Fuel Saving Thread: Motoring Tips & Tricks

[mgb_phys]

I thought it was clear from the link he posted.
Pulse and glide is a standard technique for hypermilage, just not very appropriate for Highway use.

 

[starkind]

Actually I wasn't happy with the link I posted. No calculation, just handwaving. My first thought was the same as mtworkowski@o, that the cost of accelleration would surely be more than the benefit of gliding. But the thing seems to work, according to the postings. So, why?

And, how to calculate the savings from different max and min speeds?

I have actually done a little experiment, and satisfied myself crudely that there does seem to be an advantage. Also, I found that the other drivers did not seem to notice my speeding up or slowing down. This was on a 4 lane in low traffic, so they just changed lanes and passed. No one seemed irritated at all.

thanks. I'll probably think about it some more. Formulas, I want formulas.

R

 

[mtworkowski@o]

No, what's your answer? The coasting is only marginally better than the cruising. Cruising requires you to overcome all the resistances(rolling, air,). Those are still present but air is down during slow part of the coast down. Now for the next acceleration. This is like asking if it's better to feather the throttle or go as fast as you can and then coast. If you were running out of gas which would pick?

 

[starkind]

I should think to feather, being as going faster than 50 mph or so cuts the gas mileage due to wind resistance.

So if gas mileage at the steady speed is greatest near fifty mph, I could choose to experiment with upper and lower bounds at perhaps sixty and forty, so as to stay near the speed of greatest gas mileage.

The technique has two main phases...acceleration and gliding. I use acceleration here in the sense of increasing velocity. The gliding phase would be 'decelerating' due to radiation of heat energy through friction.

So there is a short burst of acceleration followed by a long period of radiation and slowing.

My eyes are blearing or my glasses are dirty. I'll sleep on it and have clean ones in the morning.

R

 

[brewnog]

The acceleration phase needn't be full throttle.

 

[starkind]

That's true. Which brings up the question of how much acceleration is most efficient.

I guess the way to a formula would be to start by identifying all the variables and setting controls. For example road surface and inclination would be variables in driving conditions, both of which could affect efficiency, but these are not part of the question and so should be held constant, by using a flat test bed with consistant surface.

mass of vehical
upper speed bound
lower speed bound
time required to accelerate
friction effects...tires, bearings, wind resistance, weather conditions.

I'm going to refresh my memory on the Carnot cycle.

 

[mtworkowski@o]

Don't bother. No acceleration is the most efficient acceleration rate. F=MA. No acceleration, no force. No force, no gas. elegant ay? This than requires us to supply just enough power to keep the car going at a constant speed. Overcome wind, rotaional, etc resistance. Now there is one more consideration. We know that air resistance increases with increase speed, so we don't want to go fast. But considering the engine power curve and the trans gearing, what is the speed that will give us the best gas milage?
Do you see how far away from your original thinking we've gotten? Have light bulbs been turning on in your head? I hope so. Learning is an amazing feeling. I should call it understanding. It's almost creepy how you "get it".
Let me know!

 

[starkind]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

 

[turbo]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

You have neglected mechanical efficiency.

 

[mtworkowski@o]

listen. you burn more gas accelerating quickly. F=MA The motor developes more hp and moves the car faster over time. sheeeesh

 

[DrClapeyron]

Open throttle gasoline engines, other than pre-ignition problems, what is holding it back?

 

[turbo]

Introducing more fuel and/or air to an ICE than it is designed for takes it very quickly out of the efficiency-range that it was designed for. Also, pre-ignition is a complex problem based on compression ratios, lean/rich ratios, and fuel grades. Are you sure you'd like to keep pushing on these subjects?

 

[mtworkowski@o]

cruise controll

verdict? Good for fuel or not? I know the answer.

 

[brewnog]

Open throttle gasoline engines, other than pre-ignition problems, what is holding it back?

Detonation, aspiration, heat rejection, scavenging, mechanical and thermal loading, control, fuel supply...

 

[OmCheeto]

hmmm... sorry I'm a bit late joining this foray.

I've been actively researching this topic for the last couple of years, and I discovered the best source of information is found by perusing the hyper-mileage forums.

There was one recurring theme that stood out: mpg gauges.

Automotive computers can do a lot to improve fuel savings, but the human mind seeing 3 mpg on the dashboard can do a lot more to change peoples habits than anything else.

Call your next president/prime minister and make it clear that this should not be an option in non-human powered vehicles.

And pump up those tires!

 

[mtworkowski@o]

hmmm... sorry I'm a bit late joining this foray.
You're right. My wife has a Prius and I'm always looking at the mpg readout. I think it makes you drive better. I said that same thing to her not a week ago.

