Freedom and utility.  Find out why the declared values ​​differ

Freedom and utility. Find out why the declared values ​​differ


The declared and actual values ​​of fuel and energy consumption, as well as the range of electric vehicles, often differ, despite being approved in regular tests. Find out the main reasons, some of which seem to be of little importance.

In the tests carried out on cars and special publications – printed or online – the reader will have already noticed that there is a difference in the values ​​​​related to the consumption of fuel or energy in the case of electric cars between what is advertised by the brand and measurements in real conditions of use.

Those differences are due to a set of completely unpredictable factors, based on a surprisingly different set of factors.

Car manufacturers are required to report the range or fuel consumption based on a typical cycle. Between 1992 and September 2017, the NEDC (New European Driving Cycle) protocol was used, which was replaced by WLTP or WLTC (the WLTP standard includes the RDE cycle in real driving conditions, as well as the WLTC cycle carried out in the laboratory . . ).

A new cycle was introduced to make the claimed level or fuel consumption figures better reflect the actual conditions that drivers can achieve. Therefore, WLTP uses a higher speed (up to 135 km/h and a higher average total speed), is more powerful and takes into account more of the actual weight of the vehicle, as well as other factors.

WLTP cycle rules

WLTP (Worldwide Harmonized Light-duty Vehicle Test Procedures) includes a set of test procedures used to harmonize vehicles, including the WLTC (Worldwide Harmonized Light-duty Vehicle Test Cycle) laboratory test and a practical driving test known as RDE.

The WLTC test lasts 30 minutes, during which the car is driven on rollers for a total of 23 km at an average speed of 47 km / h. The circuit has four phases of severity, from the lowest to the highest, in which the car exceeds the speed of 130 km / h, all at a temperature of 14 ° C.

Photo: Turbo

The need for the car to be stationary is also taken into account, so it passes 13% of the test, or more than three minutes, at rest. In addition, the WLTP takes into account the additional equipment of the vehicle, which the manufacturer must test the use and scope or recalculate as defined. The WLTP test is performed with the air conditioning switched off.

However, the WLTC test cycle remains a laboratory exercise to ensure that reported values ​​are comparable. This allows customers to compare ranges or consumption figures not only between models from the same car manufacturer, but also between cars from different brands.

When comparing two models, it is more likely that the car with the higher WLTP range will also have the higher range in practice. But also that the declared values ​​and the real ones are different.

four reasons

According to Jan Beneš, an expert in customer test cycles at Skoda, these differences can be divided into four groups: “The first is the physics of the car, that is, aerodynamics, weight and rolling resistance; the second is the environment, that is, the weather and outside temperature; The driver’s driving style is also important and of course the road profile, which can often be more important in practice than on the test track.

The official emphasizes that “smooth and early driving, without rapid acceleration, in hot weather, without wind and on a flat road with an unloaded vehicle results in lower fuel consumption and greater freedom”.

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Drivers rarely drive in such ideal conditions. If they succeed, they may even get longer and better fuel consumption than the “factory” numbers indicate.

The proof is in many tests published in automotive magazines, various ecological meetings and attempts to win driving records with the lowest possible fuel consumption. In practice, however, there are common factors that mean that the use of the car increases compared to the declared value and that the variety, logically, decreases.

Heat affects freedom

In electric vehicles, one of the factors affecting autonomy is the outside temperature, which affects the efficiency of the traction battery, but also the need for heating or cooling the interior.

Both for cooling and heating are not considered in the test cycle. “For the traction battery itself, the ideal operating temperature (inside the cell modules) is between about 10 and 35°C,” says David Pekárek of Skoda’s High Voltage Power Systems department.

“At high temperatures, the cooling of the battery will already be activated by the high-voltage air conditioning system, which uses electricity. At low temperatures, due to the slow nature of the chemical processes in lithium-ion cells, The charging and discharging capacity of the battery is gradually reduced, which reduces the efficiency of regeneration, for example.” Czech brand.

“At sub-zero temperatures, the battery needs to be recharged in an active way (using a high-voltage water heating system),” adds David Pekárek.

Spring and summer are best

For an electric car, the best conditions are observed in spring or autumn, when the sun has enough power to heat the interior at room temperature, without the need for heating or air conditioning, and the traction battery will not need heating or cooling. .

Of course, the battery is also affected by the driving style. With hard braking and hard acceleration, the battery can heat up the battery so much that it has to be cooled, even in cold weather. Therefore, the driver himself can greatly affect the experience, since in addition to demanding acceleration and deceleration, high speed also has a negative effect.