Can exhaust heat be used to reduce automotive drag?

In summary, injecting wasted heat energy into the air in front of the car supplements the kinetic energy of the vehicle itself in moving air particles out of the way.
  • #1
David Morgan
8
0
I was reading a BBC article about the automotive challenges of building a 300 MPH capable, street-legal vehicle and had a the following thought/question: Could one dissipate enough of the considerable exhaust heat generated by these supercars into the front bumper/air-splitter, so that the air passing over these surfaces would be instantly heated enough to reduce overall drag on the vehicle?

Ignoring the obvious risk of BBQing a few stray pedestrians, could this technique work?

I guess the same could be asked for an airplane's nose cone... if you intentionally increased the temperature of the cone well beyond the temperatures that would naturally occur due to air resistance, would this reduce overall drag?
 
Engineering news on Phys.org
  • #3
I'm assuming that hotter air would be less dense (or if nothing else, slightly more chaotic) and therefore easier to push a car through. I guess I'm seeing the whole "hot knife through butter" cliche in my head, with a hot car staring as the butter knife, and the air playing the role of butter.
 
  • #4
Knife through butter and the Shkval both change the phase of the medium, from solid to liquid and liquid to vapor respectively
 
  • #5
Fair enough, air will be air in this case, no phase changes... however, if we instead look at it as an energy transfer problem, would injecting wasted heat energy into the air in front of the car supplement the kinetic energy of the vehicle itself in moving air particles out of the way?
 
  • #6
David Morgan said:
I'm assuming that hotter air would be less dense
Well, the air does not vanish suddenly, you still have to push the same mass away.
And turbulence often increases drag.

It could make cooling the engine a bit more effective.
 
  • #7
Further, in air, viscosity increases with temperature, so by causing the boundary layer to be much warmer, you will increase viscous drag. Injecting the hot exhaust air is also likely to trip the boundary layer to turbulence if it isn't already turbulent by that point, so that would increase viscous drag further in those regions rather dramatically (as in an order of magnitude increase).
 
  • #8
Here's one challenge I bet the article in the Beeb didn't mention:
What happens if Jeremy Clarkson is behind the wheel of one of these 300 mph supercars and he's feeling a bit peckish? :nb)
 
  • Like
Likes Chrono G. Xay
  • #9
If you use the heat to expand air behind the car, it will push the car forward. That's how a jet engine works.
 
  • #10
Thanks for the replies everyone, it's definitely helping me better understand the error of my ways.

So looking at this from a mass-energy equivalence standpoint: injecting more energy into the air in front of the car would essentially be the same as making the air more massive. The basic F=MA principle would then indicate that the air is applying increased (drag) force against the vehicle, thus decreasing its forward momentum instead of supplementing it.
 
  • #11
Wait, is this a relativistic car?
 
  • Like
Likes ulianjay
  • #12
We can certainly neglect relativistic effects for street-legal vehicles.

If red traffic lights appear green to you, then you are going too fast!
 
  • Like
Likes Ender0183 and boneh3ad
  • #13
David Morgan said:
Thanks for the replies everyone, it's definitely helping me better understand the error of my ways.

So looking at this from a mass-energy equivalence standpoint: injecting more energy into the air in front of the car would essentially be the same as making the air more massive. The basic F=MA principle would then indicate that the air is applying increased (drag) force against the vehicle, thus decreasing its forward momentum instead of supplementing it.

I wouldn't simplify it like that. It might lead you to think about the problem wrong later on. If I had to simplify it down to something as easy as F=ma, it's more about not being able to accelerate the mass as much as you would normally with the same force because you're changing the fluid flow.
 
  • #14
Hi David. I guess I'll have to disagree with the responces so far. I don’t have a calculator to do real cars, only for cylinders and spheres so I did a calculation for a cylinder driving down the road. Something like this:
fred_flintstone_car.jpg


Drag is linearly dependent on density so if you heat the air by 1%, density decreases by 1% and drag also drops by 1%. See for example:
http://exploration.grc.nasa.gov/education/rocket/drageq.html

As Boneh3ad points out though, viscosity increases with an increase in temperature. This gets into the calculation through the coefficient of drag which changes as a function of Reynolds number. See this graph for example:

dragsphere.jpg


As temperature increases, viscosity increases, Reynolds number decreases and coefficient of drag increases so drag force increases. But the affect is very small. The affect of gas density is much more significant. Doing a calculation for Fred’s car, I find the drag is linearly proportional to absolute temperature.
 
