Cold air vs Fuel Economy

[Rollo]

In colder air temperatures, the air is denser and so should have more air in them to compress and would give you a bit more horsepower and torque.
However, in warmer air temperatures, the air fuel mix should vapourise more effectively.
In colder air temperatures, you need to run higher idle speeds, which wastes fuel.

Are there any real world tests which solve this conundrum? All I've found is opinion and cross-conjecture.

[gadjo_dilo]

This should be this day's ration of humiliation for me.

[Mark]

In general I find running the heated windscreen etc, means that colder temperatures mean slightly worse fuel consumption.

Certainly during the summer I was getting pretty consistent 72MPG on the way home and on the way back, but now I'm getting more like 67MPG on the way to work but still 72MPG on the way back. I think just because of the cold mornings.

[Wasted Talent]

The engine consumes much more fuel before it gets to operating temperature and in colder weather this takes much longer, so for short medium journies it is the main cause.

Headlamps, heated screens/mirrors etc make some difference but not as much as the cold-running factor

WT

[schmenke]

Denser air packs more oxygen molecules per volume providing a bigger bang during combustion. In some performance cars this is achieved by the injection of nitrous oxide (N2O) into the cylinders. The added oxygen molecule provides enhanced combustion whereas the nitrogen is inert and has no effect.

In your average road car the ECU reads the input from both the temperature and O2 sensors then automatically adjusts the fuel/air mixture and/or timing to provide the optimum bang-for-buck.

[janvanvurpa]

In colder air temperatures, the air is denser and so should have more air in them to compress and would give you a bit more horsepower and torque.
However, in warmer air temperatures, the air fuel mix should vapourise more effectively.
In colder air temperatures, you need to run higher idle speeds, which wastes fuel.

Are there any real world tests which solve this conundrum? All I've found is opinion and cross-conjecture.

Air doesn't "make power". The fuel makes the power. The air allows the fuel to burn.
More oxygen means you CAN---indeed you must--add more fuel..

Above we saw the classic error that leads a friend---a top fuel drag racer who runs a machine shop I have gone to for all the block boriing and valve seat work etc ( I build nice rally and hi-po road car engines)---to say "I love NOX, it the machine shop owners No1 Best Friend"
We saw
by the injection of nitrous oxide (N2O) into the cylinders. The added oxygen molecule provides enhanced combustion whereas the nitrogen is inert and has no effect.


People say this and do this, then blow them up. More O2 means lean, more O2 + more fuel means more power.

A lot of old saws from carbie days with crappy chokes still persist.
Now with the 3-4 wire O2 sensors, and decent ECUs, even cold starts are only microscopically worse in any way.

And this improved cold star---as annoying as it is when its going wrong---like my miserable car right NOW---grrrrrr GRRRR!----means enormously less engine wear from reduced bore wash down, reduced oil dilution..
It's really amazing to open a 100k motor now compared with the "bad "Good ol' days". Things sparkle now.

[D-Type]

Nitrous oxide doesn't work quite like that.

Fuel is a hydro carbon so normally the hydrogen combines with the oxygen producing water vapour, the carbon combines with more of the oxygen producing carbon dioxide. The total volume of gas is greater than the oxygen that went in and the heat of combustion expands the volume even more so the piston gets shoved down.

Put nitrous oxide in there instead of oxygen and it combines in the same way as oxygen but when it does so it also forms nitrogen which further increases the volume.

If I could remember them I could demonstrate with some chemical equations.

Going back to the original question - the colder air is denser and contains more oxygen, but as it's colder it takes more heat to expand it so the two tend to cancel out.

They always used to say that performance increased on a humid day. the extra water in the combustion chamber was vaporised into steam increasing the volume of post combustion gases.

Not a simple subject.

[janvanvurpa]

Nitrous oxide doesn't work quite like that.

