What are the thoughts on reducing the gap as shown in the pic to nothing or even protruding the inner tube about an inch into the outer tube.

Exactly the same question I was pondering. I am planning to do the same thing with my exhaust and figured when I get to that point I would experiment with different configurations.

 

Camco obviously did some R+D on what worked best, I wonder how they fair in an open windy environment.

 

If the wind becomes an issue should the Genturi be mounted outside the shed, then placing a larger pipe (maybe some 6" duct) in the area of the air gap to act as wind deflector might work. I would think the same should be true if your fan is blowing towards the Genturi inside the shed.

Its definitely going inside the shed, the upside of that is the enclosure will capture the exhaust noise that escapes from the gap between the inner and and outer tubes on the "genturi"
Iv'e hashed over some kind of deflector in the area of the gap to protect it from the force of the cooling fan.

The Genturi has no cap. So, if this is mounted inside the shed, then some accommodation for the rain water must be made.

The exhaust gases will mix with the cooling air at the roof outlet, there is a cap over that

Also, note that the upper portions of the Genturi are plastic. So, the exhaust from the gen must get significantly reduced in temperature due to the additional air intake.

Indeed, I looked at a vid on you tube of a homemade Venturi made out of PVC, they showed it running and ran a temp gun down from the top down to the base, the top was only slightly above ambient temp.

 

That Genturi design is interesting... I've used similar tech to vent non-electric composting toilets, allowing the wind to create substantial vacuum to keep air flowing through the toilet without having to use a fan. In my case, however, the exhaust stack ends about 1/2 way up inside the outer diffuser tube, about 6 feet above the roof.

What kind of thimble do you plan to use to get the exhaust pipe through the roof? Or will it just be the larger vent tube itself that goes through the roof, with the exhaust terminating inside the building? That sounds like it might indeed have a lot of exhaust leakage inside the building.

 

That sounds like it might indeed have a lot of exhaust leakage inside the building.

Yeah, you would have to be careful how you do this. If you are relying on venturi air to make the gen exhaust go up the stack, the shed cooling fan would have to blow into the area. If the fan blows air out of the shed, air would be backdrafted down the stack and all the gen exhaust with it.

 

That should work and be independent of the air flow from the fan.

 

Thanks guys
The way Tabora described it is very close to what I had planned when I finished working on the enclosure last week. I just got to thinking during the week would there be any advantage in doing it in the style of a Venturi, mainly because of back pressure in the exhaust from the extra exhaust tube Ive already added.

Looking at those Camco genturi's on RV's I wonder how many gen's have been nuked with a 2' pipe facing upwards towards the rain and the motor not running...Most of the exhaust ports in the setups Ive seen would be above the spillover point where the elbow finishes and the outer pipe starts -but not all of them!

 

Looking at those Camco genturi's on RV's I wonder how many gen's have been nuked with a 2' pipe facing upwards towards the rain and the motor not running...

I wondered this the first time I put one of these 14 foot tall venturi stacks in, since my first version had a small 12V fan inside the stack. Surprisingly, virtually no rain actually makes it to the bottom of the stack; the air flow evaporates the tiny amount that makes it into the inner stack before it becomes an issue.

For a 2" exhaust pipe, you could just put one of these on the top. Has worked well on tractor stacks for the last century.

https://www.napaonline.com/en/p/BK_7033502

 

use triple wall stainless steel setup like they do for wood stoves.
or double wall like they use for NG and LP water heater setups.
they deal with the heat issue,
and there is through wall solutions.
PM if you need links for brands and sources.
it is not low cost but it is a real good setup that will last for years.

 

Thanks to all for the replies

Tabora,
Won’t be needing a rain cap,the engine exhaust is protected under the cap of the airflow exhaust, below that there is a stainless underground muffler, the inlet and outlet of the muffler are opposed to each other - even if you stuck a garden hose in the pipe water cant enter the engine.

Genknot
The suggestion he got was to install the Genturi at a slight angle to allow the water to run down the side and exit harmlessly.
Theres no way an engine with an open two inch pipe running vertical down to an open exhaust port would survive if left unattended (not running) in a Florida summer rainstorm, the other issue I have with it is condensation doing the same thing but slower.

ZZ28ZZ
Seems like a better method would be to install a couple of 45 deg elbows connected together so vertical pipe would be offset at some point
Thats similar to what Ive done

Iowagold
or double wall like they use for NG and LP water heater setups they deal with the heat issue
Im using doublewall with fireproof insulation between them
and there is through wall solutions
As it stands my exhaust isn't actually running through the roof, its exiting within the 12’ air cooling exhaust flashing

genknot + tabora
"was planning to explore running the exhaust pipe a little further up the stack and see if the venturi effect is lost"
Actually, it would likely be enhanced. Wind moving across the top of the wide pipe will draw air up around the reduced cylinder area between the two pipes at an accelerated rate.
I’m setting mine up with the inner tube further up the outer than the Camco product, I will test it and see how it operates, I can easily shorten the inner if it fails, at the top end I will be cutting the end at an angle for more exposure to the outgoing air cooling “vacuum” hopefully it will enhance the Venturi effect.

As it stands my exhaust is 1.5' from the engine into the muffler then 4' insulated stovepipe from the muffler through the air cooling exhaust vent with a Venturi setup top and bottom of the stovepipe, hopefully it will capture the remaining exhaust noise within the enclosure. If it doesn't work I have other options.

Rather than using the aluminum flexible exhaust I'm trying two of these between the engine and muffler, I was surprised at the build quality and price of them, two layers of double braided stainless.

