Negative Pressure Supercharging on Rotary Engine?
 

If this has already been discussed, let me know which thread please. Wondering if anyone here has some knowledge of or experience with negative pressure supercharging. This is using a supercharger to pull from exhaust side instead of pushing from intake side of combustion process. If you have never seen this, go to www.impulsengine.com. They claim that in sucking instead of pushing, it creates a more fuel eficient torquey monster of an engine. I also think that they claim it lowers the temperature of the engine quite signifigantly, to the point where they must use a hot air induction system. I thought this would be an interesting concept, so I "googled" it and found the above website. From what I can tell, it has only been used on big American muscle V8s. I was thinking the lower rpm torque this might offer with the high rpm hp a rotary can offer combined would make one sweet powerband. On my 13b, my ports are about the size where my power band starts at about 4000 - 4500 rpms. Since its so high, I was starting to fantasize about it starting lower with some wicked torque (for a rotary engine anyways). Any feedback is greatly appreciated, especially since this is my first time starting a thread

I'm a bit fuzzy on how pulling exhaust out will help much. Exhaust has a way of making it's own way out of the engine in one heck of a hurry. You'd need a significantly larger supercharger to handle the flow (read, more power to turn).

Also, with the rotary engines (at least prior to the Renesis), you have a good deal of port overlap between intake & exhaust. You'd probably end up sucking a good deal of intake charge right out the exhaust ports.

I'm still somewhat curious about it, though. If anything, I'd think it would end up functioning more like an exhaust driven turbine - using the exhaust energy to help turn the engine. Some of the WWII aircraft engines did this, and gained some pretty impressive power & fuel economy from it.

Anyone got the funding to give it a shot?

-=Russ=-

Ah, guys? Read that site a little bit more closely, in the "how it works" section.

What they're selling is in two parts. The first is mucking with the valve timing, port areas, and header design to get a high exhaust gas velocity, then closing the exhaust valve early and having a dwell period where neither valve is open. Supposedly this then captures a vacuum in the combustion chamber, which will then work like the scavenging effect from valve overlap without the possibility of short circuiting the air/fuel mixture.

... so that sort of makes sense, kinda. Except, reducing the exhaust duration, running smaller ports and less lift... restricts exhaust flow, no matter what you do. This is alluded to by all the comments of massive low end torque, and the parts where they explicitly state the powerband ends at 4500rpm.

Now, the next part, about the hot air induction, is kind of funny too. They're doing various optimizations to increase burning speed of the air fuel mixture and burn it as hot as possible... and in the process requiring really overkill cooling systems (they say the engine runs cooler... if you read much, it runs cooler because it has to to avoid detonation) and runs less spark timing for the same reason. I suspect the heating of the intake charge is mostly a bandaid for carbs.

Overall, they're touting a lot of optimizations that would possibly make a decent truck motor, but have no place on anything that belongs in something that goes fast, except possibly for the first 100 feet from a standing start. Their talk about average torque is silly, comparisons of fuel usage of high and low horsepower engines is ignorant, and http://www.impulsengine.com/performa...rsepower.shtml is just hilarious.

The rotary is a small displacement engine. Like any small displacement engine, peak torque figures are limited by the amount of air and fuel that go in. This can be increased with forced induction, or it can be compensated for by making that torque up to higher RPMs, and taking avantage of that to run taller gears. Limiting the powerband to half of its normal size means that your gear ratio has to be half as high to get to the same speed in any given gear, which means that unless you make twice as much torque at the engine you're making less torque at the wheels. You're not going to be making double the peak torque without forced induction.

i'd assume that thing would absolutely KILL your low-end if you've got much porting done... won't it suck the intake charge right out?

Syonyk: Never thought of the reverse supercharger, where the power of exhuast could be reapplied to the eccentric shaft, but thats interesting. Although I would picture that being about as efficient as using a batterey to power a battery charger. I'll research that though.

Kenku: You know, when I was reading the technical page on that website, I thought it was kind of hoakey. I don't think that there would be a problem with needing to add hot air induction, because rotaries can run smoothly without pinging with very low octane gasoline. I wonder if there is any way to increase the torque characteristics of the engine when it is in naturally aspirated form. I also think it would not be worth changing the port timing, because then you would have to move it, not just change its size.

GUITARJUNKIE28: It might suck some fresh intake charge out the exhuast, but its not like the intake charge is one set volume, and that part of that volume will be taken away. Its that some might be sucked through, but who cares if this means a 100% fresh intake charge comes in. I don't care about smog, do you? lol

I think it's a pipe dream.
You're subjecting a supercharger to stupid high temperatures.

A turbo already does that.

By the time you make an Inconel SC, you could've went a bunch 'o Garrett GT-series turbos!


-Ted

i read a lot on what they were doing there.. i misunderstood it at first.
ted, go read the stuff.

the only place i found any numbers were peak hp (250) at 4k rpm, and 370 lb tq all around...they've effectively given it a diesel powerband.

i wonder what the fuel economy is like--now that i understand more about what they're doing.

From http://www.unlimitedexcitement.com/M...0%20Engine.htm

Also, I believe some of the larger turbine-powered helicopters do something similar. The actual power for driving the helicopter is taken from turbines in the exhaust stream of a normal turbojet - the actual engine isn't connected to the rotors.

-=Russ=-

That engine put out about 3000 hp with forced air induction. It weighed about 1600 lbs. How much does a rotary weigh, any where close to a tenth of 1600 lbs, because a naturally aspirated modified rotary can put out one tenth of 3000 hp. One thing that is cool is the 3200 rpms for that much hp. Dang. There has got to be something to either sucking air instead of pushing it, or reapplying force from the exhaust back to the eccentric shaft

I believe a short block weighs in somewhere around 180-200 lbs - you can move it easily with 2 people. The full installed engine is around 250 or so for a NA, closer to 300 for a turbo.

-=Russ=-

Some of the new volvo trucks use a compound turbo. They add 50hp or so to a 400hp engine with a good increase in torque and fuel economy. I think it would be worth developing for a rotary considering the amount of exhaust gas that can be taken advantage of.

This is all interesting.. but read about what they are doing to the timing of valve opening and closing etc..

Go download/buy the Desktop Dyno for boingers.
Play with it by loading a bunch of different camshafts on a given motor design. (Like porting the rotary a slew of different ways... including ADDING material to give LESS port timing than stock)

Now you will see within 10-20 minutes of playing with it.. you can take a 350 Chevy and have 500+ FtLBS of torque that ENDS at 3K rpm.. and only makes a hundred HP or so... OR.. you could have a motor that makes 500 HP and 100 FT LBS of torque.. or anything in between.

Basically.. the timing is EVERYTHING. That is why porting is such a science in the rotary world.. and people scream DONT MAKE IT SOOO BIG!! when people are all hogging out the ports and not getting the power they thought they should.... or getting different results than they thought they should... or a powerband that starts at 7K etc.

You can radically effect the performance of the engine by changing the opening timing.. and the duration between them. What it appears they are doing.. is restricting things a TON.. and using the supercharger to bandaid the problems they are creating to enable it to function "properly"

Yep Torque.

WOuldnt it be easier to change engines to something that meets the different requirements you have?

IE... Hmmm.. Id like to tow my BOAT with my 1st gen....... my 30 FOOT BOAT!... well.... you will NOT get the torque to do that.. drop in something different. Like a truck in the driveway.

i'll be able to tow shit with my fd :D

this was done on WW2 aircraft to get higher hp, it does work really well but it was intended for engines with less overlap than a rotary if i'm not mistaken. Like anything else, the application has to be right, IMO turbo's and rotaries go well together.