Wednesday, August 24, 2011
How Far Does Your Electric Car Go?
How far do you want it to go?
Almost every discussion of electric vehicles STARTS with the concept of range. This is because it has always been the defining limitation of electric cars. In the early days of the Internet, electronic bulletin boards, and online communications, it was always "how fast is your modem connection."
Fast is relative. Incremental advances are NOT all created equal. The jump from 300 baud to 1200 baud was an ENORMOUS improvement. The jump from 1200 to 2400 was great. The jump from 2400 to 9600 was a good improvement. To 28.8 kbps was good too. And to 56.7k was a bit of a yawner.
Range similarly. The difference between 25 mile range (and falling) provided by Pb chemistry cells and the 80-100 mile range offered by LiFePo4 cells is ENORMOUS. From 80 to 200 would of course be very good. After that, I doubt that it matters to most.
Indeed, the average daily use of automobiles in the United States is 39.4 miles for about 14,500 miles per year. And over half the driving population averages under 26 miles per day. People just don't drive as much as they think they drive. But of course, owning a car means being ABLE to go as far as you like, not so much that you DO.
The internal combustion engine driven vehicles of course vary in range depending on how you drive the car. And they are about 12% efficient at translating some 33kWh of energy in each gallon of gasoline into forward motion. By far the majority of it is converted to HEAT and blown out through the radiator in the front of the car - each vehicle contributing to our warmth and comfort during the winter.
Electric vehicles are more like 85% efficient. But that efficiency is precisely what makes them MORE sensitive to just how they are driven.
In this weeks show, we begin a series of tests to demonstrate the factors in the vehicle design that affect this range for a given driving scenario. And we compare two cars that SHOULD have the same range, and somewhat mysteriously do not.
In my own opinion, we need to strongly alter our relationship to our vehicles and how we view them and relate to them. I do NOT actually fall into the camp where we should "suffer" for the sake of the planet. Technical problems usually can be attacked with technical solutions and this one is no different. Keep your Yukon or Denali for towing the boat to the lake or the trip to the nearby city to pickup someone at the airport. Take your long distance trips across the country as much as you like in your Escalade or Suburban. You aren't hurting ANYTHING because you are not doing enough of all that to move the decimal place. If typical, 95% of your driving is to the hardware store, the lawn center, the grocery store, the soccer field, and to school and work. And the mileage you are ACTUALLY driving in any one gulp is trivial - in some cases within Pb chemistry range.
But cars have become more expensive. In the 1960's, $4000 was a LOT for a car - picture a new Vette or a Cadillac. Today, $50-$80K is NOT stratospheric. That's the cost of a house in years past.
Now you wouldn't send a house to the junkyard after 9.6 years and build a new one. You might remodel the kitchen. You might do a little paint and carpet. But the very deliberately planned obsolescence from our friends at the automobile factory has led us to some ridiculous concepts. Today, you can TOTALLY REPLACE the drive train in any automobile - engine, transmission, and all belts and hoses, for LESS money than the SALES TAX is typically on a new vehicle. You COULD put an entirely new interior in a car for about $3000 - done by professionals. A paint job? if you want to be the rage at the next car show- $6000-$7000. But Earl Scheib DID make a fortune with his "ANY CAR - $49.95" paint job.
And so my intent is to pick cars I really like, and then just renovate them as needed. That pretty much precludes the use of steel bodies. I like aluminum and fiberglass because they don't try to become biodegradable and return to the earth as rust while I drive them.
With the advent of these LiFePo4 cells and electric drive trains, I can see simply continually upgrading a car forever. We'll not build it. We'll build it for life and we'll keep building it. As batteries become available, or controllers, or chargers, we will upgrade. If the paint fades and accumulates some scratches, we'll have it painted.
Back to the main topic of this week's show. Range. It does require the application of power to move a car at all. How far it moves is a function of how much power. There are several factors that determine just how MUCH power is required. Since your battery pack offers a FIXED amount of power, your range is the result of your relationship to those factors.
INERTIA. The tendency of a body at rest is to remain at rest and for a body in motion to remain in motion. Without other factors, if you had a car going 25 miles per hour, it would go 25 miles per hour forever with NO additional power. Friction, and other factors cause it to decelerate - eventually to a stop.
You have to apply power to accelerate to any speed, and other resistances will cause you to DECLERATE from that speed to zero. The amount of power required is a function of MASS. The greater the mass the more power to accelerate.
This is entirely separate from gravity. But when we combine gravity and mass, we can derive a very measurable quantity - WEIGHT. And so the amount of power required to accelerate to a given speed can be though of primarily as a function of weight.
GRAVITY is indeed one of the factors in electric car design. All roads have some incline. To climb a hill, you must move the mass in elevation against the force of gravity. In a downhill run, gravity can actually decrease the amount of power required to a negative number, and indeed you can recover energy through regenerative braking - converting both inertia and gravity into electrical power to recharge your cells.
