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  1. #1
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    Questions about Spark Timing Advance

    So I have a question and I'm sure someone has a good answer. It involves spark timing, specifically spark advance for more power.

    So, when I was learning about combustion engines in college, I was taught that the optimal point for a combustion chamber to achieve peak pressure after ignition is 15-20 degrees ATDC. I was also taught that an AFR or 12.5-13.5 is optimal for maximum flame front speed which results in more complete combustion.

    When spark timing is advanced, the burn starts and ends sooner. in order to maintain peak pressure timing of 15-20 ATDC, a lower AFR must be used to slow down the burn. The idea is that more fuel burning longer increases the power, when in reality you're tuning the AFR to slow the flame front speed in order to maintain peak pressure being in that 15-20 ATDC range. The pressure in the combustion chamber reaches higher levels closer to TDC than a equivalent, non-spark advanced engine. this early pressure increase reduces efficiency due to the mechanical leverage the con rod has on the crank, which (im assuming, correct if wrong) increases the heat. produced.

    My question is this, why not tune (retard) the spark to maximize the relationship between mechanical leverage on the crank (less pressure pre-TDC) and combustion chamber pressure? (Doesn't pressure buildup before TDC cost power?) All while keeping the AFR in the optimal 12.5-13.5 range (fast, efficient, complete burn of all the fuel). Wouldn't focusing on complete burn of fuel reduce the chances of knock due to less residual fuel being present from the previous combustion cycle? I feel like this would result in a smoother, more efficient engine.

    Im sure there are holes in my logic here. I wont get upset if someone chimes in and says "you're full of $#@!" so long as they explain why.

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    I think it's a good topic but we need the tuners to chime in on it.
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    The 15 to 20deg is the math that gives highest average net positive pressure on the piston. Doesn't matter burn rate as long as you hit that window, without detonation of course... this will result in max torque. Typically faster burn rate is better for detonation resistance, but also the net pressure math is higher i believe (although I would never be able to show this on paper, without lots of help).

    AFR gets a little tricky. Generally X ratio target is best considering mixing dynamics and power versus detonation for a given setup, but a stoich reaction will create the most energy.

    I like @V8Bait responses to these types of questions. Calling the smart guy.

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    So we're down to the nitty gritty I see haha. Hmm, this sounds like a great question for a multiphysics software suite. Anyway, there's a few major reasons for the 15-20 degrees, but in tuning you typically go for MBT on a dyno, and that can vary based on many things (fuel type, for reasons like flame speed, engine RPM, for things like piston speed related to flame speed, etc etc). But in general it sounds like you are talking somewhat about the efficiency, which comes down to thermal efficiency for a gas engine (they are heat engines after all). The mechanical efficiency and leverage is a part of that, but I honestly don't even have a guess as to how big of a part it plays.

    If you get the timing right you can really maximize thermal efficiency. That point is when the gas is expanding fastest as the piston is at it's uppermost point, converting the energy of the expanding gas into energy pressing the piston down. If you miss that sweet spot, the expanding gas generates heat instead of kinetic energy. Well, technically it always/only generates heat because heat is kinetic energy, but if you miss the sweet spot the heat energy gets put into the cylinder walls, head, exhaust, etc... vs if you hit the sweet spot the heat (kinetic) energy gets transferred to the combustion chamber and piston directly. Transferring more of that energy to the piston means less heat wasted into the cooling system and exhaust, more heat into the piston yes but that means more energy transferred to the wheels ultimately. This is where flame speed comes into play... the 12.5-13 AFR is mostly for flame speed. For others reading, think of it like this... if you had a fuel that had infinite octane and nearly instant flame speed (aka explosives and a explosives proof block lol), you'd want the timing set to zero. Your theoretical fuel is instant, by waiting until TDC, you minimize expanding gas pushing back on the piston on the upstroke, and maximize thermal expansion/kinetic energy transfer to the piston right when it wants it (lets go play with acetylene to fuel our car!). But since we don't have that, we deal with fuels that are much slower (we have to control peak pressures to not blow engines apart, acetylene would be super efficient yes, but would peak thousands of PSI and break stuff). We have to get it burning first (advance timing) so that it can slowly expand, but doesn't kill engines, and then keeps expanding to push the piston down on the downstroke (holds a high average pressure during power stroke, less efficient than a huge peak pressure, but safer on engines).

