Engine Room Air Pressure, Ventilation, and Engine Performance

[ttmott]

I was reading an article on engine room ventilation then thinking about the air needed to run the engines and how well the Sea Ray design really is.
Inboard engines, gasoline and diesel, require significant oxygen to operate at loaded conditions. Do our boats provide that air sufficiently?
Carbureted and mechanical injected engines would be particularly susceptible in performance loss relative to fuel use if not enough oxygen was available to "breathe"; they will tend to run fuel rich. Conversely, computer-controlled engines would dial back performance and fuel consumption with the loss of oxygen; they have manifold air pressure sensors.
We concern ourselves with engine loading and controlling that loading by adding or removing pitch from the propeller, but what we don't know is the engine getting enough oxygen to produce the power to establish the load conditions (RPM relative to throttle position).
So, I'm curious what the air pressure is in my engine room at various temperatures and power settings, and if the air inlet system on the boat is really sufficient for the engines' demand. We know SR really missed the mark on propeller sizing usually causing overloading conditions so did they do a good design in the air intake system? Hmmmm...
I'm considering installation of a differential pressure sensor and temperature sensor in the engine room just to see how low the engine room pressure drops below atmospheric when running the engines and generator and even the blowers.
Has anyone else looked at this?

 

[dtfeld]

I remember coming across the required cross sectional area required for the CAT 3126's at some point and thinking the same thing (sorry can't put my fingers on the reference at the moment). Its just not possible they designed in enough inlet air! I know I am able to lift the engine hatch while running, so there cant be too much differential pressure, else it would be very difficult to get that large a hatch open. However, improvements could theoretically be made, but the design doesn't leave much room for improvement if you also want to keep water out of the inlets.

I have run the 410 with the hatch up and down and I cant tell the difference looking at my dashboard gauges.

 

[Skybolt]

Tom, Your too funny with this and as usual posting the right concerns. I have two stroke Detroits and when I run at WOT to exercise them for a few minutes, if I try and lift the engine hatch it opens with resistance. I know for a fact there isn't enough air getting in there. The air inlets on the side(s) of the boat are way too small.

Sea Ray cut their teeth with my boat in regards to Diesels and the amount of air and to your point prop to use. I am going to add Turbo Boost sender this spring as I want to see the if there is a change. Given what I know already and if I run with that hatch open, I don't really see any difference.

 

[my3sons]

I don’t have any instrumentation that would be accurate enough to show a difference but when I tried running wot with the cockpit hatch open in my 400db, it made no difference in speed, rpm, boost, egt or engine temperature.

 

[ttmott]

Tom, Your too funny with this and as usual posting the right concerns. I have two stroke Detroits and when I run at WOT to exercise them for a few minutes, if I try and lift the engine hatch it opens with resistance. I know for a fact there isn't enough air getting in there. The air inlets on the side(s) of the boat are way too small.

Sea Ray cut their teeth with my boat in regards to Diesels and the amount of air and to your point prop to use. I am going to add Turbo Boost sender this spring as I want to see the if there is a change. Given what I know already and if I run with that hatch open, I don't really see any difference.

Some quick numbers show my QSM11's need 912 Cubic Feet of air per minute at 2300 RPM and 30 psig boost loaded. That is a lot of air flow...

 

[ttmott]

I don’t have any instrumentation that would be accurate enough to show a difference but when I tried running wot with the cockpit hatch open in my 400db, it made no difference in speed, rpm, boost, egt or engine temperature.

I remember our discussions on EGT's.

 

[SeaNile]

My gut tells me there is no way the vents on my boat provide enough air for the QSMs. The vents are long to the eye but when you look up in there you'll see only 3 holes which are about 4" in diameter. This is from memory, maybe it's 4 holes. Either way I have a feeling it's not enough air.

When I redid my 31 Bertram and put Cummins 6BTA 300hp engines in, the vents I cut into the side of the hull were significantly larger than what my current DA has.

 

[PlayDate]

I agree with @dtfeld & @SKybolt .....open a hatch and see if it makes any difference. In regards to duct sizing.....this is a chart we use in HVAC systems.

