Engine room air vent size

[jim rosenthal]

No kidding. When I still had AirSeps, I noticed that my CAT diesels ran a lot better with the AirSep filters off. When I switched to CCVs, I got MUCH larger air filters for exactly that reason.

I'm afraid to run with the engine room hatches open. I'm afraid they'll like it so much I won't want to put them back in place if I have any issues with vent area I am going to try to figure out a way to pull more fresh air in without changing the side vents. Maybe in the aft bulkhead under the cockpit coaming, there's some access there. I could get another 100 sq in easily there, but the noise would go up.

 

[Genesis]

One thing anyone with Detroits - especially 6V92s - can do is trash the Airseps and buy the Racor CCV filters.

They flow a LOT more air and it makes a huge difference. I dropped my EGTs by 100 degrees doing this - that's not a small change - and I changed NOTHING else!

 

[Boss Lady]

Even then they are still the equivilent of sucking air through a soda straw. I am going to research the amount of airflow through the filter material per square inch. we may find out that they are still too restrictive.

 

[MikeP]

According to the formula provided by K&N, to support an 8V71TI you need around 35 SQ IN of filter area for each filter mounted on the turbo inlet. One 9" cone-shaped K&N has more than 4 times that much and you will have two on the engine. So there isn't any restriction imposed by that sized air filter mounted directly to the turbo. If you were to design a system where there was an airbox with a tube connecting it to the inlet (don't know why you'd do that in this application) you would have to size the pipe/filter larger to avoid restriction.

The Formula is: A (effective filtering area) = CID x RPM/20,839

As discussed before - if this formula does not account for the two stroke needing more air, than doubling the total will. The 9" cone filters are still much larger than necessary.

 

[TedZ]

Has anyone thought that if two strokes required twice the air they would burn twice the fuel. The fuel burn a a two stroke Detroit of similar horsepower as a four stroke of the same vintage is basically the same. We know to "light," the fuel air ratio is again, about the same for all diesels.

Before you get the chain saw out, measure the pressure/vacuum in the engine room. Many things effect the amount of air available to the engines, vent design, infiltration etc.

I'm not saying that many engines rooms don't get enough air, that is a common problem but using formulae incorrectly and starting to chop...

Ted

PS If you don't like data loggers get one of these.
http://www.omega.com/ppt/pptsc.asp?ref=HHP-90&Nav=pred02

 

[Boss Lady]

The twice the air rule has to do with the way a 2-stroke purges the exhaust with pressurized air versus a 4-stroke. This is why Detroits have a blower and say the international diesel (4-stroke) engine used in ford trucks does not. The blower does not pressurize the combustion chamber to make more power, it is used to blow/purge the exhaust out of it. This is why it is called a blower, and when used on other applications is called a supercharger.

 

[TedZ]

Understood. With no exhaust stroke, hence no manifold "pressure" the two stroke blower gives exhaust gasses a push.

Are you suggesting that the blower push is twice as much air as it needs to be for a given fuel burn? That Detroit gives it one amount of air for combustion and then doubles it for purging exhaust gasses during port overlap? (which in this case is two intake valves and a port).

I think the happiest way to calculate required air is to figure max fuel burn and the use a fuel-air ratio of say 28 to 32 to figure the amount of air required. Yes, Detroit goes overboard and adds a little air to make sure the purge is complete but it is not double...there isn't time given the blower pressures.

This is my knowlege base anyway. Happy to be proven wrong if someone has air flow numbers. At any rate a portable manometer or a datalogger removes all of the guess work.

Ted

 

[Genesis]

Yep. The proper way to check this guys is by measuring engine room vacuum and temperature at full load. Its NOT just about air intake restriction - its also about excess airflow to keep temperatures under control.

If your temperature rise and vacuum level is ok, then it is. If its not, then its not. Before you get out the sawzall you need to check this stuff!

 

[Boss Lady]

Hi Karl, I subscribe to the "measure twice", "cut once" philosophy and that is why I put this to the group for comment. I was hoping someone had already been down this path and would have the 90% solution. I have been doing my homework on this. The company that I found that specializes in this area mentioned that this is one area that Hatteras did not do verly well, and he even said that according to their estimates even the new Hats are restricted by 30% on air flow. Well I brought up that the 68C did not hit the numbers that everyone expected and that he could possibly be right. It seems that Hat is pulling air from the cockpit and ducts it to the engine room. If this guy's estimates are correct, that is a significant restriction. I have built my share of race engines and understand how critical air flow is to performance. So all this is worth investigating to me and I like to eliminate variables. I also paid for a certain amount of HP and do not want to give it back. We rely on others and take for granted that everything was engineered correctly. But things do change, and what was acceptable a few years ago is sometimes considered obsolete today. Until something is investigate properly and measured how do you know? That is what I intend to do with this aspect of my boat. I leave nothing to chance if I have the time. I will calculate every component of the air supply and make sure there is adequate capacity. I think that others may be able to find that there is HP there that is untapped, i.e. they have air restrictions that are robbing them of some performance.

 

[AquaNav77]

I have the exact factory specs for these engines. What I would need from you is the exact version of the engine in question and then I would be able to dig it out of my engine library and answer your inquiry correctly. Just because it is turboed, there were many variations of this engine culminating in the DDEC version putting out 765 BHP. Each version was different in its air intake requirements. Hence, which version are we talking about here?