 

[mheslep]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

listen. you burn more gas accelerating quickly. F=MA The motor developes more hp and moves the car faster over time. sheeeesh

And you would get there faster, spending less time burning fuel. F=MA is not the reason going faster over the same distance requires more energy, it is the exponentials in the air drag and mechanical resistance equations that make it more energy expensive, even if one does arrive more quickly. That is, say air drag dominates and is proportional to velocity ^ 2. Then if the power required to move the car at velocity V through the air is P, then the power required to move at twice V is 4P, not 2P. So then even if the car has made the same trip at 2V in half the time, it has expended energy at 4 times the rate to do so. This is also the reason why the energy required to travel at constant speed through a fluid is less than the energy required for the same trip traveling at lower and higher speeds that average the same speed, and similarly hence the effectiveness of the 55mph speed limit in conserving energy.

 

[OmCheeto]

Hmmm.

Consider two otherwise equivalent cars at full stop. One starts out and accelerates slowly until it reaches highway speed one minute later. The other floors the pedal and (no skid marks) reaches highway speed in ten seconds. They both drive to the end of a sufficiently long course, and then stop. Do they both use the same amount of gas?

Moving the same mass the same distance means the same amount of work?

Then the best move would be to accelerate to speed in as short a time as possible, keeping in mind safety.

This is actually a very interesting question. The answer is of course quite complex and might have several answers.

Given that the kinetic energy of an object is equal to 1/2mv^2, one might assume that the petrol energy required to accelerate a vehicle to speed would be the same for all cases.

I'll simplify the problem by having two vehicles reach their destination at the point the slower accelerating car reaches the target speed of say, 50 mph.

Fast car accelerates at 50mph/60 seconds.
Slow car accelerates at 50mph/60 minutes.
So the slow car travels 25 miles getting to it's target speed and takes an hour to get there.
The fast car takes about 30 minutes to arrive.
In this example, the slow car would have to get twice the fuel mileage than that of the faster car in order to win the mpg race.
Now this is just my opinion, but I think the fast car would win the mpg contest.

The other extreme of course, is the quarter mile dragster, which interestingly, gets about 1/4 mile per gallon. Pitted against the slow car above, it would probably lose the mpg contest.

These are extreme cases, but I think it points out that there may be an optimal acceleration for every vehicle, when considering fuel economy.

 

[mtworkowski@o]

One thing I know does help is to use cruise control whenever possible on trips. I experimented with my truck on trips. It does indeed help.

Fred,
All the cruise controls I ever used would try to maintain speed up hills and slow down on the other side to keep the speed constant. If you're trying to save fuel, the best way would be to let the kinetic energy from the downside of hill A help you to travel up hill B. Unless you travel on pretty flat ground. That's OK too.

 

[du nuthin]

Look at Diesel exhaust gas; a very large proportion of that is water vapour. Most of this is a product of combustion, but some is straight from the fuel too. Any hydrogen/oxygen bonds which are broken during combustion are re-made again very, very quickly during the combustion reaction, and still ultimately released as water vapour in the exhaust. Why do you suggest water within the fuel increases fuel economy? Sure, you can inject water into the cylinder to improve efficiency, but this is due to charge cooling effects from evaporation, and not hydrogen/oxygen reaction kinetics.

The better fuel economy of WDE comes from its water component. Water converts heat energy into motive energy. In an ICE (Internal Combustion Engine) it is not heat, a by-product of combustion, that drives the pistons. It is the combustion gases (H2O + CO2) that provide the motive energy. Since one litre of fuel produces more or less one litre of water, a mixture containing 25 percent water and 75 percent diesel, should produce the same volume of combustion gases as that of pure diesel. There should not be any power loss arising from the use of WDE.

What is important is to ensure that the higher viscosity of WDE is addressed i.e. adding a pump to deliver WDE to the engine (to ensure the engine is not starved of fuel), preheating WDE fuel to reduce its viscosity, fuel tank agitation to ensure fuel consistency.

 

[du nuthin]

Look at Diesel exhaust gas; a very large proportion of that is water vapour. Most of this is a product of combustion, but some is straight from the fuel too. Any hydrogen/oxygen bonds which are broken during combustion are re-made again very, very quickly during the combustion reaction, and still ultimately released as water vapour in the exhaust. Why do you suggest water within the fuel increases fuel economy? Sure, you can inject water into the cylinder to improve efficiency, but this is due to charge cooling effects from evaporation, and not hydrogen/oxygen reaction kinetics.

In an ICE (Internal Combustion Engine), it is the volume of combustion gases - CO2, H2O, CO, NOX - that provides the motive energy that drives the pistons.

