I was just standing next to my car and looking at my LT headers, when I came up with an idea. In a boosted application (blower,) would it be a good idea to use a small amount of boost, injected into the header collector, paralell with the exhaust tubes, to create negative pressure at the tubes and improve scavenging, like an improvement to the 4-2-1 header system, which uses the exhaust pulses to scavenge. I'd think that it would more effectively "suck" exhaust out of the cylinders, enough that the fresh clean air/fuel mixture would burn better, better enough that HP would be improved, even though a very small amount of boost was lost.. You'd have to utilize a one way valve for when the engine is in vacuum (so it doesnt suck up exhaust), i'd think. OR, would the boost be "wasted," not going into the engine to increase intake airflow, and negate gains. Has it been done? Ideas?
olly

 

I'm no expert, but I believe it would take more power to scavange the exhaust gas than you would gain.

 

This doesn't really have anything to do with your exact question, but it has to do with something relating to it. Was just reading about something the other day where adjustable fins are used to increase exhaust velocity into the turbine at lower speeds to mitigate some or most of the boost lag at lower rpm's. At lower speeds they are adjusted to a smaller spacing to increase the velocity of the exhaust gas going to the turbine. Once the rpm's pick up to where substantial boost is created, the fins open back and allow the free/fast flowing exhaust to do its thing. Just thought I'd throw that out there.

My take is...

As for your concepts, you are correct in that creating a lower pressure area so close to the head you would increase the escape speed of the exhaust gases from the chambers. I'm not sure how much this would help scavenging but I would think it would help reversion. However, something you must look at is your idea in a low RPM setting. The exhaust is barely pushing the turbine along, thus not creating much boost if any at all so routing whatever little boost you are getting to the exhaust would just create more lag in the turbo, thus less intake velocity, thus you're fighting harder to get the air into the chambers and the exhaust out, thus creating a cycle. I think your one way valve might address this issue, but I'm not sure to what extent.

In walks my earlier idea... if you could increase turbine speed with these adjusting fins at lower rpm's, you could create the same or more boost as you would have before routing it partially to exhaust.

The idea sounds like it might work at higher RPM's where scavenging really matters most but again, the cost/benefit analysis of this would probably turn out even or of lesser help due to the extra effort you're going through and efficiency you're losing in the turbo. However, maybe the adjusting fins could mitigate the loss of boost.

Just my .02 though. There may be flaws in my logic and I'm hoping to hear what others think.

 

yeah, i wasnt thinking along the lines of turbos.. where the exhaust is important as far as the functionality of the system goes, because it wouldn't work very well. I was thinking about engines with blowers, where the exhaust is just piped away. the "vane" thing you were talking about would be pretty cool.. you could use springs to vector the air at low RPM's, using aerodynamic doors like a choke, and the volume of air at higher RPM would hold them open.

 

I'm not so certain you would gain anything. My gut says you would end up with a loss by pump more air into the exhaust system as it is just more mass that has to be expelled. Add that to the fact you have to do work to put it in there and the losses grow.

I know some cars pump a little fresh air into the exhaust before the catalytic converter to allow it to burn excess HC . I know of no power or fuel economy benefit that provides.

 

oh yeah the air pumps into the manifolds on the a i r system totally robbed power. they sucked we would always take the belt off the pump and cap off the air tubes after we smogged the car. but the little air that is pumped into the exhaust with the a.i.r. system is quite a bit less than what it would be if you turned the boost up on the blower and deverted some out the tail. i would say try it and see but i personally wouldn't spend the time until i found out for sure. of course you don't get anywhere different if you don't try anything new.wouldn't there be a back pressure issue creating a loss in the bottom end though.

 

The gas pumped in would really have to be inert. Oxygen from the boost would screw with the O2 sensors.

 

Not if it's tuned for it - e.g. AIR pump on cold starts.

 

Look up "air amplifiers". They're based off the coanda effect and use a stream of pressurized air to move large volumes of gas at lower pressure.

They have no moving parts to break, nothing to maintain, and they all tubular in nature (meaning you could easily weld one inline with your exhaust). The only problem is they're made primarily for industrial applications so the recommended input pressure is usually a lot higher than those produced in our cars.

 

The air pump runs on cold starts before the O2's start to work and before it goes into closed loop. On cold starts the engine runs on a stated map of fuel tables(not reading O2's). tuning cant make the O2 sensors read any different.

 

Exactly, and someone put those values in the table through tuning, which is my point. If you take the word "tuning" to only mean the modification of values from their stock form, then there is no tuning involved with the tables.

 

To do that you will have to run the car in open loop to ignore the O2 sensors. You will spend hundreds of hours on the dyno tweeking tables to get the car to run well and at the correct ratio without those.

 

How about using an exhaust-driven turbine to drive an exhaust-scavenging pump?

 

and it takes horsepower to create boost, much more then people realize. if you use say 10 horse to help scavenge exhaust to make 5 horse it donta worka so good. (those are arbitrary figures , but im sure it takes more power than you would make)

 

that makes sense.. like the law of perpetual motion.

 

exaclty. its a great idea, an it would make an engine more efficient, except you have to power it somehow.

 

What makes you think you can lower the pressure in the collector by forcing air into it?

 

theoretically, it would raise the velocity of the exhaust escaping the collector, creating a low pressure "pocket" in the tubes. You know how an air blow gun has notches in the side, where the high velocity air going through "sucks" more air into the stream from the low pressure (outside) air.

 

I think you're misunderstanding the velocity = low pressure relationship. That pertains to increasing the velocity by changing the area. You're talking about increasing velocity by increasing massflow, which would not decrease pressure. That's the first i've ever heard of the blowgun thing.

 

siphoning is the same thing... like pot sandblasters and paint guns.