I have read a rumor that Tesla is aiming to have a Cd for their Model 3 of under 0.20. If true, that is some excellent news and will allow for a less expensive battery to get more range. I'm also hoping it's light. Under 2,500 lbs for a stripped-down model would be excellent, and doable. An AWD, < 2,500 lb, CdA < 4 sq ft EV with a 300 horsepower drive system and AWD would out-perform cars 3 times its expected $35,000 price tag, and give double or triple the range per dollar spent over a fire sale marked-down Nissan Leaf.
http://www.greencarreports.com/news/1101046_tesla-model-3-to-aim-for-0-20-drag-coefficient-report
Hillhater said:
How much of a typical "city cycle" is a steady cruise at 30-35 mph ?.. And how much is stop/start, accelerating up to 30 , shuffling in line behind other traffic at < 20 mph, or braking to stop ?
Looking at chart showing the EPA Urban Driving Dynanometer Schedule, a simulation of 7.45 miles for urban driving in the U.S. that all new cars sold are tested against to determine their efficiency, it looks like roughly half of the driving done when the tested car is not stopped is at speeds over 25 mph. The average speed was low, just 19.59 mph, but since much of that chart has the car either stopped, or doing mostly 25+ mph while it is moving, assuming a 20 mph steady cruise would dramatically under-estimate the impact of air drag on range.
Weight will be the most significant factor in these conditions, but drag is still a significant factor, simply due to the speeds being traveled to.
Given the variety of situations, conditions, and individual drivers cars are subjected to, the reality is that the EPA cycle isn't necessarily accurate for A to B driving in the city 100% of the time, although it does provide a sort of baseline.
It is fair to say, using the EPA tests as a baseline, most drivers are probably going to be moving at at least 20 mph, or 9 meters per second, for the vast majority of the time that their vehicle is in motion and not stopped. I am going to estimate power required to maintain 9 m/s for two 2015 Leafs, one labeled as "Stock Leaf" with a 0.28 Cd, one labeled as "Aero Leaf" with a 0.19 Cd.
Here are the parameters:
Stock Leaf
Mass(M): 1470 kg
Drag Coefficient(Cd): 0.28
Frontal Area(A): 2.30 m^2
Rolling Resistance Coefficient(Crr): 0.008
Motor Efficiency(ME): 0.9
Controller Efficiency(CE): 0.98
Drivetrain/transmission Efficiency(TE): 0.95
Accessory Loads(AL): 1000 W
Aero Leaf
M: 1470 kg
Cd: 0.19
A: 2.3 m^2
Crr: 0.008
ME: 0.9
CE: 0.98
TE: 0.95
AL: 1000 W
At 9 m/s vehicle speed(V), assuming 9.8 N/kg for the force of gravity(G), we can use the following equations to determine the rolling resistance(RR), air drag(D), power at the wheels(PW), power required from the battery pack(P), to maintain 9 m/s for the two Leafs.
RR = Crr * M * G
D = 0.5 * 1.25 * Cd * A * V^2
PW = (RR + D) * V
P = AL + (PW / (ME * CE * TE))
You get the following power requirements at close to 20 mph:
Stock Leaf: 2588 W
Aero Leaf: 2430 W
Once you reach 20 mph, if you maintain it, the hypothetical Aero Leaf will from then on consume 6.1% less energy than the Stock Leaf. This is a 6.1% increase in range, at the average speed of the EPA city cycle.
Keep in mind, during the EPA test, usually the car accelerated past 20 mph, and air drag power consumption varies as a cubic function of speed, so the faster you go, the more losses your car induces pushing air out of the way, and for each stop, even with regen, most built-up kinetic energy from when the car was in motion is lost as heat, with acceleration requiring the bulk of the energy consumption.
I think it is safe to say that a very low drag car will still yield somewhere between 5-10% city mileage increase versus baseline for city driving. I've seen studies that have concluded similar 10+ years ago. Mass is a much greater factor during city driving, and the most important one, when it comes to energy consumption for this sort of driving condition. Given the variety of driving styles and conditions and the loading requirements presented by urban driving, a 5-10% difference can be certainly be reduced to "noise" when trying to predict day to day energy efficiency of your drive, but the difference still adds up, in the long run.
However, in the city, range isn't much of an issue. It would take 4-5 hours to use that Leaf's range up, driving around at city speeds, versus 1.5 to 2 hours to deplete the Leaf's 24 kWh battery, driving the speed limit on the highway. It's on the highway where this 80-100 mile range really becomes a limitation for the Leaf, and had the Leaf been more aerodynamically efficient, its utility would be greatly expanded as a result of increased highway range. City range, even a modest 5-10% gain, wouldn't matter a whole lot, but a 30-40% gain in highway range, allows the car to go a lot more places without running out of juice.