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Very good points...
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Posted by Snowmobile [Email] (#686) [Profile/Gallery] (more from Snowmobile) on Thu, 14 Jun 2012 12:13:10 Share Post by Email
In Reply to: Regenerative braking is grossly over-rated, Reality, Thu, 14 Jun 2012 10:03:09
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while I think you've made an order of magnitude typo (.558/15.7= 1/28th), I agree in spirit: that on the highway, regenerative braking should make little difference... ideally one should rarely use brakes on the highway, basically at all... and aerodynamics are most important.

Around town, there is plenty of fuel consumption involved getting cars up to speed (even if it is only 50-80% of 65mph, it adds up over and over). Using friction brakes "wastes" all of it, whereas regenerative braking restores some of it. Much of my driving is of that stop + go sort rather than freeway, so I notice it: the 9-5 especially does not get great fuel economy for me around town (it's heavier than the 900's)... though it is quite good on the highway.

I will absolutely admit to hand waving. Your point about idling and EV's turning off the engine is well taken. I have not really tried to work the math on how much fuel is spent idling (waiting for the light to change) vs accelerating (after the light has changed). My impression was that the accelerating was the bigger issue + that one sees a significant fuel savings by accelerating gradually vs hard suggests that it is important. Let's see...

Still hand waving, but with numbers... I'll use metric speeds: 100km/h of 1500kg car is 558kJ. 70km/h (20m/s) in the same car is 300kJ, so about half. Compared to a gallon (15.7MJ), that is only about 2% of a gallon, so the question is how often does that happen. I just did a quick google map of a route I frequently take. Over 3 miles, there are 17 stop lights! They seem to frequently be timed poorly so let's say I hit 6... That is over 10% of a gallon of fuel burned accelerating in 3 miles (4% of a gallon used per mile). Seemingly not overwhelming, but significant. Similarly with stop signs in neighbourhood areas: let's say 12m/s (a little over 40km/h) and 8 stop signs in a mile (yup, I'm serious! I google mapped a frequent route)... 108kJ per stop sign, so automatically 864kJ used to go that 1 mile, or 5% of a gallon right there. You can pretty easily calculate how far you'd get at 5% of a gallon burned per mile... in 20 miles you use 100% of a gallon... 20mpg sounds pretty decent for that sort of driving and is typically what I see around the city... obviously some of my driving is more open, but it's an estimate... so what fraction is wasted idling? sure it is also there, but I wonder if it is as big as you suggest? In my local driving experience, it would seem acceleration is at least on par with idling + most likely exceeds it. The reason I hand waved on this without numbers originally is I recall Swedecar posted once re monitoring fuel usage in real time (I believe on a Volvo) and every time he hit the gas, the numbers spiked hugely and all the idling was next to nil... of course it integrates over time so the idling will add up, as will the time spent cruising between lights once up to speed (at slightly more than an idle)... but how things adds up depends a lot on local conditions...

all of this is to say (as I think we agree) that ICE more or less works better on the highway and EV more or less works better around town. Fuel consumption data for the Prius seems to back that up. Having some sort of high energy density fuel source for long distance travel is pretty darn important at least for now (and probably at least a decade or so)... so yeah, I have been unclear, when I say EV, I mean EV with a fuel source, ie a hybrid...

even better around town is to ride your bike!

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