You missed the obvious reason that the husband firmly believed that he needs to alternately accelerate and coast: he is a retired currency trader. Traders make no money on stable markets, it’s all about riding the ups and the downs to make money. He obviously generalized to his driving.
This is a variation on the old theme - you lose both ways. For example, if you go up a hill you have to provide extra energy to overcome friction. Now when you go down the hill you lose some of the energy, again, due to friction. You lose both ways; this is why there is no perpetual motion machine.
When you accelerate, you lose entropy (because cars, unlike thermocouples, are not powered by fully reversible thermondynamic engines). So, you lose energy in accelerating. Likewise, when you declerate, you lose friction, drag, and mechanical friction, which you also had to overcome when you accelerated. In otherwords, it’s like going up and down a hill, you lose both ways.
I drive for work in a '97 Cavalier, and one of my routes takes me 200 miles round trip in an area with long uphill and downhill grades on 2-lane with 55mph speed limit. On this route, I coast down the long hills in drive usually hitting 65 or so downhill. Then I hold a steady throttle up the next hill to stay in 4th as long as possible. When it does shift to third I try to hold a steady 50 or so. As I crest the top, I lift and do it all over. I routinely get 37+ mpg on this trip. When I drive the southern part of the state on flatter roads, still 55mph, I usually hold a stead 60mph and get about 33mpg.
Click and Clack tell us that a fuel injected engine virtually shuts off the fuel when coasting in drive, so I think I’m probably saving more coasting down the long hills than it costs to pull the next up hill.
But the way this woman described her husbands driving sounds more like someone that just can’t hold a steady pedal. He ain’t saving anything.
First, credentials:ChE vaTech '61, car racer builder and designer
The technique for the wah-wah gas pedal is called hypermiling. And in countless economy runs it works. Simply put, the energy to accelerate from 50 to 60 for example takes only a few seconds while coasting down from 60 to 50 using no energy (neglecting friction and aero being present for both cases) takes a much longer while. When the work is computed for the short energy input compared to the almost no energy expended for much longer times the resulting cycle is reduced fuel consumption. I’ve proven it myself with my CRX SiR. For better proof, Google some of the Honda Insight gen 1’s economy runs where 50 mpg rated vehicles get 125 mpg in well documented and regulated contests.
As for the use of hypermiling, even on the 50 to 60 mph cycling only nutty people do that. She’s right, its very annoying. cheers.
You missed a chance to point out why so many cars are equipped with cruise control . . . it saves gasoline. My mileage is consistently better when I use cruise control on reasonably flat roads. The erratic driving on the part of people like this woman’s husband is really frustrating to those of us who use cruise control.
I agree with everything said so far. Absolutely. Particularly Durwin. If you get into the gas exchange processes, heat losses, combustion efficiency, and throttling, common engines are inefficient at the 10-50 hp levels we typically drive at.
If you could average 10-50 hp needed by producing 100 hp for a short time, and 0 hp for a short time, you could potentially operate at an average higher engine efficiency. (insert lots of small print here) That extra energy could go into a battery, a hydraulic accumulator, or in the kinetic energy of the vehicle. Each storage method has it’s own losses.
The main question is which is the lesser of two evils? Suffering extra losses through an energy storage path to improve engine efficiency, or accepting the status quo poor efficiency at part load? Each particular case needs to be examined.
I’m surprised the learned gentlemen did not advise the wife to tell her husband to use cruise control and let the onboard computer give him the best mileage. I use cruise control at every opportunity and get 33 mpg on an Altima.
My recollection of the hypermiling contests of thirty or thirty five years ago doesn’t quite match Tom’s (or was it Ray’s). Yes, ordinary cars were getting very high mileage, but it was not by flooring it up to 60 and then letting off the gas- it was by extremely gentle acceleration up to about 15 or 20, then coasting in neutral (engine off) back down to nearly 0. The harder the acceleration, the worse the mileage, so an acceleration of just above zero is the most efficient.
An example. If you want to travel 10 miles (and pass the 10 mile post at cruise speed) in 15 minutes, you need an average speed of 40 mph. An ICE delivers energy most effeciently with the throttle wide open (accelerator fully depressed), and least effeciently at near idle throttle. Most fuel effecient way would be to max accelerate to something a little over 40 mph (say 45 mph), then coast down (because of drag) to something like 42, re-accelerate (max) to 45. Repeat this dither until you pass the 10 mile post at a slightly higer than 40 average speed. The average speed from start will be brought down by the start-up ramp up in speed. Now whereas this is a violent profile that cars are not designed for, it is theoretically the most fuel effecient for distance traveled in a given amount of time. And none of this accounts for fuel mixture management shortcomings as the acceleration is increased (jerk).