  • #15
Q_Goest said:
Hi David. I guess I'll have to disagree with the responces so far. I don’t have a calculator to do real cars, only for cylinders and spheres so I did a calculation for a cylinder driving down the road. Something like this:
fred_flintstone_car.jpg


Drag is linearly dependent on density so if you heat the air by 1%, density decreases by 1% and drag also drops by 1%. See for example:
http://exploration.grc.nasa.gov/education/rocket/drageq.html

As Boneh3ad points out though, viscosity increases with an increase in temperature. This gets into the calculation through the coefficient of drag which changes as a function of Reynolds number. See this graph for example:

dragsphere.jpg


As temperature increases, viscosity increases, Reynolds number decreases and coefficient of drag increases so drag force increases. But the affect is very small. The affect of gas density is much more significant. Doing a calculation for Fred’s car, I find the drag is linearly proportional to absolute temperature.

Here's the problem with this assessment: the drag equation you are using to make this conclusion depends on the free-stream density, not the density in the boundary layer. The exhaust gas will only affect the very thin layer of air by the car, so drag is not going to vary linearly (or even hardly at all) with that density change in the boundary layer.

The primary effects will be viscous in nature, in which case the likely result is greater viscosity and a tripped boundary layer if it isn't already turbulent at the injection point, both of which increase drag. Also, while a turbulent boundary layer can reduce drag if it's effect on drag due to separation is larger than its effect on viscous drag, on a car the boundary layer is likely turbulent before reaching any separation point anyway. So there's no benefit there.

On the other hand, if you injected the air parallel to the wall just upstream of the separation point at the rear of the car, you can probably delay separation a bit if the separation point is in a location amenable to this approach. In that case it may be possible to reduce drag. It probably wouldn't work if the separation point is already so far back that it occurs at the sharp corner near the trunk, though.
 
  • #16
Hi Boneh3ad. You may be right about difficulties in creating a 'bubble' of hot air around the car. Regardless, the analysis isn't incorrect, the question is whether or not it is applicable. I don't think we can make that determination definitively and without a lot more assumptions about the system used to create this hot air bubble. But at the very least, my responce should be informative and provides some basis to learn a bit about drag forces.
 
  • #17
But ultimately, the free stream is still the atmosphere in this case. There isn't nearly enough exhaust from a car to create a large enough temperature "bubble" to change the free stream density used with the typical drag equation. The effect will be confined to the boundary layer.
 
  • #18
A hot air bubble around the car would seem implausible, so it must be easy to show mathematically if that's the case, right?

I think the OP has an interesting suggestion that may have some value for learning about drag on objects.
 
  • #19
Q_Goest said:
A hot air bubble around the car would seem implausible, so it must be easy to show mathematically if that's the case, right?

Even with the biggest bubble you can imagine, that bubble (moving with the car) will still have to displace the «outside» cold air from the atmosphere. If the bubble is big enough, the frontal area it creates will probably make things worst.

But let's throw some numbers around for fun to see if such a bubble is plausible:

Imagine an engine that consumes 1000 cfm. That is a very large engine (http://www.summitracing.com/expertadviceandnews/calcsandtools/cfm-calculator [Broken]), running at WOT. What comes in must come out ... only hotter. Since the exhaust temp will be about 3-4 times larger than atm temp (absolute), the density will be about 3-4 smaller; so say 4000 cfm out.

If we take a very small car (Honda Insight), you could have a 5 ft² frontal area. So how much exhaust gas will there be in front of the car (just in front)? 4000 ft³/min / 5 ft² = 800 ft/min. So, after 1 min, the engine will produce an 800 ft «thickness» of exhaust gas in front of the car.

The problem is that if the car goes at 9 mph (15 km/h), after 1 min, it will have traveled that 800 ft also, thus reaching the end of that "bubble".

And this was assuming we installed a 5L engine in a Honda Insight, and run it at 10000 rpm, WOT!

Unless I made some wrong assumptions or calculations, the hot air bubble around the car would seem mathematically implausible.
 
Last edited by a moderator:
  • #20
I like it Jack! Thanks... I think making those types of estimations come in very handy as an engineer. We often don't need to do a super accurate analysis of something if a conservative guess will get us an answer that proves something is good or not. I'll use your idea and add a few things...

