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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!
Posts in this Thread:
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- On board the UK's first fast-charge electric bus, JohnA , Wed, 13 Jun 2012 19:31:04
- Tesla Model S: 'If Apple Made A Car, This Would Be It', JohnA , Sat, 23 Jun 2012 07:17:05
- Won't work. Steering wheel is on wrong side, Used to be Chaz, Fri, 15 Jun 2012 12:43:03
- Foreign Policy Mag photoessay on electrics, JohnA , Thu, 14 Jun 2012 06:39:11
- Neil Young - e-car tinkerer, JohnA , Thu, 14 Jun 2012 12:28:20
- Show me the batteries, JerseySaab , Thu, 14 Jun 2012 07:02:30
- I got yer batteries, No Snaab , Thu, 14 Jun 2012 17:02:34
- don't be a Luddite!, JohnA , Thu, 14 Jun 2012 07:10:11
- I like electric motors, batteries are the problem, JerseySaab , Thu, 14 Jun 2012 07:44:25
- don't be so gullible, JerseySaab , Thu, 14 Jun 2012 07:18:47
- Don't be so narrowminded, ELaw , Thu, 14 Jun 2012 07:45:08
- Re: don't be so gullible, Steve T., Thu, 14 Jun 2012 07:43:49
- EV vs hybrid, Snowmobile , Thu, 14 Jun 2012 07:41:59
- The one thing I don't get:, ELaw , Thu, 14 Jun 2012 08:02:58
- Re: The one thing I don't get:, Snowmobile , Thu, 14 Jun 2012 08:14:58
- Regenerative braking is grossly over-rated, Reality, Thu, 14 Jun 2012 10:03:09
- Very poor math....what about efficiency?, Mark from Toronto, Thu, 14 Jun 2012 12:40:28
- Re: Very poor math....what about efficiency?, Snowmobile , Thu, 14 Jun 2012 13:12:17
- More like 20-30% efficient, Reality, Thu, 14 Jun 2012 13:12:08
- at WOT only!, Siegfried, Thu, 14 Jun 2012 14:57:27
- and within a certain (and limited) RPM range, Siegfried, Thu, 14 Jun 2012 15:07:49
- conversion is quite efficient....., Mark from Toronto, Thu, 14 Jun 2012 15:48:29
- Re: conversion is quite efficient....., Reality, Thu, 14 Jun 2012 19:38:28
- Re: conversion is quite efficient....., Mark from Toronto, Thu, 14 Jun 2012 21:12:09
- Strawman tactic, Reality, Fri, 15 Jun 2012 03:46:58
- Think this out please...., Mark from Toronto, Fri, 15 Jun 2012 08:54:56
- Re: Strawman tactic, ELaw , Fri, 15 Jun 2012 07:00:41
- Strawman tactic vs Triple Strawman tactic, Snowmobile , Fri, 15 Jun 2012 06:27:37
- Say what??, ELaw , Fri, 15 Jun 2012 05:13:20
- apples and oranges, Siegfried, Thu, 14 Jun 2012 17:35:26
- Very good points..., Snowmobile , Thu, 14 Jun 2012 12:13:10
- Fuel consumption during idling, Kristjan, Thu, 14 Jun 2012 13:00:43
- corrected..., Snowmobile , Thu, 14 Jun 2012 12:44:11
- bah, I made the error, not you!, Snowmobile , Thu, 14 Jun 2012 12:21:49
- Correct but biased..., ELaw , Thu, 14 Jun 2012 11:43:40
- Yes, hybrids can be practical, JerseySaab , Thu, 14 Jun 2012 07:49:41
- eAAM, Snowmobile , Thu, 14 Jun 2012 08:03:07
- Re: eAAM, Mark from Toronto, Thu, 14 Jun 2012 21:16:30
- Turbo!, ELaw , Fri, 15 Jun 2012 06:06:04
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