A naturally aspirated engine intake depends on a column of air at approximately 14.75 inches PSI rushing in to fill a void created by the piston being pulled down in the cylinder. The exhaust empties into the relative void of the exhaust system while also being pushed out by the piston. A side note: The faster an engine spins the more esoteric cam timing becomes I`ll stop here.
Exhaust and Intake Valve Diameters
- Thread starter MolaKule
- Start date
Because vacuum hurts volumetric efficiency.
It's pretty common in pumping systems that we are more afraid of a vacuum than we are of backpressure even if the pumping losses are the same.
It's pretty common in pumping systems that we are more afraid of a vacuum than we are of backpressure even if the pumping losses are the same.
Let’s just say that the biggest camshaft I bought was 306/312 @ .050 .510 lobe lift, 113LSA.
No “esoteric” involved. If the engine makes more power with a bigger cam, that’s the way you go. It’s all denial and error.
At EVO, (exhaust valve opening) the pressure in a typical normally aspirated cylinder might look like this:
Idle: 25+ PSI
Mid power 70+ Psi
Max power up to 250 PSI.
This helps explain why exhaust sound pressure levels increase as engine loads increase.
Idle: 25+ PSI
Mid power 70+ Psi
Max power up to 250 PSI.
This helps explain why exhaust sound pressure levels increase as engine loads increase.
I'm trying to understand how the cold air is denser and at the same time lower viscosity.
To add to the question the intake stroke sucks air through an air filter (if any) and a system of piping (the intake), if any. In addition to the air filter cars have resonator boxes before the filter to improve the air flow at lower speeds (below 3K rpm), however those boxes have narrower passages and make the path of the air to the cylinder longer. On the other hand there is no filter in the exhaust and its pipes are relatively straight.
Not sure if that matters in this case though.
May I also ask, why in 2-stroke engines the intake port is always smaller than the exhaust port? I'm guessing because the crank volume (below the piston) is much larger vs above the piston in the 4-stroke engines?
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MolaKule
Staff member
Smoluchowski's theory of Brownian motion<a href="https://en.wikipedia.org/wiki/Brownian_motion#cite_note-27"><span>[</span>27<span>]</span></a> starts from the same premise as that of Einstein and derives the same probability distribution ρ(x, t) for the displacement of a Brownian particle along the x in time t. He therefore gets the same expression for the mean squared displacement: E[(Δx)2]
![{\displaystyle \mathbb {E} {\left[(\Delta x)^{2}\right]}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/afe29c98b84f778ec79626e7257d545ab527e85e "{\displaystyle \mathbb {E} {\left[(\Delta x)^{2}\right]}}")
![{\displaystyle \mathbb {E} {\left[(\Delta x)^{2}\right]}=2Dt=t{\frac {32}{81}}{\frac {mu^{2}}{\pi \mu a}}=t{\frac {64}{27}}{\frac {{\frac {1}{2}}mu^{2}}{3\pi \mu a}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2fcf1b71e8974599297d1892f4edb74eee50438c "{\displaystyle \mathbb {E} {\left[(\Delta x)^{2}\right]}=2Dt=t{\frac {32}{81}}{\frac {mu^{2}}{\pi \mu a}}=t{\frac {64}{27}}{\frac {{\frac {1}{2}}mu^{2}}{3\pi \mu a}},}")
Each pipe, or conduit, air filter, or orifice that air has to flow through or around represents resistance to air flow which results in a decrease of the total pressure at the intake port.
The greater the pressure drop across an orifice, the greater the flow rate.
https://atlas-scientific.com/blog/r...7tUrV2pcg6dSSNJ4QmEuOec66DvMz9LwyiR0YOdeObfLV
"...As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:
Note that in many two-stroke engines that use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesn't simply flow right over the top of the piston and out the exhaust port...
If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston's connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can't hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn't going to last very long!"
https://science.howstuffworks.com/transport/engines-equipment/two-stroke.htm
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Thanks for the detailed explanation!
However, I still didn't get why the intake port in 2-stroke engine is always smaller that the exhaust one.
However, I still didn't get why the intake port in 2-stroke engine is always smaller that the exhaust one.
In 2-stroke engines when the piston bottoms out the intake port is always covered/closed (by the piston), the transfer ports and the exhaust port are open, as far as I know.
MolaKule
Staff member
Look at the chart again
The greater the pressure drop across an orifice, the greater the flow rate. Intake time is limited so you need a high flow rate with a slightly smaller port which increases the flow rate.
https://atlas-scientific.com/blog/r...7tUrV2pcg6dSSNJ4QmEuOec66DvMz9LwyiR0YOdeObfLV
and
https://www.supmeaauto.com/training/flow-rate-and-pressure
"As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel, as shown here:"
https://science.howstuffworks.com/transport/engines-equipment/two-stroke4.htm
In HSW, they are referring to THEIR very basic engine drawing on page 5 which is correct for that illustration. Your illustration is correct as well.
The combustion chamber in the cylinder head is only so big. It must accommodate both the diameter of the intake valve and the diameter of the exhaust valve So the engine designer must decide what the relative diameters should be, given the limited space.
Think of the ideal situation. The piston going down "sucks in" the intake air. The maximum pressure with which that intake air can enter is atmospheric pressure. However, exhaust gasses are being pushed out by the piston, and the pressure can be greater than the intake pressure. So, because the intake gas pressure is less, the diameter is bigger to allow more gas to get in. And because it is bigger, the exhaust must be smaller.
Think of the ideal situation. The piston going down "sucks in" the intake air. The maximum pressure with which that intake air can enter is atmospheric pressure. However, exhaust gasses are being pushed out by the piston, and the pressure can be greater than the intake pressure. So, because the intake gas pressure is less, the diameter is bigger to allow more gas to get in. And because it is bigger, the exhaust must be smaller.
MolaKule
Staff member
Here is a course in Fluid Mechanics for those wanting to increase their knowledge of the subject:
https://www.udemy.com/course/the-co...04e815a9e8f5e4babe624acd&couponCode=PMNVD1525
https://www.udemy.com/course/the-co...04e815a9e8f5e4babe624acd&couponCode=PMNVD1525
