Nissan 370Z Tech Forums banner
1 - 3 of 3 Posts

·
Premium Member
Joined
·
4,328 Posts
Toykilla,

The cyclinders will intake a volume equal to their displacement, but they can only intake from the supply that is available. There is a given quantity of air in the plenum. Think in terms of moles of a gas, in this case that gas is air. If we increase plenum volume, the pressure drops and the moles of gas expand to fill the volume. For ease of computation lets assume that the cylinder displaces 1 liter. If we have 100 moles of air in a 1 liter volume of the plenum above the cylinder intake, then that cylinder will intake 100 moles of air. If we increase the volume of the plenum above that cylinder to 2 liters we get a pressure drop and the 100 moles of air expands to fill the 2 liter volume. The cylinder only displaces 1 liter and now that 1 liter volume only holds 50 moles of air. By sloping the plenum we get to keep consistant pressure rear to front and a more even distribution of air to all cylinders.

A good analogy is a person exercising at sea level and then at a high altitude . There is greater atmopheric pressure at sea level than at high altitudes. Regardless of which elevation you are at your lungs will pull in the same volume each time you breath, but because of the higher atmopsheric pressure at sea level there are more moles of oxygen per given volume. This is the reason people fatigue earlier at higher elevations
This is all true if we assume that air behaves as a perfect gas, inlet pressure and temperatures are constant, and there are no abdiatic processes in effect. Also we must disregard some degree of fluid dynamics. Oh and I almost forgot accoustical effects.

Carry On
Ronin Z
 

·
Premium Member
Joined
·
4,328 Posts
Ok let's run with this. Robin you raise some generaly valid engineering concerns, but you do not explain how they may be relevant to the plenum issue.
It's Ronin. The Japanese word for a samurai that has lost his master. Robin is an androgynous name given to both sexes.

All engineering studies and designs begin with the perfect or ideal model. Anomalies that may be particular to the design application are then introduced to model real-word situations.
Really? Man, we do it the hard way at work. We use the equations that most accurately match the real world results. Even if it means it takes a bank of computers weeks to generate an answer.

"Oh Deep Thought, tell us the answer to life the universe and everything!"

Let's begin by stipulating that we are dealing with an engine that is at normal operating tempurature. Inlet pressures and temps may vary over the long term (minutes, even seconds), but this variation is going to be miniscule. Baring anything catastrophic, we will not be experiencing dramatic atmopheric pressure and tempurature changes. Our intake air flow does not move through or across a greatly varying change in tempurature. In the short term, crankshaft revolution to revolution, intake pressure and tempurature changes will be nill. I would have to conclude, that for this application, concerns about inlet pressures and temps changes are withou merit.
Unless you're talking about a diesel genset running at steady state operation, I can't agree with much you just said.

The abdiatic process is definately present. For those unfamiliar, the abdiatic process is one where is one where the system neither gains, nor losses heat as work is done. There IS heat gain and loss as as the engine goes through it combustion procces. Once the engine reaches operating tempurature thr combustion process and cooling systems allow the engine to reach and an equilibrium tempurature, i.e nornal operating tempurature. This results in a constant tempurature in and around the plenum. As a result of this, the ideal gas equation (P= nRT/V) applies without tempurature becoming a consideration. Temperature is constant, so as Volume increases, Pressure drops and the moles of air spread out to fill the increased volume.
And what about when we add those two pretty little words: Forced Induction? Hm?

Fluid dymanics will be applicable to any disturbances in the air flow. I do not see how this would have any increased effects in one type of plenum over the other. Ideally both types of plenums would would be constructed with smooth interior walls and a minimum nunber of seams and protrusions into the plenum cavity.
Airflow is all about fluid dynamics. Ignoring their effects, is like ignoring the problem.

As a side note you're not supposed to have perfectly smooth interior plenum/intake runner walls. It increases the boundary layer of your laminar flow, thusly decreasing intake volume. I laugh whenever I see some home garage tuner who has polished the inside of intake runners.

Accustical effects! In this enviornment? Typical enviornments for acoutical considerations would be space and rocket flights during take-off and re-entry and in heavy artillery combat situations. In most of these cases we are concerned about mechanical vibrations that are caused by acoustic sound waves. There are no vibrations in the car that are strong enough to disturbe the air flow through the plenum.
So there are no accoustic resonance effects at play? None? You really need to stop an think about that one. Seriously.

I believe that stock plenum uses sound engineering principles and is a valid design. I would be skeptical of non-sloping designs
Agreed, when you take into account all of the non-performance related concerns, the stock plenum is an acceptable design. But that is the story with most factory inlet systems. Those poor engineers have to factor in cost/ease of production, ease of assembly(fitment), ease of repair/replacement(service), performance, emissions/economy, not to mention NVH issues... it's amazing the thing works as well as it does. That being said, the design does have slight flaws.

Regards,
Ronin Z
 

·
Premium Member
Joined
·
4,328 Posts
dm4,

Congrats, you win. I'm done.

:clap:

Regards

RZ
 
1 - 3 of 3 Posts
Top