Therefore, the pressure in the combustion chamber remains below the atmospheric pressure, while the charge is still moving in the direction of the motion of the piston with high velocity. ![]() The entire charge cannot enter the cylinder even when the piston reaches the end of its downward stroke because the opening of the inlet valve is small. ![]() This air-fuel mixture (also known as charge) has both mass and motion. This is because the downward movement of the piston creates the vacuum (or negative pressure) in the cylinder and the air-fuel mixture gets filled in the empty space. In suction stroke, the air-fuel mixture or charge gets sucked into the cylinder very rapidly. when the piston starts to move up the cylinder in the compression stroke. It closes after quite a few degrees of piston reaching the BDC, i.e. The inlet valve usually opens few degrees before the piston reaches TDC in its exhaust stroke. Engine valve timing is the most critical process of IC engines. The timing of the opening & closing of valves is specified in degrees corresponding to the position of engine’s pistons. The engine’s camshaft opens and closes the valves at a specific interval. An automotive engine uses valves for it’s ‘breathing’ (inhale/exhale) process. The engine valves are just like the human nose. Engine designers take a lot into account when making all of these things happen so failure does not occur.Firstly, read here about how the engine valves open & close. Needless to say, valve timing is a lot more complex than it looks on the outside. Hopefully not confusing the issue, I will tell you that the intake valve remains open for a short time during the beginning of the compression cycle, as well as the exhaust valve opening before the piston gets to the bottom of the combustion cycle, but specifics of why this happens is probably best left for another writ. The explosion wouldn't be so good since there would still be ports open when the spark plug fires making poor compression?įor almost the entire cycles of compression and combustion, intake and exhaust valves are all closed, allowing for a complete sealing of the combustion area and helping to absorb all of the power which is available. If a timing belt were to slip or break while the engine is running, catastrophic effects will occur, causing major engine damage (usually requiring complete engine replacement, but at a minimum can "just" require head work or replacement). These types of motors are referred to interference motors. Also, this doesn't mean that valves and pistons don't share the same space, they just do it during different times of the cycles. valve open/close speed.) The valve doesn't just open completely all at once, it gradually opens as it follows the cam lobe profile. When I say the intake valve follows the piston, it opens slowly enough for the piston to be out of the way before any contact happens. When I say the "follows" the piston, or visa-versa, I mean to say as the piston comes up, the exhaust valve closes which does not allow for each of them to come into contact. The reason the piston and valve do not come into contact is because either the piston is following the exhaust valve, or the intake valve is following the piston. Top dead center (TDC) happens between two of these cycles: between Compression and Combustion, then again between Exhaust and Intake. ![]() First, remember that we are talking about four different engine cycles here: Intake Compression Ignition/Combustion Exhaust.
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