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-   -   How much ventilation for diesel? (https://www.cruisersforum.com/forums/f54/how-much-ventilation-for-diesel-245777.html)

Jdege 30-01-2021 14:29

How much ventilation for diesel?
 
I have a boat with a Yanmar 3GM30, that had it's engine compartment vents removed for painting.

I could simply replace what was there, but I'm not really thrilled with the location or design.

But before I do anything different, where could I find out how mugggch air intake a 3GM30 requires?

And how large a vent area would be required to provide this?

flyingfin 30-01-2021 14:55

Re: How much ventilation for diesel?
 
Look at your car engine, what size is the intake just before the air filter? Not very big!

Compass790 30-01-2021 15:17

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by flyingfin (Post 3332240)
Look at your car engine, what size is the intake just before the air filter? Not very big!


Depends if his car is petrol or diesel.
Diesels require roughly twice as much air as the same cc petrol engine, the clue is in the compression ratio.

Reefmagnet 30-01-2021 15:47

Re: How much ventilation for diesel?
 
I've been concerned (rightly or wrongly) about the effects of drawing air for the engine from the sound proofed engine bay (which also acts to insulate it) as this is a very 1970's approach with inherent lack of efficiency so the first question is are you running ducts to the vents? If so the area would need to be at least the same as the air intake on the engine, preferably a little more.



If you're simply drawing the air from the engine compartment then it's unlikely to be totally hermetically sealed so air will flow in from every gap, particularly the bilge area (which will also have plenty of subtle openings), and this is probably more than adequate for the supply to the engine. If this is the case, you may find the purpose of the vents is to actually help discharge heat from the area once the engine is stopped.

Uncle Bob 30-01-2021 16:05

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Compass790 (Post 3332261)
Depends if his car is petrol or diesel.
Diesels require roughly twice as much air as the same cc petrol engine, the clue is in the compression ratio.


Sorry old mate, I have to disagree with you on this one, The air capacity of any engine is dependant on the bore x stroke x rpm. Any two engines of a comparable size will draw similar volumes when run at similar revs, sure a diesel has approx double the compression rate, this is a function of piston crown height to cylinder head. Running higher compression ratios possibly increases internal losses down the bore than a lower compression engine, and coupled with the fact that diesels run slower than petrol engines, it is probable that the diesel in fact uses less air than the petrol equivalent.
The exception would be if the diesel was a turbo model. :smile:

Pete7 30-01-2021 16:11

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Uncle Bob (Post 3332292)
Sorry old mate, I have to disagree with you on this one, The air capacity of any engine is dependant on the bore x stroke x rpm. :smile:

Divided by 2 for a four stroke engine surely?

Jdege 30-01-2021 16:15

Re: How much ventilation for diesel?
 
It's my Herreshoff/Vaitses Meadow Lark.

The engine is under the cockpit, and there are two 4-inch holes on the outside coamings, and simple clamshell vent covers for them.

I'm thinking I'd like something that would make it more difficult for water to get in, but as I consider alternatives I need to know actual requirements.

Uncle Bob 30-01-2021 16:17

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Pete7 (Post 3332299)
Divided by 2 for a four stroke engine surely?


:biggrin: I guess that technically, yes, however the simple reality is that an engine running at x rpm will consume y cfm of free air.

Emmalina 30-01-2021 16:18

Re: How much ventilation for diesel?
 
If the mfct put vents in they were probably required they don't do things for fun !

PirateGuy 30-01-2021 16:18

Re: How much ventilation for diesel?
 
Compression ratio matters. Gasoline (petrol) engines run about 8:1 compression ratio. Diesels often run 20:1 or higher.

IE they consume 2,5 times (or more) air

Uncle Bob 30-01-2021 16:31

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by PirateGuy (Post 3332309)
Compression ratio matters. Gasoline (petrol) engines run about 8:1 compression ratio. Diesels often run 20:1 or higher.

IE they consume 2,5 times (or more) air


Then could you please explain just how a piston running up and down in a bore at a given speed can use 2.5 times more air if it is diesel rather than petrol. Yes the compression ratio is approx 2.5 times higher, however compression is a function of how close the piston crown is to the cylinder head at the top of the stroke.

