Clean Emissions = Higher Operating Temp ??

[krue0101]

Hello,

I was wondering if there is a general trend in engine designs about heat rejection versus engine combustion efficieny. I have heard it both ways = cleaner burning engine runs hotter and cleaner burning engine runs cooler.

Can anyone shed some light on this question?

Thanks in advance for sharing your expertise.

Joe

 

[Lou Scannon]

It depends...
Some design parameters tend to simultaneously affect engine out emissions, heat rejection and exhaust temperature in a favorable direction (e.g. fast burn combustion system), while others tend to move one or two of these variables in a favorable direction while altering the remaining variable(s) in an unfavorable direction (e.g. compression ratio, spark timing, thermostat setting).

 

[icrman]

The thought for years has been towards less efficent combustion temperatures, so as to reduce NOx emissions.
Using such things as EGR for dillution, to displace some of the O2.

 

[patprimmer]

My understanding is that as a general rule, higher temperatures means more NOx while lower temps means more CO.

Coolant temperatures being higher can effect fuel economy in 2 ways though.

Less heat and energy loss to the atmosphere.

Better aerodynamics as the radiator can be smaller or require less airflow over it.

Regards
pat pprimmer@acay.com.au
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[1Bluegrass]

Ford's new 3 valve 5.4L DOHC engines use variable exhaust cam timing.
The future says that they will go to the control of both intake and exhaust cams with an effort to increase power over a wider band and use cam timing/phasing change to get rid of the present EGR method by taking advantage of cam phasing to reduce NX output to the cats.

 

[drwebb]

Depends on what the definition of 'clean' is. Now that Kyoto treats CO2 as pollution also, it gets even more complicated . . .

 

[Azmio]

Engine operating temperature can cover both engine coolant temperature and exhaust temperature. Engine coolant temperature needs to reach certain temperature as soon as possible especially during cold start. This will come handy in lowering CO and HC pollution.

It has also been reported that advancing the ignition timing will increase the NOx emission. This is against the belief that higher exhaust temperature will cause higher NOx emission. Any comment about this?

 

[NeilDS]

It has also been reported that advancing the ignition timing will increase the NOx emission. This is against the belief that higher exhaust temperature will cause higher NOx emission. Any comment about this?

I will take a SWAG at that. The increased exhaust temperature, when timing is retated, is due to the incomplete combustion being pushed out the exhaust. I would guess that due to this incomplete combustion that less NOx is formed. Though, in my seach on this subject I found this paper

http://www.engin.umd.umich.edu/research/undergraduate/reports/reurpt3-2003.pdf

which claims that retarted timing gives you lower cylinder temperature. This may be true and still give an elevated exhaust temerature as the reaction continues as it leaves the combustion chamber. Someone want to check my logic?

 

[patprimmer]

NeilDS

Sounds good to me.

Regards
pat pprimmer@acay.com.au
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.

 

[dwedel]

I think your basically correct NeilDS. I was taught that Nox forms at longer dwell time at higher temperature. Thus a more advanced ignition event will result in higher temperatures and pressures, and lower exhaust temps as more of the heat will stay in the cylinder. A more retarded ignition event will result in lower pressures and temperature, more incomplete burn, and higher exhaust temperatures as the gas is still burning as the exhaust port/valve is opened. Thus your CO will be higher.

dwedel
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[schmidtj86]

I wouldn't call it incomplete combustion, just less thermal efficiency (less work obtained from the same charge, thus still has energy in the form of heat). The later timing puts the peak pressure later because of amount of charge combusted at that point vs chamber volume at that point. It'll still burn fairly completely before exiting though, but since less work was obtained from that amount of charge (wasn't expanded from a higher peak pressure at a smaller chamber volume) it still has some heat energy left.