The 30 mph challenge

[bwilson4web]

Originally Posted by 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. . . .

You may remember I'm pointing out the problem diesel, gas, and other non-hybrids have with sustaining a steady speed of 30 mph. I'm also explaining how the hybrid achieves a higher vehicle efficiency at City speeds.

I have no problem with a separate discussion of P&G. But if it is brought up, I will simply answer with the facts and data, the obvious limitations of P&G in traffic.

Originally Posted by dcb

. . .
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.

I am always interested in the facts and data, direct measurements:

• peak speed (posted speed*)
• lowest speed
• time-to-peak speed
• time-to-bottom speed

If you have a Garmin nuvi or can borrow one, we can analyze the P&G profile. This will give not only the parameters but also the average speed over the track. Then it is a simple matter to look at the relative MPG of P&G versus the equivalent steady-speed.

I've done it with my Prius and reported my results from the field. As the data shows, it is eminently reproducible but to achieve an 11% savings required a rather large dV.

I ran a second set of P&G tests whose average speed was centered about the maximum range speed of 18-20 mph. In this case, P&G came in much worse than holding the steady speed over the course.

Bob Wilson

ps. My tests were conducted on a stretch of road posted at 50 mph on a military base. About six years ago, I was stopped for going 52 mph in a 50 mph zone ... they are that strict. The actual route for my P&G tests:

34 38' 19.98" N 86 37' 44.70" W
34 36' 13.51" N 86 37' 49.49" W

Testing was conducted on a weekend morning when there was almost no other traffic.

[bwilson4web]

Originally Posted by Christ

. . . your posts about P&G mean literally nothing.

Consensus, we both agree to use this phrase.<GRINS>

Bob Wilson

[tjts1]

You guys are so smart, you just blew your own minds.

[tasdrouille]

Originally Posted by bwilson4web

You may remember I'm pointing out the problem diesel, gas, and other non-hybrids have with sustaining a steady speed of 30 mph.

That's kind of a non issue, cause I don't need to drive 30 mph. 35 mph is just fine and it works perfectly for me.

[MadisonMPG]

Originally Posted by 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?

Granny - '94 Grand Prix SE
90 day: 24.55 mpg (US)

Cara - '00 Caravan Base
90 day: 27.73 mpg (US)


GOOD JOB DAVE MAN


<shakes head>

[Christ]

Originally Posted by MadisonMPG

Granny - '94 Grand Prix SE
90 day: 24.55 mpg (US)

Cara - '00 Caravan Base
90 day: 27.73 mpg (US)


GOOD JOB DAVE MAN


<shakes head>

What's this supposed to be, and who the hell is Dave?

[MadisonMPG]

Originally Posted by Christ

What's this supposed to be, and who the hell is Dave?

Google

GJDM

[Christ]

Originally Posted by MadisonMPG

google

GJDM

It means that any attempt at gas mileage boosting you are doing is failing.

So... how exactly have I failed at getting better gas mileage? Please, enlighten me.

EDIT: By the way, Internet memes are for B-tards and people with no life. Please get one. Oh - and GJDM started on a bodybuilding forum... glad you're a member.

[roflwaffle]

Originally Posted by bwilson4web

Any vehicle that requires the engine to run at 30 mph, diesels, gas, or hybrids with little or no electric vehicle mode, will pay about 50% of their tank just turning over the engine. In contrast, a hybrid with significant electric vehicle mode gets a much improved efficiency, nearly double that of the other.

I don't think that's strictly true. It mostly depends on engine displacement/gearing. Generally speaking most engines in the states are oversized compared to power requirements at 30mph, but for the world as a whole this isn't the case as much given the prevalence of 1L-1.5L engines along w/ suitable gearing that can allow vehicles w/ smaller engines to idle along in fifth/sixth at ~30mph in the ~300g/kWh region of the engine's BSFC map.

[Deezler]

Who the heck drives steady state 30mph anyway? Speeds this low are reserved for neighborhoods and ultra urban areas where you probably ought to just ride a bike or walk anyway. Even a 35mph limit street is usually able to handle 40mph where this almost becomes a non issue.
I really don't understand how engine friction at 30 mph is a "problem". Internal engine friction is a function of speed (rpm) primarily and load second. Reductions in pumping losses and inherent efficiency increase in combustion under higher loading more than make up for the latter, too. Engine speed to maintain constant vehicle speed is dictated by gearing alone, so why would 30 mph be the "problem" spot? Whether you are traveling 30 mph or 60 mph at 2000rpm, you nearly have the same internal frictional losses (and hence fuel used to overcome those losses). Its really more a problem of poor efficiency at low speed and load requirements, and even then its a fully sliding scale.

And the comparison between engines is not moot. A small 4cyl engine will consume far less fuel due to internal friction compared to a large v8 dropped into the same vehicle. Saying 50% at 30mph is a very poor blanket statement. Downsizing the engine and focusing on internal friction reduction still offer great promise for fuel consumption improvement and the continual evolution of the ICE.

I would be interested to compare the FE of a prius steady at 30mph vs. a diesel VW allowed to run P & G in the 25-35 range as well.....