Service Information: Fuel consumption of plug-in hybrid vehicles

General notes

In order to be able to objectively compare various vehicles with each other in terms of consumption and emissions and to be able to reproduce such results, the responsible legislative authorities must define standardized driving cycles. For example, Europe and Switzerland currently use the so-called "New European Driving Cycle (NEDC)".

With the binding introduction of the New European Driving Cycle (NEDC) in line with European Directives 70/220/EWG, 80/1268/EWG and 93/116/EWG on the 1st. January 1996, a technical option was created whereby a manufacturer's vehicles could be compared with each other and the vehicles from another competitor manufacturer. For certification purposes the consumption and emission measurements are conducted here on a roller dynamometer, with a stringent, legally regularized process.

The exact driving cycle (NEDC) and the test conditions are described in Directive 70/220/EEC. Here is an overview of the most important criteria:

 The vehicle must comply with the series production configuration.

Any deviations such as, e.g. retrofitted installation of accessories, custom wheels, add-on roof equipment etc. are not permissible. Ex-factory special equipment may not to be taken into consideration either.

 The test fuel must comply with the CEC reference fuel features (use brand-name fuel).

 The ambient temperature must lie between 20 °C and 30 °C, the air pressure must be 1000 hPa.

 At the start of the measurement, the vehicle temperature (engine oil and coolant) must match the ambient temperature (20 ­ 30 °C).

 Lights, air conditioning and other electrical consumers to be deactivated.

 NEDC consists of a city (approx. 4 km) and a highway area (approx. 7 km). The city part of the driving cycle consist of defined idle, acceleration, constant speed and deceleration phases. It lasts for 780 s (13 min) and an average speed of 19 km/h is reached. The maximum speed in the urban driving cycle is 50 km/h. The highway driving cycle last for more than 400 s (6:40 min), at an average speed of 62.6 km/h and a maximum speed of 120 km/h. It consists mainly of constant driving phases with 50, 70, 100 and 120 km/h.

 In the NEDC, the journey on level ground is also simulated. In other words, gradients are not part of the driving profile.

NEDC was originally meant to be introduced solely as an emissions test (for EURO 0 to 6). The intention behind it was not to map the actual and real consumption on the road, but only to achieve reproducible and comparable emission values on the test bench.

Special feature of the consumption measurements in the NEDC for plug-in hybrid vehicles

NEDC is run through several times when measuring consumption as per the ECE Directive R101 for plug-in hybrid vehicles.

To start with, the maximum electric range is determined by running the vehicle, with fully-charged high-voltage battery, through the NEDC several times until the high-voltage battery is fully discharged. The electric range is determined on the basis of the route segments covered under electric power.

Finally, the vehicle runs through the NEDC once with fully-discharged high-voltage battery. The consumption determined here is set in relation as per ECE R101 with the determined electric range as well as an average defined distance of 25 km up to the next charging process at a power source. This results in a combined absolute consumption value, normalized for a distance of 100 km.

When conducting the cycle, with a discharged high-voltage battery at the start, brake energy can still be recuperated, and used to support acceleration. The vehicle in decel mode can shut-off the internal combustion engine to optimize the consumption level.

Procedure when handling customer complaints about fuel consumption

The requirements on the vehicle as well as the test specifications for the fuel consumption certification are extensive and legally prescribed. In comparison to the certified standard values, deviations normally occur under real driving conditions. To start with, this results in altered boundary conditions for the legally prescribed standard condition in the laboratory. This includes different temperature conditions, vehicle loads or active auxiliary consumers. For example, the power requirements for an A/C system is approx. 1500 W and that for a seat heater approx. 300 W.

This means that deviations for the fuel consumption values are possible and explainable. To the same degree, the electric range of plug-in hybrid vehicles is also influenced by these factors. In other words, the electric range display in the instrument cluster may deviate from the maximum value according to the certification even when the high-voltage battery is fully charged. The electric range display also represents a forecast based on the charge level, external influences and the customer's driving behavior.

During a personal discussion with the customer, please refer to the test prerequisite for the NEDC and explain that the test procedure here is an exactly-defined, legally-prescribed one. This is the only way of ensuring that vehicles from different manufacturers can be compared against each other. The certified fuel consumption is therefore only partly suited to confirming the consumption data achieved by the customer in his driving profile and under his conditions. If a customer should complain about significant differences in consumption data, it is very important that the customer's consumption data are subjected to a critical analysis and that technical facts are then referred to when clarifying the matter. It is necessary here to ensure that the vehicle has been run-in and that it has covered a distance of at least 3,000 km (better would be 10,000 km, because at such a driven distance, significant reductions in consumption can be determined).

The following diagram illustrates the relationship between the combined fuel consumption and the distance covered qualitatively for plug-in hybrid vehicles as well as vehicles with conventional drive train.

P07.00-2245-05

The combined fuel consumption for a plug-in hybrid vehicle is lowest for short driving distances that can be covered using electric or partially electric power, and it can even lie significantly below the certified combined consumption level. Once the high-voltage battery has discharged, the combined consumption increases slowly, as a greater portion of the driving distance can be covered with the internal combustion engine. On longer driving distances, the combined consumption may therefore lie above the certified value.

When talking to the customer, this relationship should be discussed extensively. A higher fuel consumption in comparison to the certified value under real driving conditions, insofar as a technical defect is not present, can therefore be explained.

 Politics and the automotive industry have been working for some time now on the introduction of the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). This is intended to replace the NEDC with the aim of bringing standard and real consumption as close together as possible in future. A measuring method in Europe is also to be introduced for Real Driving Emissions (RDE). Emissions here are measured under real driving conditions. Both activities will be fully and actively supported by Mercedes­Benz.