Supercharger and plenum volume 2

[red92]

Question: can power increase with an increase in plenum size only.

With a MAF throttle body and supercharger inlet the same.....only change is a bigger plenum.

An engine uses pounds (weight)/cubic feet (volume) of air to burn fuel. Is there an increase in velocity with a larger plenum as the supercharger sucking atmosphere psi
or is the increased volume nil because the throttle body outlet is unchanged and the supercharger inlet is unchanged ?


is increasing the inlet size of the supercharger seem to the only way to gain power with an increase in plenum size ?

 

[patprimmer]

Your question is really to vague, as the answer depends on whether the original size is optimum for power.

Mostly a power gain can be had by increasing the plenum on a standard production manifold, as they often compromise plenum volume for engine packaging.

Positive displacement superchargers are not so sensitive to manifold efficiency as N/A or centrifugal type superchargers, but they still improve with improved manifolds.

Just because big is better, does not mean, that even bigger is better still, as many things in engine design are compromises, and the optimum compromise is dependant on quite a few conflicting issues

Regards
pat

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[red92]

Yes, the question is vague, there are many variables
assuming the original size was optimum for power.

exploring fact based theory on expected flow dynamics after the throttle body and before the pulling supercharger, pressure drop, velocity, volume/desity....whether the end result would decrease power or increase knowing each engine is equal would be pointless ?

25% increase in plenum size being the only change on a production 5.4 liter eng. with a roots making 14psi.

A 25% larger plenum would not store air as needed for engine packing?

I had the idea to build my own plenum from aluminum.

 

[Marquis]

I am investingating this phenomenon right now on a test bed and using 1 D cylce sim code to verify.
I've already had various intake runner lengths fabricated upstream of the Lysholm Supercharger and intercoolers/plenums.

So far it has been seem that this supercharged engine isn't as sensitive to runner length variation as a Naturally aspirated engine. Im using the simulation to find out why.

Next step I'll be looking to see if enlargening the pleneums/collectors post intercoolers helps.
It MAY help a bit as there is evidence of cylinder to cylinder mal-distribution-alot due to the uneven firing (per bank) of the V8 and the exhaust manifolding- but perhaps SOME down to an inlet charge robbing effect?

 

[patprimmer]

Marquis

GregLowcock made some reference to the speed of sound being dependant on charge pressure or temperatur (I forget which)

As tuned length is dependant on waves traveling at the speed of sound, and as the speed of sound in a supercharged engine will vary a lot more than it does in a normally aspirated engine, I think you might be "pushing it uphill with a pointy stick" to use a crass euphanism.

Re charge robbing with adjacent cylinders fireing in non uniform sequence, I wonder if this could be corrected with changes to the individual runner entries, so as to try to compensate.

Regards
pat

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[GregLocock]

speed of sound = (gamma*R*T)^.5

gamma is 1.4 for air, less for fuel air mixture. R is a gas cosntant. T is absolute temperature.



Cheers

Greg Locock

 

[92strokedbird]

What if the blower is a roots pos disp blower and the intake is a plenum (no runners)if the size of the plenum is enlarged the heat of the air would be reduced since the air would not be compressed as much.Would this be a logical solution.

 

[Azmio]

Red92,
If you check the plenums for supercharged and turbocharged engines, the plenums are generally smaller if compared to the NA ones. One good reason is that the bigger the plenum is, the slower the response time during acceleration.
For the NA engines, the bigger the volume of the plenum, higher output can be generated from the same engine. However, we did notice that the response gets slower and slower.
For charged engine, since the plenum is pressurized, there is no clear need to enlarge the plenum. I would choose a plenum volume that is just enough to function as a 'reservoir' to each cylinder. Any further increase will slower the response time and lower the achievable plenum pressure.

AO

 

[patprimmer]

With a Roots blower, the air in the plenum will reach the same pressure no matter what the plenum size, but it will take longer to reach it, hence the slowed response.

This is because for each turn of the engine, it will take a certain volume of air, and the blower will pump a certain volume of air.

If the blower pumps twice as much volume as the engine uses, the pressure will build up until the density of air is doubled. Then as the engine takes it volume, it will be twice as dense as the air entering the supercharger, so mass in equals mass out, as it must unless you get a nuclear reaction going

Regards
pat pprimmer@acay.com.au
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[92strokedbird]

If you increase the size of your engine due to stroking then your stock plenum would be smaller than it should be especially if your blower is ported for more air flow and improved boost due to a smaller pulley.I believe that the optimum size of plenum for a supercharged engine is 80-90% of displacement.If the plenum size was larger then the air would not get as hot and the plenum would be able to hold more air for the engine to breathe from.

 

[Azmio]

Pat,
Good point, the plenum pressure will be the same at the end. Perhaps during the 190hours WOT test. However, I have never found a driver who drives at constant engine speed from Iwata to Tokyo, Paris to Rome or Boston to LA. Therefore, the plenum volume really makes a different when it comes to engine response during overtaking or stop and go.
Not to mention the increase in surface/volume ratio of the plenum in designing a big plenum volume with all the packaging clearance for a typical production car. This will further cause pressure loss to the pumped air from the compressor.
At the end, considering all the changes of energy state of the moving air, there is still some differences in the final average achievable pressure for the FTP or 10-15 cycles.
And yeah, I agree with you that there is no nuclear reaction involve.

