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hypermiling technique, driving with load 1

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papab

papab

Aerospace
Dec 12, 2013
14
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.

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.
 
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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
 
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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.
 
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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.
 
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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.
 
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LionelHutz said:
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.
Click to expand...
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
 
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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.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 
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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.
 
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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
 
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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.
 
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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
 
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I can see that going slowing on the climb is going to save gas, but so would slower on the flats.
Click to expand...

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.


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

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.


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?
Click to expand...

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.
 
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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.



 
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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.
 
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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
 
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