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EGT and air and fuel ratios

  1. Jan 17, 2007 #1
    I am doing a project and was researching some information and found this site and read some information as to what I am doing but it didn't have all the answers needed.
    I'm doing a project on egt readings and twin cylinder 2 stroke snowmobile engines.
    For example what effects higher or lower readings.
    Would an increase in the air mixture, enough to create a lean meltdown on the intake side of the piston would EGT readings show this.
    Would an increase in the fuel mixture actually cool things off, would this show in EGT readings.
    Would EGT readings drop or raise if DETO was happening in one of those cylinders.
    Would readings drop because the piston and wall are absorbing all the extra heat.

    If someone could explain some of these to me it would be very appreciated.
     
    Last edited: Jan 17, 2007
  2. jcsd
  3. Jan 17, 2007 #2
    I hope someone chimes in. This is how I found these forums, I seen a couple of guys talking in a topic about EGTs and A/F mixtures.
     
  4. Jan 18, 2007 #3

    brewnog

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    Ok, well since it's a project, you could share some of your findings and thoughts first? We're not a bank of answers!
     
  5. Jan 18, 2007 #4
    Lean a/f's on NA motors, such as something as high as 17:1 are going to drastically increase EGTs to the point where you would definitely notice the temperature increase. Detonation lowers EGTs, dumping fuel also lowers EGTs. People sometimes confuse too much fuel, i.e. lower EGTs, with detonation. They think they have too much fuel because of the low EGT value but the motor is actually knocking. Knock is more of a spike. If you were to log a knock sensors output there would be some background noise, which is the noise created from the motor, and then if you had some detonation you'd see a spike in the log. Should be similar when looking at EGT values if you log them.
     
  6. Jan 18, 2007 #5
    Fair enough.
    This is a high school shop project and I own a snowmobile but don't have egts on it but some friends have.
    I know that if we add more air to the ratio EGT temps will raise meaning more oxygen I guess and this is like adding more oxygen to a fire. The same in the combustion process but now add more fuel and temps will drop correct?
    Because the fuel is needed to cool the piston even though you think more fuel would mean more combustion.
    Friends say that if there sled dies with DETO that there EGTs didn't show anything but I have read that EGTs should be dropping if DETO is happening.
    I can't really test this because I can't afford doing this to my snowmobile.
    So now if you are leaning out because of lets say a air leak, temps will be getting higher and usually the exhaust side of the piston starts to melt away first or lets say a lean condition because a main jet is plugged and fuel is restricted the same will still happen on the exhaust side of the piston.
    This is some of the information I have gathered together for my report or assignment.
    Also I don't know why if Deto takes out a piston on a twin cylinder 2 stroke engine because of low octane gas why both piston would not detonate.
    Why only one and the other shows no sign of any kind of failure, is this because one cylinder will also run hotter?
    Thanks for taking the time to answer but I'm just trying to do the best I can and this is something that interest me as I do some work on my own but would like to learn more and shop is fun.
     
  7. Jan 20, 2007 #6
    brewnog
    As I looked back on past post I've read here it seems like you know alot about EGTs and A/F ratios.
     
  8. Jan 20, 2007 #7

    Danger

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    Brewnog knows a lot about a wide range of subjects. You've tapped into a goldmine, Detonator.
    Your question also sparked one in my own head. Since it's related, I'm going to piggy-back it here rather than start a new thread.
    If (for whatever reason) you have one cylinder running too rich and another running too lean, would the products of them ignite in the exhaust system? I know that you can deliberately ignite a rich exhaust, but does it ever happen spontaneously?

    edit: This is in reference to modern engines, not the 1930's Packard (or whatever) backfire.
     
    Last edited: Jan 20, 2007
  9. Jan 20, 2007 #8

    brewnog

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    Well you seem to be reasonably clued up, and I'm no expert in 2 strokes (or even spark ignition engines) so beware!