CFM Sizing Chart For Flex Round Ducts (50-1,700 CFM)
Duct Size (Inches) Flex Duct Airflow (CFM)
5 inches 50 CFM
6 inches 75 CFM
7 inches 110 CFM
8 inches 160 CFM
9 inches 225 CFM
10 inches 300 CFM
12 inches 480 CFM
14 inches 700 CFM
16 inches 1,000 CFM
18 inches 1,300 CFM
20 inches 1,700 CFM

A common question here is “What size duct is needed for 1000 CFM?”. If you’re using flex round ducts, you need ducts with a 16-inch diameter. For those of you who like math.....what is the surface area of a 16 inch circle?

201.1 square inches

 

[ttmott]

I agree with @dtfeld & @SKybolt .....open a hatch and see if it makes any difference. In regards to duct sizing.....this is a chart we use in HVAC systems.

CFM Sizing Chart For Flex Round Ducts (50-1,700 CFM)
Duct Size (Inches) Flex Duct Airflow (CFM)
5 inches 50 CFM
6 inches 75 CFM
7 inches 110 CFM
8 inches 160 CFM
9 inches 225 CFM
10 inches 300 CFM
12 inches 480 CFM
14 inches 700 CFM
16 inches 1,000 CFM
18 inches 1,300 CFM
20 inches 1,700 CFM

A common question here is “What size duct is needed for 1000 CFM?”. If you’re using flex round ducts, you need ducts with a 16-inch diameter. For those of you who like math.....what is the surface area of a 16 inch circle?

201.1 square inches

Flex ducting and to a lesser extent hard pipe have flow losses depending upon length and bends and velocity. I would assume these ratings are for a nominal twenty-foot length of the duct. The openings on the sides of my boat are pretty big but there is a bit of a torturous path through the side, the plenum, and into the engine room the air needs to follow so any water stays outside.

 

[Strecker25]

I recall a previous thread or discussion, maybe the same one @dtfeld is referring to, where it was basically agreed that there’s no way the 400/410 vent design could deliver enough engine room air for the caterpillar or Cummins at cruise speeds. Add to that the fact that one side of the vent is dedicated partially to exhaust blowers further limiting the vent size

I have to wonder if it contributes to some of the sooting various folks have chased over the years with seemingly healthy engines.

 

[becnme]

I had a 97 Sundancer 400 that ran very well but after an all day run there would be a black soot line all around the seal to the cabin door. The engines were trying to get air through the cabin.

I also had a 2001 396 Carver aft cabin with 6BTA Cummins that had a blower system that brought IN outside air and dumped it on the engines. There was a label stating to run the blowers when the engines were running and during cool down.

 

[Woody]

I don't know but....on my boat the engine exhaust is 8", if I got four 4" vent holes on each side(I don't know what it is) I'd have about equal cross sectional area for both intake and exhaust. If my intake vents were 5" I'd have about 50% more on the intake side. Restriction probably varies some boat to boat.

Here's the intake specs for my engines...clean/dirty filters. It might be a better location to measure intake restrictions.

 

[Jeepers]

Turn the vents to face forward.

 

[Gimme Time]

I’ll through a little input in here as I have the same questions and have done everything Tony A recommended. I must admit I fail to remember the exact numbers so if someone has the answer feel free to plug it in here.
First off I run the engine room blowers the entire time the engines are running and for an average of thirty minutes after Engine shut down to continue the slow cool down needed IMHO. Cast iron, steel and everything down there does not cool off very well with no air movement so my fans run.

I’ve run GT with the engine hatch wide open to see what if any differences I got with my Digital EGT sensors and find less almost no temp change.

If I recall Cummins correctly states the entering air temperatures should not exceed 30 degrees above ambient air temperature.

For simplicity sake on 90 degree day the EAT should not exceed 120 degrees. In any RPM range 1800+ the turbo has spooled up pretty good and my digital temperature sensor mounted just in front of the turbo intake filters show I’m within the 30 degree delta.

Most of my running is around the Chesapeake Bay Area except when we head south in November. I often wondered what the TD was for boats running in the Florida area during the summers with high ambient weather temperature conditions. I find the highest EGT readings I ever have are when you start running anywhere below 1800 RPMS and they become grossly worse if you’re running pretty much anywhere below there because of rough sea conditions.

