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BillKny 31-01-2021 11:11

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Hillbilly (Post 3332741)
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.

Actually... not.

The biggest source of diesel "efficiency" is just the simple fact that we buy fuel and measure its usage by volume, and diesel fuel is significantly heavier than gasoline. Diesel contains more energy per gallon than gasoline.

If we measured fuel usage by weight of fuel consumed (miles/pound or kilometer/kilogram) you'd see that diesel engines are actually very similar in efficiency to gasoline engines in converting the energy contained in the fuel to actual work.

Numbers: Gasoline: 0.75 grams/liter Diesel: 0.85 grams/liter

So making the very accurate assumption that gasoline and diesel have the same energy density available per gram of fuel burned, with nothing else happening at all you get 13% more power out a liter of diesel fuel as you do from gasoline.

adjo 31-01-2021 12:51

Re: How much ventilation for diesel?
 
It's good practice to allow for airflow around the engine to assist in cooling and to cool the engine compartment itself: especially in the tropics and warm waters. So inlet and outlet should be significantly bigger than that which is required for combustion air alone.
All the air used in combustion exits via the exhaust anyway (hopefully!).

As mentioned above, many boats suck air from the bilge which can help keep the air in this area fresh. A well designed boat, however, should have a sealed engine room/compartment that can be effectively acoustically and thermally insulated (and closed off in case of fire). Without a generous air inlet/outlet the entire compartment gradually heats to the temperature of the motor.

NorthCoastJoe 31-01-2021 15:50

Re: How much ventilation for diesel?
 
Get a probe thermometer and run it hard with vents open then try it with vents restricted. I would keep the air temp below 120 F. They put intercoolers on turbo diesel for a reason. Cool air is denser more oxygen.

I think the issue is more about cooling air than combustion air. If you close it up too much expect problems. On my boats, I have a lot of ventilation into and out of the engine compartment.

Jdege 31-01-2021 17:28

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by OS2Dude (Post 3332744)
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.

The original Herreshoff Meadow Larks had gas engines. The Vaitses Meadow Larks all seem to have been one-offs, with very little standardization. Curlew may well have had gas engines to start.

What I'm thinking I might do, one I have her in the water, is to seal the vents, run the diesel with a cockpit lockers open, then close the locker and see what happens.

It may be that there is no real need for vents.

petermck 31-01-2021 18:19

Re: How much ventilation for diesel?
 
I recently went down this route during a rebuild and was surprised by the result.
The rules of thumb I was told is app. 50"sq per 100HP on marine diesel engines and 1”sq per HP for convection cooling.

After doing some number crunching I arrived at the following-
Intake airflow for combustion of both engines is 585 CFM
Free air volume in the engine room is 3.3m and should be changed twice every minute = 110 l/s
30% volume to be added for use in tropics (Indonesia, Malaysia, Thailand) = 143 l/s or 302 CFM
The total air intake required is app. 900 CFM through 2 moisture eliminators.

9” fan will pull 600 – 700 CFM into the engine room and 1 off 320 CFM extraction blower is needed but maintaining slight negative pressure within the engine room would require 2 blowers.
Maximum airflow velocity of 3 m/s is also a consideration when sizing intakes.

The original intakes were only providing 30% of what the engines required. It appears that the vacuum created at WOT sucked air from the engine crankcase and helps understand why the air intakes and turbos had a film of oil inside and aftercoolers were clogged with shellac.

I ended up getting some moisture eliminator intakes (single-stage vane separators) fabricated by Camfil in NZ, DeltaT and a few others produce similar products.

I hope that this helps.

CarinaPDX 31-01-2021 18:56

Re: How much ventilation for diesel?
 
1 Attachment(s)
Odd how it took so long for this thread to get past just the need to feed the engine intake. Diesels require cooling air as well. I have attached Volvo's requirements from the installation manual. Attachment 231691

Greg

CarlF 31-01-2021 19:16

Re: How much ventilation for diesel?
 
Years ago I attended a Mack Boring Yanmar boat owner's diesel engine class. The instructor - who had been with Mack Boring since a young man told a story of when he was in his 20's starting out in the 1970's

One day his boss told him that a celebrity Yanmar owner - my memory is hazy but it may have been Art Carney (my apologies if I am wrong) was having trouble with his Yanmar engine. He was dispatched to the marina to find Mr. Carney onboard with three beautiful Hollywood starlets - who soon took off all their clothes.

