Beodude123

This is a stupid idea, but would it be possible to pressurize the exhaust manifold at lower rpms where a large overlap would adversely affect mileage? That way it would have a higher pressure at lower rpms, and get better power at higher engine speeds?

globi

If engines do have large overlap, they also have variable valve timing.

rotarygod

Yes

kw1k

So would headers improve this cars hp?

globi

Yes but instead of 10% more hp you might only get 3%.

If you have overlap and if you only have a slight pressure drop below atmospheric pressure (not vacuum) in the exhaust chamber, the intake gases can push the residual gases out of the chamber. Or in other words with a compression ratio of 10, this means 10% more massflow = 10% more power.

If you don't have overlap, the pressure drop will only reduce the amount of the residual gas but won't get rid of it. (Unless of course the header can generate a perfect vacuum, which is hardly feasible.)

tuj

BS. Tell that to all the V8 guys with their wild cams.

tuj

That's completely wrong. Overlap was found to be beneficial at only 200 rpms in the early days of engines. Overlap is a critical component of any N/A engine.

http://www.amazon.com/gp/product/083...books&v=glance

globi

'Ever noticed how stable and emission friendly these engines are at idle?

I guess you should tell this the Mazda engineers that designed a N/A engine without overlap.

Richard Paul

iTrader: (5)

With all do respect, and I'm not being my usual smart *** self. But are you serious?
Could you show me the math on this? Or are these numbers from out of the air?

globi

Ok here's the math:
(Displacement + Displacement/(Compression ratio)) is the total volume of the combustion chamber.
Example: an engine with 2l displacement and a compression ratio of 10 can ideally burn 2.2l of air fuel mixture. 10% more massflow is approximately 10% more power - agreed? It's not perfect and there are more factors playing a role, but it's a reasonable approximation.
Now, an engine without overlap cannot easily get rid of the residual gas which in this example takes 0.2l (since there is simply no draught). So, the power gain is obviously significantly less than 10%. 2%, 3% or even 4% - I don't know. But to reach 10% you have to come up with some header design that generates an absolute vacuum.
What is the lowest absolute pressure ever measured in an exhaust pipe? If you can give me that number I can do the entire math.

globi

Here's a easy project for home owners to understand what with or without overlap means:
On a windy day open one window on the east side of your house. Do you notice a strong wind in the house? (Probably not). Now open one window on the west side of your house and make sure all doors inside the house are wide open and simply watch what happens.

Richard Paul

iTrader: (5)

I'm kinda an AH here, I set you up. I already gave you the math in my paper on mass flow. But note it says "Theory" on it.

Refer to post 37. this thread.

The purpose of that paper was supercharging and filling the unswept volume. But note I didn't even claim the whole gain when supercharged to Pr2. I was skeptical even then. I think you might be carefull to claim that much gain just from blowdown.

Yet we know that a F1 can get 125%+ massflow. Albet in a very limited RPM range. So where am I on this? Somewhere on your side but can't give you that much gain on a road car no matter how much overlap you have.

I like seeing you thinking though. Just be carefull not to use such quick math.
If you read my paper you will see I break off and relate to the supercharged swept plus unswept volumes.

I know it's geek math but highly nessasary with all the variables involved. Reading it is all you need to do, forget doing the math. Leave that to the collage students who need the mental work out. Like Shellys or however he spells it.


One more thing, your example would yield 11 to 1 compression.

carbonRX8

iTrader: (1)

Hey, just a thought. Could you drill a hole diagonally through the face of the rotor and out through the side of the rotor so that you effectively produce overlap? Please comment on the idea, itself. I realize that this might cause more problems than worth.

rotarygod

I'm just going to comment that even at 100% volumetric efficiency on an engine you still have 10% exhaust gas dilution. Even with no overlap, there is still a small amount of exhaust gas returning to the intake side. Now lets say we had an exhaust that really scavenged good. In other words what if we had an exhaust that kept velocity high and actually pulled more air out of the engine and created less than ambient pressure inside the chamber just as the exhaust port is closing? It is possible and very feasible to do. You just can't get it at every rpm. You need to pick a spot but this is just tuning as with everything else. What if we got -1 psi in the chamber. What will happen as the intake ports open to a -1 psi pressure inside the engine? The air will accelerate into the engine and you will effectively raise the efficiency.

Overlap does not need to talk place to get intake scavenging. The commonly accepted definition of overlap states that it is when the exhaust has good velocity so that it's velocity pulls some air into the engine through the intake ports during overlap. I have even used this statement before. That is how it works on an engine with overlap though. These engines during overlap will also pull some of the intake charge out through the exhaust and lose some of what it was gaining. If overlap was important to making high rpm power, the Renesis wouldn't make more power up top than the 13B. It most certainly does and it even does it with a higher and wider usable powerband so it's obvious that conventional thinking into how overlap is beneficial is misunderstood in some way. It's not saying that overlap can't be beneficial. It most certainly can. No overlap is just better than what everyone thinks.

globi

I agree it's an ideal view, but especially with a supercharger the gains with blowdown due to overlap should be noticable.
I mainly tried to make a point why headers don't lead to the gains people might wish for and I believe this is a reasonable explanation. If there was exhaust pressure data available (with or without header) it would be possible to make a more accurate guess.

In that case the combustionchamber wouldn't be be sealed anymore (look where the side seals are).
It might be more feasible to add peripheral ports to the exhaust (drill 3 holes and fill them with tubes).

tuj

The pressure in the pipe is below atmospheric at times, and it is important that the optimized location of this point be determined by the length from the port. There are two considerations in this variable: column inertia, and scavenging.

