hypermiling technique, driving with load 1

[papab]

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-tips-ecodriving.php#58

http://www.cleanmpg.com/forums/glos...results&search=driving+w/Load&searchoptions=1

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.

 

[BrianPetersen]

... 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.

 

[Tmoose]

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_Volvo_122-Autocar4May62.pdf

The e-brake was capable of 0.37 g stops

 

[NormPeterson]

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

 

[tbuelna]

...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.

 

[Lou Scannon]

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

 

[tbuelna]

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

 

[papab]

"part throttle enrichment" ?

 

[JayMaechtlen]

"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

 

[NormPeterson]

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

 

[LionelHutz]

"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.

 

[papab]

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.

 

[ivymike]

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

 

[NormPeterson]

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

 

[Tmoose]

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=Carburetor

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-ratio-on-gas-emissions.jpg

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#ixzz2oOhWshP4

Follow us: @allparcom on Twitter | allparcom on Facebook

 

[ivymike]

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.

 

[NormPeterson]

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...EA79D93C8EDAEB28CCD5D1E848C&selectedIndex=203

Norm