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JeepEV: Electric Vehicle Conversion Project
How It Drives

« July 19th, 2005 »

     Since I finished my Jeep EV conversion, I've often been asked questions concerning how my Jeep EV, or any Electric Vehicle for that matter, drives. In particular I am asked to compare it to a typical gas-powered vehicle, and I find that many people have misconceptions about how modern Electric Vehicles drive. So that is what I'll discuss on this page...

"Starting" the Electric Vehicle

     Turning on or "starting" an EV is really no different than starting the Internal Combustion Engine in a gas-powered car. In most EVs, you simply turn the ignition switch to "run", wait a second for the main contactor switch to engage, then step on the accelerator and go. My Jeep is no different except that I must turn the ignition switch to "start" then "run" to turn on the vehicle. Turning the ignition switch to "start" is necessary because I have a Zilla motor controller, and they are designed to wait for two ignition switch signals before turning on (for added safety). Once the Zilla motor controller has turned on (after about one second), my Jeep is ready to drive. Though, you wouldn't know it as EVs are almost perfectly silent when turned-on. The only sources of noise are the electric vacuum and power steering pumps which run intermittently. Still, my Jeep is overall many many times quieter now than it was when it was gas-powered. What's also cool is that EVs don't need to "warm up" like engines in gas-powered cars do. There is no harm is just instantly driving off, even in cold weather, as the Electric drive components aren't bothered by the cold.

Shutting off the Electric Vehicle

     Shutting off an EV is once again almost the same as shutting off a gas-powered car. The main difference is that there is no immediately noticeable difference between an EV's "on" and "off" state, as they don't make much (if any) noise. However, I get feedback that my Jeep is off both by a light on the dash board which shuts off, and by the audible click of the main contactors disengaging. Once I've shut my Jeep off, I am certain to set the parking brake full on. In most EVs you have to use the parking brake everytime you park to prevent the vehicle from rolling away. Why? Well, because electric motors don't put any "backpressure" on the drivetrain when off. This means that, unlike an Internal Combustion Engine, electric motors can and do spin freely when off, instead of locking up. Thus, no parking brake = rolling away vehicle (unless you happen to park on a perfectly flat surface, and how often does that happen?) :-)

Driving efficiently to maximize range

     Although driving an EV is no harder than driving a vehicle powered by an Internal Combustion Engine (ICE), there are things you want to pay attention to when driving an EV to maximize the range you can go per charge:

     One big thing is coasting. Most EVs (excluding those with always-active regenerative braking systems) coast very good because the electric motor freewheels (spins freely) whenever your foot is off of the accelerator and brake pedals. This behavior is the exact opposite of an ICE, which always applies a notable "back force" on the drivetrain, causing the car to actively slow down, called "engine braking". Thus, it is best to take advantage of the coasting abilities instead of always driving with a heavy foot to reduce the vehicle's per-mile energy usage. This applies greatly to my Jeep which currently has no regenerative braking and, being as heavy as it is, coasts quite far on level ground.

     A second difference between driving an EV and an ICE vehicle is that you want to drive in as low a gear as possible, with the motor at as high of a speed as possible. For example, it is much better to drive with the motor spinning at 5000 RPM than at 2000 RPM. This is because most electric motors inherently draw less current and are more efficient the faster they are spinning. Also, the faster the motor is spinning, the more air the internal fan moves, which keeps the motor cooler and thus happier. I've found this to be especially true regarding the operation of the series-wound DC motor in my Cherokee. I've seen how driving for long durations with my motor spinning at low speeds noticeably increases my Jeep's per-mile energy usage. Once again, this is the exact opposite of an ICE, which would be far more efficient running at 2000 RPM verses 5000 RPM. Not to mention that most ICEs can't be run continuously at high speeds like 5000+ RPM, whereas electric motors can do so quite well. Although my Jeep's DC motor tops out a bit above 5000 RPM, AC induction motors used in EVs can operate at high as 10,000 RPM.

Using the clutch and shifting gears

     As I mentioned in the previous section, it is best to drive an EV such that the electric motor spins as fast as possible. This is not only for efficiency reasons, but to keep the electric motor happy. To allow this to happen, most EVs are driven in as low a gear as possible to keep motor speed high. Also, this means that many EVs never need to use all the gears in their transmissions for normal driving. For example, I've discovered that I never have to drive my Jeep in 4th or 5th gear. This is because I can and do drive my motor close to 5000 RPM in each gear before shifting. This means that the shift pattern I use is like this:

     Yes, that means that to go from 0 to 55 miles per hour in my Jeep I only need to shift gears one time. This is a huge improvement from when my Jeep was gas-powered because with the Internal Combustion Engine I always had to shift 4 times to go from 0 to 55 mph, using all 5 gears in the transmission. The result is that I don't have to shift much at all during normal city driving. It should be noted that drivers of smaller, lighter weight EVs can get by without using 1st gear at all. Many of them can do city driving without ever having to take the transmission out of 2nd or (in some cases) 3rd gear. This is possible thanks to the extreme low-end torque produced by DC motors (used in most EVs). The only reason why I have to use 1st gear when I drive my EV Cherokee is because of it's heavy weight. I can start out in 2nd gear just fine, but doing so causes the motor to pull lots of current, which is wasteful and can cause the motor to get excessively hot if done repeatedly.

     Now for a few words regarding the clutch. Although not absolutely necessary, I retained the stock clutch system in my Jeep when I converted it to an EV. (The reasons why I chose to do this instead of removing the clutch as some EV owners do is discussed in detail on the Motor, Clutch, and Drivetrain pages.) Even though my Jeep has a clutch, I don't use it all that much. This is mainly because of the fact that I shift gears much less frequently now that my Jeep is an EV. However, the clutch is also used less because I don't have to use it to change gears when I'm stopped or going slowly. For example, I can shift from 1st gear to Reverse and vice-versa without using the clutch. Also, if I need to shift gears after comming to a stop (such as rolling up to a traffic light), I can do so without making use of the clutch. Shifting sans clutch is possible because the electric motor in my Jeep freewheels and doesn't "idle" when my Jeep is stopped, thus there are no heavy spinning objects to deal with in the clutch system when the vehicle is stopped or slowly moving. Of course, I still have to use the clutch if I want to shift gears while moving at any notable speed, in order to not abuse my transmission. Either way, the less frequent use of the clutch in my EV (or any EV), certainly gives the old left foot a break. ;-)

-- A Work in Progress --