Consider the gas coming out of the exhaust and mixing 100 to 1 with the air. Instead of some very high temperature, the temperature will be just slightly above atmospheric. For the sake of argument, let's have the gas coming out at 1500 R and assume atmosphere is 500 R for a dT of 1000 F. Mix 100 to 1 and the temperature is down to 10 F above atmospheric, so 510 R. Round numbers. But we now have 100 times the amount of air the engine puts out. That's another issue we could play with, but let's just assume what comes out of the engine is a gasseous mixture of air (mostly unburned nitrogen?) with plenty of water and carbon dioxide. To make it easy, let's just assume it's air and see what happens...

Now assume a more modest engine, around 2.8 L (0.1 cubic foot) and 2000 RPM. Assuming a 4 stroke (only 1 exhaust per 2 revolutions) that's 100 cubic feet times 100 because of the mixture, so around 10,000 cubic feet of 510 F air. For a frontal area of 5 square feet, that means we can sweep a volume 10,000 ft3/5 ft2 = 2,000 ft long in one minute. And all the air in that bubble will be 10 degrees warmer than atmosphere. That's a rather slow 23 mph, but we could cut the temperature in half and go 46 mph.

Given the drag is roughly proportional to density (proportional to absolute temperature), then for a 10 F rise, that's a 2% reduction in drag. For a 5 F rise, that's a 1% reduction in drag. Nothing to brag about but we're getting closer to a realistic answer. Did I do the math right? What else? What about the method of exhausting the gas in front of the vehicle (instead of behind it)? And does the 5 square feet frontal area make sense? Doesn't that mean we have 5 square feet of hot air ALL AROUND the car? It's an interesting question.
 
  • #21
Here are my thoughts.
One needs to look at the 'bleed off' of secondary fluid from the frontal volume of a thickness t around the perimeter P, ensuring that the volume of frontal secondary air stays constant. The value of t should be similar to the boundary layer thickness, or a few multiples of, to calculate the skin friction from the secondary fluid. From there, the amount of secondary fluid injection into the frontal volume should equal the 'bleed off' volume, using rate values of course.

If the viscosity of the secondary fluid is lower than the free stream fluid, the skin friction drag should be deceased.

The frontal pressure drag shouldn't change as the same amount of free-stream fluid displacement should be the same, or roughly similar.
 
  • Like
Likes boneh3ad
  • #22
:smile: Setting aside the theoretical exercise for a moment...unless there is some amazing heat exchanger in play here, the vehicle is being wrapped in exhaust gases. At what point would someone like to consider how the engine continues to run without fresh air being available? Or the driver, for that matter! Just a thought. 8) Back to the theory!
 
  • #23
Highspeed said:
:smile: Setting aside the theoretical exercise for a moment...unless there is some amazing heat exchanger in play here, the vehicle is being wrapped in exhaust gases. At what point would someone like to consider how the engine continues to run without fresh air being available? Or the driver, for that matter! Just a thought. 8) Back to the theory!
If you think about it, we're already making 'hot air bubbles' around cars every day because that's what happens to someone following a line of other cars. We don't have any problems with exhaust gases from cars in front of us so there shouldn't be any fresh air problems with venting our own exhausts in front of us as we're driving.

But then again, if we already have all those cars in front of us putting their heat energy into the air before we drive through it, then the air we're driving through is as warm as it'll get and adding our own exhaust won't do much to increase the heat. I think what this comes down to is there simply isn't enough exhaust heat to make a difference. But it would be interesting to simply test that somehow.

Here's one more thought - asphalt generally absorbs more heat than grass, even without cars driving down them. So they should have slightly warmer air above them than in the surrounding area.

Instead of asking about exhausting gas from our car in front of our vehicle, we might ask how does this increased air temperature around a car change drag?
 
Last edited:
  • #24
Q_Goest, I would like to go over your idea in post #20.

One assumption you made that is wrong, is the estimation of volumetric flow rate; You estimated it at WOT, which will never happen, as a lot less power is needed to maintain a cruising speed. Here's the appropriate math:

The BSFC is defined by:
[tex]BSFC = \frac{\dot{m}_f}{P} = \frac{\frac{\dot{m}_a}{AFR}}{P} = \frac{\frac{\rho \dot{V}}{AFR}}{P}[/tex]
or:
[tex]\dot{V} = \frac{AFR (BSFC)}{\rho} P[/tex]
Where [itex]\dot{V}[/itex] is the volumetric air flow rate that enters the engine, [itex]AFR[/itex] is the air-fuel ratio, [itex]\rho[/itex] is the air density, [itex]P[/itex] the engine output power. [itex]\dot{m}_f[/itex] and [itex]\dot{m}_a[/itex] are the mass flow rate for the fuel and air, respectively.

