How much fresh air does an engine need?

[Babylon]

If an engine-bay is well-insulated (for noise), how will a typical naturally-aspirated small yacht diesel engine, say a Beta 25 or similar, get enough air for the combustion process?

On my Vancouver 27, the engine bay is open at its aft end to the cockpit lockers (the lids of which are sealed against seawater ingress when closed). On one side of the engine bay is the sealed quarter-berth, on the other an open-fronted oilies locker with a 3 inch gap at the bottom to the base of the engine bay. This gap currently allows air into the engine-bay, as well as providing a source of heat to dry wet oilies when under power. But it also allows a lot of engine noise to escape through the locker into the cabin.

If I seal the gap then I lose both fresh and warm air flows. If I cover it with a baffle made from ply faced with sound-insulation - thus reducing the gap - will I starve the engine of sufficient air?

 

[sailorman]

If an engine-bay is well-insulated (for noise), how will a typical naturally-aspirated small yacht diesel engine, say a Beta 25 or similar, get enough air for the combustion process?

On my Vancouver 27, the engine bay is open at its aft end to the cockpit lockers (the lids of which are sealed against seawater ingress when closed). On one side of the engine bay is the sealed quarter-berth, on the other an open-fronted oilies locker with a 3 inch gap at the bottom to the base of the engine bay. This gap currently allows air into the engine-bay, as well as providing a source of heat to dry wet oilies when under power. But it also allows a lot of engine noise to escape through the locker into the cabin.

If I seal the gap then I lose both fresh and warm air flows. If I cover it with a baffle made from ply faced with sound-insulation - thus reducing the gap - will I starve the engine of sufficient air?

4stroke diesel -1 suction stroke every 4 cycles X 1000cc @ 2000rpm = air volume consumed
change figures to suit your requirements

 

[Ubergeekian]

4stroke diesel -1 suction stroke every 4 cycles X 1000cc @ 2000rpm = air volume consumed
change figures to suit your requirements

ie every 2 revolutions.

 

[nimbusgb]

4stroke diesel -1 suction stroke every 4 cycles X 1000cc @ 2000rpm = air volume consumed
change figures to suit your requirements

1 suction stroke per cylinder every 2 cycles. ( piston down, suck in or induction, piston up compression, ignition at top of stroke ( 1 crank revolution ) piston down expansion/power, piston up exhaust ( 2 revolutions ) .... repeat ad infinitum )

3 cylinders on a beta 25 so actually 9 sucks every 6 rotations and 3 sucks to total the swept volume so at 2000 rpm = 666.6* 900cc = 600 litres of air per minute. Not inconsiderable! At the rated 3600 rpm ......

If you DO restrict airflow to the extent that there is a partial vacuum in the cockpit locker you could be exerting perhaps 5 to 10 pounds per square inch on the locker tops, sides and floor. Air pressure is powerful stuff over a large area!

 

[oldsaltoz]

I run a flexible tube about 6 inched across to feed a 39 HP Volvo, no other ventilation at all. Oh I have another in the other hull both set up the same, both tubes run to the engine compartment floor.

 

[Latestarter1]

If an engine-bay is well-insulated (for noise), how will a typical naturally-aspirated small yacht diesel engine, say a Beta 25 or similar, get enough air for the combustion process?

On my Vancouver 27, the engine bay is open at its aft end to the cockpit lockers (the lids of which are sealed against seawater ingress when closed). On one side of the engine bay is the sealed quarter-berth, on the other an open-fronted oilies locker with a 3 inch gap at the bottom to the base of the engine bay. This gap currently allows air into the engine-bay, as well as providing a source of heat to dry wet oilies when under power. But it also allows a lot of engine noise to escape through the locker into the cabin.

If I seal the gap then I lose both fresh and warm air flows. If I cover it with a baffle made from ply faced with sound-insulation - thus reducing the gap - will I starve the engine of sufficient air?

Engineering rule of thumb is that engines consumes 200 CFM for every 100 Hp produced therefore 50 sq inches per 100 Hp is a good starting #.

25 Hp @ rated equalls 12.5 square inches. Sounds like a lot, but one of the things besides restriction that engineers want to accomplish is to slow down the velocity of the air movement in the engine room (this is very important in smaller engine rooms). This allows moisture/dirt, etc. to drop out of the air before making it to the air inlet. Eng. room temp also plays a major part is the overall equation and besides keeping the max temp below about 120 F. In practice I have found that you can cut the above #'s pretty much in half PROVIDING that you a decent Delta T.

Most engine manufacturers specify a max Delta T of 30 F between engine compartment and outside ambient. Engine compartment Delta T check is normally part of manufacturers sign off, should be in the sadly neglected installation manual.

 

[Salty John]

Assuming the 3" slot you have is at least 5" long you will get enough air.
You could, perhaps, create a labyrinth entrance to this slot. This should let air in, heat out, but retain much of the noise.

 

[Babylon]

...Eng. room temp also plays a major part is the overall equation and besides keeping the max temp below about 120 F. In practice I have found that you can cut the above #'s pretty much in half PROVIDING that you a decent Delta T.

Most engine manufacturers specify a max Delta T of 30 F between engine compartment and outside ambient.

Interesting. Converting to new money, a differential of 30 F is equivalent to 16 C.

Having posted my question, I then looked up the Beta Instruction Manual PDF, which doesn't mention supply to the engine air intake as such, but states:

"The engine compartment must be adequately ventilated as the engine will produce radiated heat - approximately equal to 1/3 of the engine output power. Also the battery charging alternator(s) create lots of heat. Ventilation is normally best with two holes; an inlet of at least 200 cm2 allowing colder air to enter near to the alternator and drive belts and a second outlet (a third bigger than the inlet) for the hot air to rise and ventilate out from the opposite side of the top of the engine compartment.

