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Coolant Temperature Override Switch, Why Is It Needed?

Discussion in 'Intermediate CJ-5/6/7/8' started by Ol Fogie, Dec 9, 2020.

  1. Dec 9, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    After reading the post by ''rusty72cj5'' I was just wondering about the diagram of the coolant temp override switch. The switch simply changes the vacuum source back and forth from ported to manifold vacuum depending on the engine coolant temperature. My question is why does that need to be done? why not just run your dist vacuum line directly to ported or manifold vacuum and ignore the CTO?[​IMG]
     
  2. Dec 9, 2020
    73 cj5

    73 cj5 Not ready for the junkyard yet

    Clinton, Mississippi
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    Ported vacuum is emissions related. I ran manifold vacuum on my 304 and set the timing accordingly.
     
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  3. Dec 9, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    Manifold vacuum at idle is high, which gives more vacuum advance. Ported vacuum goes to zero at idle, which gives less advance. When the engine is cold, more advance at idle makes it run better. Less advance gives lower emissions. The CTO gives you good running at idle when cold and low emissions at idle when up to temp.
     
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  4. Dec 9, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Ah, it sounds as if there is a definite performance advantage to leaving the CTO operational. unless there is some performance advantage to having high manifold vacuum when the engine is at full operational temp also?
     
  5. Dec 9, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Just thinking further. When attempting to set the ignition timing should all the vacuum lines be disconnected and plugged or should timing be set at full operational temp with the distributor receiving PORTED vacuum thru the CTO at idle?
     
  6. Dec 9, 2020
    73 cj5

    73 cj5 Not ready for the junkyard yet

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    Set your initial timing without the vacuum advance connected then check your total timing and when done plug in the vacuum advance.
     
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  7. Dec 10, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    Well, if you ignore emissions, you may as well leave the distributor connected to manifold vacuum all the time. The CTO only has an advantage wrt emissions. Indeed, the later engines have a second CTO called "HDC" on the vacuum diagram. This stands for "heavy duty cooling" (ha!) and overrides the spark CTO to advance the spark and prevent overheating. I expect this makes a difference mostly in the low-speed regime, ie stop and go in hot weather?

    https://oljeep.com/gw/vac/84-86/86-wagoneer-hood-vacuum-diagram.jpg
     
    Last edited: Dec 10, 2020
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  8. Dec 10, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Thanks Tim for he diagram. If I understand correctly all the ''HDC CTO'' switch does is cancels out the ported vacuum the distributor advance is receiving when at full operational temperature and reverts back to manifold vacuum to help prevent overheating during high load/and high ambient temperature situations.
    Therefore I can assume that the engine will run cooler under load with more spark advance provided by the manifold vacuum, am I correct? Thanks Jim
     
  9. Dec 11, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    Yes, when engine temperature reaches a "high" level, the HDC CTO bypasses the Spark CTO and connects the distributor directly to manifold vacuum. The HDC CTO necessarily opens at a higher coolant temperature than the Spark CTO.

    Not sure why less advance is better for emissions; likely it lowers combustion chamber temperatures which makes less NOx.

    There are plenty of online sources that tell you that retarded timing makes an engine run hot, and more advance runs cooler. I have yet to find a source that convincingly explains why. Some forum posters claim it's because of incomplete combustion and more heat in the exhaust gases. I don't think that's right; in fact it seems backwards. Less combustion and hotter exhaust gases would seem to make the engine run cooler, since less heat goes through cylinder walls and more heat exits with the exhaust. I suspect it's more about combustion efficiency. When the timing is retarded, it takes more fuel to maintain the same engine speed. More fuel means more combustion heat and more thermal load without an accompanying increase in coolant flow, fan speed, etc. Maybe.
     
    Last edited: Dec 11, 2020
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  10. Dec 11, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Last edited: Dec 11, 2020
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  11. Dec 12, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    Thanks for posting that link. I don't believe everything they printed - I think my reasoning is better than theirs. Suspect they don't understand the process deeply. I would bet it lowers the NOx rather than richens the exhaust so the fuel can be burned there by the air injection rather than the cylinder. That seems absurd.

    " ...the additional ignition timing provided by the vacuum canister and full manifold vacuum source allowed the engine to more effectively burn the air/fuel mixture. It therefore produces more power (even at idle) and rpm rises as a result." By their reasoning, the HDC CTO cutting in will raise the idle speed. Not a bad effect, but I continue to be appalled that this would be an optional feature. The Non-Linear Valve has a role here, but the TSM does not provide much insight. This does support my claim that the overheating results from lowered combustion efficiency.

    I kinda think some of their prose is overly dramatic, to improve interest in the article. Still not happy with my understanding. Maybe there's something in the texts or patent literature. Life's too short to pursue every interest.
     
    Last edited: Dec 12, 2020
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  12. Dec 12, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Tim, I agree with your reasoning. To me it only makes sense that the engine could perform most efficiently with combustion being fully completed within the cylinder by the time the piston is at bottom dead center. Retard the timing and the air/fuel mixture will not have enough time to finish completely burning and the remaining burning fuel air mixture would be forced out by the exhaust valve and into the exhaust manifold creating excessively high temperature of the exhaust valve and surrounding structures of the cylinder head and cylinder walls creating unecessarly high engine temperature. Not to mention the loss of efficiency from not all the fuel/air mixture being burned while the piston is still in power stroke as it approaches bottom dead center. Just my 2 cents worth. I very easily could be missing something.
     