Trying not to be distracted by the young ladies, he opened the engine hatches and had Mr. Carney start the engine. Ran perfectly. So they motored out of the harbor with the hatches still open - engines still running perfectly. After half an hour he was stumped and told Mr. Carney to head back to the dock - closing the engine hatches. Within a few minutes the engines died. He rushed to open the hatches and there was so much suction he couldn't open them.

He then went below and found that all of the engine vents had been blocked. He said "Mr. Carney - why are these vents covered" The answer "The girls didn't like the noise".

So one practical solution for the OP would be to cover the vents and see if the engine gains power as he remove the covers. I believe the starlets are optional.

Emmalina 01-02-2021 19:02

Re: How much ventilation for diesel?
 
Funny thing is if I run my engine with the engine room door open it runs hotter than when closed I can only presume it sucks in all the hot air out of the engine room then starts sucking it out of the bilge ! I have no engine room vents only gaps from the bilge !

Compass790 01-02-2021 20:08

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Emmalina (Post 3333820)
Funny thing is if I run my engine with the engine room door open it runs hotter than when closed I can only presume it sucks in all the hot air out of the engine room then starts sucking it out of the bilge ! I have no engine room vents only gaps from the bilge !


Have you got 3 naked starlets tho?
Sorry I couldnt resist:biggrin:

IanTrail 05-02-2021 10:39

Re: How much ventilation for diesel?
 
I think the bigger issue is keeping a good airflow through the engine compartment for cooling and removal of fumes. A car or truck engine is virtually in the open air and we know how hot and smelly they get. Imagine that sealed up in a box! My 140A alternator quickly overheats when working hard unless it is in a good airflow. So I have outside air blowing into the back of the alternator and an extractor taking the hot air out of the top of the engine compartment. I probably have about 3000L/min blown into the engine compartment, 2000L/min sucked out by the engine and the other 1000L/min sucked out by the extractor. It keeps everything cool and sweet.

Perfect Ride 05-02-2021 11:12

Re: How much ventilation for diesel?
 
For what it is worth, I have had blowers with vents on 40hp diesel engine compartments. I believe it was due to regulations for gas engines that flowed over to diesel. I did not ever use them. The engines have run fine with only the air flow from the floor boards for over 2000 hours.

IanTrail 05-02-2021 11:24

Re: How much ventilation for diesel?
 
Indeed, I guess ventilation must be much more important for gasoline engines.
Whether you need blowers presumably depends to some extent to how much air 'leaks' into your engine compartment. My diesel certainly needs a good draught through, specially in hotter climates.

PineyWoodsPete 05-02-2021 12:25

Re: How much ventilation for diesel?
 
1 Attachment(s)
Quote:

Originally Posted by Jdege (Post 3332997)
The original Herreshoff Meadow Larks had gas engines. The Vaitses Meadow Larks all seem to have been one-offs, with very little standardization. Curlew may well have had gas engines to start.

What I'm thinking I might do, one I have her in the water, is to seal the vents, run the diesel with a cockpit lockers open, then close the locker and see what happens.

It may be that there is no real need for vents.


My prior Vaites '87 Meadowlark 37 had interesting vents on the the cockpit comings in the cast bronze winch base, which had screw-down caps over the ~4" vents in the base. The rounded, smooth outside of the bases made a good turning point for hauling up the leeboards, with a jam cleat termination, and freeing the jib-sheet winches/cleats. Westerbeake 25 4-cyl engine.

Jopinot 05-02-2021 16:06

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:

Absolutely correct. Intake is derived form piston vacuum (unless forced with turbo/blowers). Air requirement = Engine Cubic capacity x max RPM. Exhaust flow is the intake plus thermal volume expansion so the intake requirement is less than the exhaust. Id work on a minimal sectional area half the exhaust elbow.

Jdege 05-02-2021 21:29

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by PineyWoodsPete (Post 3336377)
My prior Vaites '87 Meadowlark 37 had interesting vents on the the cockpit comings in the cast bronze winch base, which had screw-down caps over the ~4" vents in the base.

Mine was built in '78, and the cockpit coaming is different. Molded fiberglass about 2-1/2 inches across, and the vents are on the outside.

Still too narrow to mount a winch. The boat only came with one, on the coach roof.

I've been musing about how to install winches for the sheets, despite the prior owners saying he'd never missed them.

And I've considered building some kind of platform, over where the vents are, that would combine acting as a base for a winch and a baffled vent.

What you have looks accomplishes the same thing.

