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hypermiling technique, driving with load

hypermiling technique, driving with load

hypermiling technique, driving with load

(OP)
The hypermilers have this technique they call driving with load DWL.
In a nutshell the technique is to hold a constant load on the climb, keeping the fuel consumption constant while allowing the speed to drop.
http://ecomodder.com/forum/EM-hypermiling-driving-...
http://www.cleanmpg.com/forums/glossary.php?s=&...

I'm trying to understand the fundamentals with this, but I think if one is trying to optimize the trade off between time and fuel consumed then I don't think this makes sense. Since most of the losses are aero, which is proportional to v^2, if you want to arrive at your destination in a certain elapsed time, the most efficient speed would be constant, not increasing and decreasing. So instead of slowing down on the hill it would be more efficient to slow down a bit on the flat sections and hold that speed constant on the climb. I don't think there is anything about the engine efficiency that would negate the increased losses due to aerodynamics incurred by varying the speed.
Am I missing something?
Sorry if this is too off topic for this forum.

RE: hypermiling technique, driving with load

Either approach could be optimal depending on the exact altitude profile, desired average speed, and vehicle.

It's not off topic, my first serious job was working out such things. On graph paper. With a calculator.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: hypermiling technique, driving with load

There are numerous factors involved. However, if your fuel consumption gage is reading in miles per gallon and is accurate, then you want to do whatever makes it read the lowest. Theoretically, if your power requirement goes up to maintain speed up-hill but you get over the hill faster you may not use more gallons per mile. But the mpg gage says that it does. I have no idea how accurate these gages are. That, perhaps is the real question.

RE: hypermiling technique, driving with load

The idea hinges on there being fairly equal downhill and uphill slopes and the slopes not being too far apart or too long. The engine under constant load means gravity will accelerate the car on the downhill slope storing more kinetic energy and then the car will use that extra kinetic energy as it climbs the next hill. Similar idea to a roller coaster where the speed is fast at the bottom of the hills and slow at the top.

Most people don't drive with the intent to arrive at their destination in a certain elapsed time. Generally, you decide it will take X amount of time so you leave X amount of time early. You really get there in Y amount of time. You hope Y < X so you are not late.

RE: hypermiling technique, driving with load

(OP)
I can see that going slowing on the climb is going to save gas, but so would slower on the flats.
The rollercoaster highway (Lionel's example) may be a special case where it makes sense. Most of the hills that I drive on are not like that.
I should probably run some calcs on this, but it seems to me that if the aero losses are greater by varying your speed, & you don't gain anything with increased engine efficiency or reducing other losses then its a net loss. The only gainer I can see is if you hit the top of a hill at a low speed, you can regain that energy on the downhill for free by coasting.

RE: hypermiling technique, driving with load

Quote (LionelHutz)

Most people don't drive with the intent to arrive at their destination in a certain elapsed time. Generally, you decide it will take X amount of time so you leave X amount of time early. You really get there in Y amount of time. You hope Y < X so you are not late.
That may be so but I'm not convinced. Is there not a strong incentive to arrive on or before the desired arrival time; regardless of the actual departure time, or actual elapsed time vs actual position? In which case, speed will tend to be adjusted [while respecting internal and/or external constraints] according to remaining distance and desired arrival time?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: hypermiling technique, driving with load

If they slow down too much on the climb they are never going to get there because they will become a victim of road rage.

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RE: hypermiling technique, driving with load

If you ignore how long it takes to get from point A to point B, the best fuel economy in a vehicle with a conventional IC drivetrain will normally be achieved by driving at as low a speed as practical, with minimum braking, and using a gear that keeps the engine operating at WOT and low rpm. Low vehicle speed will minimize drag losses, and low engine rpm and WOT operation will maximize BTE.

RE: hypermiling technique, driving with load

Fraid not. The lowest most efficient speed I have ever seen in the literature is the Prius at 42 mph. Almost all others are 50-70 mph.

No wind, on the flat, constant speed, obviously.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: hypermiling technique, driving with load

Fraid not. With a "conventional IC drivetrain", best engine BTE (or SFC) will occur at the point of low rpm and WOT (or full load). Drivetrain losses (manual transmission, final drive, wheel bearings, etc) are also minimized with reduced speeds. And since aero drag losses increase exponentially with speed, it would seem obvious that slower is better in this regard.

However, that does not mean that what you claim is technically untrue. If an auto with a conventional IC engine and MT must drive at 50mph in low gear to operate the engine at the WOT and low rpm conditions that give best BTE, then I would agree with you. But this not true for most automobiles.

