Why the difference in exhaust volume?

[CraigD]

Why is there such a difference in the volume and velocity of exhaust coming from engines of about the same size and rpm?

[mdsalemi]

Well Craig, I think it is engine technology, and the decades of advancement between your old Mercedes and new[ish] Honda.

I dare say, that someone I know from a long time ago attempted suicide with a Honda and its exhaust, this about 20 years ago.  There wasn't enough CO in Honda exhaust to kill him--thankfully.

Sometimes I think the driver and passenger in an old SL could get killed by CO at a stoplight.

Just my thoughts for the day.

[49er]

Cold morning here recently and I had forgotten the clouds of exhaust vapor that come out of these cars. Also another reason the original mufflers/resonators didn't last very long.

John

[CraigD]

Michael,

Yes, there are lots of differences in the engines, no argument there.  It's seems like just a question of volume... how could similar displacement engines operating at similar speeds displace such different volumes of exhaust?  The exhaust pipes aren't substantially different sizes, either, which might affect the velocity of the exhaust.  Just wondering why... 

[al_lieffring]

Answer: digital engine management and catalytic exaust reactors. You are correct in that both engines are drawing in the same volume of air to operate at the same speed, but the volume of visible exaust particulates are much lower in modern engines, due to more better engine design, tighter manufacturing tollerances, improved materials, more precise fuel metering and ignition controlls. Modern engine management systems can individually taylor the fuel delivery and timing adjustments to each individual cylinder for each combustion cycle greatly reducing the unburnt fuel that was present in early fuel systems. Any left over particulates after the combustion cycle are run through a ceramic matrix embedded with platimum in the exaust system that break these gasses and particulates down almost completly into CO2 and water vapor. The exaust system on the 280sl is practically a straight pipe out the back with no obstruction to reduce the velocity of the exaust gas. The catalyst creates quite a bit more flow restriction. The old cars exaust comes out in pulses at a higher velocity, catilyst cars the exaust would come out at an more even and lower pressure.

[CraigD]

Thank you, Al! You're helping me understand the specific reasons.  I'm hearing that a volume reduction occurs for 3 reasons: a) visible particulate reduction via the catalytic converter b) changing gasses into C02 and water vapor via the catalytic converter,  and  c) digital fuel/timing adjustments that reduce unburnt fuel.  Which of these do you think makes the largest difference in total exhaust volume?  You also mention the flow restriction in the catalytic converter, which contributes to a velocity reduction of the exiting gasses, but if it was simply a velocity reduction, then I'd expect to see larger or more tailpipes to handle the given volume.

Thanks for the info... I've wondered about this for awhile!

[SteveK]

I havn't done calculations for a number of years (20) but when I was a mechanical engineer in the process industry I use to do lots of fluid dynamic and heat trransfer calculations.   Al hit most of it; my bet is that overall its a function of compression ratio, temperature at the exhause manifold, the conversion made in the catalytic converter where overall volume will be reduced and the added pressure drop in the converter.  Lots of factors, all driven by old inefficient design compared to modern efficient design and control.  Hard to say which one is the biggest factor but I would guess it's the catalytic conversion that changes the volume and adds the back pressure to the overall process, but all of the factors contribute.

Next question is: why do you care

[hauser]

One thing to mention is when comparing the two photos, is that the SL has one rear muffler while the red car has two.  Mike mentioned that the red car is a Honda and if so Honda's' are ULEV, Ultra Low Emissions Vehicle.

[glenn]

Assuming the power portion of the fuel is not too much different in both cars....

The cat converter:
There really not too much difference in volume.  Assume the fuel is nonane, or close, C9H20.  The cat converter does the following - C9H20 + 14(O2) goes to 9(CO2) + 10(H2O) on the unburnt fuel.  That's 15 volumes to 19 volumes, if it is all vapor.  Some of the H2O maybe liquid - cutting down on the volume.  About a third of the fuel is unburnt in the cylinders???  Nonane gives a lambda of 14 or so.

So, the total is about 45 volumes to 49 volumes.

(A quick analysis!!)

[CraigD]

Glenn,

Your chemistry is better than mine!  I believe your analysis is probably correct.  But from a strictly visual observation, I'd guess the volume of gasses exiting from the two cars (both exhausts considered... the 280SL has 2 tailpipes too) is more like a 5 to 1 ratio.  (and maybe that's off... it just looks like that)  It's just interesting to see the difference, and to learn about the contributing factors.  I've learned that both modern digital engine management and catalytic converters are contributing to the difference. 

My bet is that the engine management system makes the biggest difference at idle.  I'm guessing it senses the low demand on the engine, reduces the incoming air (and fuel) to some minimum requirement, which therefore results in a much smaller amount of exhaust.  The 280Sl, with no such engine management system or converter, is purely volumetric... that whatever volume the pistons displace is the volume of air processed and exhausted.  I'd guess too, to that at full throttle, the difference between the two cars might be way less... but that's just a guess.

Appreciate everyone's input...

[wwheeler]

I wonder if the difference in valve timing and overlap has some to do with it. Also, a more efficient engine will convert a higher percentage of the fuel's energy directly into mechanical energy. That high velocity exhaust gas coming out is energy that is unused.

I also wonder if the temperature of the exhaust gas coming out of the tailpipe is lower for the new car vs. an older car given a similar operating temperature? Waste heat is also lost energy.