Brief: The U.S. Environmental Protection Agency (EPA) is the authority for environmental certification of vehicles in the U.S. and is responsible for monitoring compliance with mandatory standards for vehicle emissions. The EPA is mandated by the《Clean Air Act》to test and rate the fuel economy of all vehicles on the road, and in fact the EPA certification used to rate electric vehicles is very similar to the method used to test conventional vehicles.
BRIEF ANALYSIS OF CERTIFICATION
In order to better understand the EPA certification of electric vehicles, the following terms need to be briefly introduced:
1. The Dynamometer and Driving Cycles
The EPA tests cars on a dynamometer, also known as a dyno. A dyno is kind of like a treadmill for cars – the car is placed on rollers so the car can be “driven” while staying locked in place. Dynos can be used to gather all sorts of data about a car’s performance, such as horsepower and fuel economy. On the dyno, vehicles are put through different driving cycles, where the cars are sped up or slowed down to hit prescribed speeds to simulate different kinds of driving. The two important cycles that EVs are put through are the Urban Dynamometer Driving Schedule (UDDS) and the Highway Fuel Economy Driving Schedule (HWFET).
2. Urban Dynamometer Driving Schedule（UDDS）
The UDDS cycle simulates stop-and-go city driving by getting the test vehicle up to speed and back down to zero. It’s used to measure city fuel economy.
3. Highway Fuel Economy Driving Schedule (HWFET)
The HWFET cycle simulates sustained-speed highway driving by getting the vehicle up to speed, then having it fluctuate between various speeds in the 30–60 mph range over the course of the test. The wheels don’t come to a complete stop until the end. This test measures highway fuel economy.
4. Constant Speed Cycle
The constant speed cycle is used to rapidly deplete battery energy, and consists of a steady-state speed schedule of 105km/h (65 mi/h). The initial acceleration to 105 km/h (65 mi/h) - or 90% of maximum sustainable speed if a vehicle cannot reach 65 mi/h - must be smooth and accomplished within 1 min of the key switch being placed in the "on" position. The vehicle's speed must be held within the tolerances defined in Standard.
5. Single Cycle test(SCT)
The SCT is based on the May 1993 edition of SAE J1634 and consists of two UDDS and two HFEDS until the battery is discharged and the vehicle can no longer be driven on the driving cycle, charged to full capacity using normal AC power. The energy consumption of the vehicle (in kWh/mile or kWh/100 miles) is determined by dividing the kWh of energy charged by the number of miles travelled by the vehicle, and the EPA multiplies the initial range value by a factor of 0.7 to obtain the range value.
6. Multi- Cycle City/Highway Test(MCT)
The MCT test is based on the October 2012 edition of SAE J1634. The recommended practice is to fully charge the battery, park the vehicle overnight and then cycle the vehicle through continuous UDDS, HWFET and CSC states the next day until the charge is depleted and the vehicle can no longer be driven in the driving cycle, during which the DC discharge energy and DC discharge ampere-hours are The DC discharge energy and DC discharge ampere-hours are measured throughout the test. Once the test is complete, the battery is charged from a normal AC source using the charger provided by the manufacturer. The energy consumption of the vehicle (in kWh/mile or kWh/100 miles) was determined by dividing the kWh energy of the battery charge by the number of miles travelled by the vehicle. The charge energy includes all losses due to inefficiencies in the manufacturer's charger. the EPA will multiply the initial range value by a factor of 0.7 to obtain the range value.
7. Coast down testing
Vehicles tested on the dynamometer need to simulate road loading forces, which are the sum of the forces acting on the vehicle in terms of aerodynamic drag, tyre rolling resistance, driveline losses and other frictional influences. Determining the dynamometer setup to simulate road loading forces is divided into two stages: i. Specification of road loading forces by characterising the road operation. Two, road load derivation is carried out to determine the appropriate dynamometer load settings to simulate the specification of road load forces in a road test.
8. 4,000 miles test point
The test vehicle and tyres should have sufficient mileage to represent the road load force at the 4,000 mile test point. If the vehicle has accumulated more than 6,200 miles, the data should be adjusted to represent the road load at the 4,000 mile test point. Test vehicles should not exceed 10,000 miles. The ageing of the tyres may be carried out separately from the test vehicle.
9. Electric Vehicle Fuel Economy Label
For electric vehicles, the fuel economy label is somewhat similar to, and different from, that of a typical fuel vehicle.
The electric vehicle label also expresses the total city, highway and fuel efficiency values in miles per gallon equivalent (MPGe), for electric vehicles the EPA uses a conversion factor of 33.705 kWh per gallon of gasoline for this calculation.
The label displays text and an associated icon in the top right corner to identify it as an electric vehicle.
COAST DOWN VEHICLE PREPARATION
1. The test vehicle and tires will accumulate mileage to represent the road-load force at the 4,000-mile test point.
2. The vehicle weight will be ballasted to the ETW specification including the driver and test equipment. (40 CFR Part 86.129)
3. Target road load coefficients derived from SAE J2263 coast down testing, execute one test run. Perform road load derivation in accordance to SAE J2264 to establish the A, B, and C force coefficients for dynamometer power absorption
4. Conduct Multi-Cycle Range and Energy Consumption Test (MCT) SAE J1634 test procedure consists of four UDDS cycles and two HWFET cycles in a specified sequence including mid-test and end-of-test constant speed “depletion phase
7. Register manufacture with EPA/CARB submit all required documents for manufactures EPA Certificate of Conformity and CARB Executive Order.
INTRODUCTION TO OTHER TEST CONDITIONS
NEDC (New European Driving Cycle)
Introduced in 1970 and last updated in 1997, the NEDC consists of four repeated low-speed UDC(Urban Driving Cycle) and one EUDC(Extra Urban Driving Cycle), with speeds The variation is as follows:
The UDC period in the diagram is the first 0 to 780s and the EUDC period is 780s to 1180s.
WLTP (Worldwide harmonized Light vehicles Test Procedure)
WLTP was proposed by UNECE to replace the NEDC and to make the test conditions more relevant to actual driving habits.
The WLTP classifies vehicles into three categories according to power/weight, which correspond to three different WLTC test cycles. The test process is divided into 4 stages, corresponding to low speed, medium speed, high speed and super high speed conditions, simulating urban, suburban, rural and high speed scenarios respectively, the figure below shows the speed change of the third category of vehicles：
WLTP test is also conducted on a dynamometer, but it takes into account wind resistance and load. In addition to the above-mentioned tests, the WLTP also takes into account actual driving emissions (RDE), which shows that the WLTP is much stricter than the NEDC.
However, the WLTP is still more oriented towards measuring emissions, which is still not sufficiently relevant to the actual driving habits of the electric vehicle in terms of measuring the range of the vehicle, compared to the EPA's test conditions, EPA is more relevant and more stringent. There are of course five other EPA cycle tests, including US06 (ultra-high speed), SC03 (air conditioning) and Cold UDDS (low temperature) in addition to UDDS and HFEDS. As the EPA certification of electric vehicles currently tested in the US and Canada is still dominated by the SCT and MCT , the others are not described here.
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