Can exhaust heat be used to reduce automotive drag?

[cjl]

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).

Not that it really makes any difference to the final conclusion, but unless your 2.3L engine is a two-stroke, this number is a factor of 2 too high. Each cylinder takes in air on every other revolution, so a 2.3L engine only pulls in 1.15L per revolution.

 

[boneh3ad]

Not that it really makes any difference to the final conclusion, but unless your 2.3L engine is a two-stroke, this number is a factor of 2 too high. Each cylinder takes in air on every other revolution, so a 2.3L engine only pulls in 1.15L per revolution.

Good catch. That will teach me for posting while jet lagged.

 

[David Morgan]

This turned into a fun thread!

To clarify my original position a bit, I wasn't really imagining dispensing the actual exhaust gases to the front of the car. My original thought was somewhere along the lines of creating a front bumper out of the exhaust header.

Especially in turbo charged engines, the exhaust piping near the turbo and the turbo housing itself can glow red to orange hot under WOT conditions... the color change of the steel would indicate a surface temperature somewhere around 500C to 800C.

While certainly unrealistic to sustain such temperatures, as an academic exercise, what effect (if any) would an 8cm diameter by 1m steel cylinder at a sustained temperature of 500C to 800C have if it was the first part of the vehicle to contact the oncoming air stream?

 

[Q_Goest]

Hi David. As an acedemic excersize, it's an interesting question. As a practical application, it's highly impractical because you need to put the heat way out in front of your car as I'd suggested earlier. It won't work to create a thin, hot boundary layer next to your car. That doesn't change drag at all. The point of reducing drag by heating the air is to reduce the density of the air you need to disturb when moving through it.

 

[jack action]

While certainly unrealistic to sustain such temperatures, as an academic exercise, what effect (if any) would an 8cm diameter by 1m steel cylinder at a sustained temperature of 500C to 800C have if it was the first part of the vehicle to contact the oncoming air stream?

This is the realistic answer to that question:

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.

 

[boneh3ad]

This is the realistic answer to that question:

Given that he expanded his idea, I can expand on my answer. Generally speaking, heating up a subsonic boundary layer, as is the case of having a hot bumper, destabilizes it. The Tollmien-Schlicting waves that are usually the ultimate cause of transition to turbulence on a relatively flat surface like a car hood become more unstable when the wall is hot. This would tend to favor earlier transition with an increase in drag.

On the other hand, if you manage to heat the boundary layer up early like that without causing substantial growth (for example, by doing so in the region before T-S waves become unstable in the first place), then you may get the opposite effect. In that case you would create a hot boundary layer and before the instability takes hold, it enters a much cooler region. In that case, the wall would look like a cold wall to the boundary layer and would tend to stabilize it and quite possible delay transition and reduce drag.

How far it would delay transition (if at all, as I've never seen anyone publish any data on this so it's just a mental exercise at this point) would be the key question. If it's only delayed for a very short distance downstream, the effect on drag would be negligible.

 

[ScooterGuy]

Hi, all.

Ok, rather than adding the hot air to the front of the vehicle to reduce air density, what if we added the air to the wake to increase base pressure?

On page 228 of Wolf Heinrich Hucho's book Aerodynamics of Road Vehicles (4th Edition), he states that using an active means of wake abatement such as base bleeding has shown to increase base pressure (the vacuum behind a vehicle in the wake) by as much as 30%. He also states that using another means, actively blowing air into the wake using a blower (which was used by F1 racers and was subsequently banned, IIRC) dramatically lowered wake drag and more than offset the power requirement of the blower(s).

I'm building a new frame and aerodynamic body for my bike (a high-fuel-efficiency project), and propose to combine the two active means of wake abatement above by using the engine exhaust to power a Coanda nozzle (much like you'd find in the Dyson bladeless fan). The Coanda nozzle will pull air from the engine compartment to cool engine components, and there will be a venturi nozzle at the front of the bike to feed cool outside air into the engine compartment from the stagnation point at the front of the bike. Thus, I reduce the pressure drag at the front by draining off that stagnation point air, I provide cooling air to the engine compartment (the radiators will be cooled separately by two cross-over ducts which will also be used for cross-wind negation), and I increase the base pressure at the wake, thereby reducing drag. All with no extra power required.

What do you think? Would this be feasible?