Claimed vs. Real Fuel Consumption

FOUR litres per 100 km. We’ve all seen fuel-consumption figures bandied about by automakers in the press and advertisements, and on the new vehicles that stand on the showroom floor. Are these figures achievable? Judging by the number of complaints we receive on the matter from readers after purchasing new vehicles, we have our reservations. But first, let’s investigate how these claimed figures are calculated.

The European Standard

In order to test fuel consumption, a set drive cycle is required. The European Union (EU) has implemented the New European Drive Cycle (NEDC) as the legislated drive cycle that must be used by automakers that sell their vehicles in Europe. The NEDC’s main purpose is to gather emission data (including CO2 figures), with fuel consumption as a by-product (see page 114 sidebar). Because many of the cars that are sold in South Africa are also available in Europe, locally the NEDC figures are quoted. Other drive cycles include the EPA Federal Test in the United States and the JC08 in Japan.

In the NEDC (see figure), the speed/time profile has urban and extra-urban components.

The first part (urban) represents a vehicle operating in stop/start traffic at low speeds and loads. The fuel used during the first part of the cycle is quoted by manufacturers as the urban fuel-consumption figure.
The second part represents B-road and motorway driving, evidenced by higher speeds. Manufacturers will quote the fuel used during this part of the cycle as the extra-urban figure. Fuel used over the entire cycle will be quoted as the combined fuel figure for the vehicle.

You should note that the average speed during the cycle is low, with many steady-state sections, and acceleration is quite gentle, which implies a very conservative driving style.

The Test Setup

Repeatability during emissions (and fuel-consumption) testing is very important and therefore testing is conducted in state-of-the-art facilities, where even temperature and humidity levels can be controlled. The vehicle is strapped down with the driven wheels on rollers to mimic the longitudinal road loads seen by the vehicle through the use of a chassis dynamometer.

A human or robotic driver is employed to “drive” on the set drive cycle. A large fan in front of the vehicle is used for cooling purposes and all the exhaust gases leaving the exhaust pipe are captured and analysed by advanced gas analysers.

The Test Vehicle

The vehicle that is used for the certification process must be completely representative of the one a customer can purchase in the showroom, including the powertrain calibration. There-fore, all fluid levels and tyre pressures must be to the correct specification. Aerodynamic loads (and rotational loads of non-driven wheels) are modelled and added to the road load model of the chassis dynamo-meter, as the vehicle is stationary during the test.

The Test Procedure

All vehicles are pre-conditioned for at least six hours at 22 degrees Celsius before the test is run at a similar ambient temperature and at sea-level air pressure. It is important that the engine is started from cold at the beginning of the cycle as a worst-case emissions condition and also for repeatability. The vehicle is driven on the speed profile as prescribed by the NEDC and within a tolerance band. If the driver (or robot) deviates outside this, the test is scrapped. During the 20-minute test, no energy-sapping auxiliaries like climate control, audio system, heated seats or headlamps (except daytime running lamps) are switched on.

Differences to real-world driving

It’s clear from analysing the NEDC why fuel-consumption figures are much lower than those achieved by an owner of the vehicle on a daily basis. In summary:
• The test is always run at sea level and at 22 degrees Celsius ambient temperature;
• Low average speeds and gentle acceleration mimic a very conservative driving style;
• The steady-state nature of the speed profile is unrepresentative of real-world driving;
• No electrical auxiliaries are switched on, which lessen the load on the alternator and lowers fuel consumption.

Although the NEDC figures are very useful when comparing figures for different vehicles, they tend to be lower than figures achieved on the road (see CAR’s fuel-route figure). Manufacturers have also learned how to use a specific test cycle to their advantage (see Beating the curve) and are developing new vehicle technologies especially for the test.

A realistic test on the cards

The only way to address the pitfalls of the NEDC is to design and legislate a more representative, real-world drive cycle. One such test is already under development: the Worldwide Harmonised Light-vehicles Test Procedure (WLTP) cycle (see graph). The average speed of the new cycle will be much higher and the speed profile more transient. Another addition will be the inclusion of electrical auxiliary loads (still under discussion) to further aid real-world representation.

A problem with legislating a new drive cycle is that the official emissions and fuel-consumption figures will be different compared to the NEDC results.

This will impact current CO2 targets set for automakers by the EU (example: less than 95 g/km fleet average in 2020) and the emissions-based tax we pay. Until the new drive cycle is introduced, we will have to live with the unrepresentative figures and remind ourselves that the biggest factor influencing actual fuel consumption is driving style.

Beating the curve

Engineers have ingenious ways of achieving optimal results and this also applies to the NEDC test. As the speed points are known, engineers optimise the gear ratios, final-drive ratio, wheel sizes and the engine calibration to achieve the best results on a specific cycle. New technologies found in modern cars, such as stop/start and electric power steering, can be seen as a direct result of attempting to achieve the lowest fuel consumption possible on the NEDC. There is a large percentage of stationary time (where stop/start saves fuel) and the steering is never turned (no energy used) during the test. The problem is that fuel consumption at other speeds and load points (which are found in real life) might not be as efficient.

Determining fuel consumption on the NEDC

The complex nature of a modern vehicle’s fuel-injection system makes it difficult to measure fuel consumption by tapping into the fuel lines or using separate fuel containers on scales. An easier and accurate way is to calculate fuel consumption by analysing the emissions data. Regulation 101 from UN-ECE stipulates the following carbon balance equation for spark-ignition (petrol) engines:

FC = (0,1154 / D) x [(0,866 x HC) + (0,429 x CO) + (0,273 x CO2)] 

Where:

FC = the fuel consumption in litres/100 km

HC = the measured emission of hydrocarbons in g/km

CO = the measured emission of carbon monoxide in g/km

CO2= the measured emission of carbon dioxide in g/km

D = the density of the test fuel at 15 degree Celsius in kg/litre

 

Article written by

CAR magazine