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Old 05-09-2012, 06:08 PM   #1 (permalink)
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comments on: Tip 44 of the 100+ hypermiling tips (efficient methods of slowing down)

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44) The most efficient way to slow down

When you *have* to slow down, here's an approximate heirarchy of methods, from best to worst.

1) coasting in neutral, engine off (ie. roll to a stop);
2) coasting in neutral, engine idling;
3) regenerative coasting (hybrid vehicles)
4) regenerative braking (hybrid vehicles)
5) coasting in "deceleration fuel cut-off" mode (in gear, above a certain engine RPM)
6) conventional friction braking (non-hybrid or hybrid)
Hello everyone, my name is lbeew and I am new on the ecomodder forum. This will be my first forum post aswell.

I was reading the 100+ Hypermiling tips and when I came to number 44, something sounded counterintuitive. In modern fuel-injected vehicles, as soon as you release the throttle, the engine management stops feeding fuel to the injectors, as long as the rpms are high enough (I am guessing higher than 800-1000 rpm). This means that when you release the gas and coast IN GEAR, the wheels turn your engine, without the engine management injecting ANY fuel. Thus effectively not using any fuel. (I think this is point 5 on the list...) So this is more efficient than point 2 on the list, where you put the gearbox in neutral, the wheels aren't driving the engine and so the engine is injecting fuel to keep it idling (around ~800 usually).

On a second note: points 3 and 4 imply regenerative braking, thus winning energy back from the motion of the car. This would actually produce energy, so these two options should be the best options, way ahead of coasting with engine off and certainly better than using fuel (in neutral) and coasting.

I would suggest the order of best techniques be:

1 / 2. regenerative coasting / braking
3. coasting in "deceleration fuel cut-off"
4. coasting in neutral with engine off (Can be bad for automatic transmissions! Not so bad for manual transmissions)
5. conventional braking with car in gear
6. conventional braking with car in neutral

I put point 3 above point 4 because with the car in gear you use virtually NO fuel in fuel-injected cars and you have A LOT more control over the car than when it is in neutral. Also better for downhills. And the engine braking helps in slowing down, while putting it in neutral slows you down less fast. And the point was to slow down.

Please let me know what you guys think about this.

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Old 05-09-2012, 06:47 PM   #2 (permalink)
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Depends on if you want engine braking or not. If not, coasting in N of course is far, far better, engine on or off.
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Old 05-09-2012, 06:50 PM   #3 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Depends on if you want engine braking or not. If not, coasting in N of course is far, far better, engine on or off.
The subject is "The most efficient way to slow down".

Using no fuel is a more efficient way to slow down than using some fuel. So compared to idling your engine in neutral, leaving it in gear is more efficient to slow down.

If the aim was maximizing distance without fuel, then this might not be the case. But the subject is "most efficient way to slow down".

If the subject was to slow down with maximizing the distance you still travel, then this is another story altogether.
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Old 05-09-2012, 07:10 PM   #4 (permalink)
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Quote:
Originally Posted by lbeew View Post
If the aim was maximizing distance without fuel, then this might not be the case. But the subject is "most efficient way to slow down".

If the subject was to slow down with maximizing the distance you still travel, then this is another story altogether.
While you are correct on this technicality I believe the spirit of the subject is maximizing the distance traveled while decelerating.

Secondly many modern cars do not stop fuel flow while coasting as this often increases emissions per volume and that is more often the focus of manufacturers.
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Old 05-09-2012, 07:57 PM   #5 (permalink)
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Quote:
The subject is "The most efficient way to slow down".
Uh Huh, and the most efficient way to slow down is to coast to the stop with the engine off, as noted. If you have to hit the brakes hard or use engine braking, you're either doing it wrong or you're going down a hill with a stop at the bottom. Start the coast earlier.
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Old 05-09-2012, 08:45 PM   #6 (permalink)
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I am new here too. This is very interesting. When I read it I thought it wasn't quite right. It did make me think about my slowing down. I now use a combination of fuel off in gear coasting, and neutral engine idling. I don't shut the engine off while driving. This seems dangerous to me.

The definition of Hypermiling is...The act of driving using techniques to maximize fuel economy. With this in mind regenerative slowing, and braking are best. The next two can be switched depending on if you want to slow down. Modern vehicles with fuel off coasting does cause the vehicle to slow more quickly than neutral engine off. If your goal is to slow down then fuel off coasting will help slow down. If your goal is to go farther without slowing then neutral with engine off is better.
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Old 05-10-2012, 09:27 AM   #7 (permalink)
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Maybe the wording of the tip is confusing.

"If your goal is to go farther without slowing then neutral with engine off is better."

But unless you're going down hill, coasting in neutral IS going to cause you to slow down. I regularly plan approaches to stops & turns so that I coast 100% of the way (no brakes, no engine braking) down to the appropriate point so long as it doesn't affect other drivers.

Frank's right in pointing out that maximizing coasting distance means you have also minimized the amount of time (distance) you were burning fuel before you began coasting (decelerating).

That's what makes coasting (engine off) the more efficient way to decelerate - you're off the "go pedal" sooner than in any of the other scenarios.
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Old 06-15-2012, 04:03 PM   #8 (permalink)
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Quote:
Originally Posted by lbeew View Post
In modern fuel-injected vehicles, as soon as you release the throttle, the engine management stops feeding fuel to the injectors, as long as the rpms are high enough (I am guessing higher than 800-1000 rpm). This means that when you release the gas and coast IN GEAR, the wheels turn your engine, without the engine management injecting ANY fuel. Thus effectively not using any fuel.
While this does sound logical in theory, here's what I've noticed ACTUALLY happens (using the scangauge to verify when the car is and is not in deceleration fuel cut off):

1) Leave in gear to decelerate
2) Crap I wasn't going fast enough

OR

1) Leave in gear to decelerate
2) Count to 5
3) Deceleration fuel cut off FINALLY kicks in

DFCO is far more inconsistent than actually turning off the car. It also slows you down MORE, causing you to to have to accelerate more.

9 times out of 10, I use engine off coasting. The other times are when it's generally too short of a distance to justify EOC or when I actually need to slow down or a combination of the two.
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Old 06-15-2012, 07:30 PM   #9 (permalink)
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Quote:
Originally Posted by lbeew View Post
The subject is "The most efficient way to slow down".

Using no fuel is a more efficient way to slow down than using some fuel. So compared to idling your engine in neutral, leaving it in gear is more efficient to slow down.
Nope, it isn't.
Because you won't get nearly as far when using DFCO - a.k.a. engine braking.

Quote:
If the aim was maximizing distance without fuel, then this might not be the case.
We're all about maximizing distance and minimizing fuel.
Less about semantics.

Quote:
But the subject is "most efficient way to slow down".

If the subject was to slow down with maximizing the distance you still travel, then this is another story altogether.
Nope, that's still the most efficient way

Staying on the throttle for longer than you need to be, because you're using DFCO / engine braking, is all part of the inefficiency.
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Old 06-16-2012, 12:09 AM   #10 (permalink)
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Well stated euromodder.

I never use engine off in my diesel Beetle with an automatic transmission because the power steering effort is ridiculous without engine rotation. Even still it's always a balance of maintaining speed and coasting. I hate having to use the brakes, so I don't get out of gear unless I judge my momentum can carry me to the stop with little speed left over. Often I will coast if I can judge the traffic will start moving again before I get there, then I can stab it back in and keep going. If not, I used too much fuel, but DFCO will at least minimize fuel use while decelerating, as it's almost zero compared to idling.

It's a good challenge; I drive a lot, and this keeps me attentive.

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I'm not coasting, I'm shifting slowly.
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