Car and Driver looks at mileage myths and misconceptions
In the latest issue of Car and Driver my friend Mike Austin has taken a look at a variety of different beliefs that people have about what saves fuel when driving. As we all know by now, driver behavior does have a major impact on fuel consumption. Mike and his test colleagues went out and ran mileage tests evaluating factors like using air conditioning vs. having the windows down vs. the sunroof open, removing extra stuff from the trunk, and running with stuff tied to the roof. The A/C vs. windows test yielded some interesting results. Only one car was tested in this case. A test with a variety of cars may have turned out differently depending on the aerodynamic properties and how air flows around the car. They also talked to some engineers about the effects of putting the transmission in neutral while coasting and of course shutting off the engine while the car is stopped. That last one is not generally recommended for cars that aren't designed with auto start/stop systems. It puts a lot of extra wear and tear on the starter and lubrication system as well as causing other potential problems. Its also very inconvenient if, like most Americans, you drive an automatic transmission car. At a drive through, for example, you'd probably be better off just parking and walking inside to get your fast food fix. Check out all of Mike's results and, as always, remember that your mileage may vary.
[Source: Car and Driver]
Reader Comments (Page 1 of 1)
BoneHeadOtto 7:25PM (11/12/2008)
yeah i have a subscription to this magazine and this article ranks up there with among the worst i have EVER read in a car mag.
the one that just got me was the one about coasting vs leaving the car in gear and using teh engine to decellerate. They ask one dudes opinion and then proclaim.
"Case closed, we’d say. "
Without any testing. The expert dude they talked to said that when an engine is in gear and decelerating that Toyotas and Lexus can detect this and cut fuel to the motor. If in neutral the gas is not cut so therefore yay we figured it out.
what they FAILED to realize is that if you coast, you can go a farther distance at idle, than if you engine brake. They didnt even bother to do the easiest of tests to verify the dudes opinion before declaring "Case Closed"
Plus they only considered Autos and i know from my own experiments that this makes a very large difference in a manual. Which BTW is what most people are talking about when they mention this. The rest of teh article was equally poor. How about testing a few things before proclaiming the answer.
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philmcneal 8:24PM (11/12/2008)
well said boneheadotto, sometimes its better to coast what you got then to waste it to engine braking and find out you have to gasp accelerate again!
because in theory idling the engine in neutral burns an X amount of gas per liter/gallon, and depending on your speed that can yield good results (from 50 mpg to 200 mpg depending on the speed of the glide)
why you think hypermilers love killing the engine and coasting in neutral so much? can't beat infinite mpg or 0 L / 100 km.
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BoneHeadOtto 10:14PM (11/12/2008)
Personally what i want in a car is the ability to hypermile more. Give me a manual car that has auto on an off that i can control easily. Here is how it should work. To kill the engine all you have to do is put the transmission in neutral and let the clutch out. I want my car to kill the engine when i do this. Then if i depress the clutch, or hit the brakes, or shift into gear, the engine should quickly start back up. I guarantee i could add 10mpg to my commute if i had this auto feature. Would be even better if the steering and brakes were electric so then the engine would only come back on when i actually put the car in gear for acceleration. would be cool
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Scott K. 11:48PM (11/12/2008)
Exactly. If automakers can invent a hybrid car, they can surely invent a way to allow a more conventional car's engine to turn off when unneeded, regardless of speed. When I'm coasting in neutral with my Outback, the only reason the engine is on is to operate power steering/brakes/accessories. Just add some batteries to power those features and I'd be all set. Now say it had this "engine off" feature. If the car needs to engine brake, why not use a system that turns the engine back on once it detects a significant rise in brake pad temperature? I rarely engine brake because I rarely need to. And don't some BMWs already have a system that compensates for brake fade by checking brake temperature?Besides, if you're coasting in neutral, it should tell the car you obviosly don't need to accelerate or engine brake. I say there's no reason why this isn't a realistic idea.