Compass790 30-01-2021 16:39

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Uncle Bob (Post 3332292)
Sorry old mate, I have to disagree with you on this one, The air capacity of any engine is dependant on the bore x stroke x rpm. Any two engines of a comparable size will draw similar volumes when run at similar revs, sure a diesel has approx double the compression rate, this is a function of piston crown height to cylinder head. Running higher compression ratios possibly increases internal losses down the bore than a lower compression engine, and coupled with the fact that diesels run slower than petrol engines, it is probable that the diesel in fact uses less air than the petrol equivalent.
The exception would be if the diesel was a turbo model. :smile:


Yes quite right.:thumb:
I was assuming that capacity was volume measured at TDC.
WRONG! It is bore x stroke.
Old age brain fart.

Thank you for straightening that out.
I feel a bit like the NYYC now.:redface:

Jdege 30-01-2021 16:42

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Emmalina (Post 3332308)
If the mfct put vents in they were probably required they don't do things for fun !

It's a 40-year-old boat.

Who knows what the manufacturer did?

I do know that the Yanmar isn't the original engine.

Cheechako 30-01-2021 16:46

Re: How much ventilation for diesel?
 
I think most boat makers rely on ill fitting hatches and enclosures to provide the air. I often see no intake but often see exhaust fan outlets.
Some engine makers will specify how much open intake is required. I guess you don't want the engine sucking too hard. More suck=more velocity=more air.
But even on a small boat with two 6 cyl Cummins diesels it wasn't a lot. Thinking back maybe 8" x 48"

I imagine whatever the engine intake size is will work, if ducted maybe a little more, if free to breathe without a duct, just an opening that size..

Bill Seal 30-01-2021 16:54

Re: How much ventilation for diesel?
 
A diesel engine inhales about 1/2 its displacement per revolution, a gas engine running 7 inches of vacuum inhales only half as many molecules per revolution.

Uncle Bob 30-01-2021 17:06

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Bill Seal (Post 3332333)
A diesel engine inhales about 1/2 its displacement per revolution, a gas engine running 7 inches of vacuum inhales only half as many molecules per revolution.

Please, oh please explain just how it is so. A piston of a given size running in a the correct size bore will, on the downstroke, inhale the volume of the bore, regardless of the fuel the engine is designed to use.

Reefmagnet 30-01-2021 17:12

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Uncle Bob (Post 3332338)
Please, oh please explain just how it is so. A piston of a given size running in a the correct size bore will, on the downstroke, inhale the volume of the bore, regardless of the fuel the engine is designed to use.


Petrol engines have a butterfly valve to restrict air hence why they generate a vacuum whereas diesels don't. The cylinder will also have the same vacuum as the manifold at BDC, so therefore less air volume.


However, a petrol engine at full throttle will suck in more air than the equivalent capacity diesel as the valve restriction no longer applies and it will be running at higher RPM and any intake dimension needs to consider this maximum operating limit.

Compass790 30-01-2021 17:19

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Reefmagnet (Post 3332343)
Petrol engines have a butterfly valve to restrict air hence why they generate a vacuum whereas diesels don't. The cylinder will also have the same vacuum as the manifold at BDC, so therefore less air volume.


However, a petrol engine at full throttle will suck in more air than the equivalent capacity diesel as the valve restriction no longer applies and it will be running at higher RPM and any intake dimension needs to consider this maximum operating limit.


Do injected petrol engines still have a butterfly v/v?
Just curious.

Reefmagnet 30-01-2021 17:23

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Compass790 (Post 3332345)
Do injected petrol engines still have a butterfly v/v?
Just curious.


Pretty sure they do still do because petrol engines are air regulated and diesel engines are fuel regulated. If you try and use fuel alone to regulate the power, that would run a petrol engine too lean which besides probably not working too well also isn't good for their long term durability.


Diesel engines are the opposite in that running lean doesn't hurt them, but running rich (black smoke) isn't that great for them.

Uncle Bob 30-01-2021 17:27

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Reefmagnet (Post 3332347)
Pretty sure they do still do because petrol engines are air regulated and diesel engines are fuel regulated. If you try and use fuel alone to regulate the power, that would run a petrol engine too lean which besides probably not working too well also isn't good for their long term durability.