FORTLUFT Exhaust Flex Connector With Extension Pipes Stainless Steel | eBay

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Appreciate the input guys

 

The suggestion he got was to install the Genturi at a slight angle to allow the water to run down the side and exit harmlessly.
Theres no way an engine with an open two inch pipe running vertical down to an open exhaust port would survive if left unattended (not running) in a Florida summer rainstorm, the other issue I have with it is condensation doing the same thing but slower.

The 2" exhaust pipe does not extend into the Genturi as it is air gapped. The upper part of the Genturi is 4" pipe that is about 10' tall. So, water would simply run out onto the ground. Imagine in the photo for post #15 that the Genturi was leaned to the right. Water would come down the 4" pipe and run out harmlessly. It would look something like this...

I sounds as if the way you are planning to install your exhaust is quite different from the Genturi, so comparing it to the Genturi is not really applicable.

 

I sounds as if the way you are planning to install your exhaust is quite different from the Genturi, so comparing it to the Genturi is not really applicable.

Yes it's different however there's a lot to be gained out of cooling the exhaust temps within the enclosure and assisting the flow of the exhaust to relieve back pressure.

The attached pics are Audi r8 and Duramax tailpipes clearly showing a gap between the the inner/outer tubes, also both reducing tube sizing at the inner tip before the gap to speed up exhaust flow.
Interesting how the Venturi effect still works horizontally, I imagine the wind effect from the moving car can only assist with pushing the air through the Venturi.

And to throw some confusion into whether its better to push the inner into the outer or not a picture of an Audi s7 with no gap.

i would still do some sort of water trap at the bottom run exit of the gen shed.condensate from run and basic moisture could run back in on the small one to 2 inch exhaust from the gen set.maybe a silicone duck bill setup or a deep J trap with water in the trap.any moisture at the exhaust is a bad day in the making for the next run up.thinking engine internal rust.

Agree, condensation is a big concern when running a vertical exhaust pipe to an engine, my exhaust from the engine faces down initially into the muffler and vertical out, the muffler is below the engine, it looks something like this.

 

Interesting how the Venturi effect still works horizontally, I imagine the wind effect from the moving car can only assist with pushing the air through the Venturi.

Actually, tail pipe tips have little to do with a venturi effect... It's more about resonating and aesthetics.

As we’ve stated, unfortunately, an aftermarket exhaust tip won’t actually do anything to alter the performance of your car; however, it will change a couple of things. Most notably: the sound.​

​

Installing a new set of exhaust tips will make your car produce a stronger, more powerful sound. It’s that deep, throaty roar from an engine which petrolheads often appreciate, and installing some exhaust tips will allow you to replicate this sound. Where a new exhaust system will also achieve the same effect, merely installing exhaust tips lets you do this at a much lower price point.​

​

New exhaust tips will also upgrade your vehicle’s aesthetic, too. Since your car’s original exhaust pipe is there to perform a function – namely, removing unclean air – it’s unlikely that it looks very appealing. Aftermarket or branded exhaust tips, however, are designed to let your exhaust system achieve the same result, but look good in the process. From chrome and matte finishes to dual-forked and turndown tips, you can style the rear end of vehicle in numerous different ways which passersby can both see and hear.​

 

👆 One additional thing that the exhaust extensions do is to help prevent burns when you're at the trunk or hatch of the vehicle. The muffler can stay hot a long time, but the tips that have the vents/slots can cool off sooner.

 

My understanding was they using venturis to cool the tip of the exhaust to prevent burns as well, just got to work out how to squeeze one into my very tight enclosure...

 

My gen shed build is pretty much done, I ended up making a venturi for the stack (the stack is within the enclosure) out of triple walled stove pipe 41/2 outer to 3 1/2 to 11/2 inner, the outer pipe remains cool,I can leave my hand on it with the gen running @75 % load.

The enclosure cooling fan did mess with the venturi as discussed above (it’s a very strong fan) to get it to work I had to set it up without a gap, the internal exhaust pipe (1.5’ ) is inserted 3 or 4 inches into the stack.

The way the engine exhaust terminates at the 12’ cooling exhaust is creating a draw on the venturi as well, I had an unexpected surprise (but good outcome) when running the gen in the enclosure -I let in run for 45 mins @ around 70% load everything was good - I wanted to try it without the cooling fan so I unplugged it, without the draw effect of the fan where the exhaust terminates at the 12' roof opening CO was drawn back into the enclosure and it shut the engine down, so if the gen tripped out and the fan stopped (but continue to run without making power) the CO detector shuts down the gen before an overheat.

Pic of unfinished cooling and engine exhaust.

I had intended to use a bimetal switch on the engine exhaust connected to the engine oil float switch to shut it down in the event of an overheat or fire, easier said than done with the way the oil float switch is wired in this gen…. instead I went with a temperature controller connected to the CO power supply, using this I could easily set the parameters to shut it down and continuously monitor exhaust temps, if the exhaust temp exceeds the set point the relay opens (interrupting power supply to the CO) and it shuts down.

I did the same on the propane supply (probe inside the gen) with a second temperature controller (there is a 12v relay on the propane regulator) and I can monitor engine and exhaust temps continuously with these controllers that are setup outside the enclosure.

https://www.amazon.com/dp/B0B6HP396F?ref=ppx_yo2ov_dt_b_product_details&th=1

For anyone interested in an alternative to a conventional cooling fan this is what I used.

https://www.digikey.com/en/products/detail/ebm-papst-inc/R4E355-AF13-09/441964

A video link explaining how they work
www.youtube.com/watch?v=AcQdIAz38To&list=WL&index=14


Thanks to all those who assisted with answering my questions with my build.