How much ventilation for diesel?

[Uncle Bob]

Originally Posted by 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.

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]

Originally Posted by Uncle Bob

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]

Originally Posted by Reefmagnet

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]

Originally Posted by Compass790

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]

Originally Posted by Reefmagnet

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]

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]

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]

Originally Posted by Jdege

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]

https://www.convertunits.com/from/liters/to/cubic+feet

[BillKny]

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]

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]

Originally Posted by BillKny

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

[ggray]

I think I've seen somewhere what size of the opening(s) Yanmar says is needed.

Probably in the owner's manual.

[Compass790]

[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.
Don't think you have the right motivation but fair point otherwise.
Some are just trying to help even if mistaken.
Thanks for the formula.

[Uncle Bob]

I consider myself soundly spanked.
It was a regular occurrence, well caned actually when I was at school.
.[/QUOTE]

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