The 30 mph challenge

[Christ]

Originally Posted by bwilson4web

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.

[bwilson4web]

Originally Posted by Christ

Sorry, dude... neither of those (in the real world) will net you 30 MPH average speeds.

I prefer my "lying eyes:"

Originally Posted by Christ

Taking into account the actual pulse and coastdown rates, . . .

Do the experiment, my friend, and bring the data or get a copy of SAE 2009-01-1322 ($15) and cite chapter and verse. I'm into facts and data, not 'arguing' what can be so easily tested in the field. I have data and you bring a collection of words not even backed up by a single observation.

This is why I find absolutely no difference between a troll and a P&G advocate. A simple discussion, a model, that explains the "30 mph problem" faced by diesels is hijacked.

The only difference is I have the facts and data about P&G and have no qualms about application of a "clue by four" to the side of their heads ... an intellectual ambush. And I will continue to extend to the P&G advocates the same courtesies, kindness and respect in kind that they extend every other technical discussion until perhaps we can eventually wander back to:

So would you like to discuss the 30 mph problem?

Bob Wilson

[bwilson4web]

Originally Posted by tjts1

I'm sorry. You don't know what you're talking about.

If there is something specific you don't understand, you'll have to ask a more detailed question.

Bob Wilson

[bwilson4web]

Originally Posted by NiHaoMike

I regularly slow down to 5 below the limit or even 10 below the limit when going uphill to avoid accidental speeding when going back downhill. Other drivers just pass and I pretty much just ignore them.

This is a good practice with some vehicles. The key is keeping the engine in a peak BSFC efficiency range but that is another topic.

Bob Wilson

[Christ]

So what part of any of those P&G lines in that graph gives you an average speed that is exactly halfway between the upper and lower limits?

Answer: None of them.

[Christ]

I DO pulse and glide, occaisionally. That's how I know that you don't ever average a speed that is median of your upper and lower limits.

You do know what Median and Mean averages are, right?

[dcb]

Bob, not everyone can ditch their car and go buy a prius, but everyone can learn effective techniques for saving fuel, of which p&g is easily the most effective.

But you do have to do it with some savvy, and when appropriate, and get good at reading traffic and other obstacles. It takes skill. You should ride along with someone who knows what they are doing and see the stellar results, and note that everyone still got where they wanted to in about the same amount of time, only the efficient ones did it smartly, without being sheep about it.

Seriously, you need to experience p&g first hand before putting it down with grossly inaccurate portrays and invalid assumptions. There are plenty of actual bad drivers on the road to complain about, you are targeting skilled drivers who have to pay the most attention while driving.

[Christ]

DCB - I just assumed it was the same issue that was commented about hybrid owners on South Park.

[bwilson4web]

Originally Posted by Christ

So what part of any of those P&G lines in that graph gives you an average speed that is exactly halfway between the upper and lower limits? . . .

The "glide" begins at 43 mph with the gear engaged. Just as the vehicle passes through 41 mph, it is shifted into "N". The lower limit is 25 mph when the vehicle is shifted back into "D" and cruise control resumes. The 33-34 mph lines are the equivalent, constant speed. But I've never claimed it is always half if you would bother to read the quote:

Originally Posted by myself

. . .

• 10 mph dV - 35 mph to 25 mph
• 20 mph dV - 40 mph to 21-22(*) mph

But let's take a look at actual data:

(41+25)/2 = 33 MPH

Originally Posted by Christ

Answer: None of them.

I'm sorry you continue to post such nonsense.

I'll continue to follow the facts and data, the experimental data. You are of course welcome to your imaginations but the facts and data are unkind to your nonsense.

Would you like to discuss the diesel 30 mph problem?

Bob Wilson

[Christ]

Once again - do you know the difference between Median and Mean averages? I'm guessing not, at this point. Until you learn the difference, your posts about P&G mean literally nothing.

You might be "reading" the facts, but you're "interpreting" them incorrectly.

Yes, the Median Average of 30 and 20 is 25. The Mean average takes into account the actual acceleration and deceleration rates, and specific times at each speed.

If you were calculating the Mean average, you'd be able to figure out rather quickly that you'll not be averaging 25 MPH if you're using the method correctly, since you're accelerating much faster than you're decelerating. Your graph shows this first hand. Decel obviously takes longer than accel, which skews the average toward the higher side.