    I think the best example of the mechanical energy you're talking about is in over advanced scenario's. If you over advance, the fuel has too much time to expand, and the piston has to fight a ton of pressure to reach TDC. Not sure how much loss you get mechanically from 15-20 degrees advance vs 5 degrees, but there is definitely more loss. However, the benefit of the thermal efficiency is greater and outweighs the mechanical "leverage" loss.

    On the AFR and flame speed, flame speed slows as you go lean as well. That's part of why at part throttle you have over 30 degrees of advance, the engine is running very lean, flame speed is low, so highest thermal efficiency is with much more advance (takes longer for the fuel to expand/release kinetic energy). So, just because you are stoich, doesn't mean that's where peak power is. Peak power will be at the point where you have fastest flame speed mixed with perfect timing to utilize that, probably best determined on a dyno I think.

    Interesting subjects for sure, where do you get these things? lol
    Last edited by V8Bait; 06-01-2013 at 02:14 PM. Reason: ocd technicalities

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    Let me clarify peak pressure at 15 to 20deg and the instant explosion is a good example... if you could pick the point at which all the energy is used, this would still be 15 to 20deg. This is due to the lever arm force on the crank and i think this stays constant no matter the rod length. The higher burn rate helps with reducing negative pressure during the compression stroke.

    For AFR, my thought is that "popular" ratio for burn rate is considering more homogenous mix (less lean pockets). If you could achieve stoich throughout i believe this would give the fastest burn rate.

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    Peak pressure in advance of TDC? Are you sure about that? I'll have to check, but that doesn't make sense to me. You would be trying to spin the engine backwards at that point... you want peak pressure once the piston is in position to push (lever arm) down on the crank if I'm not mistaken... or are you talking about after tdc timing? You want the fuel to be fully combusted by about 20 degrees ATDC as well iirc. And do you mean reducing negative pressure, or reducing positive pressure during the compression stroke (after spark) with faster burns? I believe faster burns allow for less timing for MBT, lowering detonation risk, and increasing compression efficiency for a few degrees of crank rotation, could be wrong though.

    I'm not sure if stoich would produce the fastest burn rates, but definitely the most efficient burn. I'll see if I can check that out more thoroughly. Most of this is from what I've been told over time and know from general classes and reading, nothing concrete heh. Tuner input is always appreciated! I'm a medical guy :-P

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    15 to 20deg ATDC (though that was a given) is when you want peak pressure if possible depending on detonation. Faster burn rate has multiple advantages, but I was pointing out the less know pertaining to mech adv reducing pressure against the piston during compression.
    Last edited by JoshBoody; 06-01-2013 at 09:50 PM.

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    The AFR subject is interesting and I think kinda have to view this from the chem side, separate from automotive... since most tuning knowledge is based on experiences and the tools available (like oxygen sensors some distance from combustion). Like you've mentioned before there is various types of "complete" combustion. I'm curious if richer mix would have an advantage (apart from evaporative cooling) assuming perfect homogeneous mix.
    Last edited by JoshBoody; 06-01-2013 at 09:53 PM.

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    Click here to enlarge Originally Posted by V8Bait Click here to enlarge
    So we're down to the nitty gritty I see haha. Hmm, this sounds like a great question for a multiphysics software suite. Anyway, there's a few major reasons for the 15-20 degrees, but in tuning you typically go for MBT on a dyno, and that can vary based on many things (fuel type, for reasons like flame speed, engine RPM, for things like piston speed related to flame speed, etc etc). But in general it sounds like you are talking somewhat about the efficiency, which comes down to thermal efficiency for a gas engine (they are heat engines after all). The mechanical efficiency and leverage is a part of that, but I honestly don't even have a guess as to how big of a part it plays.