Diesel engines are the opposite in that running lean doesn't hurt them, but running rich (black smoke) isn't that good for them.

Yes, absolutely, and thanks for posting.

Reefmagnet 30-01-2021 17:35

Re: How much ventilation for diesel?
 
This discussion just made me realise that if you were to restrict the air to a diesel, it will screw up the air / fuel ratios of a dumb mechanical injection engine. There's often complaints of engines blowing excessive black smoke at high loads on this forum. Whilst generally attributed to hull growth/oversized props perhaps lack of engine bay air can also be a contributor in some of these situations?


Bears thinking about.

Jdege 30-01-2021 18:21

Re: How much ventilation for diesel?
 
So, given that the Yanmar has a 1.1 liter displacement, at 3600 rpm it will draw 1980 liters of air/minute?

70 ft^3/min?

The HVAC rule-of-thumb is that a 4" duct will carry 40 cfm. The two that are there will carry 80, which added to the multitude of gaps in the locker lids, and that these aren't 4" ducts but 4" holes suggests that they're large enough.

Westcliffe01 30-01-2021 18:27

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Jdege (Post 3332375)
So, given that the Yanmar has a 1.1 liter displacement, at 3600 rpm it will draw 1980 liters of air/minute?

70 ft^3/min?

The HVAC rule-of-thumb is that a 4" duct will carry 40 cfm. The two that are there will carry 80, which added to the multitude of gaps in the locker lids, and that these aren't 4" ducts but 4" holes suggests that they're large enough.


1.980m^3 is about 2 cubic meters
1 cubic meter is 27 cu ft
2 cubic m = 54 cu ft.

Jdege 30-01-2021 18:32

Re: How much ventilation for diesel?
 
https://www.convertunits.com/from/liters/to/cubic+feet

BillKny 30-01-2021 19:25

Re: How much ventilation for diesel?
 
I love how when you ask a very specific question here, you get a dozen people replying with snide, flip answers designed solely to make you believe they are witty and clever, but actually give you no useful information. Even people here who feel they can quote a number seem to have no idea what's actually involved.

Here is the actual answer: (Ref, David Gerr, "The Nature Of Boats")

For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

With that said, more is ALWAYS better.

Calculating the amount of air the engine actually sucks in is only part of the equation. You have to cool the room, and parts like the alternator. That, and the cooler the air ingested by the engine, the more efficient it is.

Far better to have blowers actively move the air, and the amount required looks like this:

(2.75 x HP) - 90 = cu. ft/min

So a 100 hp engine needs:

(2.75 x 100) - 90 = 185 cu ft / min

Again, MORE IS BETTER.

roverhi 30-01-2021 19:48

Re: How much ventilation for diesel?
 
One of my boats has a 2gm20F and the other a 3GM30F somewhere along the line the ventilation system for the original gas Atomic 4 has been removed. Have run the 2GM for 3 days straight and another day before that with no problem. Powered the 3GM for 200 miles also without issue. Only additional ventilation was opening the lazarette hatch. Took our Westsail to SoPac again without additional ventilation for the Volvo MD2 other than opening the lazarette hatch. Doesn't seem to be a problem though a ducted source of cool air wouldn't hurt but doesn't seem to be necessary.

Wotname 30-01-2021 19:50

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by BillKny (Post 3332404)
I love how when you ask a very specific question here, you get a dozen people replying with snide, flip answers designed solely to make you believe they are witty and clever, but actually give you no useful information. Even people here who feel they can quote a number seem to have no idea what's actually involved.

Here is the actual answer: (Ref, David Gerr, "The Nature Of Boats")

For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

With that said, more is ALWAYS better.

Calculating the amount of air the engine actually sucks in is only part of the equation. You have to cool the room, and parts like the alternator. That, and the cooler the air ingested by the engine, the more efficient it is.

Far better to have blowers actively move the air, and the amount required looks like this:

(2.75 x HP) - 90 = cu. ft/min

So a 100 hp engine needs:

(2.75 x 100) - 90 = 185 cu ft / min

Again, MORE IS BETTER.