NPCampbell 05-02-2021 22:11

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. 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:

I 2nd this ... with the caveat that intake valve duration has an impact. Diesel camshaft lift and duration tends to be less than gasoline engines (esp performance engines) which constricts air intake on a diesel during the intake stroke more than a gasoline engine. Excluding the fact that diesel has 15% more power per unit volume than gas, HP of the engine is not material as the ideal stoich ratio of diesel (14.5:1) is similar to gasoline (14.7:1) so for a given amount of fuel squirted into the cylinder, both engines require roughly the same amount of air for efficient combustion.

Nineteen 06-02-2021 00:35

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.

Not true. A petrol and a duesel of the same displacement will use the same amount of air per cycle. The diesel will just compress it much more.
Cheers

Tonali99 06-02-2021 09:35

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Uncle Bob (Post 3332305)
: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.

Okay ... just what does RPM have to do with volume?
So it moves more cfm in a given time frame, NOT more cfm per stroke.

MrChris 06-02-2021 12:38

Re: How much ventilation for diesel?
 
I have that engine in my Colvic 27, no external vents - the engine space is open to the boat space. Via the bilges and through space on one side of the cabin aft across the battery space.

PineyWoodsPete 06-02-2021 13:58

Re: How much ventilation for diesel?
 
1 Attachment(s)
Quote:

Originally Posted by Jdege (Post 3336674)
Mine was built in '78, and the cockpit coaming is different. Molded fiberglass about 2-1/2 inches across, and the vents are on the outside.

Still too narrow to mount a winch. The boat only came with one, on the coach roof. I've been musing about how to install winches for the sheets, despite the prior owners saying he'd never missed them.

And I've considered building some kind of platform, over where the vents are, that would combine acting as a base for a winch and a baffled vent.

What you have looks accomplishes the same thing.


That sounds like a solution.

We found the small non-overlapping jib w/boom and traveler too small and clobbered up the foredeck - replaced with a stock full-hoist Catalina 25 150% genny on a CDI Flexible furler - great improvement in performance, with about a 125% overlap, but a bit more crew work with the leeboards, and the winches def. needed for the headsail.

Uncle Bob 06-02-2021 15:49

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Uncle Bob (Post 3332305)
: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.

Quote:

Originally Posted by Tonali99 (Post 3336861)
Okay ... just what does RPM have to do with volume?
So it moves more cfm in a given time frame, NOT more cfm per stroke.

Ah, do you really need to have my quote explained? Hint, nowhere do I mention cfm/stroke, merely that an engine running at x rpm will consume y cfm of free air, or put another way, the faster it goes the more air it moves.

hpeer 06-02-2021 16:03

Re: How much ventilation for diesel?
 
Funny this thread.

Just this morning I decided to remove the ducting to the engine room.

72hp YANMAR 4JH4, original was a Perkins (4-236???). Anyway the engine compartment has a 5” diameter duct which runs through a closet and the opens inside a cockpit locker. That locker has a plastic cover. I always try to remember to olen the cover but sometimes that doesn't happen or it blows closed. Opening and closing the cover has no effect in the engine. There is no noticeable suction in the plastic cover.

That tells menthe effective air moving through this gizmo is about zero. Perhaps a practical test on a vent would be to generate some smoke (mosquito coil??) and see if the vent move it.

Anyway I am going to remove the vent tube. The engine room will still have a hole for the air to come in, just from the closet. For what ever reason the vent tube terminates into a box on the side of the engine box. It almost looks like it was intended to house a fan but there are no wires to it and no other holes in it.

Anyway, thinking this through, I see no downside of removing this vent tube and covering the opening with a screen.

Now that I think of it , this would make the perfect place to spray an extinguisher into the engine room in case of fire.

Jimmyhenry 07-02-2021 15:08

Re: How much ventilation for diesel?
 
4 stroke engines, petrol or diesel, operate on the old method of SUCK, SQUEEZE, BANG, PHEWEY.
That is the 4 strokes, 2 up and 2 down.

If the engine is 2000cc capacity, then every 2 revolutions it inhales, and exhales the 2000cc, albeit the exhaust being hot will be more volume (That's how a turbo works so well.)

Give or take, for a 2 litre engine, the air going in will be for say 2000 RPM, 1000 times the capacity, (i.e.: 1000 X 2000cc) So about 2 cubic metres of air per minute.