Since most people are familiar with the latter, I'll comment on the following; the exhaust gas coming off of the rotor face has some velocity that be utilized effectively in some engines. This velocity can be considerably more important than the resonance vacuum, but is not always. When it is, the resonance vacuum point should be relocated farther down the pipe. If the vacuum is maximized at the port, even a preferrable inertial effect can be overcome by the vacuum.

Some people might dismiss this on the Renesis because of the 90 turns in the ports, but poppet values essentially present the same problem, while scavenging is reduced by the complete absense of overlap. The problem with overlap is that the effect results in blowdown at some rpms, which is wasteful of the mixture, and is the reason why power and fuel efficiency can generally be considered mutually exclusive (and why the curve between the two isn't linear).

globi

The other issue with overlap is partial throttle operation (90% of what most people operate their cars at): At partial throttle or at idle, exhaust gas is pulled into the combustion chamber as long as the exhaust valve is open, which is especially noticed at lower rpms and usually not a very controlled process and can cause misfirings. (The piston/rotor generates a vacuum and the gases take the path of the least resistance - if the throttle is closed and the exhaust pipe is open...)
Race engines are usually not operated at partial throttle and don't need to worry about controlled partial throttle operation and can therefore deal with much larger overlaps.
Also, to some extent poppet valve engines also need to have overlap just to give the cam enough time to actually open the valve (cams with vertical walls won't work). This is an issue the rotary engine doesn't really have to deal with.

Last but not least: If the Renesis had openable peripheral ports in addition to the side ports it could have the best of both worlds. Keep in mind the 26B had the same compression ratio as the Renesis and delivered 700HP at a relatively low 9000 rpm - which would translate to 350HP with 2 rotors...
(This is actually a pretty high number compared to the F1 engines and their HP numbers at 9000 rpm and a compression ratio of 14.)

tuj

Sorry, but the part about the cams isn't right. I can easily have a DOHC engine with zero overlap. Ramp angle isn't the same thing. And the rotary does have the equivalent of a cam lobe's ramp in the port shape's leading edge. I certainly agree that race profiles run much more overlap than street cams.

Second, you seem to suggest that overlap is only beneficial at WOT, which is not true. Air is flowing significantly through the engine at many throttle openings. Even during crusing there is benefit to some overlap in conventional engines.

Finally, the 787 was a full race motor. Typically, all-out motors can see roughly double their production hp levels. An N/A 13b in that era making 175 hp wouldn't be unreasonable. I'd like a peripheral port motor that could pass emissions too , but I doubt that will ever happen.

globi

When you already throttle the airflow why would you need overlap for increased airflow? First you throttle it and then you open it up - why not close the throttle less in the first place?

Actually if you limit rpm (and compression ratio) on an engine, power of a race engine is not very impressive. A 3.0l F1 engine makes 900HP at 19000 rpm, which translates to 280HP at 6000 rpm. If the 3.0l engine in the 6 had a compression ratio of 14 it probably wouldn't be far off from 280HP.
If the peripheral ports would only open at high loads and rpms I believe emissions could be passed, but I also doubt that this will ever happen simply because I don't believe that there's much R&D in the rotary department and it would add a lot of complexity for a relatively small power gain (compared to FI).

Richard Paul

iTrader: (5)

[QUOTE=globi]


Actually if you limit rpm (and compression ratio) on an engine, power of a race engine is not very impressive. A 3.0l F1 engine makes 900HP at 19000 rpm, which translates to 280HP at 6000 rpm. If the 3.0l engine in the 6 had a compression ratio of 14 it probably wouldn't be far off from 280HP.
QUOTE]


Globi, Globi, Globi, Pleeeeezzz you cannot extropolate like that. There is no comparing these things. An F1 engine idlles at 6000 and doesn't make near 280 hp. It can't even pull the car into motion with that RPM. It would stall.

Compession ratio does not act linear. From 9 to 1 going to 10 will yeild more then going 10 up to 11 and so forth. Notice that NASCAR lowered their engines from 14 down to 12 and did not slow down a drop.

rotarygod

Static compression ratio is the number that we know as 10.0:1 but the effective compression ratio is what determines power output. The effective compression ratio changes with engine efficiency. When you have 100% volumetric efficiency, you have the same static compression ratio as you do effective copression ratio. If your volumetric efficiency is less than 100%, your effective compression ratio will be less than 10.0:1.

Richard Paul

iTrader: (5)

Excellent RG, This is why you can run hi ratios on the likes of Buick nail heads and May combustion chamber heads. If nothing gets in you can compress it all you want.

TeamRX8

iTrader: (25)

the whole thing is much more complicated than some people here have made it out to be, the wankel concept adds another level of complication, and the Renesis ups it another notch still. This application is not easily modeled using the the typical engine analysis techniques available

Based on my research you're likely looking for a total pipe length to the collector in the 18" - 24" range, pipe sizing from low end to high end power in the 1-5/8" to 1-7/8" range (probably larger still for super high boost applications), you can step up to the next larger size pipe about half-way to the collector to get a wider powerband between the two, and if you want to split the siamese port flow and keep the gases separate between the two rotors in that port then half the area of the center port works out to about a 1" ID pipe. Really high end power will likely require porting and the other usual mods, not very well documented territory for the Renesis yet.

Aseras

well this might sound stupid but what about a reverse turbo, something that literally sucked the exhaust out. The only thing is that the energy needed to make it work would probably be more than the gains it could acheive...

TeamRX8

iTrader: (25)

yep, you're talking about perpetual energy, not going to happen, the losses will exceed the gains in the real world