The Electric Car
in [Design]
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From: Eeyore on 1 Aug 2007 22:58 MooseFET wrote: > Eeyore wrote: > > > The current hybrids also use skinny tyres to reduce rolling resistance. I hate to think > > of the adverse effect on road holding. > > There is basically no difference in the traction. Wide tires look > cool but below a certain amount of force per unit area of rubber give > no traction advantage. The traction loss in thing tires is because > the force on the surface is high enough to rip bits of the rubber or > the road loose. They modern materials in the tires are less subject > to this problem. The cars we are talking about are quite light too. I'm not talking about traction, I'm talking about road holding / handling. I know American roads don't have corners (or at least from the way US cars drive you'd think that was the case) but we do here. Which reminds me... why do Americans seem to want such big engined over-powered cars when most of them fall off the road so easily if you use it ? Is it just a macho thing ? Graham
From: Nobody on 2 Aug 2007 00:35 On Thu, 02 Aug 2007 03:53:28 +0100, Eeyore wrote: >> > In city driving it's regenerative braking that can make a huge difference. The >> > complexity of shoving electrical and ICE motive power through some combined >> > transmission seems plain daft though. The series hybrid (in which the ICE simply >> > recharges a battery) seems far more sensible all round. >> >> No, I disagree. The dual electrical machine design beats the series >> system hands down. Having the engine go straight to the wheels when >> it makes sense to do so makes the demand on the electrical system way >> less. > > Why's that an advantage ? It also means you can't have 'meaningful' true electric only > operation of it, plus it requiresa gearbox which otherwise may not be needed at all. The disadvantage of an all-electric or series hybrid vehicle is that the maximum total power is constrained by both the ICE and the electrical system, while a parallel hybrid can potentially combine the two, or at least provide a more powerful ICE for motorway cruising without having to size the electrical components to match. A secondary consideration is that the generator losses subtract from any efficiency advantages of the electrical drivetrain. But mostly it's a case of having to size the electrical system to the power requirements for motorway cruising even though it has fewer advantages over a mechanical drivetrain in that context (i.e. regenerative braking is of no benefit).
From: Nobody on 2 Aug 2007 01:25 On Wed, 01 Aug 2007 07:15:06 -0700, John Larkin wrote: >>A tad but not enough to make the difference. The energy per mile >>increases as the square of the speed. > > Not in this graph: > > http://www.fueleconomy.gov/feg/driveHabits.shtml The energy which a particular vehicle consumes (in fuel) also depends upon the efficiency of the engine and drivetrain, and the efficiency of ICEs varies wildly with both speed and load. The part of the curve where economy increases with speed is primarily due to increasing engine efficiency. There isn't any source of opposing force which decreases as speed increases. Apart from increasing engine efficiency with speed, the only other factor which can produce a positive slope on the economy curve is for constant power sinks (anything which consumes power while stationary: alternator, water/oil pumps etc). > I've seen a few other mpg-vs-speed curves, and they all look similar. > Looks like aerodynamic drag starts to seriously kick in above 55 MPH. It kicks in way before that. Even at cycling speeds, you get a very noticable reduction in effort when riding in the slipstream of a truck.
From: Nobody on 2 Aug 2007 01:45 On Wed, 01 Aug 2007 17:21:24 +0100, Eeyore wrote: >> >Drag DOES depend on body shape. >> > >> > ...Jim Thompson >> >> For a given shape, at automotive speeds, I think there is a square law >> relationship between air speed and drag. > > You are correct at higher speeds. The relationship isn't simple it seems. > http://en.wikipedia.org/wiki/Drag_(physics) Note that Stokes's drag isn't meaningful for vehicles. Where it meantions "small objects", "small" means particles whose size comparable to the gap between the molecules. IOW, it's useful for modelling a cloud of dust settling, but not for modelling "objects". > Drag coefficient can make collosal differences too. > http://en.wikipedia.org/wiki/Drag_coefficient > > Unfortunately current US styling (the trend for truck like 'slab fronts' ) is producing > cars with greater rather than lesser drag. It isn't just the coefficient. Drag is also proportional to (effective) cross-sectional area; large vehicles have more drag than small ones. The sheer size of an American SUV is > The current hybrids also use skinny tyres to reduce rolling resistance. I hate to think > of the adverse effect on road holding. Tyre width only matters when you get close to the limits of traction. It doesn't affect total friction until the tyres start to distort significantly and the force/area ratio exceeds the strength of the rubber.
From: Nobody on 2 Aug 2007 01:52
On Wed, 01 Aug 2007 18:55:41 -0700, MooseFET wrote: >> For a given shape, at automotive speeds, I think there is a square law >> relationship between air speed and drag. > > No thats a cubic law. The energy per mile is the square law one. Energy = force * distance. Drag force is proportional to speed squared, so energy per unit distance is proportional to speed squared. Distance per unit time is (by definition) speed, so power (energy per unit time) is proportional to speed cubed. F = k*v^2 E = f*d E ~ k*v^2*d W = E/t W ~ k*v^2*d/t v = d/t W = k*v^2*v = k*v^3 |