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FACT SHEET: Coastdown Facts

Overview

MPG 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 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 size and 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 80 miles per hour (mph), after which it is put into neutral and allowed to coast until its speed drops below 9 mph.
  • Special devices in and on the vehicle accurately measure environmental conditions (ambient temperature, humidity, wind and barometric pressure), performance data, and speed during the vehicle’s deceleration.
  • In order to eliminate the effect of wind speed and direction, the test is performed multiple times in both directions on the track (a minimum of 5 sets consisting of a total of 10 runs, half in one direction and half in the other) on a flat, straight and dry track. Analysis of the recorded speed and deceleration 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 economy and emission values.
  • EPA began to publish fuel economy results, which were produced as a by-product of emissions testing.
  • Manufacturers began to increase fuel economy in response to public demand for more fuel efficient cars.
  • The coastdown test is the standard industry technique to determine vehicle road load, which coupled with dynamometer testing, enables the manufacturer and EPA to measure fuel economy and emission values under simulated driving conditions.
  • The Society of Automotive Engineers (SAE) developed a standard procedure (J2263- Dec 2008) to perform road load measurement using coast down testing.

Resources
The current test procedure used by the government for road load measurement using onboard anemometry and coastdown testing techniques is SAE International Standard J2263, which was most recently updated in 2008.

October 12, 2014

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