Engine off coasting?
 

How long do you need to coast before you actually gain increased mpg? Engine off for more than 6 seconds? At what point does it actually cost more to start (fuel enrichment) the engine back up vs just engine on coasting?

It depends on how long and how hard you used the gas pedal before you started the coast, and how long your trip is. Typically with a Scangauge or Ultra-gauge, you'll see immediate mpg gains when you start coasting, within a block or two. Get instrumentation like that, and eliminate the guesswork.

If you bump start does it change the mixture? (Specifically Mazda F2 engine)

I have heard that the engine if left off to long (eoc) then when fired back up the ECM goes into fuel enrichment ... I am not sure..just curious..

Again, it depends on the ECU software. My 2002 Sentra went into open loop (enrichment) for a full 4 minutes following any EOC. My 2006 Scion stays in closed loop most of the time when restarted, even after a 5 mile EOC.

5 miles is a long coast, those are definelty worth turning the engine off, but for anything under 30 seconds i don't think it's worth turning the engine off

Similar question: What's the shortest time where turning off and re-starting your engine is a net gain over idling? Or, at what time is it the same or better to idle, if any?

The automotive pundits say 7 seconds is the magic number with modern fuel-injected cars. Shut your engine down at any stoplight where you'll be idling >7 seconds. You'll use less gas to restart it than you'd waste idling. The EPA says $.17 of every gas dollar is wasted idling.

Thank you. Now does that figure change with kick-starting?

Probably not. The 7 second comparison is strictly fuel used. It doesn't consider electricity to crank the starter. Bump starting is more efficient than using the starter, but you may pay by replacing your clutch sooner.

Have to ask this question and will probably get jumped on.
Is it really worth it, I'm sure it does save fuel, but what about increasing rear and tear on the vehicle?
It was always said that Taxi Cabs in the old days could easily do 1,000,000km because they were only started and shut down a few times a week, so engine conditions are constant and wear is minimal.
There are many areas that may be negatively affected by starting, even with a hot engine, the biggest loads on any component are initial forces required to bring them in motion, so that goes all the way from starter, belts, pumps, bearings, all the moving parts basically. Every time you stop the engine the oil galleries drain away and it takes a second or so to build up oil pressurse.
Does anyone know of any studies done on longterm effects of repeated engine starting?
I generally only shut down when I think the delay will be longer than 2-3 min.

What do your roads look like that you can coast for 2-5 minutes? What speeds are we talking about at these distances?

I drive 90% highway for my commute. At most I do about 30 seconds of coasting before I hit below 50 mph. I find that the effort needed to do this just isnt worth it. At speeds above 50 the air resistance is really holding the car back. Even with pumped up tires you need a pretty nice hill just to keep your speed. The wear/tear on the cluth and engine just is not the MPG in my opinion. Just pay close attention to your foot and don't use more pedal than you need. Allow the car to slow down going up hills and gain the speed back when going downhill.

As for city driving I can see the benefits. One time on a 30 mph road I was able to coast for about 1.2 miles of pretty hilly terrain and finally ended up at a stopsign. Probably a good 2-3 minutes of coasting it was pretty cool.

You big up some very good points. The wear impact of EOC is what concerns me too. I'm sure some work was done on frequent starting and stopping for the Prius and other hybrids, but I have yet to see the data.

Diesel Dave what gear do you have in that 11.5 aam axle? 3.42? Us gear vendors overdrive? 3.42 gears would be super beneficial to your setup I believe... I have a 2005 dodge 2500, g56, 5.9 cummins, stock injection pump, lift pump, turbo (he351cw), bigger injectors, south bend 3600 dd clutch, aem brute force intake, 4 inch straight pipe, bully dog triple dog (scary Larry tune), edge ez, .... 551 hp- 1074 lb. Ft. At the wheels.. I have considered running the 3.42 gears....

I have 3.73 gears, but I've been looking at swaping them out for 3.42's. Currently all the gearing is bone stock.

What rpms do you try to maintain for best fuel economy?

Air intake? Performance exhaust? What kind of oil do you run? Another way to lessen your cost per mile is to install a fs2500 bypass filter. I change my oil and engine filter at 20k miles vs 7500 miles. I change the fs2500 filter at 10k miles. I also run Valvoline blue 15w40 premium blue oil and one bottle of Lucas oil stabilizer...

We're getting a little off topic here, I'll PM you.

Ive just started here but I think using the starter is alot easier on things than bump starting over and over again. Seems more natural for the vehicle.

It is not bad when you bump start and don't feel the bump... Don't just let (dump) out on the clutch.. Know what gear you need to bump start in so that you don't over rev the engine and ease out on the clutch ....

My opinion is that bump starting is the best thing to do. Whether you are using the starter or bump starting the process does the same thing. Spins the engine with the ignition on until it fires up. The difference is whether you wear on the starter or the clutch. When bump starting I always use the highest gear possible in order to minimize the "lunge" which makes it easier on the clutch. Of course with using the starter you avoid the lunge if you rev match.

