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The EPA fuel economy test consists of two driving routines, or schedules, performed in the laboratory.

City: Represents urban driving, in which a vehicle is started with the engine cold and driven in stop-and-go rush hour traffic. The driving cycle for the test includes idling, and the vehicle averages about 20 mph.

Highway: Represents a mixture of rural and Interstate highway driving with a warmed-up engine, typical of longer trips in free-flowing traffic. Average test speed is about 48 mph and includes no intermediate stops or idling.

Trip Type
Low speeds in stop-and-go urban traffic
Simulated Distance 11 miles
Time 31 minutes
Average Speed 20 mph
Top Speed 56 mph
Stops 23
Idling time 18% of time
Engine Temp. at Startup* Cold
Lab temperature 68-86 ºF
Vehicle air conditioning Off

Free-flow traffic at highway speeds
Simulated Distance 10 miles
Time 12.5 minutes
Average Speed 48 mph
Top Speed 60 mph
Stops: None
Idling time: None
Engine Temp. at Startup*:Warm
Lab temperature 68-86 ºF
Vehicle air conditioning Off

* A vehicle's engine doesn't reach maximum fuel efficiency until it is warm.



Fuel economy is measured under controlled conditions in a laboratory using a standardized test procedure specified by federal law. Manufacturers test their own vehicles—usually pre-production prototypes—and report the results to EPA. EPA reviews the results and confirms about 10-15 percent of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.


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In the laboratory, the vehicle's drive wheels are placed on a machine called a dynamometer that simulates the driving environment—much like an exercise bike simulates cycling.

The energy required to move the rollers can be adjusted to account for aerodynamic forces and the vehicle's weight.


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On the dynamometer, a professional driver runs the vehicle through a standardized driving routine, or schedule, which simulates “typical” trips in the city or on the highway.

See video


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Each schedule specifies the speed the vehicle must travel during each second in the test.

Right: The driver watches a computerized display that shows his driving statistics compared to the specified schedule.

See video


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A hose is connected to the tailpipe to collect the engine exhaust.

The amount of carbon in the exhaust is measured to calculate the amount of fuel used during the test.

This is more accurate than using a fuel gauge.


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Adjusting Estimates

In the 1980s, an EPA study found that drivers were typically achieving lower fuel economy than predicted by EPA laboratory tests. As a result, EPA required the laboratory-derived city and highway MPG estimates posted on the labels of new vehicles to be adjusted downward by 10 percent for city estimates and by 22 percent for highway estimates to better reflect the MPG real-world drivers can expect.



...What I think is driving the city number up so high on the Prius is the fact that the ICE shuts off instead of idling for extended periods of time. The emissions based fuel useage estimate is very interesting. The Prius fuel computer I'm quite sure does not give us a little boost in mileage numbers based on how long the car is stopped and the ICE not running and fuel not used. I think this is why some of our own observations can show numbers slightly higher that what the Prius is telling us its getting.

More information can be found at the following website:
http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

Dave Allen
2005 Prius Package 4
 
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