Strategies to keep cats from melting

[WildHorse]

Well that’s true to a point.

That's the safety factor. For various reasons.

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[engineermike]

The whole adding fuel to cool the cats seems a bit out of sorts to my mind. Sure the liquid fuel will cool from evap but won't the cat react the fuel causing the temps to rise?

I’ve thought this myself but I understand it much better now that I have actually temp data. At low flow rates the mid-bed temp is higher than the collector temp. This, of course, is due to the reaction of unburned combustion byproducts. At wot, the collector temp and mid-bed temp converge to unison because the high exhaust flow becomes the governing temp. When you let off, the collector temp drops immediately but the mid bed temp stays high, or even rises further because now the reaction is again governing the temp at the lower flow rate.

 

[Angrey]

I’ve thought this myself but I understand it much better now that I have actually temp data. At low flow rates the mid-bed temp is higher than the collector temp. This, of course, is due to the reaction of unburned combustion byproducts. At wot, the collector temp and mid-bed temp converge to unison because the high exhaust flow becomes the governing temp. When you let off, the collector temp drops immediately but the mid bed temp stays high, or even rises further because now the reaction is again governing the temp at the lower flow rate.

I was wondering when someone would rectify this. We generally know that cat's do their job and catalyze the oxidation (aka combustion) of hydrocarbons, which is an exothermic reaction, so dumping more unburnt fuel will have a physical reduction in exhaust gas temp, but ultimately produce additional heat in the combustion downstream.

 

[engineermike]

Thinking out loud here….the majority of exhaust gas is nitrogen, followed by co2, then water vapor. The un or partially reacted components are pretty small percentage-wise. The listed above opponents do not react in the converter because they’ve completely reacted already. These components are at the engine’s EGT when they reach the converter and they pass through without taking part in the goings on in there. Therefore, the majority of the exhaust gas is not reacting, but rather a coolant that absorbs heat from the reaction. So, cooling the egt by way of enrichment makes the “coolant” significantly cooler, but also adds more reactants that fuel the reaction. The exhaust only spends about 1/1000th of a second in the first cat bed so perhaps that isn’t enough time for the reaction to take place completely, so the heat moves on downstream.

 

[K4fxd]

So, cooling the egt by way of enrichment makes the “coolant” significantly cooler, but also adds more reactants that fuel the reaction. The exhaust only spends about 1/1000th of a second in the first cat bed so perhaps that isn’t enough time for the reaction to take place completely, so the heat moves on downstream.

I'm sure this is the principle, but I do wonder if it all reacts, IE: does it pollute more when in cat protection? Would this account for the bad smell after a WOT run?

 

[markmurfie]

Combustion takes oxygen.
All three reactions in the catalytic converter are exothermic.
The first part of the cat takes NOx and breaks them apart again into nitrogen and oxygen. The second part of the cat takes that oxygen and any oxygen from a lean combustion event or a misfire and burns any CO and hydrocarbons.
A catalyst( expensive metals) are used to assist and speed up these chemical reactions. It will ensure all of the oxygen that gets taken in by the engine, is used to combust as much hydrocarbons as completely as possible in a rich combustion event, or vise versa in a lean one. What ever energy doesn't get released in the combustion chamber is going to be released in the catalytic converter. Ideally leaving just CO2, water, nitrogen and the other trace gases in our atmosphere. No criteria pollutants or particulate matter. PE rich combustion will have excess HC emissions, that is why OEMs are forced to only doing it above 90% throttle and even delay going into PE at WOT in higher gears at low RPM where the driver might hold PE for a longer time period than if they downshifted to accelerate. Larger engines in trucks are even know to run stoich at WOT with no PE.
The richer mixture of PE already ensures no oxygen is left over in the combustion event, but all the oxygen doesn't only combust with the fuel and NOx(where the oxygen is "stored") still developes. Going even richer ensures less NOx, as those increase the hotter combustion events get. About 1.1 lambda is peak flame temperatures and peak NOx emissions. lean burn engines have an emissions downside. Either side of that would make less NOx. Going richer just gets you even further from that lambda so cooler flame and less NOx. More oxygen and fuel react in the combustion chamber and not oxygen and nitrogen(endothemic reaction) storing energy to be released in the catalytic converter later. Flame temperature is not to be confused with the exhaust gas temperature, which depends on how much heat from the reaction goes into work on the piston and how much doesn't. Less spark advance than MBT, increased ineffeciency, and exhaust gas temperature increases. Flame temperature depends on charge mixture density. Denser air charge, more cylinder pressure, hotter flame, more NOx that gets made.

