I've read several posts that say NOx emissions are produced because of high combustion chamber temperatures. I've also read in several posts that GM ran these engines hot to reduce emissions. Can someone explain the apparent discrepancy?

 

Three letters:

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In conditions where NOx is most likely to be formed, a little exhaust gas is added to the mix to cool the combustion temp down a tad.

Oh, higher coolant temps were more for fuel economy than they were for emissions.

 

Youre right, NOx is created by high combustion temperatures, but is also caused by efficient combustion in general. You could say NOx production is a necessary evil for an efficient engine. I dont believe GM wanted a hot running engine, but rather an engine that got hotter faster. A car in open loop pollutes more than an engine in closed loop, the faster the engine gets into closed loop, the less pollutants it will emit. (especially hydrocarbons) Also, the engine must be in closed loop in order for the AIR to divert oxygen to the converter to help break down the NOx. So you could say that high temps create NOx, but they are also necessary to control it and other pollutants. (Like hydrocarbons which are also a big issue with fuel mileage standards)

 

1) How do higher coolant temperatures help fuel economy? Is it just to get the car warmed up and in closed loop faster?

2) I think I had read that a higher octane fuel burns slower and therefore cooler. Is this the case? And if so, then all things being equal, would running a higher octane gas reduce NOx emissions?

 

The closed loop thing is part of it, but higher coolant temperatures also help the fuel vaporize better. Liquid fuel wont burn in an engine and in worse case scenarios the liquid fuel in the chamber not only wont burn, but can increase the compression ratio to the point of detonation. (This isnt likely in EFI, but it gets the point across) The more vaporized the fuel is, the more the engine can use and the less goes out the exhaust port as a pollutant. As far as higher octane fuel lowering emissions, not likely. The higher octane fuel may have less volatile end gasses and burn slower, but it makes no less BTUs than any other gasoline, so id say that it burns just as hot.

 

If hotter temps give better mileage and vaporize the fuel better, then why do people switch to lower temp thermostats?

 

Because even though the engine needs to be hot to vaporize the fuel, the intake charge needs to be cool to make maximum power, its a comprimise just like everything else. The low temp thermostat issue has been a debate for quite awhile and the archives are full of interesting threads. Personally, I chose a 180* thermostat because its a good comprimise between the factory 195* and the common 160* 'stat.

 

well i may be talking out of my back side.....but also remember a hotter engine means less dense air coming in which will hurt your performance....

 

Coolant temperatures and economy:

It's about energy. Heat is energy. The energy/heat of combustion is converted to motion by forcing the piston down. If some of that energy/heat is sucked through the cylinder walls because the coolant on the other side is at 180 degrees vs. 195 degrees, you've lost some of the energy to move the piston down.

It really is that simple.

There is some contribution to improved economy by improved vaporization of the incoming fuel. That effect is reduced with fuel injection vs. carb, though.

Transfer of heat to the incoming fuel/air mixture will reduce the density of the cylinder charge, reducing power. But, since economy is a part-throttle thing, anyway, that doesn't play a very important role (except for the vaporization mentioned earlier).

FWIW, LT1 and LS1/6 engines don't use EGR. They've controlled NOx by other means, and the combination of power & economy those engines produce is fantastic.

 

LT1s do use EGR.

 

Isn't it the LT4s that don't use EGR? How does the LS1/6 manage NOx? I've read that cam overlap will have the same effect. Is this the only method they use?

 

If Im not mistaken, all Gen II engines have EGR. Generally, the engines that dont use EGR utilize different cam overlap techniques to trap exhaust in the chamber, this and tuning contribute to the engines ability not to pollute without EGR

 

Hmmm, I might have mixed LT1 & LT4.

In the later versions of the now-discontinued GM HO 350 Conversion Kit, which used the ZZ4 engine with the carb'd system, they eliminated the EGR.

 

<b>The higher octane fuel may have less volatile end gasses and burn slower, but it makes no less BTUs than any other gasoline, so id say that it burns just as hot.</b>

Actually I read somwhere that higher octane fuels provide less BTUS, but since they burn slower you can up the compression ratio and advance the timing to produce more power.

I think CrossFireTA was the one who said that. Just giving him the credit.