View Single Post
Old 07-18-2009, 02:16 AM   #11 (permalink)
Christ
Moderate your Moderation.
 
Join Date: Nov 2008
Location: Troy, Pa.
Posts: 8,919

Pasta - '96 Volkswagen Passat TDi
90 day: 45.22 mpg (US)
Thanks: 1,369
Thanked 430 Times in 353 Posts
Quote:
Originally Posted by bwilson4web View Post
P&G works especially well in traffic. <GRINS>

After all, no one would exceed a posted speed limit so the urban P&G wizard pulses to the speed limit, 30 mph, and then 'coasts' or 'glides' down to some lower value ... to the great delight of the other drivers. Then they pulse again to the 30 mph speed limit and again 'coasts' or 'glides' or 'feathers' or whatever down to their preferred lower speed. Again, to the great delight and enjoyment of the other traffic. But in no case is the P&G driver achieving an average speed of 30 mph because the glide is always under 30 mph.

So what are the pulse and glide parameters to sustain 30 mph?
  • 10 mph dV - 35 mph to 25 mph
  • 20 mph dV - 40 mph to 21-22(*) mph

In the first case, the driver has to be on a 35 mph street to average 30 mph (see title of thread.) In the second case, the driver has to be on a 40 mph street to average 30 mph. ... Have I missed something?

Surely no one is advocating exceeding the speed limit since that would risk getting a ticket. But driving at an average speed of 30 mph on roads posted at 35 and 40 mph is the great delight of other drivers who will announce their joy with horn salutes and raised fingers. <GRINS>

In all seriousness, the hybrid traction battery is another way of storing energy that does not depend upon the vehicle velocity. Unlike changing the velocity with other traffic around, the hybrid battery is an 'electronic' system that allows the vehicle to work like all of the cars surrounding it and not pose a very real collision risk or traffic obstruction.

Bob Wilson

* - due to non-linear drag effects, primarily aerodynamic, the lower glide limit has to be raised to compensate for the higher speed pulse.
Sorry, dude... neither of those (in the real world) will net you 30 MPH average speeds.

Taking into account the actual pulse and coastdown rates, you're likely to go from the lower speed to the higher speed much faster than the other way around, meaning that you're spending less time at lower speeds while accelerating (since acceleration is a curve, not linear) and longer times at higher speeds.

Inversely, you'll slow down slightly faster (due to drag) at higher speeds than you do at lower speeds, but not enough to balance the equation totally.

So P&G from 30-20-30 will intuitively net you an average speed of 25MPH, but in reality, you'll average more like 27-28MPH, which is still legal.

Include with this the fact that almost no speedometer is 100% accurate 100% of the time, and chances are, the people around you only have the slightest clue that you're not actually going 30 MPH average.

I've found in my time of P&G (however admittedly short) that when people figure out that I'm not maintaining a set speed, they back off... better for safety (both perceived and real-world) for both of us, which I prefer anyway.

Once they have the opportunity to pass, they do so, and I pay no mind.

As far as taking 5HP to keep the engine running at 30 MPH, well, I don't necessarily disagree, but I won't vouch that claim, either. You're leaving a lot out in the open by saying this, because you're not actually accounting for engine losses or speed, you're just throwing a number out there in the open, waiting for a nice big fish to bite. (Thanks, TJTS1.)

Realistically, in order to make a claim like that, you'd have to back it up with engine speed, volumetric efficiency, temperature (which affects frictional losses), and several other factors which would eventually give you an idea of how many HP it would actually take to keep the engine running. I think if you sit down and look at some formulae, working the math for different engines at different speed/load ratings, you'll find that 5HP can be way too high, and can also be way too low of a number to work with.

I might be inclined to agree with it as a basic number, though. A "reference" number, of sorts.

__________________
"ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"

  Reply With Quote