Tuesday, July 3, 2012

How Do Electric Vehicles Perform Compared to Gas Powered Cars?

          With very few electric vehicles in the marketplace, there are many presumptions about how they stack up compared to “regular cars,” which many refer to as gasoline powered internal combustion engines or hybrids.  Through walking into various dealerships we have found the sales reps say, “They are great off the line,” and of course, their EV is the best on the market… Through various conversations we have heard many things about EVs and investigate some of the most common statements:

 “I Wouldn’t Want to Go on the Freeway, Electric Cars Are Wimpy and Have No Power!”
Driving feel is related to torque, which is the turning force available at the tires. Take for example a vehicle powered by a standard internal combustion engine. When you step on the accelerator, the engine needs time to increase speed to generate torque and power, and then that has to go through a transmission to the wheels. All through this drive-train, energy is lost as heat, friction, and noise, meaning that you, the driver, will have to wait for that power. Who wants to wait?
Consider the 2.0 liter internal combustion engine in a Ford Focus, which reaches it's peak torque of 146 pound-feet at 4,450 rpm, while it's electric counterpart has available torque of 181 pound-feet at zero rpm. What it means for you – It IS great off the line! In the electric model, there is no waiting for the engine to get up to optimal speed and torque, and since there is no transmission, none of that energy is lost, meaning that as soon as you step on the accelerator, there's no wait, no lag time, instant torque, instant fun. Unlike the compact 35-mpg Chevy Sonic with 148 pound-feet of torque, the Chevy Volt's 273 pound-feet at will put you in your seat, without a drop of gasoline.
“OK, fine, I can get a bigger engine that has the same or more torque than the electric motor.” Yes, and you'll pay for it, too. If you want similar performance to the electric motor in Ford Focus' EV, but in an internal combustion engine, then you'll have to upgrade to the Fusion's 2.5 liter engine with 172 pound-feet torque available at 4,500 rpm, sacrificing 24% of your fuel economy and the time lag waiting for the engine to get up to speed. Want to match the Chevy Volt's torque? Prepare to upgrade to the Impala's 3.6 liter's 262 pound-feet at 5,300 rpm, and experience the 40% drop in fuel efficiency.
 “What about speed? Won’t I get creamed merging onto the highway!”
As we've just discovered, given the benefits of the highest torque available at a dead stop, instant acceleration isn't an issue with an electric vehicle. What about top speed, though?  
One of the benefits of an electric motor, besides it's low-end torque being so high, is that the motor itself has only one moving part, so it can be dynamically balanced finer than an internal combustion engine with hundreds of moving parts. The Mitsubishi I MiEV electric vehicle motor can run up to 9,900 rpm with no discernible vibration while gasoline vehicles have their rpm limited by a device called a rev limiter. Electric motors achieve their peak power, measured in Kilowatts (KW), not horsepower (HP), at half their maximum rpm, so as peak torque is starting to fall after initial acceleration, it's just starting to reach it's peak power. Only requiring a one-speed transmission means very little energy is lost. 
The EV’s excellent torque characteristics and high running speed means that electric motors offer the best of both worlds, good acceleration and good highway speed. Nissan LEAF boasts speeds up to 90 mph, and the Chevy VOLT and Toyota RAV4 EV up to 100 mph, which means that even if you have a commute on the highways in New Jersey, where the speed limit seems to be a quaint notion, or California, where freeway speed limits are sometimes more like guidelines, you'll have no trouble keeping up, even in the fast lane. However, although there is instant torque, not every EV is a Tesla Roadster when it comes to 0-60 acceleration (The Tesla Roadster model does 0-60 in under 4 seconds):
 How about overall horsepower?
           “What's all this talk of kilowatts? Isn't car performance measured in horsepower?” Kilowatts and horsepower are both just measurements of power, in scientific terms, the amount of work done in a given time. The Watt has generally been reserved for electrical systems, such as a 100-watt bulb or measuring the output of a generator, while horsepower has generally been reserved for engines, probably since horses pulled the first wheeled vehicles and equipment. 
            Still, power, or work over time, can be expressed as watts or horsepower, which we will use since we are more likely to comprehend horsepower. To avoid trying to compare kilowatts and horsepower, we used the conversion factor of 746W per 1 hp to get a better idea of how the electric vehicles compared to the gasoline vehicles. Comparing the overall horsepower of three vehicles in the Chevy line-up, note that while the Impala has twice the horsepower of the Volt, it has less than a third the available torque accelerating from a dead stop. The Sonic has about the same horsepower as the Volt, but notice less than a quarter the available torque on acceleration.

How does this stack up to what you expected?
The Author, Benjamin Jerew, has been a ASE, Toyota, and Lexus, Master Certified Technician for over ten years. Branching out now, he's been investigating alternative fuels and how they impact the vehicles, their drivers, and the environment. Wherever the wind takes him, his heart will always be in upstate New York.

No comments:

Post a Comment