Effects of NA vs. Turbine Plenums on Turbocharged Cars

[rolls]

I have a question about using NA (naturally aspirated) intake plenums on turbo charged cars and the detrimental effects it may have.

Here is a standard NA plenum for an rb30 engine

Here is a turbo plenum for an rb30 engine

The difference is with the turbo one the plenum (large chamber) is above the runners, apparently this causes the air to break on the wall and disperse evenly into the cylinders, causing air mixtures to be static between them all.

If using the first NA plenum in a turbo charged application would it cause the pressure it be higher at the middle two cylinders, hence they get more air and in turn run leaner (hotter, more detonation prone) ?

The second question is some people cut and shut the turbo plenums to be front facing like this (originally entered from the centre like the first two pictures, now enters from one end).

In this case would the air create a high pressure zone at the rear of the plenum causing the rear cylinders to get more air and run leaner?

Note all these questions are assuming a turbo charged application, eg air is being forced into the plenums, not under vacuum like in a naturally aspirated engine.

 

[yus310]

Theoretically for the NA plenum, the air distribution should be uniform throughout all the ends of the plenum, given the flow rate through the plenum is uniform, steady state. If the flow rate is fluctuating then, it is possible for the air to run leaner, that is dependent on your conditions, etc. I would find the second design less optimal, in comparison to first, as theoretically the suction caused by the engine, should not allow optimal air flow through the rear, flow would dominate through the beginning.


You can prove this through pure experiments, modeling, but it should make sense and help out.'

Best.

 

[rolls]

There is no suction in a turbo charged engine though, the air is being forced into the cylinders, this changes things.

 

[cjl]

There is suction in a turbo engine. Suction is simply the flow of fluid from a higher pressure region to a lower pressure region. The turbocharger makes the higher pressure region even more pressurized, but it doesn't change the overall flow behavior (other than the fact that there is a substantially larger pressure differential driving the flow). This higher pressure differential does mean that the flow velocity will be higher, but there shouldn't be anything fundamentally different about the flow behavior.

 

[yus310]

there is always suction in the engine, via a pressure difference, simply from the combustion of gases, the increase in volume gas, and release of gases through the exhaust, etc.

Simply put, pressure differential drives the flow, and thus dictates the flow velocity! If you are similar with navies stokes, this makes sense, but intutitively, if you have a higher pressure differential, this means faster flow right? I'm currently designing a venturi with a plenum and runner, and I see the velocity being greatly affected by geometry (as you said), heat, vibration from the engine, which dictates air quality, etc.

Again it all comes down to how fast the engine creates combustion, then how the fast the gases exhaust, and these effects on the plenum and air quality.. these can be done experimentally (by yourself) or equally numerically, theoretically..

Best

 

[cjl]

In general, yes, a higher pressure differential means faster flow. There are limiting cases (choked flow), but I don't believe they apply here.