Quote:
Originally Posted by Efthreeoh
Thanks for having a great discussion on this subject; it's why I come here to discuss cars with other car geeks (no disrespect intended).
I do need to tell you that I'm no stranger to the concept of electric vehicles. In the early 1970's, General Electric decided to get into the garden tractor business. and manufactured a garden tractor called the "Electrak". We had the E15 model (I still have it, defunct as it is). It was by far a better garden tractor than the gas-powered models of the early '70s. But it was range limited too, which killed it in the market place; GE sold it for just 3 years during the 1st gas-crisis. It could cut about 2.5 acres of lawn on a full charge (which was just enough for our home), but it had the mower deck out front (for easy maintenance) and had all sorts of electric (36V DC) ancillary equipment like a weed eater, chain saw, snow thrower, and welder.
So I ran some more numbers from the data you posted (when I have time I'll look for my range vs. weight calculations). So based on the efficiency ratios the Volt runs .29KW per mile and the i3 runs .21KW per mile; more efficient by 27%. BUT look at the weight of each vehicle. The published weight I could find on the Volt was 3,781 Lbs. The weight of the i3 I found was from BMW's press release and was 2,630 Lbs. The press release was not clear if the weight spec was for the EV version or the EV + range extender version. I'd suspect it was for the non-range extender version, so I threw in an estimated 120 pounds to i3 weight for the gas engine and generator (a pure guess). Comparing the weights of the two cars with comparably functioning drive trains (EV + extender) the 2,7500-pound (estimated) i3 is 27% lighter than the 3,781 pound Volt, so the percent-better efficiency of the i3 matches its percent-better weight advantage; 27% vs. 27%. Which makes me not see the great engineering feat the i3 is. Throw in the fact that the total range of the Volt is 380 miles vs. 160 miles for the i3, I view the Volt as the better engineered car that better fits the consumer's needs.
|
The i3's range extender adds a total of 330lbs so with the REx it is 2,960lbs. You need to realize that weight is only a part of the cars efficiency, and it seems like you are using weight as the sole metric. In stop and go low speed driving yes weight has a lot to do with efficiency and since BMW does envision a lot of the i3 customers to drive in city environments weight savings was a high priority. However the EPA ratings use a blend of city and highway driving to arrive at their efficiency ratings. Driving along at a constant 65 mph, weight has very little to do with efficiency. The cars drag coefficient is what is most important here. The Volt has a cd of .28 and the i3 is .29 so the Volt has a slightly better drag coefficient. The i3's short length and tall height make it difficult to be very aerodynamic although .29 is pretty good considering its odd shape. GM clearly saw the Volt as being more of a highway cruiser than BMW sees the i3 and they probably put more emphasis on the getting the cd down as low as possible because of this. In fact, the reason the original Volt concept that was scrapped and revised for the current production form was because they couldn't get the cd down low enough in the original concept (which everyone really loved).
If both cars were tested side by side in city environments with all the driving at lower speeds, stop and go, the i3 would probably be much more efficient then your calculations show and conversely if both cars were driven at a constant 70 mph the gap may close a bit. Weight is much more important for the i3 than it is for the Volt because of the driving environment it will be most likely used in.