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Why the engine spins faster when I accelerate?

  1. Aug 15, 2011 #1
    Hello friends,

    I just wanted to know what makes the engine turns faster when you open the valve throttle.
     
  2. jcsd
  3. Aug 15, 2011 #2

    DaveC426913

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    The firing timing is advanced, which means spark plugs fire a little sooner, valves are activated a little sooner, and ultimately the engine's crank turns faster.

    There is quite a bit of science involved in where in the cycle the events occur. You want the inject the fuel, and fire the spark prior to the piston reaching top dead centre (TDC), so that it provides full force on the piston at the right time. Tweaking the timing of these events is what performance tuning is all about, but even the base model car engine requires this to be fine tuned so that the engine speeds up when you hit the accelerator.
     
    Last edited: Aug 15, 2011
  4. Aug 15, 2011 #3

    berkeman

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    Just to add to Dave's answer a bit... Say your vehicle is moving at a constant speed, and the engine is turning at the corresponding constant RPM. When you open the throttle, that lets more fuel/air mixture into the cylinders for each power stroke, which generates a more powerful explosion for each power stroke.

    This extra power gives you extra torque to the drive wheel(s), which accelerates the vehicle to the speed corresponding to the new higher power output. The advance techniques that Dave outlines are used to help optimize the ignition of the fuel/air mixture at the higher RPM that is ramped up to.
     
  5. Aug 15, 2011 #4

    brewnog

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    Sorry Dave, this isn't correct. The engine accelerates because the throttle opens. More air is available, and more fuel is added (the mechanism depending on the type of system; vaccuum in a carburettor, injector pulse width with modern FIE), there's more energy in the system, and the engine accelerates. Any timing advance is an effect, not a cause of engine speed increase. This is triggered by the increase in speed through either a centrifugal mechanism (in a traditional distributor), or by electronics in modern vehicles.

    The valve opening is independent of spark timing and in most engines is fixed.
     
  6. Aug 15, 2011 #5

    DaveC426913

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    Oops, I actually thought it was the other way around. Advancing the firing is the cause and the valves letting more fuel in and faster is the effect.

    You see why I'm in computers and not cars...
     
  7. Aug 15, 2011 #6

    brewnog

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    In approximately this order:
    The throttle opens. More air enters the cylinder. More fuel is added. The 'bang' is bigger. The crankshaft speeds up. The ignition system senses the increase in speed (and possibly the increase in inlet manifold vaccuum/boost), and may (or may not) advance the timing to assist acceleration.

    With almost all cars, the valves won't open any earlier during any of this. With the exception of very high performance engines, the valve timing is mechanically fixed by a chain or belt. Even those 'variable valve timing' engines you may have seen will open their valves at the same time, they just might close them a little later.
     
  8. Aug 15, 2011 #7
    So the bang has to be bigger.

    Let's imagine one cylinder: for each crankshaft speed, when the piston is in bottom dead center, the volume of air/fuel mixture is the same, but not the quantity of the mixture. Right?
     
  9. Aug 15, 2011 #8

    Mech_Engineer

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    Not necessarily, most engines at or near wide-open throttle (WOT) will richen the mixture to something like 12:1 to enhance power (and sometimes to reduce exhaust gas temperature EGT). For slow to moderate acceleration, the engine will stay at the stoich. ratio of 14.7:1.
     
    Last edited by a moderator: Aug 15, 2011
  10. Aug 15, 2011 #9
    In practice, that's not quite accurate. You must also consider the velocity of the incoming air/fuel. The numerous variables make it difficult to generalize, but at times there will be sufficient velocity to pack more mixture into the cylinder than the volume at static pressure. Conversely, sometimes the exhaust stroke meets sufficient resistance ("back-pressure") to cause more than the typical quantity of gases to remain in the cylinder, thus diluting the fresh incoming mixture. (I'm likely not using the correct terminology, but perhaps someone can correct me.)
     
  11. Aug 16, 2011 #10

    Ranger Mike

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    Lets look at a slug of air and fuel going through and engine...Basically, two forces act on the incoming air/fuel charge: the negative pressure exerted by downward moving piston and the positive force pressure present by atmospheric pressure (14.7 psi @ 60 degrees F). Together these two provide impetus for the inlet charge. If a cylinder is left open to atmospheric pressure (with piston at BDC) a given volume of gas (by weight) would occupy the void. During normal engine operation, another volume of gas by weight, will be "drawn" into this same cylinder. By comparing the two mixture weights just discussed, a number can be found which when presented as a percentage, represents the so called volumetric efficiency of the engine. A value of 100% represents the volumetric efficiency when the volume (by weight) inducted during actual engine operation exactly equals the volume( by weight) that would fill the engine if it were stopped and left open to the atmospheric pressure filling.
    Following combustion, the spent gas must be removed from the cylinder before a fresh batch makes its appearance from the intake system. An exhaust valve opens ands the burned mix (still at pressure higher than atmospheric pressure ) exits out the exhaust manifold, through the muffler and out the tail pipe. This is a base line point of reference.
    As Brewnog pointed out, it is an accepted fact that increased quantities of fuel and air are a function of increased quantities of engine output.