The reason I chose bump starting is because its a simpler process. Let off gas, clutch in, turn key back, wait a second, key on. Coast. When my speed gets down simply let off the clutch and I am back driving. With starting its the same except when you need to start again you need to turn the key. Its just an extra step.

I suggest doing it whatever way is more natural. The key is to get a rhythm that is smooth and effective so you can do it over and over to maximize your coasting time and MPG.

It would be interesting to see long term results with 2 identical engines driven the same way but one being shut down more frequenlty. Although it would be pretty much imposible to do an identical test. It's pretty much guaranteed that an engine that's shut down all the time will have more issues, if not with engine internals, but for sure the belts, clutch plates, etc.

Has anyone had issues with the fuel pump or other devices that turn on and off with the engine? I've got a lot of friends telling me I'm killing the Probe.

I have not had any issues with my car yet... Every car is different though, just like every tank of gas lol.

I just bought a 92 civic hatch its bone stock. I got 41.3 mpg my first tank and second tank was 46.2 mpg.. I did a few mods and cant wait to see if i gained a little but I tried the engine of coasting a few times today. Normally its at 60 plus mph so i pop the clutch in 5th gear once it looses too much momentum. But it does worry me as well that maybe it will be more costly for clutch repairs and so on by doing this compared to the gas savings. And by popping the clutch i mean just bumping it over instead of using the starter not actually "popping it". My question is for those that have been doing alot of engine off coasting. How long have you been doing it and if u noticed anything out of the ordinary from doing it..

3+ years, 80,000+ miles of using EOC. I average over 10% of my miles per tank as EOC miles, and have no issues so far with my clutch. I would guess I bump start about 10-20 times per days commute (not so much if at all in the winter though). One of these days I will make a video of my bump start technique.

Unless it considers the fuel cost of the cranking electricity then it's not a comparison at all.

The wear and tear issue of frequent cold starts doesn't really come into play with EOC because the engine doesn't cool down much. In the winter I shut off the blower fan to keep the coolant temp up, but none of my EOCs are long enough to drop the temps below the operating range even if I had the heat blasting.

I don't consider the wear on the clutch at all because you either excessively wear it or you don't. A good bump start is easier on the clutch than even the best standing start- you're using the clutch to move a few pounds of well lubricated engine innards just enough for the ECM to throw a spark at them. From a stop, you're using the clutch to move a whole car with a bunch of rolling and environmental resistance. If you can do that well, then the clutch will never notice that you now bump start. If you can't bump start well, then it's probably your regular driving that's killing the clutch and the bump starting is going to get the blame because you started doing it at around the same time you started paying attention. At that point your efficiency/value question should be whether you're better off with your own sloppy clutch work or an automatic's sloppy clutch work. You probably are- swapping a clutch costs a lot less than overhauling an automatic.

what would be your reasons for not using it in the winter time

We get a lot of snow and ice here and it is not worth the possible loss of control by sliding tires trying to do a bump start.

I don't do it in the winter time because I own a car for convenience, I find heat to be convenient. That and the slush on the roads makes the coasts pretty short.

Just my 2 bits.

As others have indicated a specific number will vary depending on conditions.

If you wanted a specific number ... you would have to define the specific BSFC for the engine in question and all the other driving context conditions ... can be done if you are that interested ... but different vehicles will have different numbers under the same conditions ... and the same vehicle will have different numbers under different conditions.

As a crude General Rule ~10 Seconds is a safe point ... but I think learning the methodology itself is a better method than trying to pick one specific number.

Properly used engine off coasting is an FE asset ... over used it is a FE negative... I'm not just referring to too short of a engine off period... too long of an engine off period is also a FE negative.

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I usually recommend those interested first learn Conservation of Momentum , and driving awareness ... leaving engine off coasting for later development.

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Then if they still want to go even further ... I recommend the BSFC chart for their engine ... and to make an effort to operate the engine more often in it's higher efficiency points.

If current driving conditions warrant it using a engine off coasting event can be an effective method of avoiding those less efficient parts of the BSFC.

An additional Benefit of Engine off coasting beyond just avoiding low BSFC points , is that it also avoids pumping losses ... something the Engine has to spend energy/fuel for even at it's best BSFC.

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The too long Engine Off Coast aspect is a factor of the exponential influence of aerodynamics ... meaning it is more aerodynamically energy efficient to travel at a steady average speed than it is to achieve the same average speed while fluctuating the speed up and down... no matter what average speed you want to travel at.

So there is a point where further engine off coasting will actually produce worse FE and lower MPG.

Although I suspect traffic conditions would often make it unsafe to fluctuate one's speed that much before you reached the FE loss point.

:confused: Traveling any distance and burning no fuel is always going to be more efficient than traveling the same distance with the engine running.

It is called "paralysis through analysis".

But there will be a point of diminishing returns, which is what I think IamIan is getting at. Accelerating to 70mph and coasting to, say, 20 and then accelerating back up to 70, repeatedly--would this be as efficient as simply maintaining 45-50 mph?

