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How much air does a bmc 1.5 need


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Looking at rebuilding my deck boards soon and soundproofing my engine bay as good as I can by boxing the engine in with soundproofing.

Thinking of putting in a vent with a baffle arrangement how big mm2 does the engine need to have to to run happily?

Raw water cooled.

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Looking at rebuilding my deck boards soon and soundproofing my engine bay as good as I can by boxing the engine in with soundproofing.

Thinking of putting in a vent with a baffle arrangement how big mm2 does the engine need to have to to run happily?

Raw water cooled.

 

 

To breathe for combustion it needs a free area the same as that for the inlet manifold connection at the airfilter, plus a bit for losses between the inlet and the manifold if you use a trunking.

 

However you need to keep the incoming combustion air cool- that means either an insulated trunk from outside the box to the engine air inlet or vents to supply cold air at the bottom and let the hot air out of the top. Sizing these depends on how hard you intend to work the engine or long periods.

 

N

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Had envisioned a vent in the deck boards at the rear of the system to provide the air to the area around engine and allow the engine to breathe normally as it does now through a air filter.

Would this not be sufficient?

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Looking at rebuilding my deck boards soon and soundproofing my engine bay as good as I can by boxing the engine in with soundproofing.

Thinking of putting in a vent with a baffle arrangement how big mm2 does the engine need to have to to run happily?

Raw water cooled.

 

I have boxed my Bukh 1500CC engine in with soundproofing on the inside of a 12mm ply box (saves on soundproofing and you can ensure good adhesions).

 

The back (stern end) of the box simply sits across the engine beds so there is about a 6" gap below it for air to be drawn in. It has not caused any problems I have detected including during a 12 hour, flat out run on the Thames in flood.

 

In theory completely boxing in and fitting baffled air intakes would have been a quieter solution but than what I did but then the box would have obscured the stern gland or there would have been problems with drip tray liquids attacking the soundproofing.

 

I have also put close cell neoprene sealing strips under the deck boards where they sit on the metal supports (hatch tape). This also helps reduce the sound.

 

This made a great deal of difference but how much was down to the soundproofing, the "near hospital" silencer, or moving the exhaust outlet to point backwards (stops exhaust noise bouncing back off hard banks). We can now hear the birds singing and do not have to raise our voices on the rear deck.

 

PS just seen Richards post.

 

JennyB has a (say) 5" x 2" vent on the inside of the cockpit upstand. For maximum quieting I should really baffle these but it is simply not worth it.

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I am planning to put soundproofing on the undersides of the deck boards as not sure how much headroom I have to put a box over the whole engine. The sides of the engine I think that a ply section might be doable and was planning on glueing soundproofing to the bulkhead between cabin and engine as much as I can.

Contemplated a silencer for the wet exhaust at some point but unsure of how easy that would be to fit?

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If my maths and understanding of the engine is right....

 

At 1000 RPM, it's 1,500 cc of air x (1000 / 2) = 750,000 cc per minute, or 0.75 m^3.

 

At 1500 RPM, it's 1,125,000 cc per minute, or 1.125m^3.

 

 

Yes, but that takes no account for volumetric efficiency. There is no way it will get the full 1500cc or air into the cylinders and the faster it runs the less the volumetric efficiency because of resistance in the inlet tracts and air cleaner. Most diesels are far better than petrol engines but I doubt their volumetric efficiency is much above 80% so your 0.75 cu. m. comes down to about 0.6 Cu. m.

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If my maths and understanding of the engine is right....

 

At 1500 RPM, it's 1,125,000 cc per minute, or 1.125m^3.

 

Bearing in mind it could get up to 3000 rpm, then it would be double, however, despite it being water cooled, some circulation is also needed, and convection around the engine compartment will help to keep batteries, gearbox and other items cool!

Otherwise, you have a big radiator in a small room with the door shut!

 

1.5l per rev., at 3000 rpm is 750l per second, I would then double that and try and make sure it comes in the clean side (away form the exhaust) and goes out the other!

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Bearing in mind it could get up to 3000 rpm, then it would be double, however, despite it being water cooled, some circulation is also needed, and convection around the engine compartment will help to keep batteries, gearbox and other items cool!

Otherwise, you have a big radiator in a small room with the door shut!

 

1.5l per rev., at 3000 rpm is 750l per second, I would then double that and try and make sure it comes in the clean side (away form the exhaust) and goes out the other!

I make that 75 litres per second, I think...? ( 1.5 * 3000 / 60)

 

But it would be half that, I think, as it's a four stroke engine and so it needs two revolutions to ingest and expell the air equivalent to the engine capacity.

 

Either way that assumes its 100% efficient which, as Tony pointed out, it's not.

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I make that 75 litres per second, I think...? ( 1.5 * 3000 / 60)

 

According to my calculator its 750? (or with below 375!)

 

But it would be half that, I think, as it's a four stroke engine and so it needs two revolutions to ingest and expell the air equivalent to the engine capacity.

 

Well done, full marks for spotting that one!

 

Either way that assumes its 100% efficient which, as Tony pointed out, it's not.

Its not the efficiency I was concerned about, but the total air flow, since what gets sucked in, ends up pushed through the exhaust, whether involved in combustion or not!

I am never clear whether more air is sucked in or less (due to the opening and closing of the valves which flushes the exhaust?)

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Its not the efficiency I was concerned about, but the total air flow, since what gets sucked in, ends up pushed through the exhaust, whether involved in combustion or not!

I am never clear whether more air is sucked in or less (due to the opening and closing of the valves which flushes the exhaust?)

 

 

We have only talked about volumetric efficiency not thermal efficiency. Volumetric efficiency is all about how much air is actually drawn into an engine as a percentage of its cubic capacity. If you want to know the total air flow you have to know the volumetric efficiency.

 

It is not easy to find typical volumetric efficiencies abacus on any engine it will vary with speed and air cleaner type. On engines using different manifolds for different applications the volumetric efficiency will also alter. Engine speed and valve timing also has an effect. Diesels with no throttle butterfly normally give a higher volumetric efficiency than petrol engines and the petrol engine will rarely reach 80% VE unless pressure charged. A diesel might get to 90% but seeing the 1.5 uses siamesed inlet ports and one exhaust port I plumped for the lower figure.

 

Happy to be corrected by anyone in engine design for their day job.

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