RE: hypermiling technique, driving with load

Most cruising economy figures in literature I've seen are for arbitrary speeds of 50+ mph, not speeds optimized for efficiency. Anything below 50 is generally urban, with stops, starts, braking, etc. Cruising below 50 is tedious.

- Steve

RE: hypermiling technique, driving with load

Quote:

I can see that going slowing on the climb is going to save gas, but so would slower on the flats.

The first rule of hypermiling is to slow down. But I think your point is that it's rather difficult to determine how much of the savings is due to going slower up the hill vs using less throttle to climb the hill.


Quote:

The lowest most efficient speed I have ever seen in the literature is the Prius at 42 mph.

Who believes everything that is published by manufacturers? Being in Canada, all the published fuel economy literature is complete BS. There are class action suits due to the overinflated fuel economy claims and many vehicles that would never achieve their published economy unless they were pushed off a cliff.


Quote:

That may be so but I'm not convinced. Is there not a strong incentive to arrive on or before the desired arrival time; regardless of the actual departure time, or actual elapsed time vs actual position? In which case, speed will tend to be adjusted [while respecting internal and/or external constraints] according to remaining distance and desired arrival time?

I will repeat myself worded differently. People do not drive with the intent to make the trip within a exact time or to drive at an exact average speed. I was assuming this is what was meant by the use of "certain time". Are you really meaning to arrive within an approximate time or drive at an approximate average speed?

certain
adjective
1.known for sure; established beyond doubt.
2.specific but not explicitly named or stated.

approximate
adjective
1.close to the actual, but not completely accurate or exact.

RE: hypermiling technique, driving with load

WOT and low rpm conditions that give best BTE

Near the low point for BSFC, in other words ... from my experience on a hybrid vehicle design team in college, most stock production cars had a peak fuel economy point at around 40-50mph. This corresponded to loading the engine as much as you could to get near min BSFC with whatever transmission was in the vehicle, while not wasting too much energy due to aero drag. Lower speeds would have given a better figure if the engines had been much smaller - but at 45mph you're looking at an engine load of 17-ish horsepower to move down the road, which is really not much load and not an efficient operating point. Running faster than ~60 mph starts to eat a whole lot of fuel compared to 45mph.

If you could alternately run the engine and shut it off completely (speed up then coast), you'd probably see that average speeds lower than 45mph look attractive.



RE: hypermiling technique, driving with load

Vehicle fuel economy depends on the total net of all systems, including engine efficiency, aero efficiency, drivetrain efficicency, rolling losses in the tires, vehicle mass, etc. With an IC engine, best BTE will occur at the combination of WOT, VE, and low rpm. Of course, this best engine BTE condition may also occur where the engine is turning 1800 rpm with the car driving in 3rd gear at 45mph.

RE: hypermiling technique, driving with load

WOT at 45 mph at 1800rpm...which will cause the car to accelerate rapidly.


Basically car engines are so powerful that you cannot get near the max efficiency point for the engine at the low speeds you would otherwise want.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: hypermiling technique, driving with load

... or you can do it momentarily by accelerating (usually lugging) in top gear and then shut off the engine and coast. The hypermilers call it "pulse and glide". I call it extremely frustrating and annoying. The transmission, main crank bearings, turbo (if equipped), and engine mounts will hate those operating conditions.

RE: hypermiling technique, driving with load

Looks like the minimum highway consumption of a British 1962 Volvo 122 was a squeak below 40 mph when in 4th OD ( 21 mph/1000 rpm).
Or, for a fleeting moment in 4th non-OD below 33 mph.

http://volvoamazonpictures.se/documents/mish/M004_...

The e-brake was capable of 0.37 g stops

RE: hypermiling technique, driving with load

Quote (papab)

In a nutshell the technique is to hold a constant load on the climb

It sounds like what they're trying to do is minimize the time spent operating with part throttle enrichment due to a drop in manifold vacuum, ideally (to them) not invoking it at all.


Norm

RE: hypermiling technique, driving with load

Quote (GregLocock)

...Basically car engines are so powerful that you cannot get near the max efficiency point for the engine at the low speeds you would otherwise want....

I agree with you on that point. Due to the design compromises that production auto drivetrains demand to make them acceptable to the general public, the engines and transmissions are never optimized for the maximum fuel economy possible. Instead they are designed to provide the best compromise in fuel economy over the entire range of driving conditions of the typical buyer.