    Generally, when in-cylinder temperatures increase, so do exhaust port temperatures. The exceptions to this are caused by things like having a lot of blowthrough at TDC following the exhaust stroke, where you get a lot of fresh air flowing through the top of the cylinder and straight out of the exhaust port. This obviously has a cooling effect on the gases being scavenged, and is most pronounced in 4-strokes when using a wide-overlap camshaft (often used specifically for this purpose), and in 2-strokes where the combustion system geometry has been designed to optimise scavenging.

    Now, the effect of mixture on EGT:

    With a very rich mixture, exhaust gas temperatures will be low. This is down to a number of factors, but the absence of sufficient air for complete combustion is the main factor. The flame front simply dies out. Also, the excess fuel has a high specific heat capacity, and is able to 'mop up' further heat, particularly in something like a Diesel engine where there are still fuel droplets present (the evaporation of these droplets requires energy input, thus lowering the surrounding temperatures). CO rather than CO2 is produced due to the partial oxidation of carbon in an oxygen deficient environment. This CO is then further oxidised to CO2 later on in the combustion process (say, when both valves are open, and there's a sudden influx of fresh oxygen). This resulting oxidation of CO to CO2 requires energy input, so temperatures are again reduced.

    Moving on to leaner mixtures, more air is available to sustain a more complete combustion, and more fuel is burned, and temperatures rise, to a peak, when all the fuel is burned (obviously all the chemical energy held within is released as heat). The effects mentioned previously decrease, particularly as the oxidation of carbon is complete. Leaner still, there's excess air, which serves to dilute the hot combustion gases, and temperatures again start to decrease.

    This is all quite high-level; there's rather a lot more with regard to heat release rates.


    As for why detonation generally only takes out one cylinder:
    The line between detonation and no detonation is extremely fine. It can take just a few degrees extra in cylinder temperatures, or a few tenths of a degree crank angle too advanced (for spark/injection) to cause detonation. As a result of cooling channel designs, and slight imbalances of flow between different inlet tracts, (not to mention different AFRs cylinder-to-cylinder caused by tolerances on the carburettors), all the cylinders very rarely operate at exactly the same timing/mixture/temperature. If you have a bad detonation, and take off the top of the piston on one cylinder, your engine will probably catastrophically fail pretty quickly on one cylinder before the others have even had chance.

    As for what EGTs would do to warn you of detonation, well, I wouldn't rely on using them as an indicator. Firstly check your mixture. Secondly make sure you're not running at some silly advance anyway, and thirdly listen out for detonation. If you've set the engine up properly in the first place then there's no reason for it to start detonating during use. However, whilst leaning out during setting your engine up (and we're talking sensible AFRs here), you'll notice the exhaust temperatures rise, peak, and then start to fall. As you said, when they're falling, there's a good chance your detonating. This is because the shock wave caused by the detonation breaks up the boundary layer of relatively stagnant mixture which normally 'lines' the cylinder walls. As a result, the thermal transfer between the combustion gas and the (normally rather cool) cylinder walls (and by proxy, piston) rocket, until you start siezing little chunks of piston land to the bores and ripping them off. The exhaust gas temperature decreases when detonation occurs because some of the heat which usually goes out of the exhaust ports goes straight into the cylinder walls. So it's important to distinguish between exhaust gas temperatures (which essentially indicate mixture; the peak temperature is at stoichiometric) and 'general engine temperatures' (coolant and oil) which are drastically affected by any changes in friction between the piston and bore/liner.

    Let me have more of a think and I'll try and rearrange that into some bite-sized chunks.
     
  10. Jan 20, 2007 #9
    Thank you brewnog
    That is more than I ever wanted or needed and I will try to digest that and will probably be back for more.
    Thank you again.
    I've got another question that I have no answer for or searched for many times.
    What always comes first.
    Detonation first than Pre Ignition.
    Does Deto always precede Pre Ignition.
     