 

[patprimmer]

Azmio

I didn't say the average pressure would be the same. I said there would be a delay in reaching the final pressure, which would eventually be the same.

The final pressure or equilibrium will be approached quite quickly in an engine with a Roots blower. It will normally take substantially longer with a turbo. That is why Roots blowers have been so popular with drag racers.

I have tried to keep my recent answers as general and simple as possible, as 92strokedbird show little knowledge of the subject and supporting science.

Re plenum size and charge temperature. I would expect a barely measurable reduction in charge temperature on final balance.

The larger volume will not build pressure, and therefore temperature quite so quickly, and the larger surface area will dissipate OR absorb heat slightly faster.

The type of vehicle and the use thereof is not mentioned, so I made no presumptions re packaging, nor temperature in the proximity of the outside of the plenum.

Also, so long as the throttle is wide open, as it often is in racing applications or power boats, or heavily laden trucks, the pressure will stabilise no matter what the speed, so long as the VEs of the engine and the supercharger stay relative to each other.

As we know very little about the engines in question, we don't know where they are in relation to optimum, so any comment is speculation.

92strokedbird

If the original plenum is well sized, an increase in displacement may well require a proportional increase in plenum.

If you increase the overdrive (OD) of the blower, you will increase boost pressure and temperature up to the point were the tips of the rotors exceed the speed of sound. At that point, the blower efficiency may start to lag behind the VE of the motor, and the charge density will fall of as the temperature increases disproportionately.

Charge temp is a lot more important on a petrol fuelled car than on alcohol or nitro methane, as the latter fuels cool the manifold to the point that further reductions, while increasing charge density, might Also decrease the vaporisation of the fuel to the point that power is lost. It's all a balancing act and varies from case to case.

Also a longer stroke will reduce the rpm of the engine, if all other things remain constant. Note, they usually don't remain constant, as the stroke to rod ratio varies surface area to volume varies, valve size and port size to displacement varies, etc etc.



Regards
pat pprimmer@acay.com.au
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[92strokedbird]

I am sorry that i did not provide enough info on the engine.The engine is a 3.8 Supercharged Ford,it uses a roots M-90 blower.The blower in its stock form flows about 500cfm at max boost levels,my ported blower flows 700cfm.The motor has been stroked to a 4.2l displacement,the major restrictions have been removed and the one that is left is the intake plenum.I was only going to open the inlet of the plenum then thought about making a modification to the plenum size.I was going to drop the plenum down into the lifter valley between the heads,then install a plate to help to keep the heat out of the plenum and coat the plenum with a heat coating to also aide with heat soak.I am not a automotive engineer so i decided to post here to see if this might help performance.If enlarging the plenum size would not be of benefit then i would abandon the idea.I also read somewhere that the optimal plenum size for a supercharged car is 80-90% of the cid of the engine.Thanksd for trying to explain this fully for me,all is appreciated.

 

[NeilRoshier]

Marquis could you please let us know if you find anything...within the realms of confidentiality of course.
92strokedbird perhaps a full rundown of the engine may help as I can only assume that other traditional tuning enhancements (e.g. reshaped in valves etc) have been completed as well. The reason I ask is that there may be another way to improve performance that will have a better return on your investment rather than attempting to increase volume of the plenum.
Whipple blower?

 

[Marquis]

Initially I changed the firing order to another well known cruciform crank V8 firing order,-changing the cams and exhaust manifolding to suit- but this engine still exhibited a massive cylinder to cylinder mal distribution- but the order the cylinders were effected was redistributed-the closed firing cylinders being effected most adversely again.

I then vented the exhaust ports straight to atmosphere-which was quite an undertaking on the dyno- I can tell you, with 8 individual equal length pipes exiting the cylinder heads but converging on a damping voulume before doing so. The results correlate with cycle simulation code results- showing that there is still a gross air flow mal-distribution effect.
I can only conclude from this that the mal distribution is predominantly NOT down to exhaust effects from this.

Post intercooler intake "runner"/plenum geometry effects to be investigated next.

 

[NeilRoshier]

Thank you for the information. I (and I assume the others) look forward to hearing the results. What effect would individually throttled cyls/port have if any to the plenum design and would this in any way assist?

 

[Azmio]

Neil,

The individually throttled cylinders remind me of the high performance engine like sportbikes, sportcars and race cars.

With the use of individual throttle, you can size up the airbox or plenum up to more than 10 times the engine displacement volume without having much effect on the engine response during acceleration.

However, the problems of cost, idle calibration (cylinder to cylinder COV during idle but not a problem at high load), production applications, etc. are something that we need to be aware of.

 

[NeilRoshier]

Azmio I wondered if the individual throttles would assist the design of the plenum in any way that may assist the maldistribution Marquis noted. I am aware of the added complexity/cost etc, however it has been done on forced induction production vehicles before e.g. Nissan SR20DET from GTiR pulsar and Rb26DETT from GTR.
In truth I have great admiration for Marquis enquiry and willingness to share the information that he does so generously and partly expected that he would have already tested such a setup