FIFY :flowers:
:biggrin:

ggray 30-01-2021 20:18

Re: How much ventilation for diesel?
 
I think I've seen somewhere what size of the opening(s) Yanmar says is needed.

Probably in the owner's manual.

Compass790 30-01-2021 22:59

Re: How much ventilation for diesel?
 
[QUOTE=BillKny;3332404]I love how when you ask a very specific question here, you get a dozen people replying with snide, flip answers designed solely to make you believe they are witty and clever, but actually give you no useful information. Even people here who feel they can quote a number seem to have no idea what's actually involved. ( QUOTE )



I consider myself soundly spanked.
It was a regular occurrence, well caned actually when I was at school.:facepalm:
Don't think you have the right motivation but fair point otherwise.
Some are just trying to help even if mistaken.:peace:
Thanks for the formula.

Uncle Bob 30-01-2021 23:38

Re: How much ventilation for diesel?
 
I consider myself soundly spanked.
It was a regular occurrence, well caned actually when I was at school.:facepalm:
.[/QUOTE]

:biggrin: Go on, admit it, you love a good spanking don't you. :biggrin::biggrin:
:flowers:

Jdege 31-01-2021 00:10

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by BillKny (Post 3332404)
For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

(2.75 x HP) - 90 = cu. ft/min

It's a 27hp engine, so the minimum passive would be 8.2in^2. which is far less than the 25in^2 that the two 4in holes provide.

And as for forced air, the formula yields -15.75ft^3/min, which suggests that it doesn't need forced air.

Plus, given where the engine sits, it can pull air through all of the cockpit lockers, none of the lids of which are air tight.

All of which suggest to me that restricting the flow of the two existing holes a bit, by adding a baffle to make it harder for water to get in, would not cause problems.

Thanks.

Jim Cate 31-01-2021 00:27

Re: How much ventilation for diesel?
 
Well, we've had the theory now ( and I agree with those who say that a diesel will suck in about half its displacement per revolution, averaged out.

But the nitty: make up a simple manometer with some clear plastic tubing. One end goes to the engine room/box, the other to the cabin air. Run engine at various speeds and see how much pressure drop there is in the box with various vent schemes.Now you have data which you can send to the engine mfg for evaluation.

But I strongly suspect that you will find that it gets plenty of air with only one 4 inch vent plus all the other sources that exist in most boats.

Jim

Compass790 31-01-2021 02:33

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Jim Cate (Post 3332490)
Well, we've had the theory now ( and I agree with those who say that a diesel will suck in about half its displacement per revolution, averaged out.

But the nitty: make up a simple manometer with some clear plastic tubing. One end goes to the engine room/box, the other to the cabin air. Run engine at various speeds and see how much pressure drop there is in the box with various vent schemes.Now you have data which you can send to the engine mfg for evaluation.

But I strongly suspect that you will find that it gets plenty of air with only one 4 inch vent plus all the other sources that exist in most boats.

Jim


There's a major source of hot air in our boat.....
Funnily enough it's related to beer, not sure of the exact formulae but its a logarithmic progression:whistling:
Whoops I mite be heading for a thrashing again.:popcorn:

GordMay 31-01-2021 02:53

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by BillKny (Post 3332404)
... Here is the actual answer: (Ref, David Gerr, "The Nature Of Boats")

For natural ventilation of an engine room without blowers, the minimum recommended area (in sq inches) is (Engine HP) / 3.3 Increase by at least 20% if the hose are long or convoluted.

So a 100 HP engine should require a MINIMUM of 100/3.3 = 30 sq in of vent.

With that said, more is ALWAYS better.

Calculating the amount of air the engine actually sucks in is only part of the equation. You have to cool the room, and parts like the alternator. That, and the cooler the air ingested by the engine, the more efficient it is.

Far better to have blowers actively move the air, and the amount required looks like this:

(2.75 x HP) - 90 = cu. ft/min

So a 100 hp engine needs:

(2.75 x 100) - 90 = 185 cu ft / min

Again, MORE IS BETTER.

