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Coastdown Facts

Coastdown Facts


Overview

A coastdown test is a procedure that determines key metrics used to calculate a vehicle’s fuel consumption values or “L/100kms rating.”   The litres per 100kms ratings are established using a machine called a “dynamometer.”   A dynamometer is like a treadmill for vehicles, enabling vehicles to be operated indoors on a stationary platform to simulate real-world vehicle operation.  The level of resistance on the dynamometer is adjusted for each specific vehicle model tested to simulate the level of resistance that the vehicle would encounter if operated outside on the road.   Coastdown testing is used to determine the appropriate resistance levels (or “road loads”) to use on the dynamometer for a given vehicle model.  Coastdown testing is used to measure all types of resistance encountered by a given vehicle model during real-world operation, including:

  • Vehicle aerodynamic resistance, a factor affected by the vehicle’s shape, which determines how much air the vehicle has to push out of the way as it moves. The more resistance, the more energy has to be expended.
  • Tire rolling resistance, a factor related to tire design that determines how much energy the vehicle has to use to overcome the resistance caused by the interface between the tires and the road.
  • Driveline and powertrain mechanical resistance, a factor of the vehicle’s drivetrain and how much energy the vehicle has to use to overcome internal friction to drive the wheels.

Coastdown Method

  • A vehicle that has been properly broken in prior to the test (generally the mileage, fluids and fuel, tires and vehicle warm-up) is driven up to a certain speed, typically around 128 KPH, after which it is put into neutral  and allowed to coast until its speed drops below 14 KPH.
  • Special devices in the vehicle accurately measure environmental conditions (ambient temperature, humidity and barometric pressure), performance data, and speed and distance traveled during the vehicle’s deceleration.
  • In order to eliminate the effect of wind speed and direction, the test is performed multiple times (a minimum of 5 runs) on a completely flat, straight and dry road in both directions of the track.  Analysis of the recorded speed and distance information provides the vehicle’s road load force.

Background

  • A dynamometer is an electric motor that is connected to the vehicle and simulates standard highway and city driving cycles, enabling generation of simulated fuel consumption and emission values. 
  • EPA and NRCan publish fuel consumption results, which are a by-product of emissions testing.
  • Manufacturers began to reduce road load in response to public demand for more fuel efficient vehicles.
  • The coastdown test is the standard industry technique to determine vehicle road load, which coupled with dynamometer testing, enables the manufacturer, the EPA and NRCan to measure fuel consumption and emission values. 
  • The Society of Automotive Engineers (SAE) developed a standard procedure (J2263- Dec 2008) to perform road load measurement using coast down testing.
  • The Society of Automotive Engineers (SAE) developed a standard procedure (J1263- Mar 2010) to perform road load measurement and dynamometer simulation using coast down testing.

Resources

  • The current government-approved standard for road load measurement using onboard anemometry and coastdown testing techniques is SAE International Standard J2263, which was most recently updated in 2008. 

November 15, 2012

Author
Hyundai Auto Canada Corp.

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