So if the engine is running, it will suck all the air in the engine area quite nicely, and then suck in air from outside the room. Conveniently removing engine smells out eh exhaust.

Remember, diesels like to have plenty of air, so if the engine room is tightly sealed you will end up with negative pressure inside that room, not a good thing.

Pete

Compass790 07-02-2021 22:29

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Nineteen (Post 3336710)
Not true. A petrol and a duesel of the same displacement will use the same amount of air per cycle. The diesel will just compress it much more.
Cheers


Not quite right but yes I was wrong:facepalm:. See post #12, I've already taken my punishment.
Need to read the whole thread to get the whole story.

Icarus 07-02-2021 23:42

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.


The amount of air sucked into the combustion chamber is the same regardless if it is a diesel or petrol engine if both are of the same displacement.
Compression has nothing to do with it.

Uncle Bob 08-02-2021 01:29

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Icarus (Post 3337806)
The amount of air sucked into the combustion chamber is the same regardless if it is a diesel or petrol engine if both are of the same displacement.
Compression has nothing to do with it.

Hi, he has already admitted his confusion, been whipped and beaten, (and loved it :biggrin:) and as he has said, reading the whole thread would have revealed this. :flowers:

mikesu2016 08-02-2021 03:26

Re: How much ventilation for diesel?
 
To answer the OP's question, I would say, that I have a Volvo 3cyl 29hp diesel (about same size/output as the Yanmar) driving my 13.5t gaffer and it is under the cabin steps - a fairly closely fitted sound-proofed box (not very efficient for sound, as there is no bottom to it!) All the combustion air is therefore sucked from the space under the floor. This under floor space runs almost from end to end of the boat and contains 6-7 tons of lead, iron sash weights and some pig iron (all the Ballast in fact) held on cradles between the frames clear of the hull planking. So no restrictions on air flow at all.

Most days it gets used twice for 20mins or so to get out of the marina, down the river to where we can sail as the point is to sail as much as possible.

Unless the OP's engine space is an airtight box, I would guess that he doesn't need to worry about air vents at all.



Here's hoping we all get to sail again this year !


Mike

Nineteen 08-02-2021 04:53

Re: How much ventilation for diesel?
 
All good - I was speaking generally and on the fly. I wasn't trying to factor in density, dew point and mixture strength[emoji6]
Quote:

Originally Posted by Compass790 (Post 3337797)
Not quite right but yes I was wrong:facepalm:. See post #12, I've already taken my punishment.
Need to read the whole thread to get the whole story.


Compass790 08-02-2021 05:01

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by Nineteen (Post 3337890)
All good - I was speaking generally and on the fly. I wasn't trying to factor in density, dew point and mixture strength[emoji6]


Hell, I was only on the beer when I got into trouble, dont wanna start on the fly! :biggrin:

Nineteen 08-02-2021 09:13

Re: How much ventilation for diesel?
 
[emoji1360][emoji23][emoji23][emoji23][emoji23][emoji23]
Quote:

Originally Posted by Compass790 (Post 3337894)
Hell, I was only on the beer when I got into trouble, dont wanna start on the fly! [emoji3]


FPNC 09-02-2021 15:29

Re: How much ventilation for diesel?
 
With a small engine on a sailboat, combustion ventilation is usually not a problem. You need to worry about ventilation for removing heat. Both engine heat and alternator heat. As far as the engine is concerned, if your 155 F or less, your fine. Cooler is always better with diesel. Get a thermometer.

Jdege 09-02-2021 15:41

Re: How much ventilation for diesel?
 
Quote:

Originally Posted by FPNC (Post 3339370)
With a small engine on a sailboat, combustion ventilation is usually not a problem. You need to worry about ventilation for removing heat. Both engine heat and alternator heat. As far as the engine is concerned, if your 155 F or less, your fine. Cooler is always better with diesel. Get a thermometer.

Another helpful tip.

I'd planned on covering the vents, closing the hatches, and seeing how the engine ran. Adding a thermometer to the mix should be simple enough. It's the engine getting enough oxygen? Is it getting too hot?

It's something I need to understand, as I determine what needs to be done to this boat.

Of course, it will have to wait. Temps here, this week, are below zero fahrenheit.

FPNC 10-02-2021 06:34

Re: How much ventilation for diesel?
 
If the engine has the ability to reach and maintain wide open throttle underway without overheating and the engine compartment is not too hot, then the engine installation should be fine. Keeping the engine compartment as cool as possible so as not to heat soak the living quarters is another matter.


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