engine room ventilation

[incognito]

(repeated on Scuttlebutt)

I had a bit of a problem with my engine, so got the engine expert down on Saturday to have a look.
In the event, he suddenly turned round and pointed out that the well insulated (noise and fire) 'engine room' had no ventilation.
I thought about this a bit, and came to the conclusion that engine rooms generally do not have ventilation - am I wrong? Is there any way of letting a draught through that will not let water in?

 

[bedouin]

Engines most definitely need ventilation!

A 2 litre 4 stroke engine at 2000 rpm will require roughly 2000 litres of air every minute (about 2 cubic metres).

 

[incognito]

Supplementary then... how does yours get ventilation?
Mine sucks air in, and it exhausts air and combustion products out the exhaust, but what about general engine room ventilation?

 

[bedouin]

So your air intake to the engine is ducted straight in from outside?

My engine compartment (I can't really call it a room). Has ventilation ducted from a grill on the inside of the cockpit coaming (high up) to the compartment underneath the engine.

The engine air intake is from the engine compartment itself, and so will draw air in through that vent (and probably through the bilges as well). The airflow generated therefore also ventilate the engine compartment on its way through.

Given the location of the vent, some water might enter the pipe if the cockpit were swamped and filled to the coamings, but this would drain harmlessly to the bilge. In the event of a knockdown the vent

 

[Guest]

I assume that:

o - We are talking diesel engine here?!

o - The "engine problems" aren't due to the engine being starved of air!!

If so then I don't think an unventilated compartment is a problem - as long as:

o - Enough air is getting into the compartment to satisfy the engine's requirements.

o - There is a small access hole to the compartment to "fire off" a dry powder or CO2 fire extinguisher.

If there is enough air coming into the compartment to satisfy the combustion requirements of the engine then "supplementary air" only serves to keep the compartment dry, free of fumes and cool(er).

If the engine has enough air to work properly (and not fed through a sealed duct into the engine) there MUST be some supplementary air getting into the compartment from somewhere. (i.e. it can't be so finely balanced that there is only just enough air for the engine.)

The only thing to be wary of is opening up the compartment too much. If there is bags of supplementary air available it could make it impossible to extinguish a diesel fire in the engine compartment.

As well as that, air borne noise (as opposed to vibration noise transmitted through the hull) will travel along any unbaffled route where air is being sucked into the compartment.

Best regards )

Ian D

 

[johnsomerhausen]

My boat being US built had two ventilators on the stern (compulsory for petrol engines in the US) to exhaust the petrol fumes before starting the engine. Since it's a diesel that's sitting there with no explosion danger, I replaced the clamshell ventilators with Vetus mushroom vents which can be closed in bad weather
john

 

[Guest]

Engine rooms must be ventilated for two reasons.

The first is that engines consume their capacity every other revolution if they are 4 stroke (every rev if 2 stroke) so if you use my boat as an example with twin 2 litre engines cruising at 4000 RPM, they will ingest 8000 litres of air! To make it easier to envisage, that's like using the volume of 40 45gallon oil drums every minute!!

The second reason is your engine(s) are running at roughly 90 Deg C and have a fair old mass, so that is like having a group of central heating radiators down there and you need ventilation to prevent the other stuff (batteries and the usual gash items) from overheating.

For petrol engines, there is a third need. 5 minutes before I start engines until 5 minutes after I stop engines, I turn on my bilge blower. This is a powerfull spark-proof electric fan which is ducted to draw from as low a point as is practicable & vent out of the transom. It is there because petrol fumes are heavier than air. In a car, they simply fall out from under the car but a boat holds them in the bilge. The blower purges the petrol fumes before they can build up to an ignitable mixture! As a spin off, it also purges any butane or propane from cookers, should you have a ring blow out!

I hope this helps........