AquaNav77

 

[Boss Lady]

I have the exact factory specs for these engines. What I would need from you is the exact version of the engine in question and then I would be able to dig it out of my engine library and answer your inquiry correctly. Just because it is turboed, there were many variations of this engine culminating in the DDEC version putting out 765 BHP. Each version was different in its air intake requirements. Hence, which version are we talking about here?

AquaNav77

I have the 1988 8v92TA 735 HP version (twin turbo), I can get the serial numbers if you would need those.
Thanks for your help!
Chris

 

[Genesis]

Just measure temperature rise and vacuum in the engine room. Its either in spec or its not. All the handwaving doesn't tell you a thing; you can compute until you're blue in the face but unless you account for every means by which air can get into the engine room, you're not looking at the right thing - and boats are notoriously LEAKY environments in that regard.

I heard all this BS from multiple people about my 45, and from looking at the side of the under-coaming intakes I'd tend to agree. Then I got out the gauges - no more than 10 degree F temperature rise from ambient at cruise and the vacuum was almost unmeasurable with a manometer.

That was all the evidence I needed - its in spec, I leave it alone.

 

[Boss Lady]

Karl, I am doing this for a repower, since I have more than doubled the HP and went from naturals to twin turbos, and the original air inlets were not adequate for the naturals. With the naturals the engine room was rather warm and the access door was sucked tight if we were underway. I have time to do this is now before the boat is put back together. I have the opportunity to calculate and make sure that I have some overkill designed in.

 

[Genesis]

Well yeah, if you had naturals and it wasn't good enough, then obviously you need more

Thing is I'd calculate and then add some, but its not just "dimensions" - as was noted its also restrictions - any turns, any obstructions, spray filters, etc.

This is the problem with the "ok, we'll just do X square inches" computations. If you're going hullside with the vents you MUST engineer them to keep spray out of the engine room or you're going to have a very expensive problem a couple of years down the road with corrosion issues, and its a royal pain to fix that once it gets started.

If you're going from naturals to turbos and doubling horsepower figure the natural's requirements, double that, then add 50% again to that figure. Its not just airflow into the intake, its also ambient temperature control. If you can't get there reasonably with free air ventilation consider powered vents. They're not cheap but they absolutely do work.

 

[Boss Lady]

I have almost enough open area with existing vent openings, but the airboxes are inadequate. I will be putting new ones with baffles and spray filters. I will add some powered ventilation to keep the engine room cool after shutdown. Go back and read the other posts on this thread and I have discussed all of this.
thanks Karl,
Chris

 

[Genesis]

No problem at all... sometimes I get lost in all the threads

Looking forward to seeing the new and improved Boss Lady....

 

[Boss Lady]

Karl I have decided to build the airboxes after getting the $5,000 price tag from the pros. Geez, we are only talking about some $150 fans and some louvers! I am going to use an air impingment demister design (corregated fiberglass roofing panels cut and stacked with 1/2 inch spacing so that the air has to make "S" turns) just like the big boys. We should only have $100 in materials plus Paul's labor to make the boxes and paint them. $300 bucks for fans and $100 for a temperature fan controller and we are done! What a bunch of rip off artists. I will have enough cash saved to splurge on some stainless steel outer louvers to complement my new stainless window frames. We are going with 66" long air boxes to compensate for the 1/8th inch thick airflow louvers thickness reducing the overall surface area. The fans free flow about 1700 cfm each unpowered so the fans will not be a choke point. What do you think of my redneck engineering?

P.S. THANKS AQUANAV77 for the Detroit Data and Fuel curves!

 

[Genesis]

Sounds good to me!

 

[REBrueckner]

Maybe this is too simple minded, but I always thought engine manufacturers and production boat manufacturers worked together to insure engine manufacturer specs are met for engine warranty purposes. For example, engine manufacturers specify air intake volume and exhaust pressure requirements and these SHOULD be tested for each model boat.

As Genesis, I think, pointed out, engine intake air vacuum pressure is important and limiting temperature increase helps retain specified engine HP. The warmer the engine intake air the lower the HP output....not a big difference per degree of temperature rise, but something that should ALWAYS be engineered,designed and tested against specs.

In any case, if HP in an engine room is increase by,say, a factor of two, the air intake openings should be increased by at least that amount and some additional increase, if desired, can't hurt.

In any case, has anybody talked with Hatteras to see how this stuff is determined? I'd be more uncertain, and concerned, about exhaust size than air intake requirements.

 

[Boss Lady]

Detroit's published data is 2010 cfm per engine for the 8v92ta or 4020 cfm for the pair. This is changing the volume of air in a 20'x20' room with a 10' ceiling every minute and it all goes out the tail pipe. My particular engine room is about 600 cubic feet, so the air gets changed about 7 times per minute. I am installing powered fans on one intake vent, so when the engines are not pulling maximum air the other vent will become the exhaust and expel engine room heat. Like Karl pointed out in an earlier post stock engine rooms are notoriously leaky and will pull air from every crack. Likewise heated engine room air finds a way to get where you might not want it, so we have tightened up the engine room air leaks during reconstruction and are insulating the entire boat from the environment, and insulating the salon from radiant heat from the engine room. We have put some insulation on the hull walls already and you can feel the difference inside the boat, since radiant heat from the hullsides is undetectable now. There are real cool insulations available now. I used a fireproof double faced aluminum foil insulation that has a R14.7 rating and the foil is a tremendous radiat heat barrier. It is about 1/4 inch thick and comes in 5 foot wide rolls. I can post what I used if anyone is interested.