Since 1 litre of diesel produces approximately 1 litre of water during combustion, the total volume of combustion gases produced by an emulsion of diesel should give the same power as pure diesel even though it contains less diesel.

Another benefit of water diesel emulsion is the effect of secondary atomization of fuel droplets that result in a more complete combustion. More complete combusion means less fuel wastage.

There is absolutely no reason why any ICE should be using pure diesel. Converting waste combustion heat to motive energy should be the way forward.

Waste heat is currently dumped into the atmosphere via the engine's radiator. Such a wasteful use of a scarce resource should be immediately stopped.

 

[mgb_phys]

My wife has a Prius and I'm always looking at the mpg readout. I think it makes you drive better.

It makes me drive badly! I rented a prius recently - I had to keep reminding myself to look at the road instead of the watching the computer as I experimented with what it was doing.

 

[mtworkowski@o]

The display is definitely bad. It's not the fuel thing so much but the radio and heat/ac are all touch screen. You could go of the road. But we got 58 mpg on a trip. That's more than a Smartcar.

 

[brewnog]

Waste heat is currently dumped into the atmosphere via the engine's radiator. Such a wasteful use of a scarce resource should be immediately stopped.

How do you propose that this waste heat is all recovered and used?

 

[mgb_phys]

I think at least one of the US Army's tanks powered by gas turbines uses peltier stacks to generate electrical power from the waste heat.

 

[OmCheeto]

I think at least one of the US Army's tanks powered by gas turbines uses peltier stacks to generate electrical power from the waste heat.

There are two devices that make me utterly crazy in my research on fuel savings: peltier generators and sterling engines. Finding the efficiencies are almost impossible. The wiki entry on sterling engines looks like it was written by someone trying to sell them.

Todays search for peltier stack generators yielded the following:

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JAPIAU000101000002023704000001&idtype=cvips&gifs=yes
The maximum energy conversion efficiency DeltaW/Winput obtained at an optimum period is 0.37% for the generator

Now is it my imagination, or are peltier stacks the least efficient of any device on the planet?

And don't get me started on sterling engines. Ok. Let me. I once read that a 4hp sterling engine weighed 2 tons. (Both these numbers may be off by a factor of two as I cannot find the original reference.)

 

[du nuthin]

How do you propose that this waste heat is all recovered and used?

In an ICE (Internal Combustion Engine), it is the volume of combustion gases - CO2, H2O, CO, NOX - (NOT HEAT) that provides the expansion energy that drives the pistons.

Since 1 litre of diesel produces approximately 1 litre of water during combustion, the total volume of combustion gases produced by an emulsion of diesel (containing say 33.33 percent water and 66.66 percent diesel) should be about the same. It should, therefore, give the same power as pure diesel even though it contains 33.33 percent less diesel.

Waste heat of combustion converts WDE's water component into superheated steam.

Another benefit of water diesel emulsion is the effect of secondary atomization of fuel droplets that result in a more complete combustion. More complete combusion means less fuel wastage.

Preheating WDE fuel to 60 deg C using an engine's coolant has two benefits. First, it reduces its viscosity for a better fuel spray quality. Second, it minimizes the heat absorption rate of WDE's water component. Since diesel engines rely on the heat of compression for ignition, preheating WDE will minimize its impact on the engine's designed heat of compression.

 

[turbo]

In an ICE (Internal Combustion Engine), it is the volume of combustion gases - CO2, H2O, CO, NOX - (NOT HEAT) that provides the expansion energy that drives the pistons.

Since 1 litre of diesel produces approximately 1 litre of water during combustion, the total volume of combustion gases produced by an emulsion of diesel (containing say 33.33 percent water and 66.66 percent diesel) should be about the same. It should, therefore, give the same power as pure diesel even though it contains 33.33 percent less diesel.

Waste heat of combustion converts WDE's water component into superheated steam.

Another benefit of water diesel emulsion is the effect of secondary atomization of fuel droplets that result in a more complete combustion. More complete combusion means less fuel wastage.

Preheating WDE fuel to 60 deg C using an engine's coolant has two benefits. First, it reduces its viscosity for a better fuel spray quality. Second, it minimizes the heat absorption rate of WDE's water component. Since diesel engines rely on the heat of compression for ignition, preheating WDE will minimize its impact on the engine's designed heat of compression.

How can a liter of diesel possibly produce a liter of water when it is combusted? Doesn't any of the carbon in the diesel get combusted to CO₂ or CO? How about some references to back up this claim?

 

[du nuthin]

How can a liter of diesel possibly produce a liter of water when it is combusted? Doesn't any of the carbon in the diesel get combusted to CO₂ or CO? How about some references to back up this claim?

