get to L.A., and never have a problem getting on the 405 via Century, Santa Monica, or Wilshire blvd.

Bud Johns

The point is there are times when you need plenty of power. Other advantages to the prius are silky smoothness due to no shifting and a very quiet driving experience. The acceleration is very linear, no peaks and valleys.


The point is this car could potentially be running on B100 in this country but its not! The only question we have to ask it why aren't we doing more to promote the sale of these kinds of vehicles here. Spare me all the EPA regulation excuses. We need more of these cars here now.
And the C4 looks a hell of a lot better than the Prius.

Rafael Seidl

Justin -

"Spare me all the EPA regulation excuses."

Carmakers are not going to promote vehicles they are prohibited from selling by law. How is that an excuse?

Among other tasks, EPA is charged with implementing the Clean Air Act. Five states (CA, NY, MA, VT, ME) representing about 1/3 of the total vehicle market have chosen to adopt emissions regulations so strict no diesels have met them in over 20 years.

Unlike their European counterpart, neither EPA nor CARB have a mandate to differentiate emissions limits by engine technology. The fact that EPA Tier 2 Bins 9-11 would be dropped after MY 2006 has been known for some time. EPA stuck by that decision, even though US consumers have recently warmed up to the idea of driving a modern European turbodiesel car. Fuel economy is the purvue of the Dept. of Transportation, which administers CAFE and of the IRS, which adminsters the gas guzzler tax.

Btw, biodiesel is not a cure-all miracle fuel. While it contains no sulphur and reduces engine-out PM by 50%, NOx is as high or even slightly higher than with mineral diesel. And therein lies the rub.

Gasoline engines are fitted with three-way catalysts that simultaneuosly clean up HC, CO and NOx once the device has raeched its light-off temperature. The snag is that it only works if the engine is operated within a very narrow window 2% either way of the stoichiometric air-fuel mixture.

Diesels cannot be operated on mixtures that rich, they would produce too much PM and fuel economy would be no better than for gasoline cars. There are NOx aftertreatment technologies for lean burn concepts, including diesels and stratified GDI. However, they are expensive and require either extremely low - preferably zero - fuel and engine oil sulfur content (NOx store) or, a distribution infrastructure for an urea additive and robust OBD functions against operator defeat strategies (SCR catalyst). A third option currently under intense scrutiny is HCCI combustion.


"Carmakers are not going to promote vehicles they are prohibited from selling by law. How is that an excuse?"

Who told you that diesel cars are prohibited by law? Did somebody forget to tell DaimlerChrysler about this law? They don't seem to be bothered by the current state of diesel car regulation in this county. They sell the E class, GL and soon Jeep Grand Cherokee, ML and R class 2007 diesels. Are they breaking the law?

Rafael Seidl

Justin -

carmakers at least try to make a profit. Note everyone can afford a E-class Mercedes, and right now, the technology involved in meetting US emissions laws would price e.g. a VW TDi out of contention.

What I meant is that the carmakers aren't going to market their *existing* diesel solutions because the law no longer permits their sale as MY 2007 vehicles. I did not mean to imply that EPA prohibits diesels outright - just the ones that Joe Average might be able to afford.


I often drive a 2005 Ford Focus 1.6 Tdci (check for details), which is powered by the same engine as the C4, but with a 5 speed manual. The engine is smooth and powerful enough (with 110hp,and 240Nm of torque). In real world driving I can get from between 42 and 51 US MPG depending on the type of roads. In the city a Prius would probably beat that, but on the open road a Prius wouldn't stand a chance.

Larry R

In the real world driving that I do [relatively flat Midwest],I routinely average 44-49 mpg at 70 mph in my "04 Prius. If I stay at the 65 mph speed limit,the average approaches 51 mpg. Many factors besides types of roads affect my mileage however...wind
and weather also play a prominent role . In winter I'm lucky to average 38.5 to 42.5 mpg,and a 20 to 30 mph headwind will knock it down similarly. Rain also increases the drag and it stays in the 42-44 mpg range.
I seriously doubt that most Prius owners here in the US drive as hard as their European counterparts...[from what I've witnessed in the Czech Republic],so I suspect a Prius driven similarly to your Focus would not do as well in the urban environment.But the numbers are closer than "doesn't stand a chance".

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