The alternative is to slowly accelerate to a slightly higher speed (say 50 mph), and then hold that speed with a steady state throttle position. (We need a higher cruise speed because it took us longer to get there.) Slow acceleration means less effectient operation of the engine, and longer time to get to cruise speed, therefore requiring a higher cruise speed for a fixed average speed of 40 mph. Higher cruise speed means greater aerodynamic drag (square law). Sustaining a constant throttle for the cruise speed means delivering power to the wheels in a more ineffecient operating regime of the engine. All things said, however, this is how we like to operate because it is better for our necks, our inner-ears, our equilibrium, and in general our sensibilities.
So there is more to life than fuel effeciency. And Wah-Wah is best to rock-n-roll.
The simplest explanation for it burning more gas is the following. The amount of gas burned is proportional to the power output of the engine. The primary need for power at cruising is to overcome wind resistance. Wind resistance is proportional to the square of the velocity, and therefore the power required to overcome is proportional to the cube of the velocity (since power is force times velocity). Therefore, if you are oscillating between 55 and 65 mph to maintain an average velocity of 60 mph, you’re going to spend more power moving the car in the 60-65 range than you save when the car is in the 55-60 range.
Andrew
Pressing on and off the gas pedal is a brain dysfunction. My husband and I have worked with other engineers in mining that do the same thing. Along with those that like to drive down the middle of the road. It’s semi-autistic and they can’t control it. They try to justify it, but that’s a cover story. It would take a psychologist specializing in bad driving methods to better explain it. I agree with other writers that say cruise control is the ultimate solution. My theorem: “everyone’s neurotic in their own way.” Our neuroses are just different from these peoples. --Kluck
First, I think Durwin and jbabb (the first two posters) were both absolutely on target.
I always relate problems like this one to purely mathematical optimization problems like this one:
“If x, y, and z are positive numbers such that z is fixed and x + y = 2z, which x and y produce the largest possible product xy?”
By symmetry (x and y are interchangeable) and the fact that xy tends to zero as x (or y) tends to zero, the answer cannot be anything other than x = z and y = z.
It depends on the situation. The thing is, unlike a 50,000+ lb semi with 10+ speeds, most cars aren’t geared to operate the engine at maximum efficiency under all conditions, so if someone is actually accelerating then coasting, not just letting it slow down while it’s idling in gear, then maybe they can increase their fuel economy a little bit. It all comes down to engine efficiency versus load. If you look at any BSFC map, from a 1L gas engine to a 15L diesel engine, they will tend to be more efficient at high load, so it’s possible to change how efficiently an engine operates by changing how you drive if it isn’t already geared to operate efficiently under all conditions.
If someone is just accelerating and letting off the gas, even under the right conditions, then it probably won’t do anything because most engines will still be injecting a small amount of fuel even after they let off the gas if the engine speed is under some pre-defined figure (say 1000 to 1500 rpm), and having the engine still connected to the trans means they won’t coast as far either.
In order to do it right the car actually needs to be in neutral, and in order for it to be worthwhile the driver generally needs to do it on a slight downhill grade (Not on a steep downhill grade!), and/or at low speeds, where they can coast for a while, because otherwise any gain in engine efficiency with an automatic may be offset by a lost in transmission efficiency as it’s being engaged and the torque converter clutch is initially unlocked.
The best situation is a manual transmission or an automatic that’s built to be flat-towed costing with the engine off, but even then ya gotta be sane about it since you’ll loose power steering on cars w/ hydraulic PS systems with the engine off. You’ll also loose power brakes after about 5+ seconds of application, but if you’re doing it and then riding up someone’s ass you’re doing it wrong anyway and you shouldn’t be driving the first place (Don’t tailgate!).
I also doubt it’ll be tremendously hard on the engine if it’s done correctly. It may not provide enough gas savings for someone to offset paying for an automatic transmission rebuild at ~150k instead of ~200k if you’re paying someone to do it, but if you work on your own car it can save you thousands in gas over the years, but you have to be realistic about the conditions it’ll work in. Otherwise you’re just pissing in the wind.
All that said, this is only a band aid for not having the right transmission to maximize fuel efficiency. If your transmission is geared properly you’ll be able to select a gear that will allow you to run the engine at it’s most efficient load, and you’ll get the best mileage possible with your car in those conditions without any kind of weird driving.
Anyway, the moral of the story is to drop in a transmission that will maximize your fuel efficiency, and if you can’t because one isn’t available, get a car that either has a decent transmission for fuel economy, or can at least accept one.
While visiting family in PA I was stuffed in the back seat on a 90+ degree day. The car a.c. was not great, and the sun was on my side of the car and through the back window. The driver was a wah-wah driver. As we exited the car, my husband quietly said to me “man, I feel queasy from the wah wah driving”. I informed him he should have been in the back seat instead of the front. After a day of popping into shops and then coming back home with the same driver at the wheel, I finally upchucked my entire day’s food stuffs…and I had eaten lightly! Not a pleasant experience. Next time we go there, I told my husband to drive. He is an excellent driver.
