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Ignition Timing vs. Heat Distribution in the Engine

  1. Jul 17, 2013 #1
    Hello all,

    I am studying four stroke ICE engines (as a mechanic) and there is something I can't fully understand about the combustion process.

    It's repeatedly documented in books that retarding the ignition timing will reduce cylinder temperatures / avoid engine knock.

    Though less consistently it seems to also be said that further advancing ignition timing will lead to greater power output.

    It's also said that approximately 1/3rd of energy from the fuel is lost through the exhaust and 1/3rd through the cooling system.

    So my problem is I don't understand why it seems that advancing ignition timing increases cylinder temperatures and heat to the cooling system and retarding timing appears to increase the heat in the exhaust.

    What I believe is that the sooner the mixture is ignited the higher the pressure in the cylinder will be during the time when the piston is rising against and pushing down from the combustion. Whereas if the ignition is later then less work is done because the piston is already more headed down and the remaining fuel air mixture burns as it travels out the exhaust.

    Some others have claimed it has to do with the cooling passages around the area of the combustion, like whether the flame burns more toward tdc or lower in the engine block.

    So my question is how does ignition timing affect the heat distribution and useful work achieved, does it split the energy between exhaust and engine and why?

    Thank you,
    Andrew
     
  2. jcsd
  3. Jul 17, 2013 #2

    SteamKing

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    You don't want to ignite the fuel-air mixture before maximum compression is reached, otherwise, the engine is working against itself. Likewise, you don't want the spark to occur after max. compression is reached, because then you lose efficiency because the mixture is not fully compressed when ignited.
     
  4. Jul 17, 2013 #3
    If you forget the charge for a minute, and just consider the piston compressing and decompressing air, then you'll know that as the air is compressed, it's temperature rises.

    The temperature and pressure traces would look a bit like this.

    engtemp1_zps6900a423.jpg

    Igniting the charge earlier means the mixture is burning as the piston is still rising up the cylinder on the compression stroke. The burning increases the temperature, which increases the pressure, but at the same time the piston is also increasing the pressure which increases the temperature!

    Here's how the temp and pressure traces look when the charge is ignited 45° before TDC.

    engtemp2_zpse250d62f.jpg

    We can see that the two effects combined increase the pressure and temperature dramatically. The fuel is still burning past TDC, so the temperature continues to rise even though the piston is now descending down the piston which expands and cools the gas slightly offsetting the increase in pressure and temperature from the fuel burn.

    Now consider what would happen if we ignited at TDC. The piston is now descending down the cylinder, again expanding and cooling the gas. This effect works against the increasing temperature of the fuel burn, meaning although the temperature from the burn is still high (still the same energy going into the same mass of air) the overall pressure is dramatically reduced.

    engtemp3_zps9cae35e9.jpg

    You can see that the pressure and temperature are already dropping before the fuel burn really gets going. Notice also that because it takes longer to get going, there is more residual heat left at the end of the stroke, which increases EGT and also transfers more heat to the head and cylinder walls, which in turn will go into the water jacket.
     
  5. Jul 18, 2013 #4

    Ranger Mike

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    So my question is how does ignition timing affect the heat distribution and useful work achieved, does it split the energy between exhaust and engine and why?
    Ignition effects heat distribution. Does it split the energy between the exhaust and engine. You could say that. Let us look at what we got going on. We take a fuel air mixture and light it. It does not explode..it burns very rapidly. To get it to burn as clean as you can, you want the fuel atomized to as small a droplet configuration as you can. You want these droplets suspended in the air mixture so you get the maximum flash when you spark the ignition. In todays advanced CD ignitions you have a module that wil cause multiple sparks at low rpm ( idle) and this module will make one long spark over 10 degrees crank shaft travel at high rpm to ensure the mix is well lit, Not so in the old days when you had breaker points and one weak anemic spark.