Fuel is a hydro carbon so normally the hydrogen combines with the oxygen producing water vapour, the carbon combines with more of the oxygen producing carbon dioxide. The total volume of gas is greater than the oxygen that went in and the heat of combustion expands the volume even more so the piston gets shoved down.

Put nitrous oxide in there instead of oxygen and it combines in the same way as oxygen but when it does so it also forms nitrogen which further increases the volume.

If I could remember them I could demonstrate with some chemical equations.

Going back to the original question - the colder air is denser and contains more oxygen, but as it's colder it takes more heat to expand it so the two tend to cancel out.

They always used to say that performance increased on a humid day. the extra water in the combustion chamber was vaporised into steam increasing the volume of post combustion gases.

Not a simple subject.


Since you are a moderator, I will be obsequious and grovelling and obediant like you Moda all demand but sorry while you're of course 100% right, I will trust a man who has made a living building top fuel drag engines running crazy ass fuels for longer than I am willing to bet you have been alive. And who has one AHRA National title--I will trust he's more 100% righter than you are 100% right.
Further:
HowStuffWorks "How does nitrous oxide help an engine perform better?" (http://auto.howstuffworks.com/question259.htm)


Y*ou may have read our article about how sodium chlorate acts as a way to store oxygen (http://auto.howstuffworks.com/question258.htm). You release the oxygen in sodium chlorate by heating it. It turns out that nitro*us oxide (N20) works exactly the same way. When you heat nitrous oxide to about 570 degrees F (~300 C), it splits into oxygen and nitrogen. So the injection of nitrous oxide into an engine (http://auto.howstuffworks.com/engine.htm) means that more oxygen is available during combustion. Because you have more oxygen, you can also inject more fuel, allowing the same engine to produce more power (http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/fpte.htm). Nitrous oxide is one of the simplest ways to provide a significant horsepower (http://auto.howstuffworks.com/horsepower.htm) boost to any gasoline engine.

*Nitrous oxide has another effect that improves performance even more. When it vaporizes, nitrous oxide provides a significant cooling effect on the intake air. When you reduce the intake air temperature, you increase the air's density, and this provides even more oxygen inside the cylinder.
The only problem with nitrous oxide is that it is fairly bulky, and the engine needs a lot of it. Like any gas, it takes up a fair amount of space even when compressed into a liquid. A 5-liter engine running at 4,000 rotations per minute (rpm) consumes about 10,000 liters of air every minute (compared to about 0.2 liters of gasoline (http://science.howstuffworks.com/gasoline.htm)), so it would take a tremendous amount of nitrous oxide to run a car continuously. Therefore, a car (http://auto.howstuffworks.com/car.htm) normally carries only a few minutes of nitrous oxide, and the driver uses it very selectively by pushing a button.




And a little "general" and poor choices of terms but:
How does Nitrous Oxide work? (http://paultan.org/2006/02/23/how-does-nitrous-oxide-work/)

How does Nitrous Oxide work?

So how exactly does injecting nitrous oxide into the engine help? Combustion is basically igniting a mixture of oxygen and fuel in a combustion chamber with a sparkplug. The resulting explosion produces power. What happens is a nitrous oxide injection system pumps nitrous oxide into an engines intake system. At 296 degrees Celsius, the nitrous oxide breaks down into nitrogen and oxygen. The loose oxygen atom is burned together with the combustion. More oxygen = more power. (!)Its as simple as that.

Of course there are some issues with the proportion of fuel and air and pre-mature detonation that well look at later. Also, why not just inject oxygen instead of nitrous oxide since what we need is the oxygen anyway? Pure oxygen would result in too wild a combustion, resulting in a blown engine. This is because the nitrogen atoms in the nitrous oxide have a function as well. The nitrogen atoms released have a cooling effect on the combustion temperature. It absorbs the heat and helps carry it away.



The rest of your stuff I am sure it is also 100% right, and fortunately I don't have the time to go point by point showing how others are 100% righter.. :rolleyes:
Moderators are always 100% right.

