Getting air to motors

So I was thinking the other week about how my motors get air. It all started when I was considering a cold air kit for my truck. Why do our boats just pull warm/hot air from the engine rooms with the only connection to the outside being slotted vents on the side of the hull? Wouldn’t motors run much better if we plumbed a path to grab cold air more directly from the outside? Maybe not a sealed system, but maybe just a draft of cooler outside air directed toward the motor’s intake.

It seems to me that a measurable gain in performance (HP or efficiency) could be had by doing something like that. Or am I missing something?

Discuss... and thanks!
Tom-

The vents are a direct path to outside air. If designed properly by the OEM, no more is really needed. I suppose you could run blowers if really paranoid for some reason.

caltexflanc said:

The vents are a direct path to outside air. If designed properly by the OEM, no more is really needed. I suppose you could run blowers if really paranoid for some reason.

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I think he may be referring to the fact that the outside air has to sit in, and travel through, the 115 degreeF engine room to get to the air cleaner intakes.

My boat and truck engines are turbo-diesels. Cold air wouldn't make a (noticable) difference.

Gasoline engines have a fixed stoichiometric ratio, that's why they benefit from cold air induction.

At high power settings, there is so much air entering turbos that it is in fact rather cool at the actual air filter.

At low power settings, it does get hot at turbo suction, but at low power settings there is no downside to that air being warm.

At high power the turbo compressor is more efficient if the air entering is cool, but only by a small amount. It roughly follows the ratio of temps using an absolute scale. So comparing 80F to 115F is really comparing 540R to 575R, or about 6% reduction in mass air flow.

Actual cylinder charge air temp is mostly governed by the charge air cooler (aftercooler) and that is not very sensitive to turbo suction temps, maybe by a few degrees. The 6% loss of mass air flow due to turbo suction temp will be reflected in a little less flow and little less manifold pressure, but since diesels run with such a high amount of excess air, not worth fiddling with it in my opinion. If exhaust is clear of smoke, still plenty of excess air.

Cooler intake air is certainly better, but not worth clogging your tight engine room with all sorts of large diameter plumbing.

And if you route it outside, the noise would be nuts.

You would also need some provision to coalesce out any entrained sea mist.

Unless your ER blowers are like 3hp or more, they won't do diddly compared to the airflow going into a turbo at power.

The "general rule of thumb" is that the engine room temperature should be no more than 30 deg F above the ambient air temp.
If you are within that parameter, you should have enough air flow to maintain efficiency.
Take some measurements and see where you stand before adding anything.

Another caution is that the air intakes on boats are placed to limit downflooding under severe heel. Anything you do should not compromise that geometry.

In general I agree with Ski and others that it doesn't make enough difference as long a Jay's rule of thumb is met.

David

Go for a test run with your hatch covers off and see if you notice a difference.

So I WAS missing something. Thanks for the education y’all. My thought was that cooler air might make a difference, but didn’t know most of the higher physics of the turbo motors.

Tom-

"It seems to me that a measurable gain in performance (HP or efficiency) could be had by doing something like that. Or am I missing something?"


The HP gain from denser cold air comes at full throttle , nothing gained at modest cruising loads .

Commercial tugs have large blowers to bring in large volumes of air. Usually a smaller exhaust blower to remove some of the air. Incoming air is dispersed right over the diesel intakes. Even with really big diesels at full power the engineroom is only slightly warmer than the rest of the inside.
As long as you have sufficient incoming air vents and you have a turbo diesel with an aftercooler, a cooler engineroom probably won't make a noticeable difference. But a turbo w/o an aftercooler or natural engine might benefit.
In a hot climate, a cooler engineroom makes less for the AC to overcome.

djmarchand said:

Another caution is that the air intakes on boats are placed to limit downflooding under severe heel. Anything you do should not compromise that geometry.

In general I agree with Ski and others that it doesn't make enough difference as long a Jay's rule of thumb is met.

David

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Indeed do not do what the PO of my boat did once... looking at the hull vents he had a sudden “bright” idea... why no to reverse them, being mouth open directly to the air flow will make more air coming in... until the first storm when he realized he was taking water in from rain and waves...

[emoji15]

L

My IG has a blower,it is near the vent on one side but not connected by tubing to the vent.
If I forget to run it, the ER will be noticeably hotter. At the vents I can feel air entering on one side and exiting the other, no great pressure but enough to be noticeable.
FLs, so no turbos to feed.

We are fortunate to have opening portlights port and starboard in the engine room. I typically have those open when underway. During the summer. When it's calm. Seriously, most of the time they are open when underway. I have only closed them on a few occasions when sea conditions were really nasty.

BruceK said:

My IG has a blower,it is near the vent on one side but not connected by tubing to the vent.
If I forget to run it, the ER will be noticeably hotter. At the vents I can feel air entering on one side and exiting the other, no great pressure but enough to be noticeable.
FLs, so no turbos to feed.

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Which way does it blow, in or out of the boat?

A little off topic: My Hatteras had two OEM sets in each ER, both of which sucked air out of the ER. One set was DC and vented the batteries, with intake low along the battery boxes. The operating manual said not to run these at or above "cruising speed" which meant about 80% throttle which we never did for extended periods of time. The other set were AC blowers specified for cooling the ER after the engines were turned off or to vacate CO2 if the fire system had discharged. These vented at the ceiling.

The engines, Detroit 8v92tti (twin turboed) were fed air from large side vents next to each engine. During the winter, we used to stuff towels in the intake vents so more of the the warmth generated by the block heaters would stay inside the boat.
A few times I forgot to remove the towels, until I put that on the checklist. At hull speed and below it didn't seem to make any difference and I don't recall running the engines up with them in, but if I did I didn't notice a difference then either. But then again they were never in there more than an hour or two .
So that there's my story, or most of it on that subject. I looked into putting a Delta T system in, but never could make a "business case" to do so.

if you have seawater cooled aftercoolers they will control charge air temp more than air temp IMO.

bayview said:

if you have seawater cooled aftercoolers they will control charge air temp more than air temp IMO.

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That is, indeed, what we have. Thanks!

Along these lines I often wondered if the engine on Possum was getting enough air. I thought about putting a recording barograph in the engine compartment. If the pressure dropped when the engine was running it would indicate the compartment needed more ventilation. Never did get around to trying it.

What do you guys think of that idea?

HopCar said:

Along these lines I often wondered if the engine on Possum was getting enough air. I thought about putting a recording barograph in the engine compartment. If the pressure dropped when the engine was running it would indicate the compartment needed more ventilation. Never did get around to trying it.

What do you guys think of that idea?

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Your cellphone may already have a barometer in it.

https://www.phonegg.com/list/303-Cell-Phones-with-Barometer

HopCar said:

Along these lines I often wondered if the engine on Possum was getting enough air. I thought about putting a recording barograph in the engine compartment. If the pressure dropped when the engine was running it would indicate the compartment needed more ventilation. Never did get around to trying it.

What do you guys think of that idea?

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Just do the temperature measurements per my post above and that will tell you.

Take a short piece of vinyl tubing, put one end outside the ER, and then have a loop down and back up inside the engine room. Fill it with a bit of colored water and see what vacuum / pressure you have on your ER. That's the cheapest vacuum gauge you can get.

If you are curious about E.R. temps then get an inexpensive [??], battery operated electronic thermometer with a remote accessory sensor. Radio Shack and others sell them for ~ $25-40.

Mount the remote where you want and see what it reads. Move it around to get an idea of what the E.R. temps are. THere will be cooler and warmer spots.

Many of those thermometers can use more than one remote so consider that too.