Engine Ventilation: Skip it?

[daddle]

My engine compartment exhaust blower just quit. Burned out. Doesn't seem to make any difference. Seems to me the diesel itself is continuously exhausting from 50 to 10 CFM just running. Why would I need a little more?

(But sometimes I do warm my hands by the outlet)

[GordMay]

The maximum temperature of the engine room should never exceed 140 degrees F, within 3/4" of any electrical equipment (ie: Alternator), and should never exceed 115 deg. F anywhere in the engine compartment.

The diesel itself is exhausting the products of combustion, but is not ventilating.

[lorenzo b]

A good way to check if your engine needs more air for combustion is to see if you notice any change when you suddenly open the hatch under power. I have a large engine room and installed blower fans but found that the room stayed under 90 degrees and was well ventilated without mechanical help.

[daddle]

Originally Posted by GordMay

The maximum temperature of the engine room should never exceed 140 degrees F, within 3/4" of any electrical equipment (ie: Alternator), and should never exceed 115 deg. F anywhere in the engine compartment.

The diesel itself is exhausting the products of combustion, but is not ventilating.

Excellent. Some actual temperature numbers. I can work to those. However the diesel is indeed a huge air pump. Its intake draws about the same amount of air into the engine compartment as a blower and blows it out the exhaust.

[YourOldNemesis]

In my humble opinion:

A diesel engine sucks in all the air it needs. The suck from the engine itself is far stronger than any electrical fan you might install (how many electrically driven turbochargers have you ever seen?).


If your engine is starved of oxygen, then it needs a bigger vent, not a bigger fan in the vent. If you're saying that you normally have an electrical fan running blowing air out of the engine bay, this doesn't seem to make sense - it's unlikely to make any headway against the amount of air being sucked in to the air intake of the engine itself and, if it did, would starve the engine of oxygen!

Your engine is water-cooled, not air cooled, so if it's getting too hot, you either need to clean the heat-exchanger, unblock the raw water filter, fix the water pump(s) or get a bigger heat-exhanger.

[jkleins]

The only reason I added the blowers was to cool the alternators down some at the advice of the Balmar folks. The engine was probably fine but the alternators supposedly functioned better at a lower temp. I suspect my installation is overkill but it does cool off the boat when the engine is running.

Jim

[Stevens 47]

Do you blow air into your engine compartment or do you blow it out? My boat has very limited access to a outside the boat vents. I was going to run a 4" pipe to a external vent and blow the engine air out through it, hoping that that would remove some of the heat. I also had planned on installing blowers to circulate air inside the engine compartment and directs the flow to the exit. I will install vents that will allow air into the compartment from inside the boat. I would appreciate comments. Jack

[jkleins]

I do both. In one side and out the other. The "in" side blows right over the front of the engine where the alternators are and the "out" pulls from a little further back. They are both attached to vents in the cockpit comings. The only problem is that you can hear the air flow in the cockpit when the engine is quiet like idling or low RPM's. I am thinking of putting them on a switch but haven't decided.

Jim

[daddle]

Some good points here. I think I'll replace the blower but only use it after shutdown to keep the cabin cooler. I'm going to use the blower to exhaust the compartment since the cabinet does not seal perfectly and the hot air and smell will blow out the cracks if I blow air in. That and check the air temp as GordMay suggested.

[David M]

For my Cummins engine installation, I was allowed only a 30 degree difference above the ambient air temperature. This was confirmed by using a thermometer with a remote sensor.

If your engine space temperature difference goes above what your engine manufacturer allows, then you may need to put in larger or more engine space ventilation ports or less ideally, install continuous duty forced draft blowers. The noisy blowers that you already have that you run for 5 minutes before starting the engine are probably not continuous duty.

Be sure you have adequate air inlets as far down as what is practical.

[lorenzo b]

A water cooled diesel also dissipates heat to the surrounding air and out the exhaust. All three cooling systems are needed for your engine to run properly. As the engine runs it draws air from around the room that has already absorbed some of the engine heat, so you may not need the either blow more air in or suck it out to keep the air from overheating. If you do, you'll know it.

[LtBrett]

Originally Posted by YourOldNemesis

If your engine is starved of oxygen, then it needs a bigger vent, not a bigger fan in the vent.

Absoultely correct.

Originally Posted by YourOldNemesis

If you're saying that you normally have an electrical fan running blowing air out of the engine bay, this doesn't seem to make sense - it's unlikely to make any headway against the amount of air being sucked in to the air intake of the engine itself and, if it did, would starve the engine of oxygen!

The rationale for an exhaust fan is simple: if you run a fan to cool the engine compartment after shutdown, an intake fan would push hot air and engine smell through any cracks in the bulkhead and into your cabin. Much better to suck out. During engine operation, an intake fan may be better for cooling because you can aim it at sensitive components like the alternator. Of course the optimal set up could be both intake and exhaust fans, but small boats might not need that expense.

Brett

[Hud3]

After I installed a 210 Amp alternator on my Yanmar, the heat build-up in the engine compartment was noticeable. I installed a duct fan with the outlet high up on the transom, and the inlet right over the alternator. There's an opening between the engine compartment and the main bilges which allows cool air to enter, blowing over the alternator. Heat will derate your alternator, so keeping it cooler is a good thing to do. The fan made a big difference.

[YourOldNemesis]

Originally Posted by lorenzo b

A water cooled diesel also dissipates heat to the surrounding air and out the exhaust. All three cooling systems are needed for your engine to run properly. As the engine runs it draws air from around the room that has already absorbed some of the engine heat, so you may not need the either blow more air in or suck it out to keep the air from overheating. If you do, you'll know it.

Water is really good for cooling things compared to air - think about how quickly water cools down red-hot ironwork at the blacksmiths. How long would it take to cool the same object down by wafting air at it?

The exhaust is (normally) water-cooled too, which is why the final outlet to the air can be made of rubber tubing rather than metal. Your engine does dissipate some heat into the air, but this is more by accident than design.
A water-cooled engine will always be hotter than the ambient air temperature when it is running so you are technically correct to claim that it dissipates (some) heat to the air, however, the heat exchanger is the component specifically designed to dissipate the engine's heat.

If your engine gets too hot, then (as above) find a way to dissipate more heat - you can either do that by wafting some air at it, or by using the components designed to efficiently carry heat away from it.

The temperature inside the combustion chamber is around 1800 degrees F, I doubt that the intake temperature even being 200F would make a great deal of difference to the engine (although of course lots of other things would have melted by then).

If your engine bay is getting too hot for comfort - eg. alternators overheating, cables melting etc... then you need to carry heat more efficiently away from the engine bay. Where does the heat come from... the engine? Which system did the engine manufacturers specifically design to carry heat away from the engine...?

[lorenzo b]

Quite right. According to Nigel Calder, only about 10% of an engines heat is lost to friction and radiation from the block,while 27% goes to the cooling system and 27% goes out the exhaust, the rest is useful power output ( 38% )