    Last edited: Jan 20, 2007
  11. Jan 20, 2007 #10

    brewnog

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    As you obviously appreciate from your question, there's a subtle difference between the two. In fact, they're two distinctly separate phenomena, but the effects can be similar.

    Pre-ignition is when combustion (the rapid progression of a flame front through the end gas) occurs before (in time) the spark. This can be due to a number of factors, generally by 'surface ignition' where the charge is ignited by a hot surface (often a hot valve, the spark plug, or a hot spot on the bore or head, and occasionally by a glowing carbon deposit somewhere in the chamber). Note that surface ignition can also occur after the spark event; this is known (surprisingly enough) as post-ignition.

    Knock (or detonation) is spontaneous combustion of some of the charge ahead of (in distance from the spark plug) the advancing flame front. (I'm assuming here that you understand (or can imagine) how a flame front propagates from the area around the spark plug, if not please shout!). As this flame front propagates, the gas which has not yet been ignited, is compressed, and as a result its temperature and density increase. If the conditions (temperature, pressure, density) of this unburned portion of mixtue are right for combustion, then ignition will occur. This resulting combustion advances MUCH (5 to 25 times) faster than the 'normal' flame front, and is sometimes severe enough to cause catastrophic failure (as I described earlier). This rapid combustion causes a high-pressure shock wave which gives knock its characteristic (onomatopoeaic!) sound. Knock occurs when this secondary, spontaneous ignition occurs before the flame front has reached it. In effect, there's a race between the normal flame front (propagating from the spark), and any auto-ignited areas of charge.

    So, surface ignition (usually in the form of pre-ignition) can cause knock, by causing an abnormal combustion event ahead of the flame front. If the spark has already occured, and a normal combustion event taken place, but knocking occurs, this is known as spark knock, i.e. it has not been caused by surface ignition.
     
  12. Jan 20, 2007 #11
    Yes some of that went right over my head :surprised but I'm still wondering what comes first, sorry.
    Can you not have pre igniton without detonation?
    Is Pre Ignition a by product of Detonation or do you need Pre Ignition first then Deto happens?
     
  13. Jan 20, 2007 #12

    brewnog

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    Think of them as separate issues. You can have pre-ignition without detonation, and you can have detonation without pre-ignition. You can also have detonation caused by pre-ignition, or more correctly, by surface ignition.


    Pre-ignition: Any ignition which occurs before the spark event happens (in time).

    Detonation: Shock wave caused by ignition in area beyond advancing flame front.
     
    Last edited: Jan 20, 2007
  14. Jan 20, 2007 #13
    Thank you again some of it or most of what you said I understand and can visualize the combustion process and I still read it more than once because it helps to understand it.
    Another question on Detonation.
    Why is it if you have a a high compression 2 stroke motor that needs a high octane fuel but if you use a lower than required octane that Detonation will occur.
    I know that the higher octane helps keep things cool but is this the only reason deto occurs using a low octane gasoline.
     
    Last edited: Jan 20, 2007
  15. Jan 21, 2007 #14

    brewnog

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    I don't really understand the chemistry behind combustion, but all octane number measures is the fuel's ability to resist knock/detonation.

    Octane number has nothing to do with the quantity of octane in the fuel. Octane number is expressed relative to a mixture of isooctane, and n-heptane. For example, a fuel with an octane number of 80 has the same ability to resist knock as a mixture of 80% isooctane, 20% n-heptane. Any octane number above 100 is merely an extrapolation of this system.

    I believe that the mechanism by which a higher octane number limits knock is its higher energy of activation; ie more energy is required to provoke autoignition. As a result, your end gas can be hotter, and at a higher pressure, without detonation occuring. Any more info you need I'd advise you to go and find a chemist!

    It is a common misconception that higher octane number fuels give more power. This is not inherently true. A fuel with a higher octane number is more resistant to knock, and as a result the timing can be advanced; which itself can produce more power. Alternatively (in a supercharged engine), higher levels of boost can be employed with a higher knock limit.

    Hope this helps.
     
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