Yes, proven many times by countless P&G,ers.

Not 'paralysis through analysis' at all ... there is a point of diminishing returns... and past a certain point it is less efficient... the point is that it can be useful to know how it works in order to maximize benefits and minimize disadvantages.

It takes energy to get moving from a stop ... you already burned the fuel to get the kinetic energy you are consuming during the coasting portion ... you consume that invested kinetic energy to over come aerodynamic losses and to over come rolling resistance losses ... there is no traveling a distance without spending energy / fuel to do so ... there is no free lunch... having spent the the fuel/energy at a prior point doesn't change this , you still had to spend it.

It is more aerodynamically efficient to achieve a given average speed by a steady speed than it is to achieve the same average speed by fluctuating speed... like it or not ... that's just how aerodynamics works... when it comes to aerodynamics there is a bigger hit to going faster than there is a benefit to going slower... and yes aerodynamics has been tested over and over and over again ... this relationship is well known ... it is just the way it.

Example of concept:
As I already wrote ... there are pros of the engine off method ... maximizing time / fuel spent in better / best BSFC ... and avoiding unnecessary pumping losses... but there are cons as well... there is a point of diminishing returns ... and a point where it breaks even ... and yes ... even a point where taken too far it is a net lower efficiency , and lower over all FE.

I see weather here is pretty good in the winter. Im going to try it out some this tank as my tank only netted 43 mpg this time around. I coasted from 65 down to 55 today once and it did take some effort to get back up the hill maybe negative idk but i coasted a good 30 secs or so.

You can't produce numbers like these engine on.
http://i1179.photobucket.com/albums/...2/100_0222.jpg
http://i1179.photobucket.com/albums/...228Small-1.jpg

Just keep in mind the break even point isn't a simple static point for all vehicles ... or even the same vehicle under all conditions... and the concept is at the same average speed.

Aerodynamic drag changes with wind speed , air pressure, temperature, humidity, etc.

Different Engines have different BSFC , different pumping losses, etc.

Different driving contexts consume different amounts of energy ... all other things being equal slower is less energy than faster ... energy conservation is more efficient than energy conversion ... etc.

For example of the 75MPH and 25MPH example of aerodynamic concept of that example vehicle 0.25 Cd and 20 Ft^2 ... while it increased the Aerodynamic drag energy consumed by ~2.5 kwh ... that would only be a negative to net FE if that ~2.5 kwh is larger than the gains from reduced Pumping losses and improved % of time at better or best BSFC.

I am not trying to propose a specific set thing ... like 75 to 25 is always too much of a speed variation ... that is not my point ... my point is the reality of the pros and cons ... and it is better to understand how things actually work than a hard set rule that will be wrong in some contexts.

I am referring to the known scientifically confirmed pros and cons of the method ... the break even point will not always be a 50 MPH speed variation ... it will not always be 25 MPH speed variation ... etc.

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For Example:
If the steady state vehicle in the previous example achieved 50 MPG ( so the math is easy at 1 gallon consumed ) and averaged of 28% ICE efficiency ... from average ~36 kwh / gallon gasoline... using ~10.1 kwh per gallon of the chemical energy of the gasoline.

Even with the additional ~2.5 kwh of aerodynamic losses would not result in a net lower FE if the speed varying method can achieve an average ICE efficiency of at least ~35% ... including the efficiency benefits of both time and fuel at better ICE BSFC points and reduced time spent paying for pumping losses.

At that point there would be no net negative FE effect.

But if the increased average ICE BSFC only went from 28% to 32% efficiency ... that is not a large enough gain to offset the aerodynamic penalt in that example.

In different context with different aerodynamic CdA, different conditions, etc... the break even point moves ... it is not static.

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Can't produce those numbers if engine is just off either... use 0 fuel/energy travel 0 Miles... no free lunch.

It isn't the engine off itself that achieved those numbers ... it is how the engine was operated when it was on to push the vehicle , by converting fuel chemical energy into vehicle kinetic energy , and the context of the conditions to achieve whatever average speed you had over that distance.

Simply I understand that what your saying is everything has to be factored in but the equation is kinda over my head. My guess is that if i make sure i still have enough momentum before i hit the next hill from the last coast and don't gas it too hard i'll be ok. Or if my fuel mileage goes up, maybe im doing something right lol. I bought a vacuum gage. just waiting on that to come in. I'm unsure what i'll be looking for there as well when using it.

The equation is simple: terrain dictates. What works best at one spot today might not be the best in the same spot tomorrow. I find that EOC from 70 to 20 works wonders for me... when there's a tollbooth in my future. Except that one of them is best hit from 60. Usually. Only one of my four tollbooth glides has a clearly defined start point, and even that doesn't have a clearly defined starting speed.

A good rule is to never go faster than you have to, and experiment to find out if starting a particular glide from a higher speed pays off by getting you farther enough to matter. Ain't hypermiling fun?