We already said that the exhaust volumetric flow rate [itex]\dot{V}_{ex}[/itex] will be related to the inlet volumetric flow rate by their temperature ratio:
[tex]\dot{V}_{ex} = \frac{T_{ex}}{T}\dot{V} = \frac{T_{ex}}{T} \frac{AFR (BSFC)}{\rho} P[/tex]
The power needed to maintain a constant speed is mostly due to aerodynamic drag and you have to assume some drivetrain losses ([itex]\eta[/itex]):
[tex]P = \frac{0.5\rho C_d A v^3}{\eta}[/tex]
Or:
[tex]\dot{V}_{ex} = \frac{T_{ex}}{T} AFR (BSFC) \frac{0.5 C_d A v^3}{\eta}[/tex]
The velocity over the frontal area - as we calculated in our previous post - is:
[tex]\frac{\dot{V}_{ex}}{A} = 0.5 \frac{T_{ex}}{T} \frac{AFR (BSFC) C_d}{\eta} v^3[/tex]
And comparing that velocity to the car speed:
[tex]\frac{^{\dot{V}_{ex}} / _A}{v} = 0.5 \frac{T_{ex}}{T} \frac{AFR (BSFC) C_d}{\eta} v^2[/tex]
If that ratio is less or equal to 1, then the car is always «catching up» the front of the bubble. The minimum speed to break that pattern, such that the car is inside the bubble, is:
[tex]v \geq \sqrt{\frac{\eta}{0.5 \frac{T_{ex}}{T} AFR (BSFC) C_d}}[/tex]
Typical value for the temperature ratio is 3; a good air-fuel ratio at cruising speed will be at most 15; Typical drag coefficient is 0.325; Typical BSFC is about 250 g/kW/h (7 x 10-8 kg/W/s) and you can put a drivetrain efficiency of 0.85.

This gives a minimum speed of 1289 m/s (over 4600 km/h or 2800 mph). No comments necessary.

But you said «let's mix it with some fresh air». That will change the following:
[tex]\dot{V}_{ex} = (X+1)\frac{T_{ex}}{T}\dot{V}[/tex]
Where [itex]X[/itex] is the ratio of fresh air you will mix with the exhaust (mass-wise). Furthermore, to get that extra flow rate, you will need to take some energy somewhere, which will come from the engine power somehow. The minimum power required will be the pressure differential across the pump ([itex]\Delta p[/itex]) times the flow rate, which is [itex]X[/itex] times larger than [itex]\dot{V}[/itex]. So:
[tex]\dot{V} = \frac{AFR (BSFC)}{\rho} P \left( 1 + \frac{\Delta p \left(X \dot{V}\right)}{P}\right)[/tex]
Or:
[tex]\dot{V} = \frac{\frac{AFR (BSFC)}{\rho} P}{1 - X \frac{\Delta p}{\rho} BSFC}[/tex]
Where we could even replace [itex]\frac{\Delta p}{\rho}[/itex] by [itex]R\Delta T[/itex], which is the specific gas constant for air and the temperature difference across the pump.

All of this modifies our speed equation to:
[tex]v \geq \sqrt{\frac{\eta}{0.5 \frac{T_{ex}}{T} AFR (BSFC) C_d} \left(\frac{1 - X (R\Delta T) BSFC}{X+1}\right)}[/tex]
Even if we assume the addition of air flow is free ([itex]\Delta T = 0[/itex]), and that [itex]X = 100[/itex] like in your post #20, then our previously calculated minimum speed drops by a factor of 10, i.e. approx. 460 km/h; Still way too high.

Even if we consider the pumping losses, we can drop it to zero by setting [itex]X (R\Delta T) BSFC = 1[/itex]. But in that case, [itex]\dot{V}[/itex] becomes infinite; Cleary, the fuel consumption will also be infinite, defying our first goal of decreasing drag in the first place.

Conclusion: As others previously mentioned, you will only end up with a thin layer of hot air across the car - no matter what - because the car goes too fast for the quantity of exhaust gas that comes out.

Q_Goest said:
Instead of asking about exhausting gas from our car in front of our vehicle, we might ask how does this increased air temperature around a car change drag?

