You’re correct, Mike. Perhaps I should have explicitly stated that the air density I was mentioning is the air density at the intake port – by saying manifold air density – I left some vagueness on where in the manifold I was referring to.    However – it really doesn’t matter what you measure or where you measure it – the engine doesn’t know nor care.  What matters is the actual manifold pressure/air density in the intake manifold at the intake port as that is what shoves the mixture into the combustion chamber (plus a little help from inertia of the air mass flow and FAW waves and just a few other things).  The point is regardless of what your TB intake design is like once you reach ambient pressure/air density in the intake manifold (or as close as your are going to it  get due to TB or intake losses), that’s it.  You have maxed out on power regardless of Throttle plate position or where you are measuring it.

 

If you are measuring ambient right after the TB but something less in the intake manifold next to the intake port – then as you suggested that would be a reflection of intake losses.  But regardless you still have manifold pressure/air density = Ambient Pressure/density - intake loss. As you know,  intake loss may be caused by such things as air filter, throttle body partial throttle, intake runner loses, port intake losses and others such as interaction of FAW inside the intake.  But again the engine doesn’t know or care – all it knows is the air density at the manifold intake port. Once that is ambient or as close as the intake losses permit then additional throttle opening will not do anything for you.  Now this all refers to a “Normal” intake set up.  There are things that you can do to change this - like turbo charging  or using Dynamic Effect Intake design.

 

Yes, a well designed DIE/DEI intake manifold temporarily results in a localized high pressure region at the intake port for a few milliseconds – what happens is the extremely powerful (but very short duration) pulse generated in the intake when a intake port opened by the exhaust gas pressure still present due to the intake/exhaust overlap case of the old 13B.  This pulse then hit the opposite end of the intake (the other rotor’s intake ports) and locality pressurize the area of the intake port just as that  rotor was closing the intake port.

 

Reversion is a well known engine process, whereby as the piston/rotor moves from its maximum volume area on intake into the compression stroke – some of the intake charge previously drawn into the combustion chamber is forced out before the port closes.  Estimates I have seen puts this “lost” charge at around 15% of the total intake charge. Perhaps just a coincident, but  that turns out to be the % power increase at 6000 rpm that the Mazda engineers reported using the DIE intake on the older 13B.  The Renesis also makes use of the DIE effect, but it is different in several aspects – Dennis has worked all of that out.

 

The DIE effect does not in itself directly increase the density in the combustion chamber – but if timed right – arrives at the intake port just as its starting to close and in effect stops or minimizes this revision effect. So I guess since the end result is there is more air inside the same volume – it has the effect of retaining the combustion chamber air density rather than having it drop due to Revision.   But,  if you have a normal manifold pressure sensor even at the intake port – you would probably not see any increase in pressure – its over in a few milliseconds.  Now with special sensors you can indeed detect the temporary increase in localized pressure there.

 

But, my original intention was simply to try to explain – perhaps poorly – how you can get throttle travel without an increase in power and why.  And some of the factors that can cause that effect which are many.  But, the two dominate factors are rpm (load on the engine) and volumetric efficiency.

 

I am apparently not doing a very good job making the point about the relationship between throttle body size, throttle travel and engine power clear.  So here is another try at it - using extreme examples and no math.  Let’s assume your volumetric efficiency is 100% (no losses).

 

IF you put a ½ ” dia TB on you engine.  You could have it wide open and your engine (under normal prop load) probably wouldn’t turn 4000 rpm.  That is because even wide open, the ½” dia TB restricts airflow sufficiently - that the manifold air density never approaches the ambient air density.  Since we know that the engine power is directly proportional to the density of air in the combustion chamber – and this density is limited in this case to less than ambient, you engine is not going to produce much power, certainly not full power.  So this bit of information tells us “Bigger Lithium Crystals, Scotty!!” – i.e  try a larger throttle body.

 

Now if you keep enlarging the diameter of the TB you would find that at fully open -your engine would be producing more power than it was, but perhaps still not the maximum power it is capable of.  This is because the air density in the manifold has increase due to the less restrictive flow, but is still below ambient.  This is due to the better, but still restrictive effect of the TB size on the air flow.  Now if you continued enlarging the TB size, you would reach a point where with the TB just reaching fully open - your manifold air density is exactly ambient and your engine is producing all the power it is going to.

 

Now if you enlarge the TB even further, you will simply find that you can cause the manifold pressure (air density) to reach ambient without opening the TB fully.  Its simply large enough that all the air the engine can use (ambient air density point)  is met at partial throttle opening.  In fact , you can certainly continue to advance the throttle thereby opening the throttle plate even more – but, you are not going to increase the air density in the manifold and therefore you will not produce any additional power for that additional throttle travel.

 

Bigger Throttle body’s result in more power only up to the point the airflow they permit causes manifold air density to reach ambient.  Beyond that point, the only thing they do is provide frustration – by having all that throttle travel remaining which does nothing to produce more power {:>).

 

Now if you can somehow  lighten the load on your engine, then engine can turn faster providing more “suction” on the manifold volume reducing the air density below ambient, now opening your “oversize” TB a bit more will produce more power because you are increasing the airflow again to the point where the equilibrium point between ambient air density in the manifold and rpm is again reached.  Lighten the load further and you can again increase engine power by opening your TB more. Etc, etc.

 

 

Ah, ain’t this hobby wonderful {:>)

 

 

Ed

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

 

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