RE: hypermiling technique, driving with load

True. Technologies such as supercharging [combined with downsizing], VVT, cylinder deactivation, and, arguably, hybrid powertrain, are helping engineers tilt the optimization more toward overall fuel economy.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: hypermiling technique, driving with load

Yup. Turbocharging/downsizing, VVT, and variable displacement all allow an SI engine to operate with lower throttling losses over a greater operating range.

RE: hypermiling technique, driving with load

(OP)
"part throttle enrichment" ?

RE: hypermiling technique, driving with load

"part throttle enrichment" refers to ancient carburetor technology - but incorrectly?
the mix is usually (intended to be) richer at higher throttle openings, and maybe at idle, but leaner at part throttle, right?

Jay Maechtlen
http://www.laserpubs.com/techcomm

RE: hypermiling technique, driving with load

Carbureted only makes the approach to mixture enrichment fully mechanical, either by a so-called "power valve" (Holley) or my stepped or tapered metering rods (Q-jet). It's still present in fuel maps/lookup tables, where injector open time at any given rpm is varied electronically in accordance with load.

The term may be a bit archaic.


Norm

RE: hypermiling technique, driving with load

"part throttle enrichment" ?

A power enrichment mode when the engine runs with the fuel richer then stoimetric ratio or with lambda < 1.0.

People will often say with the air fuel ratio < 14.7:1, but that's not necessarily true. Different fuel blends require different air fuel ratios.

RE: hypermiling technique, driving with load

(OP)
I know what enrichment is, but why would you do that at part throttle? I can understand wot enrichment, but seems like less than wot would be stoichiometric.

RE: hypermiling technique, driving with load

I agree with papab, waiting to hear an explanation...?

RE: hypermiling technique, driving with load

Increases in load imply that the driver needs more power, which a little enrichment from stoic provides. As determined from manifold vacuum or MAP sensors or a TPS, there is no difference between requesting acceleration and climbing a hill, so enrichment is what happens either way.

The hypermilers generally don't want to invoke downshifts (higher revs) either.


Norm

RE: hypermiling technique, driving with load

I think some level of mixture enrichement is commonly applied a little before WOT.
Holley suggests selecting a power valve that opens about half way between manifold vacuum at idle and zero.
http://www.holley.com/TechService/FAQ.asp?category...

Stoichiometric is the target (what we are stuck with) for cars with O2 sensors and demanded by catalysts needing to meet NOX emissions limits.
http://ect.jmcatalysts.com/images-upload/air-fuel-...

I think in the old days running leaner than stoi was attempted as there are some BSFC and unburned HC advantages to doing so.
The Chrysler lean burn system of the late 70s said this -
"The control system is particularly advantageous when used in an engine which uses a leaner fuel-air mixture (17-18:1, for example) in comparison to presently used mixtures (15-16:1). Improvement accrues by way of reduction in exhaust emission products and better fuel economy.”

Original is at The Chrysler Lean Burn engine control system http://www.allpar.com/mopar/lean-burn.html#ixzz2oO...
Follow us: @allparcom on Twitter | allparcom on Facebook

RE: hypermiling technique, driving with load

there is no difference between requesting acceleration and climbing a hill, so enrichment is what happens either way.

I think maybe there is a terminology difference here. To me "enrichment" means increasing the proportion of fuel in the charge (ie, changing the air-to-fuel ratio in the direction of being "richer", or having more fuel). Using that definition, there is no need to reduce AFR to increase power output. When the throttle opens wider, more air gets through, so more fuel is added (per unit time) to maintain the air-to-fuel ratio at roughly the same level. More fuel is used, but the mixture does not become more rich. Using my terms, there would be no "enrichment" at part load. At/near full load, having excess fuel can reduce the peak temperature in the cylinder and help to keep hot parts from exceeding relevant limits, so temporary enrichment (increasing of the amount of fuel relative to air) is used under the highest-load conditions. Most of the time the three-way-catalyst needs to be near stoich to do its job, and its job must be done to meet emissions requirements.



RE: hypermiling technique, driving with load

I understand the meaning of enrichment, and having worked with a laptop-tunable aftermarket multiport fuel injection system I have some idea what EFI maps look like.

Operation outside the range of rpm and load that covers emissions testing need not operate at stoic, and from what I understand about OE fueling they may intentionally be set to run a little fat at higher rpms and loads to protect the converter(s) from overheating.

Within emissions certification load and rpm for a car requiring it you would of course want to remain close to stoic, and closed loop operation with O2 sensor feedback ensures that you stay there.


http://www.bing.com/images/search?q=EFI+fuel+maps&...


Norm

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