See also, some earlier CF discussions:


Engine Ventilation: Skip it?
https://www.cruisersforum.com/forums...-it-37561.html

Engine room ventilation
https://www.cruisersforum.com/forums...n-85829-2.html

Engine Room Blower
https://www.cruisersforum.com/forums...wer-42656.html

Engine ventilation
https://www.cruisersforum.com/forums...tion-7269.html
Engine Bay extractor fan
https://www.cruisersforum.com/forums...an-158145.html

Caterpillar has an excellent guide, which seems to go missing (links don’t work), requiring a search, each time.
“Engine Room Ventilation” [Caterpillar publication # LEBW4971-03]
https://www.scribd.com/document/1061...on-LEBW4971-03

Squanderbucks 31-01-2021 09:51

Re: How much ventilation for diesel?
 
:thumb: GordyMae. There is more need for an engine room than just what the engine consumes. Anyway when engineers do all the fancy calculations they often end up after their all done adding in a + fudge factor to be safe. Safe bet the route or re-route what you have as long as yo go no smaller. When I added a larger genset and repowered from 427 cui gas to 5.9L diesel I added additional passive and active air intakes as well as fan exhausts. I installed a temperature probe and watched what the engine room temp was with various configurations and went from 145 deg F down to 115-118F. Engine room designers recommend max engine room temps below 120F for engine and accessory life expectancy. Always avoid the minimum of most things and see what the maximum is you can reasonably provide - all things considered.

walbee 31-01-2021 10:03

Re: How much ventilation for diesel?
 
1 Attachment(s)
The attach Cat Guide I found helpful, although a bit math intensive, it does reveal variables I had not considered.


Quote:

Originally Posted by Jdege (Post 3332221)
I have a boat with a Yanmar 3GM30, that had it's engine compartment vents removed for painting.

I could simply replace what was there, but I'm not really thrilled with the location or design.

But before I do anything different, where could I find out how mugggch air intake a 3GM30 requires?

And how large a vent area would be required to provide this?


jdaltonpe 31-01-2021 10:25

Re: How much ventilation for diesel?
 
In my experience more air is better all the way around to keep temp down and alternators happy, especially if it is via natural aspiration like dorad vents. One advantage that I donít see mentioned is air exchange at anchor which seems to help keep corrosion at bay in the engine room.

GordMay 31-01-2021 10:40

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Squanderbucks (Post 3332705)
:thumb: GordyMaeGordMay. There is more need for an engine room than just what the engine consumes ...
... Always avoid the minimum of most things and see what the maximum is you can reasonably provide - all things considered.

Indeed.

Hillbilly 31-01-2021 10:54

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Bill Seal (Post 3332333)
A diesel engine inhales about 1/2 its displacement per revolution, a gas engine running 7 inches of vacuum inhales only half as many molecules per revolution.

Bill Seal finally gets to the heart of it. It's not compression, it's air/fuel ratio.
In order to get reliable spark ignition and burn properly with a mixed incoming charge from the carburetor or equivalent, a gas engine must maintain an A/F ratio within narrow limits. This is done by "throttling" the air inflow to match the load on the engine. The resulting manifold vacuum requires the engine to consume extra energy in order to act like a vacuum pump. This probably is the main source of diesel efficiency advantage.

The diesel runs with hardly any inflow restriction so its air consumption is greater and depends almost entirely on rpm. A/F ratio doesn't matter because fuel is introduced directly into the combustion chamber as a mist and self ignites. The fact that there normally is an excess of air promotes more complete combustion. Notice the over-sized exhaust pipes on diesel pickups.

OS2Dude 31-01-2021 11:04

Re: How much ventilation for diesel?
 
Our 1985 Catalina 30 has similar vents. To my understanding, it is because originally Catalina used Automic4 gasoline engines which required venting with blowers to remove any vapors from around the fuel tank (Stbd vent) and the engine (Port vent). Our boat has a Universal M25 diesel (which does not require venting for vapors) but still has vents. I use the engine vent & blower to pull heat and smell from the cabin. (Engine is under the settee and galley cabinet in main saloon.)

If your boat has vestigial vents like ours, you could likely close them off. It may be hard to get a good gelcoat color match and finish though.


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