How much ventilation for diesel?

[Jdege]

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]

Look at your car engine, what size is the intake just before the air filter? Not very big!

[Compass790]

Originally Posted by flyingfin

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]

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]

Originally Posted by Compass790

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.

[Pete7]

Originally Posted by Uncle Bob

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.

Divided by 2 for a four stroke engine surely?

[Jdege]

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]

Originally Posted by Pete7

Divided by 2 for a four stroke engine surely?

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]

If the mfct put vents in they were probably required they don't do things for fun !

[PirateGuy]

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]

Originally Posted by PirateGuy

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]

Originally Posted by Uncle Bob

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.

Yes quite right.
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.

[Jdege]

Originally Posted by Emmalina

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]

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]

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.