    We want this combustion to have maximum possibility to do the most work..Force x Distance over Time = Power..in this case horsepower. To do this we don’t want to light the candle before the mix is ready and we don’t want to light it too late. It does no good to have combustion when the piston is half way down the cylinder on the power stroke. Nor do we want to light it ahead of time as this is called detonation. Imaging the poor piston and connecting rod trying to reach top dead center and running into a just ignited wall of combustion..bent rods, cracked piston ring lands, even blown out cylinders in some cases. Now today it is known that gasoline has a finite burn rate. This burn rate can be coordinated ( timed) to happen when we get the most out of the combustion. This is not a perfect scenario..never claimed it was. We can expect to get 1/3 energy out of the gasoline but that is about it. When gasoline and air are compressed the mix can detonate simply by the high temperature of the compression. This compression rate denotes the octane rating of the gasoline. The higher we compress it, the more octane is required to suppress the premature detonation. Most street cars today have 8 or 9 o 1 compression ratio so you run 87 octane..9 to 1 performance cars need 89 octane..we run aviation gas because we have over 11 to 1 compression ration.
    So we have a piston and a cylinder and a fuel air mix and light it. the mix expands and pushes the piston down the cylinder and this power is achieved over a very small degree of crank shaft rotation. Again, it is not perfect. The energy not converted to power is changed to heat. it is removed from the engine by two systems. The water / antifreeze in the cooling system and the engine oil. You will find that 33 % of the heat goes to each. And the rest goes out the exhaust. Much overlooked and missed is the heat removed by the oil system. Pressurized oil squirts out of the crankshaft connecting rod bearing and sprays all over the cylinder walls and the bottom of the piston. This is what oils the piston wrist pin. The oil is like taffy..it is a thick stream that slings every where. We add windage trays to the oil pan to keep the oil in the pan. A crankshaft whirling around at 6000 rpm makes a lot of wind and unless we cover the top of the oil pan the taffy will be blown up on the crank throws and suck horsepower. We add crankshaft scrapers to scrape excess oil off the rotating mass to kill off parasitic drag. long story short..oil does two things, lubricates and carries away the heat. Most real race cars have oil coolers and figure the engine oil temperature is about 100 degrees F higher than the water temperature.
    one final note= detonation is a real bummer. Avoid it at all costs. wrecks a good race weekend.
     
    Last edited: Jul 18, 2013
  6. Jul 21, 2013 #5
    Follow-up

    Thanks to everyone who replied.

    I've been doing more reading and research and thinking about this one.

    The impression that I'm getting especially by looking at the graphs Kozy supplied is that the burning mixture can't perform much of any work if the container for it is too large. In the last graph showing late ignition timing it seems like the piston has run away from the flame. I don't know much about their relative speeds but it looks like the temperature gets very high because the fuel is still burning, but little pressure is achieved because the effective chamber has grown so much that for the same volume of gas there is less pressure.

    So if we lose some of the pressure we would have gotten on the piston then we lose a portion of the energy contained in the fuel. But it has to go somewhere so it seems the residual heat goes out the exhaust and finishes burning out there. So I would expect more heat transfer to exhaust valves, manifold, guides, and catalyst with late ignition timing as it seems we are effectively burning fuel in the exhaust stream.

    As for advanced ignition timing because of the compression effect that works against the expansion of the burning charge we will have more heat energy, and because the burn takes place in a smaller more confined effective area we will get higher pressures and therefore higher temperatures. I would expect more heat rejection to the combustion chamber, valve faces, and cylinder walls with advanced timing.

    I keep studying this topic because it's very important to understanding modern engine controls, for example it seems that the timing is often retarded at startup to help achieve a rapid catalyst warmup. During idle conditions the timing may be controlled in order to vary the torque output to maintain a given speed. Which I think has to do with the relationship of the Location of Peak Pressure and the crank angle for maximum mechanical advantage, which seems to be dependent on the ignition timing and engine speed.

    Ignition timing is also used in traction control systems I believe to modulate engine torque.

    Thanks for help with this topic and I appreciate any further insights,
    -Andrew
     
  7. Jul 23, 2013 #6
    You seem to have a good handle on what to expect and why. I would reccomend googling 'mass fraction burned' to give more insight into combustion behavior. It becomes very clear that unburned charge can make its way out of the chaimber with too little ignition timing. The time for complete charge burning varies with mixture strength, cylinder pressure and temp, and the octane of the fuel. To get the whole shooting match just right is a delicate balance. It is easy to see with a wideband lamda sensor and display, while tweaking ignition timing realtime. Idle mixtures can change from 13:1 with 12deg timing to >18:1 by adding 10-15deg of timing(that is without touching fueling!). Also, one of the reasons that retarded ignition timing is used at idle is to ensure a steady solid idle. The engine will pick up quite a bit of torque and rpm if the idle timing is advanced from 5-10 degrees, bit this comes at a cost of instability as engine temps change and altenator loads fluctuate. So a trade-off is struck with less efficient and ecconomical timing, for the benefit of a rock solid idle.
     
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