How does an engine “suck” in air? Like during the intake stroke it goes down which creates a low pressure area and the air rushes in. But how does all that work? How does the piston going down allow air to be pulled in? LOL please explain it as simple TY!
In: Engineering
If you’ve ever put your finger on the end of a syringe hole trying to pull the plunger back you can feel the force on your finger. That’s basically the exact same as the piston(plunger) pulling back and creating that suction force.
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Have you ever heard the saying “nature abhors a vacuum”? That’s because the laws of physics dictate that differences in pressure will always try to equalize.
So when the piston moves downward, the volume of the cylinder increases. Put another way, the space inside the cylinder becomes greater. If you were to seal the cylinder off and do this, the pressure inside the cylinder would drop considerably. But the cylinder isn’t sealed. The intake valve is open.
So as the piston moves downward, the space inside the cylinder increases, and air rushes in to fill it.
In a manner of speaking, the piston isn’t really “pulling” the air in. The piston is moving down, which creates more space. The air from outside actually pushes its way in.
An engine can only suck in air if there is fuel in the fuel system and the ignition was already turned, which a security system determines the ignition security and the fuel system may have at least two portions, an inflow and a injector. A dynamometer vehicle is directly hooked up to the fuel tank which is virtual for a dynamometer vehicle while a vehicle deployed in the field is used with an actual fuel tank. What causes the engine to aspirate air when this is turned is the motion of the crankshaft which drives up to three separate piston per cylinder with internal combustion, three piston in a W12 engine with high torque or mainly just one cylinder even in an I16 engine. The piston motion implied by the crankshaft, which the speed of crankshaft is determined by the throttle control and throttle wire thus the speed of piston oscillation and also implied is the inflow of air from an availible feed of pure oxygen or another oxidant which flows into the internal combustion section before impinging on the piston and also the powertrain which after the piston is the powertrain and this provides the gearing of the system which may go as low as one gear or three in a motor trike or five or six in a standard vehicle, which a labor time on a standard vehicle internal combustion engine is 30 days or less mainly due to the effect of the powertrain and is only 5 days for a 1 gear engine like in a gocart.
This is a fundamental rule of physics: Nature abhors a gradient.Wherever a difference exists, nature will try to eliminate that difference.
When the piston is on the intake stroke, the volume of the air-side of the piston is increasing. Air will try to fill the new volume, but can’t since there isn’t enough air available in the piston, so the pressure decreases. The pressure of the atmosphere outside the engine is now higher than the piston interior, and the weight of the atmosphere itself forces air into the piston.
The intake valve opens due to the camshaft rotation causing the lob to push down on the spring. When the intake valve opens the piston goes down at the same time drawing in the air due to vacuum. This creates what is know as manifold vacuum. Depending on how far open the throttle valve is will allow a certain amount of air for the “vacuum”, know as mass air flow. Depending on these values the engine control module feeds a signal to the injectors to deliver a certain amount of fuel, this in combination with correct spark timing causes combustion.
Edit: source; automotive tech for 20+ years
In a 4-stroke IC engine, remember: “suck” (intake), “squeeze” (compression), “bang” (ignition), “blow” (exhaust).
”Suck”: Intake valves open and piston moves down, creating negative pressure in the cylinder, so air rushes in to fill the vacuum (fuel is either injected or mixed with intake air in a carburetor during this stroke).
”Squeeze”: Intake valves close/piston moves up and compresses the air/fuel mix.
“Bang”: When the piston reaches the end of its travel at the top (top dead center) and maximal compression is achieved, the spark is triggered, the air/fuel ignites, and the rapid increase in pressure due to expanding gases forces the piston down, producing the engine’s power output.
“Blow”: When the piston reaches the bottom of its travel, the exhaust valves open and the piston travels upwards, ejecting the exhaust gases from the cylinder.
(Then, the exhaust valves close, the intake valves open, and the cycle repeats at ”suck”.)
Some people call 4-stroke ICEs “suck, squeeze, bang, blow” for this reason. Piston travels “down” (not necessarily down from the perspective of a person standing on the street, but away from the head of the cylinder); the ensuing vacuum draws air into the cylinder. In a carbeurated engine, this also draws fuel into the air stream in the carburetor, while in a fuel injection engine, the fuel injector meters in a precise amount of fuel at a specific time, right into the cylinder.
This is for a naturally aspirated engine. In a turbo or supercharged engine, there is a blower (just a high volume air compressor, really) that forces more air into the cylinder than what would naturally be drawn in (asperated).
Fun experiment you can do at home! Take a 2L soda bottle and cut the bottom off and take the cap off. Put the bottle in a bucket of water until it the water almost reaches the top. As you pull the bottle out of the water air will be sucked into the bottle through the cap. The water is basically the piston but in this case the bottle moves and not the water.
There are at least one each intake and exhaust valve per cylinder. On the intake stroke the intake valve is open and the piston goes down, drawing air fuel mixture in. The following up stroke both valves are shut and the piston comes up, compressing the mixture. When the piston is near the top the mixture is ignited, driving the piston down and making power. The exhaust valve is open as the piston drives the exhaust gas out.
Suck, Squeeze, Bang, Fart.
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