If there are so many other cars around, turbulences will probably be a bigger factor in drag (or if you tailgate like in Nascar racing :biggrin:).
 
  • #25
Hi Jack. I appreciate you trying to put some numbers to it. Thanks for that.

First, let’s clear up a few things. I have little doubt that the heat produced by the engine is insufficient to make a significant difference. I do however have no doubt that hotter air will reduce drag on the car and that IF the engine’s heat could somehow create that hot air bubble, then depending on how large that ‘bubble’ is, the drag should be reduced. Also, viscosity has little impact on the analysis as the slight rise in viscosity changes the drag only by some tiny fraction compared to the density of the air. A couple of quick calculations per the analysis I gave earlier is all that’s needed to prove that.

This question isn’t a bird flying inside a box and the box gets ligher kind of thing as near as I can tell. Also, I think this is an interesting question that needn’t be dismissed out of hand so I thought I’d chime in.

I’m not sure you were following what I wrote in post 20 because quite honestly, I can’t follow what you wrote. I’m not understanding why you want to add energy to mix the hot air with ambient. You wrote: “Where Xis the ratio of fresh air you will mix with the exhaust (mass-wise). Furthermore, to get that extra flow rate, you will need to take some energy somewhere, which will come from the engine power somehow.” When I say the hot air (coming from out of the exhaust and also out of the radiator for example) mixes with the air, I meant that it simply disperses to heat the surrounding air just as it would for any car driving down a road. I'm imagining an exhaust pipe sticking 10 or 20 feet in front of the car with a header on it to disperse the hot exhaust as the car drives through it for example. The radiator could also be stuck way up ahead of the car like that. It really doesn't matter. The point would be only to try and imagine dispersing the amount of heat into the atmosphere ahead of the car somehow. How that's done isn't important. Then, assuming a constant specific heat, the temperature of that air will vary linearly depending on how much cool air mixes with hot air.

We might change the original question just slightly to help clarify what I’m thinking. Imagine for example, we have a single car driving down a straight road with another car behind it and ask if the car behind it is going through air that is any warmer than the one ahead of it.

Anyway, we can check how much heating we might get in a variety of ways. I think you’re trying to come at it from BSFC, but at least to me, that seems a bit more difficult than to simply consider how much energy is being rejected to atmosphere for a typical car. If a car needs 10 hp to get it down the road, it’s rejecting about 30 hp as heat so we can take that energy, assume some volume of air and calculate the increase in temperature per the first law of thermo. That much power in 1 minute, will warm about 7000 cubic feat of air to a temperature 10 degrees hotter than atmosphere which is roughly what I got before. So the estimate in post 20 isn’t off by that much.

I'm surprised how much the air is warmed up by a car. I wonder if I'm making a mistake here, but I don't see it.
 
Last edited:
  • #26
Q_Goest said:
I’m not sure you were following what I wrote in post 20 because quite honestly, I can’t follow what you wrote. I’m not understanding why you want to add energy to mix the hot air with ambient.

You're right, I misunderstood. I thought you wanted to mix fresh air through the exhaust pipe to increase the flow rate. No biggy, I had fun doing the math.

As for the car warming the air for the car behind, I don't know how the «7000 ft³» of air could be delimited. The real sink is the entire atmosphere. So it depends a lot on the convection heat transfer (hot air going up, replaced by colder air getting down, for example). How fast is that process? Also, don't forget that part of the heat rejected by the engine will go through the cooling system, not only through the exhaust gases. Even the exhaust gases cool down a lot along the lengthy exhaust pipe. That complicates the heat transfer calculations (No, I'm not going to even try it!).
 
  • #27
Q_Goest said:
I do however have no doubt that hotter air will reduce drag on the car and that IF the engine’s heat could somehow create that hot air bubble, then depending on how large that ‘bubble’ is, the drag should be reduced. Also, viscosity has little impact on the analysis as the slight rise in viscosity changes the drag only by some tiny fraction compared to the density of the air. A couple of quick calculations per the analysis I gave earlier is all that’s needed to prove that.