 

[SpeedLu]

It really is too bad that the Variable Compression tech like Nissan has in their VC Turbo engine isn't more mainstream as it seems like it would help solve some of these issues. I just don't forsee an OEM putting in the necessary R&D to do so as ICE's aren't long for this world.....

ICE vehicles will be around for a long time yet.

 

[engineermike]

More interesting epiphanies as I churn through data...

When running a 4" pulley (9-10 psi), I was able to run spark timing in the 22-23 deg range. On pulls from about 50 to 135, the COT kicked in at 105-108 mph. Peak mid-bed cat temp measured with a thermocouple was 1530 deg F. COT appears to have flattened the curve.

When running a 3.625" pulley (~13 psi), the spark timing was 18-20 deg. The COT kicked in earlier in the pulls and the EGT continued to rise even with COT flogging it with fuel. The peak mid-bed cat temp measured at 1630 deg F, or about 100 deg hotter than the large pulley.

So, with the smaller pulley the COT came in sooner and still wasn't able to keep temps under control.

 

[IamCDNJosh]

ICE vehicles will be around for a long time yet.

Maybe, I sold my mustang and bought a tesla. It's quicker and a more enjoyable drive. If there's a breakthrough in battery chemistry which allows for a lighter and quicker charging battery, its game over for ICE.

 

[K4fxd]

If there's a breakthrough in battery chemistry which allows for a lighter and quicker charging battery, its game over for ICE.

No, too many people live in apts or houses with on street parking only. Plug in EV's are never going to be a majority.

 

[engineermike]

Don’t forget grid capacity. Or lack of it.

 

[IamCDNJosh]

No, too many people live in apts or houses with on street parking only. Plug in EV's are never going to be a majority.

Do you get gas at your house or apartment? Likely what I see happening is gas stations start offering L3 charging. Petro Canada stations have already started doing this.

 

[IamCDNJosh]

Don’t forget grid capacity. Or lack of it.

More likely solvable than trying to develop renewable liquid fuels on a worldwide scale. I will agree though that this is a significant issue.

 

[SpeedLu]

Maybe, I sold my mustang and bought a tesla. It's quicker and a more enjoyable drive. If there's a breakthrough in battery chemistry which allows for a lighter and quicker charging battery, its game over for ICE.

I disagree with EVs being a more enjoyable drive. I've been in a few, and even ridden in one around a track and they have no soul at all. Quiet, way too refined, all the joy is sucked out of the experience. What's the point? Boring. And mass adoption will never happen, the infrastructure doesn't exist nor can the electrical grid handle it, especially considering they don't want to use fossil fuel or nuclear for power generation.

 

[markmurfie]

Maybe, I sold my mustang and bought a tesla. It's quicker and a more enjoyable drive. If there's a breakthrough in battery chemistry which allows for a lighter and quicker charging battery, its game over for ICE.

I think we all have been using smart phones for nearly 15 years now. Batteries have not gotten better in that time. With my experience of having to replace an expensive phone every 1-2 years because the battery won't hold it's charge for a complete 8 hour day with mild use and the manufacture choose to not make a known failure point easily replaceable... I'm not all too excited about going through the same thing but with a 40k$ car.

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