Sizing exhaust hose and muffler to engine

[Snotter]

I'm installing a 7.5kw generator that has a 2-cyl diesel. I'm at the point where I need to purchase a muffler and exhaust hose. The little 2-cyl has an mixing elbow that accommodates a 2" ID wet exhaust hose. A 2" exhaust hose and muffler seems over-kill to me. What do you all think if I went with 1.5" ID hose and a water-lift that also accommodates 1.5"?

 

[woodster]

what kind of diesel is it .....maybe you should check with the manufacture.....

regards

woody

 

[kloudie1]

So that is about a 12-15 HP diesel.. 2" hose has about 3.14 square inches of open area; 1.5" hose has about 1.77 square inches. A 43% reduction in flow area.. Gut feel is a 1.5" for that size engine is probably OK, especially since it won't be running at full throttle very much. As Woody notes, ya might want to bounce it off the tech side of the manufacturer if you can get to talk to them. just an observation, a diesel tractor with about that much engine will have a 1.5" pipe but it does not have to pass water like the genset ..
If it is too small, it won't develop full power..

 

[Bill Roosa]

Exhaust gas and steam

Don't forget you have a diesel. It breathes 100% every cycle so your gas engine eye-ball may be out of tune. The manufacture put a 2" on it for a reason. You have to pass all the exhaust AND the steam from the water that is being boiled to keep the temp down. Most of that steam condenses before it gets out the "tailpipe" but between the engine and the exit there is more volume of gas to pass than would normally be expected with a dry exhaust.
I'd keep it at 2"

 

[Snotter]

My Morgan OI has a perkins 4-154 (62 hp) that has a 2-1/2" mixing elbow, but Morgan reduced it to 2" and it's been working fine for decades. My gut feel is that 2" is over-kill for a 12 or 13 hp engine; however, the diesel operates at 1800 rpm 100% of the time when it is running. I wonder what Maine Sail would say.

 

[bria46]

If the 2 cylinder generator uses 2" plumbing from the manufacturer, stick with it! My Westerbeke 10-2 engine uses 2" plumbing and that's 10 HP

 

[Ol Dave]

What exactly is the downside to sticking with the manufacturer's specs? Why would you willfully void your warranty? What is the benefit ... a cheaper hose run and lift? How much are you 'saving" against the $$$ and performance you are risking?

Ol' Dave ... the 'fraidy cat

 

[Snotter]

No warranty. The genset was purchased from ebay. The genset was manufactured by Onan which has been absorbed by Cummins. I would trust this forum more than Cummins tech support.

If I can use 1.5" hose with out incident, I would much rather work with 1.5" hose than 2" hose. The large stuff is very unflexible. I can't see using the same size exhaust hose and muffler as on my 60 horse auxiliary. However, I'd be the first to admit I am no expert in diesel aspiration.

 

[RichH]

Exhaust 'size' is calculated based on 'displacement volume' per unit time. cc's or cubic inches per minute, etc. This is to keep the 'velocity' below certain limits to prevent 'resonance', increase the 'noise', and lessen 'back-pressure', etc. ... a 'pressure drop' calculation.
A correlation is possible - Assume that the 2" exhaust 'size' corresponds to max. rpm (volumetric capacity at MAX rpm or per minute). Select or estimate the normal rpm during 'normal' power generation then multiply the 2" Ø 'cross sectional' area (3.14 in sq.) by the ratio of: operating rpm divided by MAX rpm .... and that will give you the 'cross sectional' area of 'piping' required for operation at 'normal' rpm.

Of course you can lessen the diameter if the engine is not running 'full out', but that should be accompanied with the MINIMUM usage of elbows, especially close radiused 90 deg. elbows, and other 'transition fittings' that cause velocity increase/decrease, etc. ... 'tight bends', or anyother component that makes the fluid to 'turn a corner', etc. cause high pressure drop in fluidic mass transfer systems - avoid them. Dont be an anal fanatic 'neatnik' by putting in unnecessary elbows, fittings, etc. etc.
Inotherword if you reduce the diameter of the 'hosing, etc.' be sure to only employ loooong radiused elbows, etc. and keep the 'numbers' of hose components (nipples, unions, etc. etc. etc.) to an absolute bare minimum. Also make the exhaust 'run' as 'short as possible'.

;-)

 

[Ron20324]

No warranty. The genset was manufactured by Onan which has been absorbed by Cummins. I would trust this forum more than Cummins tech support.

You will get different opinions in response to your question, but you may never know which one is correct. That is, to assume that at least ONE of them is correct. Maybe NONE will be correct. Contact Cummins.

 

[Snotter]

I broke down and bought 20' of 2" hose and a 2" muffler. They are now selling unreinforced exhaust hose. Being a traditionalist, I went with the SS reinforced hose; still, I wonder if the new stuff is the way to go.