    Scavenging or ram effect is a technique to improve volumetric efficiency VE without the use of turbo chargers or super chargers.both mechanical means to force feed the cylinder with more fuel/ air charge. Air is probably the cheapest fuel component available, and since it is a fuel carrying vehicle, the ram trick involves increasing intake air flow. (assume we are using cylinder heads designed for maximum flow as are the intake and exhaust).
    RAM or scavenging effect is most efficiently accomplished at a given engine RPM. This is based on the fact that the velocity of pressure waves traveling in intake and exhaust passages is variable with engine speed. Consider what happens during the intake cycle. The piston begins its downward intake stroke from TDC. Its sucking action is the greatest (theoretically) at a point 90 degrees in the crankshaft rotation past TDC. Piston speed is at maximum value at this point.During the action of the piston, a pressure wave in the intake passage is created. This is a negative (rarefraction) wave that does, oddly enough, travel toward the inlet of the intake passage. The departing negative wave actually adds to the speed of the incoming fuel /air charge, increasing cylinder filling by promoting the passage of more mix per unit of time (engine RPM). As engine speed increases, the frequency and number of negative pressure waves increase, altering the ram effect. When a negative wave reaches the intake inlet, it meets atmospheric pressure head on. It reacts in the form of a positive compression wave that travels backwards down the intake passage, helping force more mix into the cylinder.
    There is a lot more that goes on and i can comment on this later but I have to close for now..
     
    Last edited: Aug 16, 2011
  12. Aug 16, 2011 #11

    brewnog

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    Volume isn't a good quantity when dealing with aspiration, because it depends entirely on temperature and pressure! At bottom dead centre, the volume of mixture within the cylinder is always the same, because it's defined by the geometry of the engine (bore and stroke). The quantity of mixture (let's use mass), however, varies a lot!
     
  13. Aug 16, 2011 #12

    Ranger Mike

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    good point Brews, does what I wrote make sense if we sub MASS of the weight in above para?
     
  14. Aug 16, 2011 #13
    Or density.

    A better way would be to think of the air as a loosely wound spring, with atmospheric pressure on one end and the intake valve at the other.

    The "negative wave" is actually a zone of reduced density resulting from the reduction in pressure on one end of the air column, causing the entire air column to start moving into the cylinder. When the piston reaches the bottom of the stroke and the valve is still open, the air column continues to move into the cylinder. The column's inertia continues to push air into the cylinder until the pressure in the cylinder brings the air column to a stop. At this point it would be advantageous to close the intake valve to capture as much air as possible.

    Playing with a partially stretched slinky is a great way to get a visual on what the air is doing. Start with one end, hold the main body about 3 inches from one end with your left hand and pull the loose end of it over an inch or two with your right hand.

    Now for the tricky part: to accurately simulate the pulsing of the air column you have to release the pulled end (right hand) just before you release the main body (left hand). To simulate the intake valve closing, regrab with your left hand before any of the slinky moves back to the left. You'll have to be fast, and it may take a little practice!

    When you finish one cycle, if you've done it right the whole slinky will have moved to the right, simulating the movement of the air column, and you can repeat the process. You'll notice that the "negative wave" doesn't actually "bounce" against the main slinky body (representing atmospheric pressure), showing that the boost effect relies on inertia rather than pressure. Just a nitpick.
     
    Last edited: Aug 16, 2011
  15. Aug 16, 2011 #14
    You're right in including the exhaust's effect on the process. The pulsing that occurs in a properly designed exhaust has a significant effect on the extent of the "over-charging" during the intake stroke.

    For that to happen the valve events must be timed to allow the pulsing generated in the exhaust to augment the pulsing in the intake, by having both valves open at the right time and amount. This is the overlap and is determined by the duration of the valve opening events and by the lobe separation angle.

    The interaction of the various events has to be considered and properly integrated when looking for that extra bit of power. That's why an engine with carefully selected and assembled components can produce noticably more power in its usable range than essentially the same components in another engine.
     
  16. Aug 17, 2011 #15

    brewnog

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    I think it makes sense anyway old chap, I just thought the comment might help the OP get his head around it a little better.
     
  17. Aug 17, 2011 #16
    Thank you all for the inputs.

    I'm still studying the responses, especially that of Mike Ranger, very complex.

    I'll be back soon.
     
  18. Aug 17, 2011 #17

    Ranger Mike

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    Tonhovsk..please take time to think about it and I have another additional paragraph about what happens after cam shaft timing event and during exhaust cycle but have little time now to type,,will do it later tonight..hang in there..it really is not complicated once you know
    the " physics"..
    rm
     
  19. Aug 17, 2011 #18
    It’s exactly this accepted fact that I’m trying to understand: what happens when increased quantities of air/fuel mixture are pushed into intake manifold, so that it increases the engine speed.
    And I think that the ram effect doesn’t help to explain it, since the pure and simple engine speed increase is obviously anterior to this technique.
     
  20. Aug 18, 2011 #19
    You accelerate because the engine is outputting more power when you open the throttle.

    Getting more power is caused by more the force pushing the piston down. A bigger bang = more force.

    You get a bigger bang by burning more fuel, and to burn more fuel you need more air.
     
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