    If you get the timing right you can really maximize thermal efficiency. That point is when the gas is expanding fastest as the piston is at it's uppermost point, converting the energy of the expanding gas into energy pressing the piston down. If you miss that sweet spot, the expanding gas generates heat instead of kinetic energy. Well, technically it always/only generates heat because heat is kinetic energy, but if you miss the sweet spot the heat energy gets put into the cylinder walls, head, exhaust, etc... vs if you hit the sweet spot the heat (kinetic) energy gets transferred to the combustion chamber and piston directly. Transferring more of that energy to the piston means less heat wasted into the cooling system and exhaust, more heat into the piston yes but that means more energy transferred to the wheels ultimately. This is where flame speed comes into play... the 12.5-13 AFR is mostly for flame speed. For others reading, think of it like this... if you had a fuel that had infinite octane and nearly instant flame speed (aka explosives and a explosives proof block lol), you'd want the timing set to zero. Your theoretical fuel is instant, by waiting until TDC, you minimize expanding gas pushing back on the piston on the upstroke, and maximize thermal expansion/kinetic energy transfer to the piston right when it wants it (lets go play with acetylene to fuel our car!). But since we don't have that, we deal with fuels that are much slower (we have to control peak pressures to not blow engines apart, acetylene would be super efficient yes, but would peak thousands of PSI and break stuff). We have to get it burning first (advance timing) so that it can slowly expand, but doesn't kill engines, and then keeps expanding to push the piston down on the downstroke (holds a high average pressure during power stroke, less efficient than a huge peak pressure, but safer on engines).
    This raises a question for me. Would the use of multiple spark plugs and direct injection (able to produce a more homogeneous air:fuel mixture) allow you to use less advance?

    I think the best example of the mechanical energy you're talking about is in over advanced scenario's. If you over advance, the fuel has too much time to expand, and the piston has to fight a ton of pressure to reach TDC. Not sure how much loss you get mechanically from 15-20 degrees advance vs 5 degrees, but there is definitely more loss. However, the benefit of the thermal efficiency is greater and outweighs the mechanical "leverage" loss.
    This raises the same question about multiple spark plugs/DI

    On the AFR and flame speed, flame speed slows as you go lean as well. That's part of why at part throttle you have over 30 degrees of advance, the engine is running very lean, flame speed is low, so highest thermal efficiency is with much more advance (takes longer for the fuel to expand/release kinetic energy). So, just because you are stoich, doesn't mean that's where peak power is. Peak power will be at the point where you have fastest flame speed mixed with perfect timing to utilize that, probably best determined on a dyno I think.

    Interesting subjects for sure, where do you get these things? lol
    I get them from having too much time on my hands haha. I take a big interest in the more technical aspects of automotive performance. Once I have a thorough understanding of the engine and driveline, I'll foray into tire physics, suspension geometry, how weight transfer affects these and how to use this information to properly setup the chassis of a car. Can you tell I want a career in motorsports? lol.

    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    Let me clarify peak pressure at 15 to 20deg and the instant explosion is a good example... if you could pick the point at which all the energy is used, this would still be 15 to 20deg. This is due to the lever arm force on the crank and i think this stays constant no matter the rod length. The higher burn rate helps with reducing negative pressure during the compression stroke.

    For AFR, my thought is that "popular" ratio for burn rate is considering more homogenous mix (less lean pockets). If you could achieve stoich throughout i believe this would give the fastest burn rate.
    This is similar to my line of thinking that led me to ask the question about multiple spark plugs and direct injection.

  10. #10
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    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    15 to 20deg ATDC (though that was a given) is when you want peak pressure if possible depending on detonation. Faster burn rate has multiple advantages, but I was pointing out the less know pertaining to mech adv reducing pressure against the piston during compression.
    Briefly 're read your op and the post after. You're clear about ATDC, I'm just not used to reading like that. Makes more sense about your lever arm thing and such now. I need to stop using my phone so much on forums lol.

    So, assuming mechanical efficiency of piston force is greatest at 20 atdc, thermal efficiency is much lower there. I'm guessing it has to do with something there why it's not better exploited. I'm going to read your posts again tomorrow though and maybe see what I can dig up on it.

  11. #11
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    So what you are asking, is why do we have spark advance in the first place, right? Because if peak power is generated at 15-20 ATDC, spark advance is the wrong idea, we need better gas.

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