[call_me_andrew]

This may vary from car to car. Some models measure fuel temperature as well as ambient temperature. The IAC would be closer to closed in compensating for increased air pressure.

Assuming this engine is warmed up and naturally aspirated, there's no real change in fuel consumption in the low to mid range RPM's.

[D-Type]

Since you are a moderator, I will be obsequious and grovelling and obediant like you Moda all demand but sorry while you're of course 100% right, I will trust a man who has made a living building top fuel drag engines running crazy ass fuels for longer than I am willing to bet you have been alive. And who has one AHRA National title--I will trust he's more 100% righter than you are 100% right.
Further:
~
The rest of your stuff I am sure it is also 100% right, and fortunately I don't have the time to go point by point showing how others are 100% righter.. :rolleyes:
Moderators are always 100% right.

John, you really need to get your head around the whole moderator thing. A moderator is simply a forum member who has volunteered to take on the task of moderating, or if you prefer facilitating, a particular forum. In my case the "History" forum. If I or another moderator make a contribution to a discussion such as this, it is as a forum member and not as a moderator.

I strongly resent the way that you have taken my sincerely meant contribution and used it as a peg to hang cheap jibes at moderators on.

I also feel that this post has been written more as personal abuse than as an attempt to progress the discussion.

So, to come back to the subject in hand, which part of my basic chemistry do you disagree with? I would like to know if I have, in fact, got it wrong.

[janvanvurpa]

John, you really need to get your head around the whole moderator thing. A moderator is simply a forum member who has volunteered to take on the task of moderating, or if you prefer facilitating, a particular forum. In my case the "History" forum. If I or another moderator make a contribution to a discussion such as this, it is as a forum member and not as a moderator.

I strongly resent the way that you have taken my sincerely meant contribution and used it as a peg to hang cheap jibes at moderators on.

I also feel that this post has been written more as personal abuse than as an attempt to progress the discussion.

So, to come back to the subject in hand, which part of my basic chemistry do you disagree with? I would like to know if I have, in fact, got it wrong.


Well you're wrong, it made a slightly wry, and passing reference to what many many people here feel is an arbitrary and heavy handed dishing out of demerits or Infractions which you all seem to do at the drop of the hat. You included. Those "infractions ' are implied threats of bannation. So personally I try to avoid all interaction and conversation with any of you guys.

And I received an INFRACTION for my efforts to make a tiny wry comment how many perceive the operation of these forums works. Which you exaggerate and make it into some "cheap jibes" and "personal abuse" . Both wrong in intent and purpose.

And I will not engage you now as there is enough info in the good post above, enough links and I have race car parts to make today, I canno,t as I said, teach the entire process of how engines work and power is made.

[Kneeslider]

Back on topic...

May I suggest that when the air is colder, and denser, the quantity of drag goes up too.

Colder, denser air allows the engine to run better. Denser air allows wings and downforce to work better, but this is balanced with increases in drag.

So, as with most things, the answer depends upon rather a lot of complicated variables.

[Zico]

Back on topic...

May I suggest that when the air is colder, and denser, the quantity of drag goes up too.


Spot on...



An IC engine will have the potential to produce more power on a cold day but that alone actually has close to zero reason for increased fuel consumption on a daily cruise to work.... unless of course you drive flat out into work every day! :D . I think people will often drive more aggressively on morning rush journeys and fuel consumption will also be affected by heated windscreens etc, but if your journey is via a motorway where the majority of the journey is at higher speeds then Id suggest that this increase in air density and therfore drag is where the main part of the reduction in fuel economy is coming from.

If you look at the engineering equation for drag you can see the significance of the fact that the velocity value is squared.
Without having a greek characters/symbols input handy, in spoken terms, iirc the equation is- half the value of the density (Roe symbol) x velocity(squared) x the Cd x the area.
Its because the velocity squares, you can see that any change in density is going to have a significant effect on overall drag and therefore fuel consumption. If it can affect a car journey enough to be measurable, imagine the huge range in temperatures and air densities experienced by aircraft jet turbine engine.

