Elistan

Acording to one document I read, the S2000s intake valves are 36mm, while the exhaust are 31mm. Why is that? Initially I would have thought that the exhaust would be bigger since there's so much more air to move because of the combustion. But then I also though that maybe the exhaust is smaller because it has a lot of pressure going for it to help move the air out.

Anybody know for sure?

Slows2k

The intake valves are larger because the air and fuel volume taken in by the engine is greater than the exhaust volume.

Elistan

How can that be? You take in air and fuel. You ignite them and suddenly you have much more gas - hence the pressure that drives the piston down. Right?

Slows2k

Air and fuel take up more space than just air.

The Reverend

Conservation of mass. What goes in = what comes out. BUT, what goes out is hot. Therefore, although the mass remains the same, the volume goes UP. That increase in volume, as Elistan said, is what moves your car.

S2oooNvegas

lil help for ya here. check this, theres so much to this that id need to type a book. ill give you a tidbit. you have to think on a micro level. atmoized fuel and nitrogen and oxygen are headed down your intake port pretty quick. well, when they get to the chamber the intake valve closes and they are stuck in there. well, nitrogen (76% of the air you breathe) is at a high content in there, and cannot be burnt. all it does is expand very rapidly, the more heat the more expansion. THIS, is what pushes the piston down, the piston has to get the hell out of the way of the expanding gas or be destroyed. well, when that mixture is compressed enough, its volatility goes up, and all it needs is a baby spark to get a chain reaction going. well, the burn uses the oxygen to fuel the flame front and burns up the hydrocarbons as it goes. all you have left is some carbon monoxide, (partially burnt hydrocarbons) and some Co2 (byproduct of combustion) and water. well, that mixture thats left after the burn, is a much smaller batch then what came in. so less area is needed to get it out of the chamber. like i said, there is alot more to this, such as cam overlap, port velocity, and a whole lot of VE talk. but i wont go there. this should help you understand, or at least make you do some research.
laters. dave

RACER

The piston is forced down during the third stroke. #1 Intake, #2 Compression, #3 Spark and #4 Exhaust.

The air and fuel are sucked in as the the piston is moving downward with the intake valve open. The piston is what sucks in the air and fuel. By the time the intake valve closes, the piston should be in the downward position and ready for combustion.

steven975

also the exhaust gasses are "pumped" out with positive pressure (the piston going up) while the intake gasses fill the void of a low pressure area inside the cylinder.

no_really

OK, intake valves need to be larger than exhaust valves because you are using them to get as much air into the combustion chamber as possible, in what becomes an increasingly narrow window of time. As the engine increases rpms, the time the valves spend open decreases. Since engine output depends on the amount of oxygen in the combustion chamber, you would like to make it as easy as possible for air to enter the chamber. Exhaust will leave on its own, due to the fact that it is expanding gasses, while intake air needs to be drawn in using only atmospheric pressure as a motivator. With FI engines, the same rules apply, where getting air in is still harder than getting the exhaust out, so you still want bigger intake valves.

The air-fuel mixture doesn't change mass, but it surely changes volume when ignited, which is what pushes the piston down. Part of the increase in volume comes from the gasses created by the oxidation of the fuel, and part from the heat generated by the oxidation. After combustion, the piston is pushed downward, until at some point, depending on the cam profile, the exhaust valve opens and bleeds off the exhaust gasses. As the exhaust valve comes open, the exhaust gasses continue to expand, flowing out the opening. The crank completes a revolution, and the piston moves upward, pushing exhaust out of the chamber, driven by momentum and/or another combustion event, depending on the engine design. The intake valve typically opens before the exhaust valve is fully closed, so the momentum of the escaping exhaust gasses lowers the combustion chamber pressure and pulls air into the chamber though the intake port. The piston moving downward continues to draw air into the cylinder, and even once the piston starts moving back up, air continues to flow into the chamber, due to momentum. The intake valve closes sometime before ignition (depending on cam profile), which occurs a few degrees after the piston begins its downward motion once again.

With direct port fuel injection, getting the fuel into the combustion chamber doesn't present much of a problem, compared to the difficulties of moving large volumes of air through a convoluted intake tract. An engine is essentially an air pump, and the more air you can get through it, the more power you will make. You can always get bigger injectors, but you cannot as easily increase airflow. Heads that flow poorly will strangle output, even with forced induction.

mxt_77

Oooh... this is a good one.

I always assumed it was because the exhaust gasses were at a higher pressure, therefore they were more anxious to get out of the cylinder. Add that to the improved scavenging effect of exhaust vs. intake.

However, I'd like to hear the answer from someone that has lots of time studying fluid dynamics and the other appropriate courses.