Insulation of Car Exhaust 2

[Minimalist]

Insulation of Car Exhaust


I am out of my element here but I was hoping for some help. I just bought an underpowered import. I love the car, love the gas mileage, and the true cost of the car was one of the cheapest there was. I am wondering if there are any cheap methods of modifying the car to increase gas mileage and power.

Reviewing the internet, I have come to the conclusion that the only way to modify a car cheaply to increase power and gas mileage without radical revisions to the car itself is to change the muffler and or insulate the exhaust pipe. I will start with the exhaust insulation question:

The common explanation for the reason for hot exhaust gas being better than colder exhaust gas is that hot gas is easier to push out than cold gas. That explanation did not make sense to me so I did a quick pipe pressure drop calculation assuming 130 lbs/hr of flow from the car exhaust and a 2.5 “ tail pipe, I found that hot gas had a higher pressure drop exiting the engine than cold gas. Viscosity of the exhausts increases with increasing temperature which would also make the gas harder to push out.

I believe that pressure of the gas exiting the muffler would be lower due to Bernoulli and therefore there would be less residual exhaust in the piston chamber making the car more efficient, but I am not sure I am really understanding it right.

So I guess my first question is two parts:

1.Does insulating the exhaust help the gas mileage and fuel economy?
2.Would this practice invalidate the warranty?

 

[patprimmer]

I guess you must think the factory engineers are so stupid that they overlook cheap ways to increase both fuel economy and power. I expect they are generally quite smart and competent.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

If you increase power you will generally decrease fuel efficiency.

Pat -- good one by the way, yep us engineering professionals always miss the easy things, its the hard stuff we're really good at.

Nick
I love materials science!

 

[TheBlacksmith]

Probably you should insulate the radiator too. An awful lot of the heat energy liberated by the combustion of $3 a gallon gas is thoughtlessly lost there by careless automotive engineers.

 

[Fabrico]

In regard to your questions:
1) No, otherwise there would be at least one other person doing it.
2) Yes, insulating exhaust on any car can cause metal to fatigue and/or rust out.

 

[thundair]

Ease up.... It is a good thing to look at things differently.

Cost is the reason some of the company's will or will not do something.(Not that they didn't think of it first)
Also they have to hit a middle ground to please the masses. I am positive if your focus was on fuel economy that you could make improvements. (not that wrapping the exhaust is one of them)
If they did every thing perfect what would all the hotrodder's do to spend money.

Cheers



I don't know anything but the people that do.

 

[SomptingGuy]

Err, hotrodders don't need to pass emissions tests or go for 50k beween services.

Most motors can put out more, but if you squeeze the tube too much, toothpaste squirts out from anothr hole.

 

[thundair]

So true

I don't know anything but the people that do.

 

[GregLocock]

Just to confuse things, we do insulate the exhaust. Not much, and only for a short distance.

However, it will theoretically increase the back pressure, which will hurt power in particular. It won't have much effect on fuel economy, because the fuel economy tests are at power levels where the exhaust flow is minimal, and back pressure for a given exhaust will be proportional to power^2, roughly.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Perhaps we are confusing where to insulate. Header wrapping is well proven to work, but that's not really what most would refer to as the exhaust pipe. Google "header wrap" and you fill find a plethora of information.

Just one: "Why Header Wraps Suck...and Why Coatings are Recommended"

http://www.centuryperformance.com/heatwraps.asp

 

[rmw]

If you are trying to be a purist, insulate the exhaust only to your turbo. Save the miniscule amount of heat that is lost there so that it can become work in the turbo. Past that the heat is being rejected to the atmosphere anyway and keeping the temperature of the exhaust gas up just keeps its specific volume high and adds to pressure loss (back pressure.)

rmw

 

[SomptingGuy]

And of course pre-cat insulation (through double skinning or other) speeds up catalyst light-off, reducing the drive-cycle tail-out emissions.

 

[Minimalist]

Thanks Fabrico, I was indeed confusing terminology. The header wrap people are pretty convincing “blah blah, second generation, copper impregnated wrap, blah, blah….” The part that gave me some hope is that I have not found a negative review of the specific product since 1997. The review in 1997 that was negative was only if you spilled oil on the wrap. Usually if there is a magic gold dust idea it doesn’t last a decade with the same product and it has a lot more people upset. I have seen several people on the net who have posted on the net the use of the product, but usually it is only in conjunction with many other modifications to the car to the point where the independent dyno results were meaningless with regard to the wrapping. If the consensus of automotive folks in this forum believe that the product is not worth the risk I can respect that. I would still like to understand the principles of how it works though. So on that note,

GregLocock, Maybe I am missing something here but theoretically since the temperature of the gas is higher and the mass flow rate is the same, the density must be less therefore the velocity of the gas has to be higher. If the velocity of the gas is higher using Bernoulli’s equation the pressure of the system has to be lower. I am having to make a lot of assumptions here so I may be off a bit. But I would be interested in the basis of theory that you are referring to.

