Engine off coasting?

[IamIan]

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.

[moorecomp]

Originally Posted by IamIan

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

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".

[Vman455]

Originally Posted by IamIan

Just my 2 bits.
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.

Originally Posted by moorecomp

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?

[moorecomp]

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

[IamIan]

Originally Posted by moorecomp

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".

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:

Say you have a Vehicle with a Cd of 0.25 ... and a Frontal Area of 20 Ft^2 ... Say for example you want an average speed of ~50 MPH over 50 Miles.

Looking at the aerodynamics.

If you achieve this average speed of 50 MPH via a steady speed of 50 MPH for 1 hour your wind resistance is about ~3.37 kwh of energy consumed... 50 miles covered.

If you achieve this same average speed by traveling for 30 minutes at 25 MPH ... covering 12.5 Miles , consuming ~0.210 kwh of energy ... and the 2nd 30 minutes at 75 MPH ... covering 37.5 Miles , consuming ~5.68 kwh... over the same 1 hour time period you have the same average speed of 50 MPH ... but due to the exponential effect speed has on aerodynamic losses ... your fluctuating speed method used ~5.89 kwh to cover the the same 50 miles at the same 50 MPH average speed.

The fluctuating speed method to cover the same distance at the same average speed consumed ~2.52 kwh more energy due to the exponential effects of aerodynamic losses.

The larger the speed variation the larger the penalty to the speed varying method ... the smaller the speed variation the smaller the penalty ... but it is always an aerodynamic penalty.

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.

[shockj2000]

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.

[SentraSE-R]

You can't produce numbers like these engine on.

[IamIan]

Originally Posted by shockj2000

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.

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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Originally Posted by SentraSE-R

You can't produce numbers like these engine on.

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.

[shockj2000]

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.

[Fat Charlie]

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?