inlet ports

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

Neal 19 16v wrote:

oh yeah. i dont see why the gas speed at the valve will be different for different bore:stroke ratios.

Will two 2L engines identical in every way except the bore and stroke have different gas speed at the valve?

yes, even at the same rpm.

when the piston travels down towards BDC, the air "on top of it" (this includes the ports) is subjected to a pressure drop (obviously), hence the incoming charge travels through the head into the chamber/bore. if the piston moves faster, so does the air...including the air coming through the head/valve.
think about this...the air travelling through the head at say 4000rpm is slower than it is at say 8000rpm...all because the piston speed is higher.

and like i said, piston speed is greater with a longer stroke therefore the gas speed at a given rpm is greater.

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

theres no arguments chris, gay people dont argue!!

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

lol ok fair enough.

But stans right about the gas speed and his last explaination summed it up nicely.

If your struggling to picture it, suck through a straw (small bore), air passes through it quicker than sucking a hose pipe (big bore), both hav same capactity (your lungs). The gas speed in the straws quicker.

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

Very argumentative arent you Chris, any kind of debate or discussion and to you its automatically an 'argument', ive noticed this before Razz

i can see what youre saying but the straw not a very good analogy.

of course i can see how the gas speed will be higher through a smaller port for a given mass flow, but in terms of a fixed size valve, wont the gas speed through it be the same for 2 cylinders of the same capacity regardless of the bore:stroke ratio?

Still not arguing BTW!

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

stan wrote:

when the piston travels down towards BDC, the air "on top of it" (this includes the ports) is subjected to a pressure drop (obviously), hence the incoming charge travels through the head into the chamber/bore. if the piston moves faster, so does the air...including the air coming through the head/valve.
think about this...the air travelling through the head at say 4000rpm is slower than it is at say 8000rpm...all because the piston speed is higher.
and like i said, piston speed is greater with a longer stroke therefore the gas speed at a given rpm is greater.

are you not reading my posts??!!! im progressivly getting simpler as i post more!!

its as simple as this: longer stroke=faster piston=faster air movement

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

Yes i have been reading your posts, im not a moron.

ok for fixed capacity, longer stroke = smaller bore yes?

so, even though the piston is travelling faster, the piston is smaller... therefore in a unit time, the volume of air drawn in is the same by definition, yes?

therefore, the volume of air travelling through a given sized valve will be the same? regardless of the size of the port (where yes the gas speed will be different) it went through to get to the valve?

If its just volumetric efficiency which is at the bottom of it all, fine

I am not disputing what youre saying, you obviously know better than i do!

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

stan wrote:

its as simple as this: longer stroke=faster piston=faster air movement

so if i have a 1mm diameter piston travelling a 10m/s, it will suck in more air than a 1000mm diameter piston travelling at 9m/s?

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

furthermore, if our 1mm cylinder and 1000mm cylinder both have the same size entry hole, the airflow will be higher in to the 1mm cylinder because its little 1mm piston is moving slightly faster? The airflow speed has nothing at all to do with the volume of air trying to get through the hole ?

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

sorry mate, didnt mean you were being a moron!

yes thats about it.

but as far as volumetric flow at a given engine speed for the same size valve (for the two different bore/stroke), it wont be the same in practice, since volumetric efficiency will be different due to different gas speed. but in theory yes it would be the same

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

right, volumetric efficiency it is then Smile

God i'm pissed off tonight, gotta get out of this shit hole ASAP

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

Neal 19 16v wrote:

so if i have a 1mm diameter piston travelling a 10m/s, it will suck in more air than a 1000mm diameter piston travelling at 9m/s?

no the 1000mm bore would suck in more air...it has a greater volume. there isnt much difference in speeds for the extreme difference in goemetry, so it isnt realistic

Neal 19 16v wrote:

furthermore, if our 1mm cylinder and 1000mm cylinder both have the same size entry hole, the airflow will be higher in to the 1mm cylinder because its little 1mm piston is moving slightly faster? The airflow speed has nothing at all to do with the volume of air trying to get through the hole ?

these two cases are too extreme!!!!

Forum Moderator Joined: 18 Feb 2004 Posts: 7432

stan wrote:

Neal 19 16v wrote:

so if i have a 1mm diameter piston travelling a 10m/s, it will suck in more air than a 1000mm diameter piston travelling at 9m/s?

no the 1000mm bore would suck in more air...it has a greater volume. there isnt much difference in speeds for the extreme difference in goemetry, so it isnt realistic

This was a rhetorical question, exception that proves the rule sort of thing.

stan wrote:

Neal 19 16v wrote:

furthermore, if our 1mm cylinder and 1000mm cylinder both have the same size entry hole, the airflow will be higher in to the 1mm cylinder because its little 1mm piston is moving slightly faster? The airflow speed has nothing at all to do with the volume of air trying to get through the hole ?

these two cases are too extreme!!!!

Yeah i was being facetious here

Gay Rights Activist Joined: 07 Apr 2004 Posts: 1268

LOL...its been a long day!!

and its going to be a longer night!!!

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

fine I'll be quiet Neal... slams door, off in huff.