The true reason In/Ex valves are different sizes:

[RogerH]

Hi Folks,

              lots of good info to read in those posts (except the first post)

My simply view of the issue is that the inlet charge has only the depression of the cylinder to draw it in (ignoring supercharging etc etc). No positive push.

Whereas the exhaust has the upward moving (plus pressure) to get rid of the gases.

You need as much inlet charge as possible. Any restriction will stifle the performance.

 

Roger

[NonMember]

On 12/07/2019 at 18:36, ShaunW said:

The mass of the gas may be the same but the volume is greatly increased. That's how it works.

Err... think again. Really.

The volume of the exhaust gas, at the time that it matters, is the same as (or probably less than) the volume of the inlet gas. Why? Because its inside the cylinder. It can't expand until it's in the exhaust pipe, beyond the valve. It's the pressure that's greatly increased.

[Clive]

PV=nRT

Says it all

[ShaunW]

14 hours ago, NonMember said:

Err... think again. Really.

The volume of the exhaust gas, at the time that it matters, is the same as (or probably less than) the volume of the inlet gas. Why? Because its inside the cylinder. It can't expand until it's in the exhaust pipe, beyond the valve. It's the pressure that's greatly increased.

Yes you're right. The pressure not the volume has increased. Although we're talking about exhaust value size so "at the time it matters" (ie when the exhaust valve is living up to it's name) the volume is increasing rapidly to a volume significantly greater than that of the inlet mixture.  How much greater is something Clive knows seeing as he's the one guy who looks like he concentrated at school. 😊

But tbf we both know how an engine works so we're probably both guilty of more pendantry than actual illumination on the subject.  ☺️

[Clive]

The whole thing will be VERY dynamic. Yes, as the valve opens the gas will expand, temperature will drop, pressure reduce. And changes will continue all along the exhaust. Just think how much cooler the tailpipe is compared to the manifold, that won't be entirely due to heat loss. 

But yes, we all have the right idea. Well, most of us, and unless it is our job (an old friend now works at Williams doing just this sort of stuff, but no point in me asking as I won't follow after 2 mins) the above discussion works as it is the basic principle. Good enough for me

(And the gas equation is one that has stuck for many many years...)

[Colin Lindsay]

Just a query while we're on the subject: if the whole idea of the exhaust system is to get burned gases out of the engine as quickly as possible - and given that the tailpipe is a considerable distance from the engine - has anyone ever come up with a kind of suction device to actively suck exhaust gases away? It's the sort of thing you'd see in old ads in 1950s / 60s motoring magazines so was just wondering...

[NonMember]

In effect that's what a tubular manifold and straight-through system is. The flow dynamics mean that, under the right operating conditions (i.e. revs), the exhaust valves open onto a lower-than-atmospheric pressure. This all relies on getting decent gas flow velocity, though, so fitting a really big bore exhaust kills the benefit.