There was a good article on this on autospeed.com… basically saying that although it varies from engine to engine, generally best BSFC comes at moderate to high (but not maximum) load combined with lower (but not idle) RPMs.
http://autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_110216/article.html
I remember the days of the Shell mileage claims. An internal combustion has a maximum BSFC (brake specific fuel consumption) at wide open throttle and some RPM. An IC engine is basically an air pump and if the throttle is not fully open the engine efficiency is lowered. The game the thermodynamic nerds played was optimize the engine performance at some WOT point and collect the data across an RPM band. From this they would determine a maximum and minium speed to operate the engine. The vehicle would be accelerated to maximum speed and then shut off the engine and coast in neutral down to the minimum speed, restart the engine and accelerate at WOT to the maximum and then repeat the process. The tuning of the engine would probably make it difficult to drive in normal traffic conditions.
To relate my experience: with the manual transmission on my '96 Protege, I will slip it into neutral or hold down the clutch pedal on the downhills - without altering the traffic flow at all. I will actually have to brake sometimes as I gain on the cars who are “powering” down the hills. This strategy works great on my trips to and from work, where I am very familiar with the roads, and the best places to coast. They are fairly rural roads and I can coast for over a mile on one hill! I regularly get well over 40 MPG in this car.
By the way, this car gets 32 to 35 MPG with normal driving, so when I see new car ads bragging about their 28 MPG cars, I have to wonder what is wrong with these car companies? Surely there should have been some technical advances in fuel efficiency in the last 15 years! Do you remember the Datsun B-210 “Honeybee” that boasted 50MPG in the late 1970’s? Maybe they really don’t want us to get better gas mileage (can you say, “Big Oil?”).
The most abundant, cleanest, and fastest burning fuel in the universe surrounds us everyday. I’m speaking of hydrogen. There was an article in Popular Mechanics a few years back that told about the hydrogen engines used in aircraft during WWII (so don’t tell me, “they’re still working on the technology, they’ll have it figured out someday soon.”) and that after the war there could easily have been a nationwide (and worldwide) change to using hydrogen for fuel instead of oil.
But “Big Oil” had the infrastructure in place and was able to push hydrogen to the back burner. Can you imagine how much cleaner and healthier our environment would be had that change been made in the 1940’s? Today the oil infrastructure is more entrenched than ever! Just search “hydrogen fuel” on the internet and you’ll find BP, and Exxon, and others, in control of the so called “research” into hydrogen as a fuel - talk about the fox watching the hen house! Search on and you’ll find that hydrogen was used in the 1890’s to fuel locomotive engines in Germany - new technology? HA!
So my point is: if we really want to save fuel, and clean up our environment, instead of changing our driving habits, why not change our fuel altogether? It is said that a small engine, running on hydrogen, can be placed in a closed room and left running, and that the air will be cleaner after an hour than it was before! (Apparently, the burning process causes some free radicals already present to become bonded into harmless compounds in an hydrogen flame.)
One more point: do you realize that most people who die in airplane crashes burn to death? The liquid fuels (that are heavier than air) surround everything in the wreckage and when ignited…enough said. Hydrogen, being much lighter than air, rises so rapidly that the majority of the deadly flames occur above the wreckage - sparing many lives. Don’t let the Hindenburg disaster fool you, most of those people died from the fall, not from the flames.
A hybrid that can cruise electric only like the Chevy Volt could have its gas engine cycle every few miles between running at its optimum load and shut off.
The battery could be reduced in size to the minimum needed for short term power.
Where I work we’re about to put a tiny turbo-diesel in an EV-1:
http://www.howard.edu/ceacs/dean/EV1-Automobile.htm
http://www.howard.edu/ceacs/dean/GM-EV1.htm
It’s a shame we can’'t take it out of the garage.
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Most automotive engines will give best BSFC at somewhat less than WOT due to “enrichment circuits” designed to keep the engine cool at high power settings (rich mixtures burn cooler). But for this fact, WOT would be ideal.
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“Pulse and glide” hypermiling is basically a compensatory behavior to account for the fact that auto engines are “too powerful” for cruise. Ideally (for FE), a sedan would have an engine that would run at 75% power at straight-and-level highway speeds, like trains, ships, etc. Such a car, however, would have a 35-40HP engine and be utterly unsalable (and probably unsafe) due to an inability to accelerate/climb hills.
So, P+G “works” the engine at 75%–or 0%–and does better than working it at 25% the whole time.
- re: H2, hydrogen is a STORE of energy, not a SOURCE: it’s a “glorified battery.” The energy consumed to liberate 2H+ from a molecule of H2O is IDENTICAL to the energy produced when H2 is oxidized back to H2O. The energy has to come from somewhere else–heck, it could come from a gasoline-powered generator