In general, this is true that the density effect would be greater since drag will decrease linearly with temperature due to the density decrease but will increase with the square root of temperature due to the increase in viscosity. That is ignoring the potential effects on laminar versus turbulent boundary layers and separation, which may invalidate that generality. In reality, injecting any meaningful amount of hot air (or air of any temperature) into the boundary layer will dramatically alter the stability characteristics of the boundary layer and likely lead very rapidly to transition to turbulence if the flow was not already turbulent at that point. In that case, the viscous drag will immediately increase tenfold, not with the square root of the temperature, due to the effects of a turbulent boundary layer. That has been my point all along. If instead the boundary layer was already turbulent at the injection point (so somewhere farther down the hood), then the effect would be much, much less.

Q_Goest said:
We might change the original question just slightly to help clarify what I’m thinking. Imagine for example, we have a single car driving down a straight road with another car behind it and ask if the car behind it is going through air that is any warmer than the one ahead of it.

I am sure that air is slightly warmer behind a car. After all, if you have ever stood behind a running car, it gets mighty warm. However, consider that the mass flow of warm air coming out of the muffler compared to what is passing by the car is tiny, and there is a large turbulent wake behind most vehicles that is going to rapidly mix that air with all the other surrounding air, minimizing the effect. Consider that my car with a 2.3 L engine running at 3000 rpm at 70 mph down the highway is going to pass about 0.15 kg/s of air through it (2.3 L * 1/1000 m3/L * 1.29 kg/m3 * 3000 rpm *1/60 min/sec). At that same 70 mph, the mass flux of incoming air is right around 40 kg/m2/s. Assuming my car has a frontal area of about 2 m2, that means there is about 80 kg/s of air going around the car; about 533 times greater than what is coming out of the exhaust. That isn't even taking into account air going around the car that isn't in that frontal area but still mixes in the wake.
 
  • Like
Likes jack action
  • #28
Q_Goest said:
First, let’s clear up a few things. I have little doubt that the heat produced by the engine is insufficient to make a significant difference. I do however have no doubt that hotter air will reduce drag on the car and that IF the engine’s heat could somehow create that hot air bubble, then depending on how large that ‘bubble’ is, the drag should be reduced.
How do you create that bubble without pushing the air that's there out of the way?
 
  • #29
russ_watters said:
How do you create that bubble without pushing the air that's there out of the way?
If a car drives down the road, does the air behind it get any warmer? If so, is it impossible to reject the heat that made it warmer so that it's in front of a car instead of behind? Maybe 10 feet in front? Maybe 20 feet? The term "bubble" is a bubble only in the sense there is a thermal gradient that is warmest where the heat was released and coldest farther away. Bonehead's post is similar. See what you think.
boneh3ad said:
I am sure that air is slightly warmer behind a car. After all, if you have ever stood behind a running car, it gets mighty warm. However, consider that the mass flow of warm air coming out of the muffler compared to what is passing by the car is tiny, and there is a large turbulent wake behind most vehicles that is going to rapidly mix that air with all the other surrounding air, minimizing the effect. Consider that my car with a 2.3 L engine running at 3000 rpm at 70 mph down the highway is going to pass about 0.15 kg/s of air through it (2.3 L * 1/1000 m3/L * 1.29 kg/m3 * 3000 rpm *1/60 min/sec). At that same 70 mph, the mass flux of incoming air is right around 40 kg/m2/s. Assuming my car has a frontal area of about 2 m2, that means there is about 80 kg/s of air going around the car; about 533 times greater than what is coming out of the exhaust. That isn't even taking into account air going around the car that isn't in that frontal area but still mixes in the wake.
Agreed. The affect of this hot air bubble is going to be tiny. But how about highways where a hundred cars might pass a point at any minute.* The affect is multiplied many times so there should be some measurable rise in temperature above a highway. Since drag is linearly proportional to absolute temperature, a 10 degree F rise will reduce drag by roughly 2%.

Russ, Bonehead, I'm not suggesting we modify cars to somehow reject all their heat (radiator, exhaust) well out in front of the car, only that IF we somehow devised such a car, THEN we might expect the hotter air through which it is driving to help reduce drag, even if it's only a fraction of a percent. As I mentioned to Jack, this question isn’t a bird flying inside a box and the box gets ligher kind of thing as near as I can tell. It's an interesting question that needn’t be dismissed out of hand.


*Instead of thinking of it as a bubble, we might think of it as a tunnel above a highway of slightly warmer air. Warm air that is constantly being blown off the road by the wind.
 
Last edited:
  • #30
Certainly it could be done, but you didn't answer my question. It seems to me that the suggestion is akin to mounting a jet engine on the wrong side of the car: heating and expanding air is what they do!
 