In yachts we strongly recommend forced ventilation - using an extraction fan to draw out the hot air and limit the engine compartment /room temperature to a maximum of 60°C. You can check the temperature in the engine compartment with a thermometer - the cooler the better - maximum temperature 60°C [140 F]; this is best done on a hot sunny day. (A symptom of overheating problems is black belt dust)."

I used to get black belt dust with the old Bukh DV20 (similar capacity), so engine bay wasn't sufficiently ventilated as it was.

 

[Babylon]

Assuming the 3" slot you have is at least 5" long you will get enough air. You could, perhaps, create a labyrinth entrance to this slot. This should let air in, heat out, but retain much of the noise.

The slot is about 14" long, so plenty of scope there to create a sufficient baffle - ie two L-section lips - each faced with 12mm insulation - overlapping one another by a decent margin. Just need to make sure we don't then pile the bottom of the locker with seaboots with oilies hanging over them!

 

[BAtoo]

The slot is about 14" long, so plenty of scope there to create a sufficient baffle - ie two L-section lips - each faced with 12mm insulation - overlapping one another by a decent margin. Just need to make sure we don't then pile the bottom of the locker with seaboots with oilies hanging over them!

FWIW - my last boat - 28hp engine (Volvo2003) - had 2 x 6inch rigid plastic tubes to allow air into the engine compartment + 2 other vents without tubes + "leaks".
My current boat - 18HP(Volvo 2002) - has 1 x 6 inch rigid tube plus "leaks" to do the same.

 

[ccscott49]

Interesting. Converting to new money, a differential of 30 F is equivalent to 16 C.

Having posted my question, I then looked up the Beta Instruction Manual PDF, which doesn't mention supply to the engine air intake as such, but states:

"The engine compartment must be adequately ventilated as the engine will produce radiated heat - approximately equal to 1/3 of the engine output power. Also the battery charging alternator(s) create lots of heat. Ventilation is normally best with two holes; an inlet of at least 200 cm2 allowing colder air to enter near to the alternator and drive belts and a second outlet (a third bigger than the inlet) for the hot air to rise and ventilate out from the opposite side of the top of the engine compartment.

In yachts we strongly recommend forced ventilation - using an extraction fan to draw out the hot air and limit the engine compartment /room temperature to a maximum of 60°C. You can check the temperature in the engine compartment with a thermometer - the cooler the better - maximum temperature 60°C [140 F]; this is best done on a hot sunny day. (A symptom of overheating problems is black belt dust)."

I used to get black belt dust with the old Bukh DV20 (similar capacity), so engine bay wasn't sufficiently ventilated as it was.

You can only have an effective engine space extractor fan, if the engine combustion air is drawn from outside of the engine space. No engine space extractor fan will keep up with an engine breathing inside the engine space. I hope that makes sense.

 

[bikedaft]

in my ignorance i reckoned that what goes in must be similar to what goes out ie exhaust dia etc

will i run into trouble?

thanks for the interesting maths tho...

 

[Deleted member 36384]

You can only have an effective engine space extractor fan, if the engine combustion air is drawn from outside of the engine space. No engine space extractor fan will keep up with an engine breathing inside the engine space. I hope that makes sense.

The extractor fan is not trying to keep up with air drawn into the engine space by the engine. Its simply moving hot air out which is replaced by some of the air drawn in by the engine.

The position of the extractor duct inlet is more important than the flow rate of the extractor fan. It should be optimised to extract hot air from a position that is high and opposite to the engine air inlet.

 

[ccscott49]

The extractor fan is not trying to keep up with air drawn into the engine space by the engine. Its simply moving hot air out which is replaced by some of the air drawn in by the engine.

The position of the extractor duct inlet is more important than the flow rate of the extractor fan. It should be optimised to extract hot air from a position that is high and opposite to the engine air inlet.

It is far better to feed the engine cold/cooler air from outside the engine space and allow the extractor to draw out the hot air, it needs a vent into the engine space of course, but most of the air normally comes out of the bilges, no panels fit that well. This is basically what I was trying to say, your way is inefficient in the extreme.

 

[Deleted member 36384]

It is far better to feed the engine cold/cooler air from outside the engine space and allow the extractor to draw out the hot air, it needs a vent into the engine space of course, but most of the air normally comes out of the bilges, no panels fit that well. This is basically what I was trying to say, your way is inefficient in the extreme.

I am not saying where the engine is getting its air from at all. You make a comment about extractor fans keeping up with air flow by an engine "breathing" in an engine space.

The air drawn in by an engine "breathing" in an engine space more or less flows straight into the air intake by the shortest, least resistant route. The air around the engine gets hotter, not cooler despite what is quite a high volume flow rate of air into the space caused by the engine demand.

Placement of an extractor in an optimum position removes the hot air which must be replaced by some of the colder flowing in by the draw of the engine (and the small flow rate of the extractor fan) combined.

I interpreted your post, where you quoted Babylon as a challenge to the Beta Instruction Manual that Babylon quotes, where your implication was that the extractor is responsible for sucking in combustion air, hence my comment.

I agree with what you say about the engine source of air from vents or bilge space in the engine room, however, if the hot air can not be removed by convection through high vents, then an extractor (blower) is required to pull the hot air out, not to pull air into the engine space for combustion.

So "my way", is not what you think.

 

[PetiteFleur]

My Beta 25HP has 3 x 3" Dia Flexible hoses which lead to a vents in the cockpit. There is a fourth hose but this is just laying in the locker alongside(can't fit it onto the spigot as the gas locker is in the way!). In addition there are gaps at the rear into the bilges. Never had a problem with air starvation. The original engine was a BMC 35HP.