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Old 07-17-2008, 09:11 AM   #1
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Exhaust gas throttling

I'm not talking about a throttle plate in the exhaust gas to increase back-pressure

This is the idea. A throttle assembly free(in the usual manifold vacuum sense) engine design that uses exhaust gas to dilute the intake charge for throttling.

This is my 3rd throttle-free design and the first to actually leave the otto-cycle alone. We know that you can only lean a mixture so far before you run into ignition issues and performance / drivability suffers. However, you can do things like the HAI to decrease air density and make the engine work harder(more efficiently) to make the same power and hence increase mileage.

This plays off of that a little bit but instead of making the air less dense we dilute it with inert gasses(exhaust); this will inevitably heat the intake charge but the real benefit comes from the fact that you have no(practically) manifold vacuum. The less power you need from the engine the more exhaust gas you have it suck in

I'm thinking of building a small 5hp briggs to test this one... see if the engine will even idle with that much exhaust gas in the chamber. I'm thinking I may have to actually use some sort of vacuum to let the engine idle with a mostly undiluted mix. But I can't say for sure.
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Old 07-17-2008, 09:29 AM   #2
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The problems with lighting lean mixtures come with the amount of "stuff" in the chamber, not just excess air, so I think you're gonna have to come up with a method of stratifying the exhaust gas in the chamber to enable the mixture pocket to light off successfully.
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Old 07-17-2008, 09:48 AM   #3
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This was discussed in a couple other threads, I think probably intiated by you. As I remember it seemed like it wouldn't work, but if you could make it work it seems like a good idea to me.

If I understand correctly, it works by making the exhaust gas just use up space in the cylinder, almost effectively making your displacement smaller. It's not that the exhaust gas is used for any burning or chemical reaction, just to take up space and be compressed. Is that correct?
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Old 07-17-2008, 09:52 AM   #4
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Remember that when a piece of fuel wants to combust, it has to try to get an oxygen atom from nearby. It is one thing to have a fairly "empty" chamber with just little bits of air and fuel, but if you "pollute" the chamber with inert gasses the fuel will have a harder time finding oxygen when it's time to combust. Your combustion will probably be drastically slower and less complete (i.e. more unburnt fuel will be left over).

Sorry if that is a poor explanation but you get the idea.
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Old 07-17-2008, 09:53 AM   #5
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Quote:
Originally Posted by theholycow View Post
It's not that the exhaust gas is used for any burning or chemical reaction, just to take up space and be compressed. Is that correct?
Exactly. Most likely won't work because of low idle issues. This is the only one of the 3 that has the possibility of not working.
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Old 07-17-2008, 10:06 AM   #6
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Just thinking.... maybe it would actually be easiest to test with a throttle plate in the exhaust... Just leave the carburettor wide open, and the amount of exhaust you let out determines the amount of air and fuel it can suck in.
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Old 07-17-2008, 11:25 AM   #7
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That would work if we introduced the exhaust gases after the carb. I was just gonna build a small EFI system for the 5hp engine. In that case I just have to meter the air coming through the fresh air inlet.

A valve can be used to adjust the air to exhaust mix and a second valve can be used in the fresh intake to allow for low-idle that I honestly don't think can be accomplished with much, if any, exhaust in the mix. This would mean that your only gains are between right off idle and 60-75% throttle.

Electronic timing too so I can do real experiments with it.
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Old 07-17-2008, 11:02 PM   #8
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Is this not similar to what EGR system do? recycle exhaust gases, in the attempt to burn anything that didn't? Only, you're hoping to start with a fully burnt mixture and just use the gas to take up space. EGR ...kinda already does that, more for reducing "bad" exhaust. a 5hp Briggs doesn't, so its possible there might be gains in a small or older vehicle engine (even my dad's '85 chevy used it), but not likely to see anything in newer engines. My '90 Protege doesn't, but my '02 GP does. Emissions reasons, but I'll bet there's some FE gains as well.

Not shutting you down or anything, but I don't think you're pioneering new ground that hasn't been touched. Don't let me stop you though (I'm curious what you find out), Shadetree mechanics has won in the past, I'm hopeful for the future.
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Old 07-18-2008, 05:15 AM   #9
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Originally Posted by almightybmw View Post
Is this not similar to what EGR system do? recycle exhaust gases, in the attempt to burn anything that didn't? Only, you're hoping to start with a fully burnt mixture and just use the gas to take up space. EGR ...kinda already does that, more for reducing "bad" exhaust. a 5hp Briggs doesn't, so its possible there might be gains in a small or older vehicle engine (even my dad's '85 chevy used it), but not likely to see anything in newer engines. My '90 Protege doesn't, but my '02 GP does. Emissions reasons, but I'll bet there's some FE gains as well.

Not shutting you down or anything, but I don't think you're pioneering new ground that hasn't been touched. Don't let me stop you though (I'm curious what you find out), Shadetree mechanics has won in the past, I'm hopeful for the future.
His idea is to take the EGR system to the extreme. To dilute the intake charge with enough exhaust gas to control the engine rpm with the throttle plate being 100% open all the time. However, it appears as though at idle the throttle plate will have to be partially closed and little exhaust gas going to the intake manifold just to keep the engine running, in theory anyway.
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Old 07-18-2008, 07:37 AM   #10
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I have been interested in this as well. Here's a thread that I started- http://www.gassavers.org/showthread....&highlight=EGR

I also tried an exhaust pipe restrictor to try to increase backpressure- that was a big flop.

I'll be interested to see your results!
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