A Typical chemical formula of diesel = C12H23. When combusted the C combines with O2 to produce CO2 and the H2 combines with O2 to produce 2 H2O.

H2O = Water

 

[mheslep]

A Typical chemical formula of diesel = C12H23. When combusted the C combines with O2 to produce CO2 and the H2 combines with O2 to produce 2 H2O.

H2O = Water

More precisely, in a pure oxygen atmosphere:
CxHy + (x+y/4)O2 -> xCO2 + (y/2)H2O
so for every mole of C12H23 diesel one would have 12 moles of CO2 and 12.5 moles of water products

 

[Topher925]

In an ICE (Internal Combustion Engine), it is the volume of combustion gases - CO2, H2O, CO, NOX - (NOT HEAT) that provides the expansion energy that drives the pistons.

I disagree. Work is created from thermal expansion in both the Otto and Diesel cycles, hence why they are called Heat Engines. This can be shown with P-v and T-s diagrams. I will agree with you that work might be obtained from the restructuring of molecules during the reaction, but the majority of the work is created from the heat from the exothermic reaction.

A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work

http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heaeng.html#c3

Recovering this waste heat isn't an easy thing to do. Peltier/thermo-piles and sterling engines look great on paper, however in practice they are just about worthless. Kind of like the Carnot cycle. BMW created some device to recover waste heat from the exhaust and reuse it but I believe it ended up being a pretty big failure. Something about the added weight not being worth the gains in efficiency.

 

[OmCheeto]

BMW created some device to recover waste heat from the exhaust and reuse it but I believe it ended up being a pretty big failure. Something about the added weight not being worth the gains in efficiency.

Where did you hear that?

I can find no reference on the net that implies they've stopped their turbosteamer research program.

http://www.bmwgroup.com/e/nav/index.html?http://www.bmwgroup.com/e/0_0_www_bmwgroup_com/forschung_entwicklung/science_club/veroeffentlichte_artikel/2006/news20063.html"

But thanks for reminding me. I see now that they've published their Th & Tc (1000'F & 160'F)
Lets see, that's 810'K and 344'K which yields a Carnot efficiency of 57%.
They claim a 15% increase in efficiency with the system.
hmmm...
Someone correct me if the following assumptions and conclusion are wrong:

ICE's are 25% efficient.
Multiplying 25% x 57% yields ~15%.
Is it safe to assume that BMW has extracted all the practical usable energy using this system? Or is the 15% mathematical result just a coincidence.

 

[du nuthin]

I disagree. Work is created from thermal expansion in both the Otto and Diesel cycles, hence why they are called Heat Engines. This can be shown with P-v and T-s diagrams. I will agree with you that work might be obtained from the restructuring of molecules during the reaction, but the majority of the work is created from the heat from the exothermic reaction.


http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heaeng.html#c3

Recovering this waste heat isn't an easy thing to do. Peltier/thermo-piles and sterling engines look great on paper, however in practice they are just about worthless. Kind of like the Carnot cycle. BMW created some device to recover waste heat from the exhaust and reuse it but I believe it ended up being a pretty big failure. Something about the added weight not being worth the gains in efficiency.

Heat alone has no motive energy. A medium such as water is necessary to convert heat energy into motive energy.

Water droplets encased in diesel instantly flashes into steam upon entry into an ICE combustion chamber. This secondary atomization of fuel droplets accelerates the rate of combustion.

 

[Topher925]

Obviously a working medium is required for thermal expansion. If it wasn't, then it wouldn't be called thermal expansion. However this is not what you stated. You stated that the energy that provides the expansion of the medium does NOT come from heat which is a false statement.

In an ICE (Internal Combustion Engine), it is the volume of combustion gases - CO2, H2O, CO, NOX - (NOT HEAT) that provides the expansion energy that drives the pistons.

OmCheeto, I cann't remember exactly where I heard that nor could I find any info on it either. I believe I heard it from a german engineer from my last job. Also, all articles I could find on it were no later than 2006. There seems to be no info on it from the past couple years. So, its anyones guess what stage the project is at now.

 

[OmCheeto]

OmCheeto, I cann't remember exactly where I heard that nor could I find any info on it either. I believe I heard it from a german engineer from my last job. Also, all articles I could find on it were no later than 2006. There seems to be no info on it from the past couple years. So, its anyones guess what stage the project is at now.

Given that it was a secret project for the first 5 years, I'd say they've gone back into stealth mode. I came up with the same idea independently six months before they announced it. I've been continuously improving my design ever since, and I would imagine they're doing the same. They did say it would be another 10 years before it was to go into production.

Btw, this was the topic of my very first thread here at the forum; https://www.physicsforums.com/showthread.php?t=203654"