Tech Talk – How fuel consumption tests work?

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Tech Talk - Fuel Consumption Test

Ever wondered why you can’t match your manufacturers claimed fuel consumption figures on your car, no matter how gingerly you drive?  Chances are, you have, and chances are you’ve probably thought the manufacturers have been lying through their teeth – real world figures simply don’t match claimed consumption, and the same goes for emissions.

 

But, in manufacturers’ defence, they aren’t just thumb sucking random figures that are better than their competitors’ claims.  The truth is, they’re all subjected to a specific testing method, by law, which is used to derive fuel consumption claims as well as test emissions for both petrol and diesel powered vehicles.  It’s called the New European Driving Cycle (NEDC) test, and this is how it works:

 

Urban Driving Cycle:

 

It all starts off with an urban fuel economy run, which takes place over the first 780 seconds (13 minutes) of the test, and covers 2976.1 meters, with an average speed of 18.35km/h.  This Urban Driving Cycle (UDC) test is run at 25 °C, and starts with a cold engine from start-up of the vehicle.  The steps of the test are run as follows:

 

  • After ignition, the car stands for 11 seconds (manual vehicles stand for 6 seconds in neutral and 5 seconds with the clutch engaged and gearbox in 1st gear).
  • The vehicle slowly accelerates to 15km/h over 4 seconds, and then cruises at 15km/h for 8 seconds, before slowing to a stop in 5 seconds.
  • The vehicle remains stationary for 21 seconds (manual vehicles: 16 seconds in neutral and 5 seconds in 1stgear with the clutch pedal engaged).
  • At 49 seconds into the test, the car accelerates to 32km/h over 12 seconds (manual vehicles spend 5 seconds in 1st gear and 5 seconds in 2nd, with 2 seconds allowed for gear changes).
  • The vehicle then cruises for 24 seconds, and slowly brakes to a stop over 11 seconds (manuals disengage the clutch over the last 3 seconds).
  • The vehicle again remains stationary for 21 seconds (manual vehicles: 16 seconds in neutral and 5 seconds in 1stgear with the clutch pedal engaged).
  • At 117 seconds, the vehicle accelerates to 50km/h over 26 seconds (manual vehicles spend 5 seconds in 1st gear, 9 seconds in 2nd gear, and 8 seconds in 3rd gear, with 2 seconds allowed for each gear change).
  • The vehicle then cruises for 12 seconds at 50km/h, before decelerating to 35km/h over 8 seconds.
  • The vehicle continues to cruise at 35km/h for 13 seconds, and then brakes to a halt over 12 seconds (Manual vehicles are required to downshift to 2nd gear which takes 2 seconds, remain in 2nd for 7 seconds, before disengaging the clutch for the last 3 seconds).
  • The vehicle remains stationary for 7 seconds in neutral.

 

This takes 195 seconds in total and the vehicle covers 994.03 meters.  It is repeated 4 consecutive times; with the final consumption results being given as the Urban Consumption quote your manufacturer supplies.

 

Extra-Urban Driving Cycle:

 

The EUDC is designed to represent more ‘aggressive’, high speed driving.  The test is conducted with a maximum speed of 120km/h, but low powered vehicles are limited to 90km/h. The EUDC test takes place over 400 seconds (6 minutes 40 seconds) and covers a distance of 6956 meters at an average speed of 62.6km/h.  The test is run as follows:

 

  • After a 20 second stop, following the UDC test, the vehicle accelerates to 70km/h over 41 seconds (manual vehicles spend 5 seconds in 1st gear, 9 seconds in 2nd, 8 seconds in 3rd, and 13 seconds in 4th gear, with 2 seconds allowed for each gear change).
  • The vehicle then cruises for 50 seconds (manual vehicles are required to be in 5th gear for this), before decelerating to 50km/h over 8 seconds (manual vehicles are expected to shift to 4th gear after 4 seconds).
  • The vehicle then cruises at 50km/h for 69 seconds, before accelerating to 70km/h over 13 seconds.  It cruises at this speed for 50 seconds (manuals in 5th gear), before slowly accelerating to 100km/h in 35 seconds, and cruising for a further 30 seconds (manuals in 5th or 6th gear if available).
  • Lastly, the vehicle accelerates to 120km/h over 20 seconds, cruises for 10 seconds, and then slowly brakes to a halt over 34 seconds, before idling in neutral for 20 seconds.

 

This test gives the Extra-urban consumption figure you’ll often be quoted by manufacturers.

 

Combined Cycle:

 

The combined fuel consumption figure manufacturers quote is the overall consumption based on both the Urban and Extra-urban tests run back to back.  The total time taken is 1180 seconds (19 minutes 40 seconds), and the total distance covered is 11 023 metres at an average speed of 33.6km/h.

This graph displays the speed vs. time of a full NEDC test cycle.
This graph displays the speed vs. time of a full NEDC test cycle.

 

Adjustability of NEDC tests:

 

NEDC tests are required to be completed on a flat road at a temperature between 20 °C and 30 °C, with no wind – however since this is impossible to come across in the real world, tests are conducted on a rolling road system in a controlled laboratory environment.  All tests are run with secondary systems turned off, including air conditioning, lights, radio systems etc.

 

A fan is provided ahead of the vehicle that adjusts to mimic airflow at different speeds, and the rolling road system is able to provide adjustable rolling resistance based on the vehicle’s weight, drag coefficient, and body style.  As such, by making slight changes to the test apparatus, manufacturers can achieve results for multiple body styles (hatchback, sedan and station wagon) with only one test unit available, simply by changing the equipment setup accordingly.

 

The problem with NEDC testing:

 

Do you drive in the manner in which vehicles are tested as explained above?  No?  we thought not.  No one does.  If we all took that long to accelerate, and if we drove without air conditioning, radios, and headlights, we’d never get to our destinations, and we’d lose our minds in the process.

 

That’s the biggest flaw with the NEDC test setup – it fails to accurately mimic real world driving conditions (it was set up in 1970 originally, so is it any wonder it feels a bit archaic?).  Since emission data is also measured based on these tests, it’s also fair to assume that the claimed emission figures manufacturers supply are also grossly under rated.  Don’t blame the manufacturer though – they’re forced to test their vehicles like this.

 

Another problem with NEDC testing is that it easily gets cheated by modern turbocharged setups.  How often have you heard of manufacturers claiming wonderfully low consumption figures for their 1 litre turbo three cylinder powered cars, only for naturally aspirated competitors to be no worse in the real world?  Case in point, The Suzuki Swift’s NA 1.4-litre consumes 5l/100km in the real world, matching the consumption we achieved in the Ford Fiesta 1.0 EcoBoost.

 

This is because the NEDC tests put such little strain on the turbocharged engines that the turbos aren’t even required to spool up due to the low loads.  Consumption figures and emissions are massively reduced on paper, but in the real world they remain as bad as they were before.  Road and Track covered this beautifully in a recent rant of theirs which you can read HERE.

 

 

In the end, it’s no wonder we can’t match claimed consumption figures.  So, next time you fill up and wonder why you’re consuming so much more than the manufacturers’ claims (up to 40% more usually), know that you’re not alone, and that it isn’t the manufacturers fault they can’t be ‘honest’ – they’re just following the rules.

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