We covered the basics in Aero Eng last year in aerodynamics but also again more recently with regards to high bi-pass turbofan jet engines where there is a crossover between the turbine and fan sections between drag on the fan vs the performance improvement/efficiency experienced by the turbine section from the increase in pressure created by slowing the air down via a diverent duct (Bernouli's principle) Basically this means that the fan size and pitot divergent duct angle/ratio had to be designed in accordance with the aircrafts pre-determined optimum cruising speed (V) and altitude (Roe) to achieve maximum fuel efficiency.

[D-Type]

Mixed thinking there: Drag is proportional to the density not to its square. It may be proportional to the square of velocity, but velocity is not affected by the density of the air any more than the area of the car or its shape (Cd). And remember density is inversely proportional to absolute temperature so the change is not as much proportionally as you would think. And aerodynamic drag is only part of the resistance a car has to overcome.

Now here's a thought. At lower temperatures tyre pressure goes down. When tyre pressure goes down rolling resistance increases. So, another reason a car is marginally slower on a cold day.

[airshifter]

In theory if you discount for warm up times, use of heat, etc you could get the same efficiency on a cool day if you could control the rate of acceleration and other performance parameters to the be same. But essentially people drive with the same amount of gas pedal, and won't be able to make such things equal.

Here in the US there is a segment of the population that is into the eco thing, and they often use warm air intakes to increase MPG by reducing power output. They often just set up a heat riser from the exhaust manifold and pull the warm air into the intake upstream of the intake air temp sensor.

[Zico]

Mixed thinking there: Drag is proportional to the density not to its square. It may be proportional to the square of velocity, but velocity is not affected by the density of the air any more than the area of the car or its shape (Cd). And remember density is inversely proportional to absolute temperature so the change is not as much proportionally as you would think. And aerodynamic drag is only part of the resistance a car has to overcome.

Now here's a thought. At lower temperatures tyre pressure goes down. When tyre pressure goes down rolling resistance increases. So, another reason a car is marginally slower on a cold day.


Hmm, I just did some very rough calculations and for a temperature range of 8 Deg C at 70mph an average car would only experience a negligible difference of around 3% (around 1 mpg) due to air density increase alone. As you say tyre rolling resistance-friction will increase but I wonder if the increased oil viscosity and therefore drag in the engine, gearbox and diff would have a significant effect to be a contributing factor?

This Eco calculator allows you to have a play with some of the different temp related factors/values. - Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com (http://ecomodder.com/forum/tool-aero-rolling-resistance.php)

[Kneeslider]

Rolling resistance will be constant. It doesn't change with speed, when you negate the effects of air resistance.

It does interest me that you say that some eco-ists have warm air intakes in their cars...

Apparently this causes the engine to use less fuel? How is this accomplished?

[schmenke]

... at 70mph ...

:s hock:

[airshifter]

Rolling resistance will be constant. It doesn't change with speed, when you negate the effects of air resistance.

It does interest me that you say that some eco-ists have warm air intakes in their cars...

Apparently this causes the engine to use less fuel? How is this accomplished?

I would think that rolling resistance would in fact change somewhat with temps, as the combined outside air and road temps would create a lower "hot" tire pressure. Certainly if you adjust for seasonal temps this would be minimized, but a lot of people probably don't.

As for warm or hot air intakes, it's simply reducing the amount of air (mass) and thus reducing the amount of fuel to reach the stoich mixture. Assumed to be 14.7 to 1 for light load cruise situations, if you inject less air, you inject less fuel to maintain a proper mixture. Power goes down as well, but the eco types aren't looking for power.

It's a similar result to the removal of a restrictive airbox on a bike and fitting it with free flowing fliters. You often have to re-jet the carbs to get mixture back to normal. More air/more fuel/more power.

In theory the warm air intakes also reduce intake restrictions as well, being that he farther open your throttle (or throttle body) butterflies are open, the less intake restriction.