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  13. Dec 13, 2020
    Downs

    Downs Rattlecan All The Things!

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    The gymnastics they dealt with in the 70s as far as emissions and trying to figure out how to pass them didn't always make sense.
     
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  14. Dec 13, 2020
    Downs

    Downs Rattlecan All The Things!

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    This is how I run my 304. Runs great and when warm starts with less than one revolution it seems like. Previous owners had disconnected it and had the base timing set so far advanced when you tried to hot start it, it didn't want to crank.
     
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  15. Dec 13, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    This is what they write in the article that I most disagree with.

    The fuel burns to make heat. That amount of heat is a conserved quantity, determined by the amount of fuel, regardless of where it is burned. I have no problem with the hotter exhaust valve claim. Under any circumstance, lots of the combustion heat exits with the exhaust gas. Late ignition gives less time for the heat to diffuse through the cylinder walls into the coolant. The exhaust gas temperature rises. However, for the exhaust gas to be hotter, that has to take away from the heat going through the cylinder walls and into the coolant. You can't have both higher coolant temperatures and higher exhaust gas temperatures - it's not physical.

    As the article points out, the idle speed will drop without vacuum advance. Less advance, less power to the pistons and lower engine speed. Lower engine speed means less fan action and lower coolant flow, same thermal load with less cooling action. Ok so far. It also costs more fuel to maintain the same idle speed. Both less cooling action and higher thermal load.

    Another thing I would point out about the exhaust valve claim. If the idle mixture is not completely burned, the exhaust gas now both oxygen rich and rocket hot. They don't call them "burned" exhaust valves for nothing. I have to assume this is all done to lower NOx at idle, and the air injection is a band-aid to fix up the trade off with high HC in the exhaust. It's wildly inefficient in terms of HP per gallon of fuel, but it stays within the emissions limits set by the law.
     
    Last edited: Dec 13, 2020
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  16. Dec 13, 2020
    Fireball

    Fireball Well-Known Member 2023 Sponsor 2022 Sponsor 2021 Sponsor

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    I think Timgr is correct.

    The timing is most likely retarded at idle to reduce combustion temperatures for low NOx. This paper shows that NOx increases with advancing timing: Study and the effects of ignition timing on gasoline engine performance and emissions. Having been an automotive engineer in that past, the pragmatic reason was is only done at idle in the early 1970s is because that was the only operating condition specified in the emissions laws at the time.

    As for the effect of timing on engine temperature, I have anecdotal evidence of setting the timing correctly on my recently purchased Buick 350 powered '71 CJ-5. When I got it, it would always overheat at idle and tuned out to be retarded to a few degrees ATDC. The vacuum advance was not working and the centrifugal advance was binding. Fixing all that and setting the timing properly cured all hot idle issues (and made a lot more power).

    Again, I agree with Timgr on the reasons for this. Explanations online about hotter exhaust transferring more heat to the coolant as it exits the head doesn't make a lot of sense. If the exhaust is hotter, then *less* heat is going to the coolant for the same amount of fuel entering the engine. The only thing that makes sense is the drop in thermal efficiency as the spark advance is reduced. For the same amount of power required to idle at X RPM, with retarded spark you will need the throttle open further and will be burning more fuel at lower thermal efficiency. This creates more waste heat overall even though the peak combustion temperatures are lower. Figure 2 in this paper has a good graph of how fast thermal efficiency drops off as timing is reduced: (PDF) Effects of Combustion Phasing, Combustion Duration, and Their Cyclic Variations on Spark-Ignition (SI) Engine Efficiency
     
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  17. Dec 13, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    Another possibility - hot manifolds and pipes raise the under-hood temperature enough to affect cooling efficiency.
     
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  18. Dec 13, 2020
    Fireball

    Fireball Well-Known Member 2023 Sponsor 2022 Sponsor 2021 Sponsor

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    True.
     
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  19. Dec 13, 2020
    Ol Fogie

    Ol Fogie 74 cj5 304, 1943 mb

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    Ah, that is beginning to make more sense to me now. You guys have a much better way of explaining the scenario, also I am not nearly as good of a scientist as you all are. It makes sense that retarding the timing does not create more heat in the combustion cycle it simply changes where the heat is located as the fuel is burning later in the cycle and does not give as much time for the heat to dissipate into the cooling system before escaping. With that said the heat has to escape thru the exhaust valve and into the exhaust manifold raising the temps there to a higher level than it would be with more advanced timing. The retarded timing lowers the fuel economy by requiring more fuel to produce the same amount of horsepower because the given amount of fuel is burning later in the combustion cycle, therefore less energy is captured in the cylinder when it is most useful.
    It is no wonder that the engines produced in the late 70's produced lower fuel economy trying to meet the emission standards. I would seem that the increase in fuel usage would cancel any advances made to reduce emission levels. :confused: What am I missing?
     
  20. Dec 13, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor 2021 Sponsor

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    More CO2 and water, less "bad" stuff like NOx, unburned hydrocarbons and CO.

    The main baddies are NOx oxides of nitrogen and CO carbon monoxide. HC is just left over from incomplete burning. At high compression and temperatures, the nitrogen in the air burns along with the fuel, making NOx. The CO and HC are more about controlling the mixture and combustion so that the burn is near ideal as possible with no leftovers.

    Ideal combustion of hydrocarbons (ie gasoline) results only in CO2 and water.
     
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