BoneHeadOtto 12:10AM (11/13/2008)
not only is it realistic it would be downright easy. I have often considered modding my car in this way but i care more for safety than mileage and i would worry that my mods didnt always work.
An interesting way to kind of do this yourself...although i dont advise....you can push the clutch in while coasting and turn the engine off but immediately turn the ignition back on. Now to restart the motor all you have to do is engage the clutch. When gas was in the stratosphere i experimented with this a little when conditions were safeish and my mileage went up noticeably. With a few changes i got my WRX to go from 20mpg mixed to 27mpg mixed. I currently get 26mpg in mixed city hwy and dont do anything as drastic as what i just mentioned above.
Another idea, why not have a plugin non-hybrid. Basically make all the accessories run only on electricity, give the car extra batteries and let the owner charge them at home. Then you can commute to work using the stored electricity. In the meantime the engine it not being hit with all those parasitic losses. If batteries run low, engage the good ol alternator. could probably add that feature for $500max to a car and get a 5 extra mpg in city.
Sam Abuelsamid 6:34AM (11/13/2008)
There are a couple of issues with having the engine shut off at speed as you describe. A number of accessory drive systems rely on the engine running such as the brake booster, and hydraulic power steering pump if so equipped. If you allow the engine to shutoff you have to either add a vacuum pump, or switch to a hydraulic boost system such as used on full hybrids. You also need a heavier duty battery to provide the 12V electrical supply for lights, wipers, fan, ECUs etc when the alternator is not turning. There are also a lot of issues with passing emissions requirements during all the re-starts. It can be done, it's just much more complicated than you describe.
BoneHeadOtto 9:39AM (11/13/2008)
true. But its not as difficult as you think. Many cars these days have electric steering assist. I dont know of any that have electric brakes, but the brake master cylinder maintains pressure while the engine is off. So you can slam on your brakes and have enough pressure to stop and give the engine plenty of time to start back up. Emissions are not as much of an issue because you are not starting the engine from cold. Plus there is not lots of wear and tear like people often state because the engine is already warm and lubed. The Prius does this all the time so it is a solved problem.
I was really excited to hear about Mazdas new engine start stop tech where they use the DI to start the engine faster. BUT unfortunately they will not offer this with a manual. PLEASE give me this car with a manual and allow me to shut off the engine like i described above. If they made this i guarantee that a skilled hypermiler could get way better mileage with this setup.
Anomoly 12:23PM (11/13/2008)
I believe that this it somewhat the idea of the "mild hybrid" i could be wrong... or the American car companies could have screwed it up... but when idling it shuts the engine off and everything runs off an extra battery...
i know that in order for the engine to shut off that you have to have the brake depressed for X amount of time so there is no way to "coast" with the engine off, but like some have said i'm sure it wouldn't be too hard for them to mod it so that one could coast.
I think the problem is in the danger that the average driver can't talk on a cellphone and drive at the same time... so trying to explain how the whole coast thing works would make their head explode!
Anomoly 12:28PM (11/13/2008)
and p.s. the "mild" hybrid on average increase gas milage a whopping 1-2 MPG's great job American Car companies!!
If it was able to shut off during coasting then we might be able to see some real increase in mpg's, even if there was a way to shift into coast mode or a button that you could push -
but i digress...
Ricknplano 9:50AM (11/13/2008)
BoneHeadOtto is not so bone headed. He has my vote for chief engineer at GM (or maybe at some company with a future).
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MikeW 10:26AM (11/13/2008)
The electronics of modern engines deserve more credit.
The computer can cut the fuel injection, and then it can adjust the variable valve timing for minimal throttling losses.
on older cars, the eEGR can open up, and the engine pumps in circles.
So the throttle braking in top gear is virtually nil. So if you want throttle braking, you downshift.
Taking you transmission out of top gear to 'save gas' is bone headed.
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