The part I can’t figure out because I don’t have the right background is whether increased scavenging effect that you get when you have low backpressure makes up for the fact that you will have a higher viscosity of the gas because of the higher temperatures which causes a higher pressure drop across the pipe. The increased pressure drop did seem to be pretty minor maybe only 0.02 psi. So in the comparison to the additional scavenging effect, the additional energy to push the gas across the pipe is probably negligible. At least that's my book answer. So is that the real answer, it is all about the scavenging effect? It just didn’t seem to be that with the advertisements.

All,
So let us see $ 600 for internal coatings on the headers. I lose the use of my car for a couple days to a week. Maybe I gain 2 mpg going from 38 mpg to 40 mpg. So assuming 1400 miles per month, difference between $ 105 and $ 110 per month or 10 years to pay for it. Yuck. I would think that I would have to at least get a gain of 5 mpg to even begin to make it worth it. Is that even realistic?

 

[patprimmer]

I would think if you got a gain at all, it might be more like 0.1 MPG.

Increasing exhaust gas scavenging during TDC overlap increases power, but decreases fuel efficiency by drawing more unburnt fuel into the exhaust system.

Decreasing scavenging increases fuel efficiency, but decreases power.

Wrapping exhausts in the interests of hot rodding is an attempt to increase power. There are two theories, one I believe and one I doubt.

Theory one. The wrapping insulates the exhaust and keeps under bonnet temperatures lower, thereby preventing vapour lock and other heat related damage and thereby keeping inlet air charge cooler, thus increasing density and thereby making more power.

Theory two. The wrapping insulates the exhaust, thus keeping exhaust gas temperatures higher, and thereby increasing volume and therefore velocity. This increased velocity, somehow makes more power, but increasing velocity by using a smaller exhaust pipe somehow decreases power.

Either way, you considerably increase corrosion in the exhaust.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[cammax]

I would think cooling,the exhuast flow at the exh ports would improve scavenging, which would increase HP but decrease MPG, not sure what the torque would do.

but it would be like a condenser on a gas turbine...

thoughts? ( just me thinking outloud)

 

[TheBlacksmith]

Finally, making sure the car is properly tuned, filters clean, tires properly inflated, wheels aligned and accelerating gently and coasting up to red lights, etc. will increase the mpg more than all the "re-engineering" in the world. I do use a K&N filter and synthetic oil. By doing the above, I routinely exceed the EPA highway rating (19)of my F150 on long trips and approach it during rush hour commuting and I will drive at 65-70 mph.

 

[thundair]

Minimalist

One reason your theory (using Bernoulli’s theorem) is not representing the real condition, is exhaust flow is not easy to calculate, As you have expansion, contraction and reversion, all going on in the same pipe, at the same time.

Like the velocities from the valve start at 285fps to 1100fps, some where down the line. Then at some collector or muffler point there is a reversion wave back to the valve.

I invite the real automotive engineers to help me out here as I may be a savant' that can't add or play the piano.

Cheers

I don't know anything but the people that do.

 

[btrueblood]

Pipe wraps mean higher exhaust temps, lower density, and thus more velocity in the pipe. More velocity means more frictional pressure loss in the pipes. I'm assuming here that the engine & intake are operating at a constant speed and temperature/pressure condition, thus mass flow into and out of the engine are constant.

The pipe loss equation looks like:

dp = (K/2)*(density)*V^2

where dp is the pressure loss due to friction, and V is the mean gas velocity. For constant mass flow, the decrease in density causes a proportional increase in velocity (mdot = rho*A*V); the net result is friction losses are inversely proportional to density (as temperature rises and density drops, pressure losses increase).

The only place the pressures are the same between the wrapped and unwrapped condition, is at the air inlet and at the outlet of the tailpipe (atmospheric pressure). Everywhere else, the wrapped engine must therefore be at higher pressure, per above (although I'm ignoring pat's idea of decreased heat rejection to the intake air). The cylinders therefore are less, not more, scavenged, and thus wrapping the tailpipe would actually decrease the intake charge due to higher back pressure during exhaust stroke, all else being held equal.

 

[ivymike]

if you have tuned-length headers, changing the gas temp will change the tuning (speed of sound). Not all changes are for the better.

 

[SMOKEY44211]

Many years back I was racing in a rules restricted class that only permitted stock cast iron exhaust manifolds. I fabricated a water jacket to surround the manifolds with engine coolant. Picked up torque noticebly from 3,000 rpm to 6,500. Gained 15 hp at peak. This was a sb chev 350 that was in the 400-450 hp range. The purpose of the modification was to increase power but I also kept track of BSFC. In this case the #'s were favorable to suggest an improvement in fuel economy. Based on that I think finding some method of cooling the exhaust would have benificial effect on fuel economy in a normally aspirated engine. Although what we are discussing here is a dynamic process if you take an instant snapshot of what's happening logic would suggest that (PV=NRT Boyles law) by lowering the temperature you also decrease in this case back pressure in the exhaust. I never tested the engine refferred to at part throttle operation but at WOT there is something to be gained fuel economy wize.----------Phil