Throttle Positon Rule Change and Plenums

[BeaverGuy]

I decided to start a new thread because this really doesn't belong in the other one.

First off if you want to find info about the effect of plenum size on restricted naturally aspirated motors you can read Catching Breath in Issue 2 Volume 1 of Race Engine Technology. In short Gordon Blair set up a series of simulations to see what variables affected power output the most. The simulations were with a 1L sport bike motor, and for the plenum variations it used a 27.5mm diameter restrictor. Increasing the plenum from 10L to 16L netted an ~3% increase in steady state peak power. Not great but considering that it would be akin to the typical FSAE engine increasing plenum sizes from 6L to 9.6L when we currently see maybe a 4L max it would certainly increase power numbers. We don't run plenums that large now because throttle response would be poor with that large a throttled volume, with ITBs it would become a packaging issue only.

Okay, I made a simple spread sheet that shows the transient airflow for an engine with a given VE/RPM relationship assuming a pressure build up in the plenum. Other key inputs are plenum volume, initial plenum pressure, displacement, and air inflow rate. It is located here. (http://home.comcast.net/%7Ejoshuagillett/airflow.xls)

Assumptions:
T (temp) is constant
VE is directly proportional to Plenum Pressure.
72 g/s of air = 95 HP, I used this to give an idea of what transient power might be.
Constant supply of air to plenum at 72 g/s, I can see this being done with a couple different types of compressor schemes. I didn't try to make this applicable to NA engines because I don't have the time and an engine simulation would give a better idea with less time input.

Eqns:

Outflow=RPM/60*displacement/2*VE*P/Pref*rho

PV=nRT and variations of that equation.
delta n=(outflow -inflow)/M *(delta t)

HP=Outflow/inflow*95

Other notes:

Outflow is air going from the plenum to the engine. Inflow is air from the atmosphere to the plenum.

The VE curve I supplied was corrected back to a plenum pressure of 1 atm to simplify mass calculations. Also, this curve is for an NA motor that was setup to make peak torque in the 7000-9000 RPM range so it would vary from what a turbo application would warrant. The places where VE is 1 are points tha I didn't have data for.

I used a fixed value for the density of air this is taken care of by the fact that any time air masses are calculated the plenum pressure is divided by the reference pressure.

[BeaverGuy]

I decided to start a new thread because this really doesn't belong in the other one.

First off if you want to find info about the effect of plenum size on restricted naturally aspirated motors you can read Catching Breath in Issue 2 Volume 1 of Race Engine Technology. In short Gordon Blair set up a series of simulations to see what variables affected power output the most. The simulations were with a 1L sport bike motor, and for the plenum variations it used a 27.5mm diameter restrictor. Increasing the plenum from 10L to 16L netted an ~3% increase in steady state peak power. Not great but considering that it would be akin to the typical FSAE engine increasing plenum sizes from 6L to 9.6L when we currently see maybe a 4L max it would certainly increase power numbers. We don't run plenums that large now because throttle response would be poor with that large a throttled volume, with ITBs it would become a packaging issue only.

Okay, I made a simple spread sheet that shows the transient airflow for an engine with a given VE/RPM relationship assuming a pressure build up in the plenum. Other key inputs are plenum volume, initial plenum pressure, displacement, and air inflow rate. It is located here. (http://home.comcast.net/%7Ejoshuagillett/airflow.xls)

Assumptions:
T (temp) is constant
VE is directly proportional to Plenum Pressure.
72 g/s of air = 95 HP, I used this to give an idea of what transient power might be.
Constant supply of air to plenum at 72 g/s, I can see this being done with a couple different types of compressor schemes. I didn't try to make this applicable to NA engines because I don't have the time and an engine simulation would give a better idea with less time input.

Eqns:

Outflow=RPM/60*displacement/2*VE*P/Pref*rho

PV=nRT and variations of that equation.
delta n=(outflow -inflow)/M *(delta t)

HP=Outflow/inflow*95

Other notes:

Outflow is air going from the plenum to the engine. Inflow is air from the atmosphere to the plenum.

The VE curve I supplied was corrected back to a plenum pressure of 1 atm to simplify mass calculations. Also, this curve is for an NA motor that was setup to make peak torque in the 7000-9000 RPM range so it would vary from what a turbo application would warrant. The places where VE is 1 are points tha I didn't have data for.

I used a fixed value for the density of air this is taken care of by the fact that any time air masses are calculated the plenum pressure is divided by the reference pressure.

[Bill Kunst]

Josh-

Thanks for putting some time into this. Although you didn't say it outright, is this with ITB setup on turbo w/ 15psi boost at a steady state 8000rpm? Also, as this shows, is it .63 seconds to max hp the engines would potentially have now? Could you produce 130hp for 3.5 secs and how much volume would that take?

What would happen to power output if the turbo cars were only allowed a single tb before the plenum? Is it as I would assume, virtually the same as the engine under current rules?

I appreciate your work and hope that my questions are also the ones going through everyones heads. These, I believe, are the ones that need to be answered before this rule change discussion goes forward.

Once again, thanks-
Bill

[BeaverGuy]

The default value in the spreadsheet is 1.5 atm so about 7.5 psi boost. Yes, the spreadsheet sits at a constant RPM which is default at 8000 RPM. Though all the input values can be changed.

The spreadsheet was meant to represent any situation where the plenum or another body could act as an accumulator. This would mean any setup that allowed a throttling device to be placed any where other than some short distance from the outlet of a compressor or in front of the restictor.

With the magical 50 gallon plenum you could make 130+ HP for over 3 seconds using the "dumb" mechanism of a manual throttle. If you were to implement an electronic throttle, which as the rules commitee has stated is allowed if it doesn't interfere with the operation of the manually controlled throttle, using feedback control to regulate air flow, you could milk 105 HP out of a 10 liter plenum for ~1.2 seconds with an initial plenum pressue of 2 atm.

Frankly, as far location of the throttle goes I think if there was to be a change it would have to be located imediately after the compressor on in an NA situation resrictor for there to be no real change in power output. I might be missing something here though.

[Dan G]

Yes, it seems like after all the debate and discussion, the simplest fix for all of this is to require a single TB (post compressor) with a specified distance/volume between it and the restrictor or turbo.

The end result would be intake systems pretty similar to what we have now, and there should be no significant change on peak hp levels.

[Bill Kunst]

And this is what I have been trying to get at. The reason that I had proposed testing different setups was to demonstrate to the committee that there is a simple answer that will not affect hp, positively or negatively.

Thanks again,
Bill

[Dan G]

Semi-related, does anyone have pictures of A&M's supercharged powertrain? I didn't even hear about it until late Saturday and never got any pictures or the chance to see it in person. I assume it was a centrifugal blower, correct? I guess even if a team wanted to use a roots-type, they'd still have such a custom intake system that a post compressor TB wouldn't be much of an issue.

[iolair]

TB before plenum vs ITB after plenum.

IMO, with the TB placed in front of the plenum and/or airbox little is gained or lost compared to a ITB setup as it relates to volume size adjustment because during a normal run the throttle is open and closed several times and the volume is de-pressurized each time the throttle is closed. OTOH, when ITB's are shut, the volume and/or airbox is re-pressurized either N/A or from a compressor, thus the plenum volume and/or airbox assembly may be huge in comparison without the fear of loss of throttle/transient response. With ITB's, a relatively large volume may be implemented, resulting in a slower pressure decay rate vs rpm, etc, etc.

Neat air box assembly:

http://www.pbase.com/image/61575171.jpg

http://www.pbase.com/image/61575170.jpg

John