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Thread: Car engine

  1. #1 Car engine 
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    Purely by curiosity I'm looking into how a car engine works. And the different explanations I get about the four-stroke cycle are pretty simlple to get :

    While the piston is going downwards and making space in the cylinder, the intake valve opens and lets a mixture of air and gasoline fill that space. On its way back up, the piston compresses the mixture and the spark plug ignites it. The oxygen from the air and the hydrocarbons in the fuel then react and become vapor and carbon dioxide, which for some reason rises the pressure and forces the piston to go back down. When the piston comes back up, the exhaust valve opens and lets all of the used gases out.

    What I don't get is :
    -What makes the intake valve open when the piston is on its way down ? Is it just the drop in air pressure inside the cylinder caused by the piston's movement that "sucks" it open ?
    -What causes the piston's first down-and-up movement ? Is it kinetic energy from the previous cycle ?
    -How does the ignition happen at the right time ?
    -What makes the exhaust valve open when the piston comes up after the ignition of the mixture ? If it's the pressure inside the cylinder due to piston's movement, then it should also open during the compression of the mixture, which wouldn't serve much use...

    Thx in advance


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  3. #2  
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    -What causes the piston's first down-and-up movement ? Is it kinetic energy from the previous cycle ?
    The starter cranks engine and starts rotation process. Later, if engine is in-line four cylinders type there is always some pistons which make process of rotation unstoppable.
    -How does the ignition happen at the right time ?
    Different cars could have different models but newer cars have electronic sensors which detect cylinder head position.


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  4. #3  
    AI's Have More Fun Bad Robot's Avatar
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    The following picture gives you an example of how cam shafts work. As your cylinders move up and down the cam shaft is timed to open and close per the shape of the rotating cam.



    The next picture clearly shows cam shafts for both the intake valves and the exhaust valves.

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  5. #4  
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    The image already shown are for what are called overhead cams.

    Another method used was the push-rod rocker-arm method.

    Here's an animated gif I worked out to show how this works for a four cylinder engine. (it's based on a 1982 Corolla engine) Note how the timing of each piston is worked so that each piston is on a different stroke.



    Not shown here is that the cam shaft and crankshaft are connected by a belt/chain drive that keeps the valves and pistons timed correctly.
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    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by Janus View Post
    The image already shown are for what are called overhead cams.

    Another method used was the push-rod rocker-arm method.

    Here's an animated gif I worked out to show how this works for a four cylinder engine. (it's based on a 1982 Corolla engine) Note how the timing of each piston is worked so that each piston is on a different stroke.



    Not shown here is that the cam shaft and crankshaft are connected by a belt/chain drive that keeps the valves and pistons timed correctly.
    Cool animation. Most modern engines use 4 valves per cylinder and that's very difficult for the push-rod rocker-arm method. But the principal of the cam shaft is depicted quite well.
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    Forum Professor jrmonroe's Avatar
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    Nice video, Janus.

    The four strokes in their order — intake, compression, power, and exhaust.

    A key point is that the crankshaft drives the camshaft, but due to the gearing, at half the speed of the crankshaft. The crankshaft make two turns to turn the camshaft once.

    So, the intake valve opens the first time the piston drops (that is, the intake stroke), but it remains closed the second time it drops because the burning gases need to push the piston down (that is, the power stroke).

    Likewise, the first time the piston rises, the exhaust valve remains closed in order to compress the air/fuel mixture (that is, the compression stroke), but the second time the piston rises, the exhaust valve opens to let out the burnt gases (that is, the exhaust stroke).

    The reason why the valves open inward is because the compression and power strokes require that the gases be trapped in the cylinder in order to interact with the piston — the piston pushing against the gases (in the compression stroke), or the gases pushing against the piston (in the power stroke). Sure, other kinds of valves work (such as sliding valves and rotating valves), but almost all engines use the kind of valves (called "poppet valves") that open inward.

    One last thought — the air-fuel mixture burns, and even though it burns quickly, it does not explode. Explosions produce very different pressures dynamics that will quickly ruin an engine, so the engine's operation is arranged so these explosions (also called "detonations" and "pinging") do not occur.
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    All good stuff here! Especially like the animation. It's a long process of learning to understand and picture the various events occurring within I.C. engines. Once all those basics are absorbed, the deeper Engineering concerns are absolutely taunting, sometimes!

    Examples: Chrysler redesigned their basic small V-8 to compete with Ford's new lightweight V-8, around the mid-to late '60s, and encountered unpredicted weaknesses in the structure of their cylinder heads. This resulted in the impossibility of sealing them using the low-cost embossed "beaded steel" head gasket design. I happened to work in the Victor Gasket Development Lab then, and experienced first-hand the frustrations of a major supplier to an even more major manufacturer. Old John Kratochvil and his group changed every conceivable facet of the beading process, this requiring new die sets each time, to no avail; the damn things leaked collant water to the outside of the engine, visibly, under static no-run conditions. After many months, Chrysler relented and agreed to use composite steel-cored laminate gaskets. Then they bought them from Victor's competitor! Ford, incidentally, perhaps with more foresight, chose similar design gaskets from the beginning for their small block, which began life as a 221 cu. in. engine in about 1961, finally reaching the apex of use in 1998 in Explorer vehicles, by then called "5.0 Liter". After 35+ years, and production of probably several millions of these engines, their "pushrod" design was finally retired in deference to overhead camshaft design. Virtually everyone has heard of the Ford 302! I liked them! The best were those offered in Mustangs as 5.0L HO (High Output), these having nice little extras such as roller camshafts and roller rocker arms (in Cobras). jocular
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