  • #31
I am completely clear on what you are proposing and have been for some time. My last post essentially proves that a car does not produce anywhere near enough hot gas to create a bubble of warm air around it and so any gas expelled forward will just remain in the boundary layer anyway. That would not modify the effective shape of the car and so CD would remain the same and the density used in calculating drag would remain the same free stream density as it is without any forward exhaust.

Now, supposing that the engine actually did make enough hot exhaust to generate a bubble of any meaningful size that would actually change CD (by changing the effective shape of the car seen by the free stream), that could possibly be advantageous purely from an aerodynamics standpoint. The bubble shape would have to be just right, though, as a car typically has a lower drag coefficient than does as sphere. However, even if this was done, you still have to push all of that incoming air out of the way to make space for your bubble, so the engine is now working extra hard to eject a stream of exhaust forward with enough force to create a bubble out in front of the car. Even doing this a foot in front of a car would require quite a bit of forward momentum in the exhaust. Now your forward exhaust, which requires more engine power to generate, is also acting as a forward-acting thrust. You've now double penalized yourself.

I also have no doubt that on a busy highway, the temperature will rise above ambient by a few degrees due to passing cars. Of course, this already happens, so it's not that much of an improvement.

Finally, if your car is driving into a warmer pocket of air, the lower density means your car engine is going to run less efficiently since it's taking in less air per volume for combustion.
 
  • #32
The question raised by Q_Goest could be answered easier in another form:

Do cars driving in cold climates have more drag than cars in hotter climates?

I don't have a verifiable answer to that question, but I never heard of anything about significant differences from region-to-region or season-to-season comparisons.
 
  • #33
jack action said:
The question raised by Q_Goest could be answered easier in another form:

Do cars driving in cold climates have more drag than cars in hotter climates?

I don't have a verifiable answer to that question, but I never heard of anything about significant differences from region-to-region or season-to-season comparisons.

They do, but their engines also run more efficiently. Cars have less drag in Denver than the do in Houston due to altitude, too. These effects aren't gigantic, but they're finite.
 
  • #34
Not sure why there's so much confusion here.
boneh3ad said:
However, even if this was done, you still have to push all of that incoming air out of the way to make space for your bubble, so the engine is now working extra hard to eject a stream of exhaust forward with enough force to create a bubble out in front of the car. Even doing this a foot in front of a car would require quite a bit of forward momentum in the exhaust. Now your forward exhaust, which requires more engine power to generate, is also acting as a forward-acting thrust. You've now double penalized yourself.
russ_watters said:
Certainly it could be done, but you didn't answer my question. It seems to me that the suggestion is akin to mounting a jet engine on the wrong side of the car: heating and expanding air is what they do!
Does this help?
Q_Goest said:
When I say the hot air (coming from out of the exhaust and also out of the radiator for example) mixes with the air, I meant that it simply disperses to heat the surrounding air just as it would for any car driving down a road. I'm imagining an exhaust pipe sticking 10 or 20 feet in front of the car with a header on it to disperse the hot exhaust as the car drives through it for example. The radiator could also be stuck way up ahead of the car like that. It really doesn't matter. The point would be only to try and imagine dispersing the amount of heat into the atmosphere ahead of the car somehow. How that's done isn't important. Then, assuming a constant specific heat, the temperature of that air will vary linearly depending on how much cool air mixes with hot air.
We might change the original question just slightly to help clarify what I’m thinking. Imagine for example, we have a single car driving down a straight road with another car behind it and ask if the car behind it is going through air that is any warmer than the one ahead of it.
 
  • #35
Q_Goest said:
Not sure why there's so much confusion here.Does this help?

And we have repeatedly said that the amount of heating as a result is so minuscule as to barely register and doesn't address the fact that outside of the heated region, you are sill pushing full-density air out of the way. Further, you would need to disperse that heated air so far ahead of the car that you would either need a large amount of forward thrust to get it far enough ahead toaster or you would need a 15 for pole sticking out of the front of your car to dispense the gas. Otherwise the effect will be confined to the boundary layer, which has already been addressed several times.
 
