Engine compartment forced extraction ventilation

[brico]

Hi,
Do all of you guys use electric fan to extract heat from the engine compartment while engine is running? I have it fitted on my 1986 F36/38 but not sure if it does any good to the engine? Diesels are known to like to run hot. Sound and heat insulation is reasonably good on mine so not sure how much good does it do if it is extracting heat while engine is running? any thoughts?

[GeoffSchultz]

No. The only real reason that I can see for doing this would be to keep the alternator cooler. The heat exchanger on my 3JH2-TBE does a fine job of keeping the engine at normal operating temp.

-- Geoff

[Mike Holibar]

If I was you I would be blowing air into the engine compartment, not extracting it. The reason is the engine needs air to run and if your extraction results in any drop in air pressure, you may find the transom starts to turn black from unburnt fuel. On Fyne Spirit we had a problem with this which was caused by there being practically no air intake, the compartment was almost sealed. I put in a duct to allow sufficient air in and transom now stays clean. On ships the engine room fans always blow fresh air into the engine room for essentially the same reason, it all helps get more oxygen into the cylinders which means better combustion.

[brico]

Thanks guys. From the time I purchased the boat last year in March I had this fan and the duct disconnected until yesterday and did not notice any difference when I connected it to extract the hot air from the engine compartment. I find the compartment pretty small and well insulated so the heat transfer is not the problem. I'll try to reverse the fan to blow into the compartment to see if it will make any difference.

[jdpandlp]

There are a couple of items that need to be concidered before blowing air into the engine compartment;
1. Forced air will feed any fire in the compartment.
1.a. Any forced air will make fire supppression, such as halon, useless by reducing concentrations.
2. Preasure in the compartment will force contaminated air into the living spaces. Diesel fumes, oil fumes, anit-freeze may not be toxic at normal tempuratures but they do stink.


An exhaust fan for a diesel compartment is usefull for after the use of the engine as it will remove the hot air in the engine compartment before it soaks into the living space. It will also reduce the amount of work the cooling unit for the refer if you have that type of compressor.

I do not believe that preasurizing the engine compartment is a good idea from a saftey standpoint.

Pete

[Mike Holibar]

There are some other things about air in engine compartments which need to be considered. When commissioning a new installation, as far as air goes, we are interested in two things.
1) Air pressure. If air pressure due to lack of appropriately sized ducting is too low the engine cannot breath properly. If pressure is below the manufacturer's spec the engineer will insist that the airflow to the compartment is improved, usually by cutting a larger intake.
2) Air temperature. High air temperature causes the same problem as low air pressure, a lesser weight of oxygen in each firing stroke reducing engine performance. Same fix as above.
3) There is a very good safety reason for blowing fresh air into the bilge/engine compartment, especially if you have propane on board. The dc motor of the blower is not usually flame or spark proof, and there is a high likelyhood of sparking at the commutator. If you suck up a duct full of propane... good night Josephine. Hence the term "bilge blowers" not bilge extractors.
Bilge blowers are good for keeping the air fresh and ventilating spaces that don't usually get much air.
As far as an engine room fan goes, perhaps you can get the operating specs for your engine and see if the temperature and pressure are within the manufacturer's specification. You may not need to use the blower.

Of course the other obvious flaw with a suction fan is that it is competing with the engine for air. If the air supply is already fairly restricted, the fan will be totally ineffective.
Mike.

[Castaway]

From an engineering point of view, Mike is no doubt correct, but Pete's point about safety is more important. A leak from the exhaust could fill the accommodation with carbon monoxide, which does not smell, but often is fatal. If the original fan was an extractor, and the engine compartment was fed from the interior of the boat, reversing it may have extremely serious consequences. A blower system needs an engine compartment which is isolated from the accommodation and with an adequate outlet which cannot be blown back on board.

As for the heat of the engine, at 60º North, we hang onto every Joule! Things might be different where you are.

[GeoffSchultz]

From an engineering point of view, Mike is no doubt correct, but Pete's point about safety is more important. A leak from the exhaust could fill the accommodation with carbon monoxide, which does not smell, but often is fatal.

Diesel engines don't generate enough CO to cause a problem.

-- Geoff

[jdpandlp]

Don't usually get involved with these threads as most of the advise given is benign. In this case though it is not.
If the engine is not breathing properly then increase the passive air flow with larger ducts.
At the end of the day if you wish to reduce heat and fumes for the engine compartment then run an exhaust fan.
Proper exhaust fans are designed and tested for operation in explosive fumes.
Although propane leaks were not the subject of this thread, any leak should be mitigated by airing out the compartment either by natural means or properly rated exhaust fans. Any action that increase preasure, even by a few pounds per sq inch will increase the concentration level increasing the risk of concentrations that could ignite in an open space.

Forced air into the engine compartment will increase the hazard of fire by increasing the supply of oxygen and diluting the concentration of fire suppresent. ABYC is pretty clear on this point.

Pete

[Mike Holibar]

For safety reasons I just ran this discussion past a retired Lloyd's Surveyor, the result being:


a) having an exhaust or suction fan operating has the same effect on a fire and fire suppresant as both circulate air to and from the engine compartment. The exhaust sucks air into the compartment from the inlet duct and the blower blows air into the compartment.
b) firefighting procedure using fire suppressants such as CO2 invloves first shutting down all fans and closing all ducts and shutting off fuel supplies. Then the fire suppressant is released. The suppressant takes away the oxygen which extinguishes the fire, but as heat and fuel may still be present, the space must be kept sealed until it is certain it has cooled and there is no risk of re-ignition.
c) The standard and safe practice for ventilating and making safe voids, tanks and spaces that may contain dangerous fumes is to dilute the fumes with air and gently blow them out of the space in diluted form with blower fans. It is highly dangerous to draw concentrated fumes out of a space with an exhaust fan as the concentrated gas is likely to explode from any source of a spark, even if the exhaust fan itself is flame proof.
d) He used as an example a local incident where some work was to be carried out on a fuel storage tank. The worker intended to use a blower to clear fumes from the tank but inadvertently placed the blower the wrong way round resulting in a huge explosion.

Exlposion on small boats arising from petrol or propane concentrations in the bilges are not uncommon, unfortunately. It appears that attempting to evacuate concentrated fumes from the bilge via an extractor fan can be highly dangerous.

Blower fans, due to the fact that the engine draws air from the comparatment, and any excess pressure can excape through the inlet ducting, cannot unless of massive size, increase the air pressure in the compartment by even 1 psi.

Mike.