The 30 mph challenge
 

Hi,

This article got me thinking about the 30 mph problem:

Power can co-exist with fuel efficiency

The 30 mph problem is it takes about 5 hp (3.7 kW) to sustain a modern sedan. But this is about the same amount of energy needed by the engine alone to turn over at low power settings. So about 10 hp is needed of which about 50% is used to turn over the engine and 50% used to keep the vehicle moving.

In contrast, a Ford or Toyota hybrid will run the engine for a short period of time to generate about 20 hp with 5 hp the engine overhead, 5 hp to the wheels, and 10 hp (7.45 kW) being stored in the traction battery. Roughly 25% of the energy runs the engine and 75% is used for traction power either when the engine is running or later from the traction battery.

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. Now granted, these numbers are approximate but they also match the specifications of the Toyota, Ford, and Honda hybrids and starter motor power needed by diesels and ordinary gas engine vehicles.

So I can admire BMW's efforts to improve vehicle, transmission and engine efficiency. But until they understand the 30 mph challenge, their results will remain as disappointing in the future as they are today.

Bob Wilson

Isn't that a reason why P&G works?

Yes.

I think BMW understands the problem very clearly but these are 300hp engines they are talking about. This isn't the base model poverty spec 3 series. If you are just interested in fuel economy then get the 4 cyl diesel with 116hp.
BMW UK : The new BMW 316d Saloon
Obviously Americans buying bmws couldn't care less about FE so the model isn't offered in the US. But believe me, BMW gets it.

316d 52.3 mpg US combined city/highway
116d 53.5 mpg US combined city/highway

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.

The vehicle efficiency model works the same regardless of the engine size. Although the larger engine is likely to have greater internal drag and require more than 5 hp (3.7 kW) for internal engine operating losses, it still has to turn to maintain 30 mph and generate the fixed internal energy losses as well as the 5 hp needed to sustain 30 mph.

Bob Wilson

And don't forget hills. It's possible to use them to your advantage.

And when you get your hill, be sure it is a good one and use your 'hill hook' to hold it front:
http://hiwaay.net/~bzwilson/prius/coasting.jpg

Since you've proposed cartoon physics:
http://hiwaay.net/~bzwilson/prius/pri_mag.jpg

Of course if you want something more practical:
http://hiwaay.net/~bzwilson/prius/pri_tow_140.jpg

Bob Wilson

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

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.

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.

I prefer my "lying eyes:"
http://hiwaay.net/~bzwilson/prius/pri_PnG_020.jpg

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

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

Bob Wilson

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

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.

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?

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.

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

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:
But let's take a look at actual data:

(41+25)/2 = 33 MPH

http://hiwaay.net/~bzwilson/prius/pri_PnG_020.jpg

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

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.

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.

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.

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

Bob Wilson

You guys are so smart, you just blew your own minds.
http://imagecache.allposters.com/ima...nd-Posters.jpg

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.

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>

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

Google

GJDM

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.

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.

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.....

The average speed of the EPA City cycle is 28 mph so 30 mph happens to be very close.
Because of the low vehicle power demands, ~5 hp, it can be treated as a constant for engines of similar displacement. So we don't really have to bother with "a function fo speed (rpm) primarily and load second." A steady speed eliminates another red herring.

Actually it is fairly accurate in the case of vehicles that must run their engine to maintain a constant speed. As pointed out, the 5 hp starting motor power gives a good approximation of the engine, internal power loss at low power settings. To provide an equal amount of motive power, 30 mph requires pretty close to the extra 5 hp needed.


Actually I would like to see steady state, MPG vs mph for a diesel similar to what I already have for the Prius:
http://hiwaay.net/~bzwilson/prius/pri_2010_800.jpg

In the meanwhile, the EPA City ratings tell the tale when identical driving profiles are used:

• 29 MPG - Jetta diesel automatic
• 51 MPG - Prius 2010 automatic

But since P&G seems to posses cache (regardless of the traffic issues that limit its application,) I would be just as happy to see the Prius vs Diesel P&G contest. The reason is we have already run the NHW20 Prius at P&G averaging close to 30 mph:
http://home.hiwaay.net/~bzwilson/pri..._MPG_Rev_B.jpg

Seriously, do diesel advocates really think they'll get in excess of 100-115 MPG as the Prius team did in August 2005?

Bob Wilson

Exactly, I said steady state, and you are talking about average speed. Even the city drive cycle consists of multiple periods of acceleration followed by cruising at speeds a good bit higher than 30 mph. Furthermore I still think that driving a car in an ultra urban environment is a pretty stupid way to get around.

Playing with the aerodynamic calculator on this site just now one can find required road load power levels at 30 mph of anywhere from ~2.5 HP for a small car to over 10 HP for a truck or SUV. But I was referring to engine speed (rpm) here, not vehicle speed.

Again you are lumping all IC engines into one number. I measure internal engine friction at work (auto industry supplier dyno lab). A large V8 engine we just ran consumes 17 HP just to spin at 2000 rpm. This engine of course has the low end torque capability to drive most cars at less than 2000 rpm, so its FE performance is going to depend entirely upon vehicle gearing. A small 4 cyl engine will often show less than half that much power loss.
So your average engine / vehicle approximations are crude at best. The balance point of vehicle power requirements to engine friction power consumption will vary.

Probably not (although 80+ mpg wouldn't surprise me). I hadn't seen your data, and it is impressive. How long a period of time do you obtain your data for? I would imagine once the battery is drained your mileage plummets. Hence the advantage gained in city driving with regen braking. But long stretches of road where one can drive steady state at 30 mph don't really exist, so I still find the whole conversation kind of moot. For the same reason I'm not going to offer to test my own vehicle. I bought it for highway commuting.
Oh and please refer to AMCI obtaining 38 mpg city for the new VW diesel. EPA is a pile of Sh*t.

You really have no clue what the hell you're talking about. This is a random number you pulled out of your ass and you keep repeating over and over again in the hopes that it will stick as 'fact' in people's minds. This is pure fantasy.

Apples and oranges.

The Prius was designed and marketed to be the ultimate fuel economy production car, which it is.

The TDI is just an engine option in a brand of cars designed around driving passion, but it just happens to have pretty good fuel economy too! There was no exceptional attention paid to RR or aero with the TDI.

Put a diesel ICE in a Prius-like car and the diesel wins hands down.

BTW, according to Wayne, who's got to try pretty much all hybrids and the Jetta TDI, over 60 mph the TDI starts to pull away.

About how much power does it actually take to maintain a speed of 30MPH in a Prius on level ground with no wind? I'm thinking it might be possible to install a bicycle generator in front of the passenger seat and use that provide a fair share of the power needed. (And of course, I really like the idea of Lauren Stanhouse acting as an APU for the Prius. Enable EV mode, let Lauren pedal to keep the battery charged, and at some impractically low speed, it can maintain an "infinite" MPG for a very long time. Add some more bicycle generators and fat chicks and it might actually be able to maintain a constant 30MPH on a track...)