AERODYNAMIC DRAG is actually one the most significant forces involved in range. As anyone who has put their hand out the window at 60 mph knows, the force of the relative wind can be felt as quite a strong force. That is aerodynamic drag experienced across probably 20 square inches of hand. In the case of the Spyder or the Speedster, that same force is multiplied by the 1.76 square meters of frontal area if you can imagine it. It is an enormous force.
The aerodynamic drag force can actually be given by the formula Fd=1/2 pv2CdA.
Fd is the resulting drag force.
p is the density of the fluid - in this case air and is slightly variable by temperature.
Cd is a "drag coefficient" typically derived from wind tunnel testing to account for the fact that for a given frontal area the drag will vary according to the shape of the vehicle and the resulting laminar flow of air across the body.
A is the total "frontal" area of the car. How big is the hand.
v is the culprit - velocity. It is squared. In this equation, whatever v is is multiplied by itself. If it is 10 mph, it equals 100. And if it is 100, it equals 10,000. This cruel fact is what caused such an enormous difference in power at 70 mph than at 40. Recall Jimmy Carter's 55 mph speed limit. It was not invoked to save lives - but rather to save gasoline. And as I mentioned, electric cars are MORE sensitive to such things than the gasoline cars because they are MORE efficient in the first place.
After inertia, gravity, and aerodynamic drag, we have rolling resistance. If you put your car in neutral, and push it out of the garage by hand, you are experiencing it's rolling resistance first hand.
The biggest component of rolling resistance is probably the tires. Large fat squishy tires give us a very comfortable ride. But the flexing of the sidewalls to accomplish this generates heat loss and makes the car difficult to roll. The larger the contact area of the tires with the pavement, the larger the rolling resistance. If you don't have pavement, and are driving on sand for example, it gets worse.
But there are drive train resistances as well. The friction of the gears in the transmission and differential add a bit of drag to the system. The bearings in the car support the entire weight of the car on the wheels while allowing wheel rotation. Whatever your car weighs, each wheel has to carry part of that and it does so primarily through the wheel bearings. They have friction, and give off heat. If you have one that goes "dry" and is not lubricated, it will generate enough heat to melt steel.
This week, we did some testing of the 1957 Porsche Speedster Electric we call DUH, because it was our second speedster build - part deux. And we also tested a 1955 Porsche 550 Spyder electric replica that we consulted on when it was originally built by Duane Ball and Scott Smith, and subsequently we bought the car.
We tried to eliminate INERTIA and GRAVITY from the test. We did this by driving a substantial distance of about 20 miles at a steady speed. Not accelerating and not decelerating pretty much eliminated the application of power for the purposes of inertia - just what we needed to get up to speed in a few hundred yards out of the 20 miles. And even that was largely cancelled by the recovered energy or lack of force required for the distance we decelerated.
All roads have slope. We divided the 20 miles into two halves and did them in opposite directions on the same road. This should pretty much cancel the effects of gravity.
Leaving aerodynamic drag and rolling resistance. We tested both cars on the same route and nominally at the same steady speeds of 40 mph, 50 mph, 60 mph and 70 mph. Let's look at the results.
If we look at these results, a couple of things jump out at you. At 40 mph, the Speedster gets 19.69% greater range at 158 miles than the Spyder at 132 miles. Also curious is that this advantage decreases to 9.3% at 70mph with the Speedster coming out to 94 miles and the Spyder at 86.
That brings up two questions: Why does the Speedster have greater range? And why does this disparity vary with speed?
And these questions jump from curious to bizarre when we look at the two cars.
Coefficient of drag:
Spyder : 0.20
The Speedster should be LESS efficient than the Spyder by quite a bit. I confess the references to Cd on the Spyder are slim. There were only 90 of these vehicles built originally and wind tunnel testing wasn't a big deal in those days. But just looking at the two shapes, you would expect the Spyder to be dramatically more slippery through the air. Both vehicles have a frontal area of 1.76 square meters.
As we took the weight largely out of the picture by eliminating Inertia and Gravity, that's not a big mystery.
The cars are similar in other ways as well. They both have 36 China Aviation Lithium Battery Company cells of 180Ah per cell. They operate at the same voltage. They have the same High Performance Electric Vehicle Systems AC-50 motor. They both used the Curtis 1238 three-phase controller. In this, the Spyder features the 650 amp -7601 controller variant while the Speedster has the 550 ampere -7501 controller. But we never went over 200 amps in any event. Both controllers were set for 5% neutral braking and 500 rpm taper. These are controller configuration items controlling the amount of regenerative braking.
They both now use the same hall effect accelerator.