<h2>1. Can exhaust heat really be used to reduce automotive drag?</h2><p>Yes, it is possible to use exhaust heat to reduce automotive drag. This concept is known as exhaust gas recirculation (EGR) and it involves redirecting some of the exhaust gases back into the engine to reduce the amount of oxygen in the combustion chamber. This leads to a slower and cooler combustion process, resulting in less energy being wasted as heat and a more efficient use of fuel.</p><h2>2. How does using exhaust heat reduce automotive drag?</h2><p>Exhaust heat can reduce automotive drag by reducing the amount of energy wasted as heat in the combustion process. This leads to a more efficient use of fuel, which can result in better fuel economy and reduced emissions. Additionally, by redirecting some of the exhaust gases back into the engine, the EGR system can also help to reduce the amount of oxygen in the combustion chamber, which can help to lower the overall temperature and reduce the amount of work the engine has to do to maintain speed.</p><h2>3. Are there any downsides to using exhaust heat to reduce automotive drag?</h2><p>While using exhaust heat to reduce automotive drag can have many benefits, there are also some potential downsides. For example, the EGR system can increase the amount of soot and other particles in the exhaust, which can lead to increased maintenance and potential engine damage. Additionally, the EGR system can also reduce the overall power and performance of the engine, which may be undesirable for some drivers.</p><h2>4. Is using exhaust heat a common practice in the automotive industry?</h2><p>Yes, using exhaust heat to reduce automotive drag is a common practice in the automotive industry. Many modern vehicles are equipped with EGR systems, and some even have advanced systems that can control the amount of exhaust gas recirculation based on driving conditions. Additionally, some manufacturers are also exploring other ways to use exhaust heat, such as using it to power a turbine and generate electricity for the vehicle.</p><h2>5. Are there any other ways to reduce automotive drag using exhaust heat?</h2><p>In addition to EGR systems, there are other ways to reduce automotive drag using exhaust heat. One example is the use of turbochargers, which use exhaust gases to spin a turbine and compress the air entering the engine, resulting in increased power and efficiency. Another method is the use of exhaust gas heat recovery systems, which capture waste heat from the exhaust and use it to preheat the engine or cabin, reducing the load on the engine and improving fuel efficiency.</p>

1. Can exhaust heat really be used to reduce automotive drag?

Yes, it is possible to use exhaust heat to reduce automotive drag. This concept is known as exhaust gas recirculation (EGR) and it involves redirecting some of the exhaust gases back into the engine to reduce the amount of oxygen in the combustion chamber. This leads to a slower and cooler combustion process, resulting in less energy being wasted as heat and a more efficient use of fuel.

2. How does using exhaust heat reduce automotive drag?

Exhaust heat can reduce automotive drag by reducing the amount of energy wasted as heat in the combustion process. This leads to a more efficient use of fuel, which can result in better fuel economy and reduced emissions. Additionally, by redirecting some of the exhaust gases back into the engine, the EGR system can also help to reduce the amount of oxygen in the combustion chamber, which can help to lower the overall temperature and reduce the amount of work the engine has to do to maintain speed.

3. Are there any downsides to using exhaust heat to reduce automotive drag?

While using exhaust heat to reduce automotive drag can have many benefits, there are also some potential downsides. For example, the EGR system can increase the amount of soot and other particles in the exhaust, which can lead to increased maintenance and potential engine damage. Additionally, the EGR system can also reduce the overall power and performance of the engine, which may be undesirable for some drivers.

4. Is using exhaust heat a common practice in the automotive industry?

Yes, using exhaust heat to reduce automotive drag is a common practice in the automotive industry. Many modern vehicles are equipped with EGR systems, and some even have advanced systems that can control the amount of exhaust gas recirculation based on driving conditions. Additionally, some manufacturers are also exploring other ways to use exhaust heat, such as using it to power a turbine and generate electricity for the vehicle.

5. Are there any other ways to reduce automotive drag using exhaust heat?

In addition to EGR systems, there are other ways to reduce automotive drag using exhaust heat. One example is the use of turbochargers, which use exhaust gases to spin a turbine and compress the air entering the engine, resulting in increased power and efficiency. Another method is the use of exhaust gas heat recovery systems, which capture waste heat from the exhaust and use it to preheat the engine or cabin, reducing the load on the engine and improving fuel efficiency.

Similar threads

Replies
1
Views
1K
Replies
3
Views
560
Replies
12
Views
4K
  • Mechanical Engineering
Replies
2
Views
2K
  • General Engineering
Replies
8
Views
22K
Replies
14
Views
1K
  • Mechanics
Replies
3
Views
1K
Replies
131
Views
4K
Replies
5
Views
3K
Replies
19
Views
3K
Back
Top