# Predicting EV performance



## bwilson4web (Mar 4, 2019)

Hi,

Comparing EPA metrics for electric vehicles (EV) can be confusing. The EPA can publish 5-test metrics, 2-test metrics with an 'engineering factor', and/or manufacturer sandbagged value. For example, Tesla reports the 5-test metrics, the same as ICE and hybrid cars use, and is often criticized for 'too rosy' range. Fortunately, the EPA mandated, roll-down coefficients provide a single approach for head-to-head comparisons.

The roll-down coefficients measure the drag force as a function of speed:









The vehicle must supply an equal amount of thrust, the power, to maintain a constant speed:









These roll-down coefficients are for a "Standard Day". One could take an EV to a high altitude, hot summer day and make a benchmark with 'too good to be true' claims. For example, there was a Honda Insight team that did laps in the summer for bragging rights to an abnormally high range. More practical, cold weather can really hit performance such as my benchmark with a Gen 1, 2003 Prius, in freezing weather whose performance loss matched the density of the cold air. There are 'density altitude' techniques (and web pages) to predict these effects:









So here is an example of a spreadsheet comparing four EVs:









This approach unifies EV performance. Combined with the usable or available battery kWh, one can accurately predict range and electric operating costs. Yet is also works with ICE and hybrids.

The operating line and thermodynamic efficiency predicts the fuel consumption at any given speed. For example, the Toyota hybrid transmissions are very good about keeping the engine at peak thermodynamic range and banking or drawing excess power from the traction battery. So I've attached a ZIP file with both this OpenOffice presentation and spreadsheet which you can play with.

Bob Wilson


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