They DO have different transmissions with the Spyder having a 3.88:1 ring and pinion and the Speedster featuring a 3.44:1 ring and pinion. But the other gears result in almost identical final ratios between the two cars. Alll the testing was done in fourth gear. There was a 100 rpm difference at 60 and 70 mph but almost identical rpm at 40 and 50 mph.
The Spyder features a GPS speedometer and the Speedster features a cable driven speedometer calibrated in MPH. Or should I say miscalibrated. There is a 2.3 % difference from the GPS which we assume to be true distance and speed. This is undoubtedly part of the mystery, but doesn't quite cover the question in degree I'm afraid. It is less than 2 mph and that kind of gets down into the noise level of what I can maintain as far as a steady speed on an Interstate highway with traffic. We used the distance measured by the Spyder for all calculations.
The clue is in the disparity between the 40 mph results and the 70 mph results. If aerodynamics accounts for 1/2 the difference between the two, and rolling resistance the other half, as we increase in speed the aerodynamics should increase as a percentage of the total as aerodynamic drag increases. Rolling resistance may increase, but not as a square function and really not by much.
In this case, the difference between the two vehicles decreases from 19% at 40 mph to 9% at 70 mph. What this tells me is that as aerodynamic forces become a LARGER percentage of the total problem, the difference between the two vehicles DIMINISHES. And so the difference is NOT attributable to aerodynamic difference at all. It must therefore be attributable to rolling resistance.
The Spyder has a set of 165/65 R15 Khumo tires on it. The Speedster features a new set of Michelin Energy Saver All Season 185/60 R15s. It also has some Moon Eye wheel covers which might lend a bit of aerodynamic assistance.
The Spyder started life as a WIDE FIVE option from Beck Speedsters. This is historically accurate but problematical. There aren't many wheels drilled to FIT the Porsche wide five pattern. So Duane had put a set of 911 aluminum wheels on the car with an adapter that moved the wheels out about an inch. This caused the wheels to rub on the body fender in front when steering. So they jacked up the front of the car - probably hosing up the aerodynamics in the process.
In any event, I never liked the setup in the first place. And there were "noises" from the rear of the car. I kept telling Brian I had a brake dragging but he never could really find any evidence of that.
In any event, we contacted AIRKEWELD. They make a very interesting aluminum brake rotor that has a billet aluminum hat that bolts on - allowing them to do any wheel pattern and offset desired with a very lightweight rotor. They couple that with a Wildewood aluminum caliper. This little puppy would reduce our unsprung weight by about 18 lbs. Aluminum rotors and calipers are not nearly as durable as steel and we would not normally recommend their use on a car. But this car features regenerative braking anyway. We don't really use the disk brakes much at all. So we think it would be a good upgrade for this particular electric car.
We add a lightweight Weld aluminum wheel in a Ford 5 by 4.5 pattern. And on this, we'll mount a low rolling resistance tire. The Michelin Energy Saver A/S curiously is SOLD OUT NATIONALLY. You can't get em - anywhere. Brian has confirmed with Michelin directly that they do not exist at the moment. They are a little mysterious as to why this might be. We suspect they are redesigning for even lower resistance.
Tirerack.com tested low rolling resistance tires on a Prius and Michelins got the highest mileage at 53.8 mpg, compared to the Goodyear Insight that comes standard on the Prius at about 51.5 mpg. But the Bridgestone Ecopia rated just a little behind the Michelin and is $25 per tire less AND they are not unobtanium.
The Bridgestone and Weld combination is about 2 lbs lighter than the earlier Porsche style aluminum wheel and Khumo. The Bridgestone Tire weighs 17 lbs and the wheel a little over 12 lbs.
Unfortunately, AIRKEWELD is an American Company. ANd like all non-Chinese American companies, they cannot manage to get an order in a box and shipped from Arizona to Missouri without at least three tries. We got the long axle version on the rear hubs when we had specified the short axle. We got the wrong brackets for the front. And we go the wrong brake lines for the rear. ANd we got the wrong brake pads for the Wildewood calipers in the front. It has been a total comedy and we have now received our THIRD shipment from these guys. The parts were ordered in June and received in late August (on three different dates of course). And they were $1800, which approaches 10% of the cost of the original vehicle roller.
But they were very nice and the units are just gorgeous. We should have them on this week, and be ready for retesting. It will be very interesting to see the results.
We are also installing some Silicon Nitride ceramic bearings on the rear axles of the Spyder. This is a standard 6306 bearing which normally costs $12.95. The ceramic version is $325 each. But they are good up to 2550F and have significantly lower friction. I guess I think we are stringing the bounds of reality here at a $650 upgrade to avoid bearing friction. But the Illuminati team claim it is their secret sauce on their vehicle which gets very good efficiency on the highway and is nearly 3000 lbs to boot.
We don't do very well with secrets at EVTV. If you have any, tell them to someone else if you want them to remain a secret.