Thursday, 22 July, 2021

Automobile: why my consumption exceeds that announced by the manufacturer

A given hybrid for less than 6 liters of super per hundred kilometers? A beautiful Diesel SUV for less than seven? If only to approach the official consumption values ​​printed on advertising catalogs, you have to travel the roads of a decidedly very flat country and comply with the strictest respect for the precepts of eco-driving. Do not see in this gap between real and official consumption any trick on the part of the manufacturers: they are only taking advantage of the leniency of European regulations.

Over the years, in fact, engineers have devised a thousand ways – all perfectly legal – to optimize the fuel consumption and polluting emissions of the vehicles they present for homologation tests. The values ​​recorded in for the new model presented are valid for each of the thousands of copies produced identically.

Since September 2017, however, the rules of the game have been tightened considerably. Firstly, engines are tested harder, in order to get as close as possible to the reality of driving. On the other hand, each of the variants of the same model must be subjected to two test protocols: the first on a laboratory bench, where the movement of the road is simulated by the rotation of rollers under the wheels of the immobilized vehicle; the second outdoors, when the moving vehicle is equipped with miniaturized measuring instruments. In either case, the technician behind the wheel is responsible for reproducing a codified sequence of very precise accelerations and decelerations. On his control screen, he observes the movements of a cursor which moves according to the pressure of his foot on the accelerator: it is up to him to follow a predefined curve as closely as possible.

Until 2018, the official consumption was that of the lightest model

The importance of the gap between official and actual consumption is also explained by the fact that the manufacturer was authorized until the fall of 2018 to submit the homologation tests to the the most stripped-down variant of its models. The consumer, for his part, almost always buys a car that is better equipped – and therefore heavier – and fitted with wider tires that consume more energy, because it offers greater resistance to forward movement. This practice ended in September 2018 with the entry into force of the WLTP regulation, which requires manufacturers to submit to homologation tests each variant of their models. A painstaking job.

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The Association of European Automobile Manufacturers (ACEA) indicates that the fitting of 18-inch rather than 17-inch wheels results in a increase of 2 g / km of the average CO2 emission value. The penalty is still 1 g / km of CO2 for vehicles which do without a spare wheel and run with four puncture-proof tires (known as “run flat” or “run flat”). This is how each piece of comfort or safety equipment weighs on the scale and inevitably increases fuel consumption.

Large wheels and wide tires increase fuel consumption

This is why manufacturers encourage their customers to go through the accessories box rather than options: the idea is to order a rather stripped-down variant of your sedan or SUV, then entrust your dealer with the assembly of the coveted large wheels, roof bars that increase aerodynamic drag or even the motorized hitch that weighs down the car.

Although legal, this practice runs counter to the legislator’s objective of delivering the most realistic possible hundred kilometer averages to consumers, personalized for each model. Above all, this drift deprives the consumer of any chance to equal the official average consumption value, calculated for a vehicle by definition less heavy.

Difficult to reproduce the reality of the road in the laboratory

Even made more realistic in 2018, homologation tests can only provide an approximation of the reality of road traffic conditions in Europe. The contingencies are too numerous, the Too varied route and user profiles. It is obvious that one does not ride in the Alps as on the polders; that the cars trapped in sprawling cities consume more fuel than those that cross peaceful countryside.

The topography and the behavior of the driver have a particularly noticeable influence on the real appetite of a plug-in hybrid car, capable of covering 40 to 60 kilometers in all-electric drive mode. As long as you leave with a fully charged battery!

Battery empty, the consumption of a plug-in hybrid soars

The nature of the old NEDC certification tests outrageously favored this type of plug-in hybrid. So much so that Volkswagen could announce a truly incredible official average consumption of 1.6 l / 100 km for its large Passat GTE sedan. Switching to the WLTP standard (which provides for measuring both empty and full battery consumption) makes consumption values ​​much less flattering. So much so that Volkswagen has decided like other manufacturers to suspend marketing of its plug-in hybrids, the time to optimize their mechanics by transplanting a larger battery and a more fuel-efficient gasoline engine.

The owner of a plug-in hybrid car will be well inspired to take the trouble to plug in the power outlet every evening. Because gasoline consumption soars once the battery is exhausted. Our colleagues from the weekly Auto Plus established an average fuel consumption of 3.6 l / 100 km behind the wheel of a Volkswagen Golf GTE plug-in hybrid. On the first hundred kilometers only because, beyond, the average climbs to 6.9 l / 100 km. Compare to the 7.7 l / 100 km of the Golf TSI petrol and the 6.6 l / 100 km of the Golf GTD Diesel.

WLTP test cycle sequence

With the trend towards urbanization, the legislator wanted to include more stops and starts in its consumption test cycles, with more frequent gear changes. These are now calculated according to an algorithm taking into account the mass of the vehicle, its power, the number of gear ratios. What to put an end to the “long boxes”, unpleasant in use but favored by the old NEDC cycle.

The car harnessed to the test bench is rolling for 30 minutes at an average speed of 46 km / h (against 33 km / h previously, according to the NEDC cycle) according to four successive phases standardized by the WLTC test protocol. A first phase at low speed lasting 589 seconds, a second at medium speed (433 seconds), a third at high speed (455 seconds) and a fourth at very high speed (323 seconds). What respectively simulate journeys in town, on the road, on the expressway and on the motorway. In the latter case, the maximum speed reached during the cycle is 135 km / h.

Although imperfect, the WLTP standard guarantees a fair comparison between vehicles.

It is undoubtedly worth remembering that as fuel consumption increases with the cube of speed, fuel economy is much more noticeable when the driver reduces his speed from 130 km / h to 110 km / h, than when it travels at 80 km / h rather than 90 km / h.

At the time of writing, the application of the WLTP standard to all models on the market is still too recent to verify whether the official consumption values ​​are closer to reality than under the rule of the rules. NEDC. The affirmative would result in greater ease for the motorist to match in real traffic conditions the three official values ​​given for the average fuel consumption in town, on the road and in the combined cycle.

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