Shape of intake plenum for Single Cylinder Engine

[anonymous]

Hello guys,

I am currently involved in the design of intake system for our single cylinder engine. I have calculated dimensions of runner based on Induction wave theory. But I am struggling to decide shape of plenum. I have read somewhere that volume of plenum equal to 1.5 times engine displacement works well most times. But I couldn't get anything on shape of plenum.

I have few shapes in mind such as Horizontal cylinder, Vertical Cylinder, Spherical shape with bellmouth shaped entry (i.e. restrictor outlet) and exit (i.e.runner inlet). I tried for transient simulation of intake system with cylindrical shaped plenums, in Ansys fluent. What I am observing is Vortices created during suction stroke of engine. So now, my approach is to try out different shapes and select the one with minimum creation of vortices. But is this logic correct? What I think is, there should be minimum restrictions to flow during suction stroke & less the vortices, more smooth flow, less the restriction.

Can anybody tell me about my logic, whether its correct or not? Or should I focus on something else in the results of transient simulation & choose it as design criterion?

Thanks in Advance. :o:o:o

[Mbirt]

I would suggest not worrying so much about exact plenum geometry and instead turning your analytic focus to plenum volume. Using an academic rule of thumb for plenum volume will not test favorably with design judges or your power curve--really digging into the matter will have you arrive at a plenum volume much, much larger than 750 cc.

However, common sense applied to the shape of the plenum to achieve a decent discharge coefficient to the intake runner will get you close on the shape aspect of the overall design. I always found the following RET article by Prof. Blair to be helpful: www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

[apalrd]

Yes, I'll go with what Birt said about plenum size. It's way more important to analyze the size of the plenum and runner geometry (length/diameter) than the shape, as long as you aren't too terrible in shape.


I once worked on a calibration project for a 5.7L V8 engine. I was working on airflow calibration and cam phasing among other things (not designing the intake), but we had issues when the plenums got bigger than 2.5x the engine displacement (for the 5.7L V8, this is 14L). With a plenum larger than that, throttle response was poor, especially when raising the manifold from the e.g. ~25 kPa idle at 600rpm to WOT. Engines like this is where the '1.5' rule of thumb comes from. Since the engine has induction events that all overlap, there is always a nearly continuous draw on the plenum and the pressure is very constant, even without filtering.


Our FSAE powertrain 2012-2014 had a 4.5L plenum, which is 10x the engine displacement. Simulation suggested diminishing returns after about 3.3L. Since there is a single draw on the plenum (one cylinder) for about 180 degrees, and then about 540 degrees of dead time, the plenum pressure will be drawn down significantly by the induction event, then slowly climb back up (possibly to barometric, even at part throttle!). This is true for engines with extremely odd firing orders (all singles, and some twins, or engines with multiple plenums or individual throttle bodies), but the single is bad (odd fire inline twins are arguably worse - The second cylinder gets vastly different air intake!).


tl;dr do your 0-d (spreadsheet) math first, then your 1-d simulation (WAVE or GT-POWER), before you do 3-d simulation. More dimensions will just confuse you early on.

[Jay Lawrence]

Just keep in mind the quite substantial vacuum that you can generate, and how that might be dangerous for certain shapes of intake. While the 'shape' may not be terribly important for power (within reason as noted above), it can be disastrous for structural integrity.