The 30 TD between ODT & entering intake air temperature number is what I remember from a Cummins document I’ve seen & read somewhere but memory fails exactly where I read that. As the sea water gets hotter the Higher RPMs I tend to run to keep the EGT <1,000.

 

[ttmott]

I’ll through a little input in here as I have the same questions and have done everything Tony A recommended. I must admit I fail to remember the exact numbers so if someone has the answer feel free to plug it in here.
First off I run the engine room blowers the entire time the engines are running and for an average of thirty minutes after Engine shut down to continue the slow cool down needed IMHO. Cast iron, steel and everything down there does not cool off very well with no air movement so my fans run.

I’ve run GT with the engine hatch wide open to see what if any differences I got with my Digital EGT sensors and find less almost no temp change.

If I recall Cummins correctly states the entering air temperatures should not exceed 30 degrees above ambient air temperature.

For simplicity sake on 90 degree day the EAT should not exceed 120 degrees. In any RPM range 1800+ the turbo has spooled up pretty good and my digital temperature sensor mounted just in front of the turbo intake filters show I’m within the 30 degree delta.

Most of my running is around the Chesapeake Bay Area except when we head south in November. I often wondered what the TD was for boats running in the Florida area during the summers with high ambient weather temperature conditions. I find the highest EGT readings I ever have are when you start running anywhere below 1800 RPMS and they become grossly worse if you’re running pretty much anywhere below there because of rough sea conditions.

The 30 TD between ODT & entering intake air temperature number is what I remember from a Cummins document I’ve seen & read somewhere but memory fails exactly where I read that. As the sea water gets hotter the Higher RPMs I tend to run to keep the EGT <1,000.

John - on the QSM11 the higher the Intake Air Temperature, fuel is dialed back by the ECM. The higher the air temperature the less dense it is and the less oxygen is available for fuel burn. Pressure and Temperature is what the ECM uses to tailor fuel at a load (throttle) setting. Quite possibly the less dense the air (higher temperature) the higher the EGT's as a given RPM - total unverified speculation here though. In my thinking, if the engine room vents are choking air the situation would be similar as there would be a less dense volume of oxygen available. Of course, this is specific to diesel engines. I haven't installed EGT instrumentation but based upon consensus I try to run the boat between 1800 and 1900 RPM as cruise settings; plus, the boat just simply seems happy there at 37 GPH.

 

[techmitch]

John - on the QSM11 the higher the Intake Air Temperature, fuel is dialed back by the ECM. The higher the air temperature the less dense it is and the less oxygen is available for fuel burn. Pressure and Temperature is what the ECM uses to tailor fuel at a load (throttle) setting. Quite possibly the less dense the air (higher temperature) the higher the EGT's as a given RPM - total unverified speculation here though. In my thinking, if the engine room vents are choking air the situation would be similar as there would be a less dense volume of oxygen available. Of course, this is specific to diesel engines. I haven't installed EGT instrumentation but based upon consensus I try to run the boat between 1800 and 1900 RPM as cruise settings; plus, the boat just simply seems happy there at 37 GPH.

How about a simple M.A.P. sensor ER mounted and wired up to your Maretron display along with that head system vacuum sensor. Sorry but I love this stuff.

 

[bfernald0]

I was reading an article on engine room ventilation then thinking about the air needed to run the engines and how well the Sea Ray design really is.
Inboard engines, gasoline and diesel, require significant oxygen to operate at loaded conditions. Do our boats provide that air sufficiently?
Carbureted and mechanical injected engines would be particularly susceptible in performance loss relative to fuel use if not enough oxygen was available to "breathe"; they will tend to run fuel rich. Conversely, computer-controlled engines would dial back performance and fuel consumption with the loss of oxygen; they have manifold air pressure sensors.
We concern ourselves with engine loading and controlling that loading by adding or removing pitch from the propeller, but what we don't know is the engine getting enough oxygen to produce the power to establish the load conditions (RPM relative to throttle position).
So, I'm curious what the air pressure is in my engine room at various temperatures and power settings, and if the air inlet system on the boat is really sufficient for the engines' demand. We know SR really missed the mark on propeller sizing usually causing overloading conditions so did they do a good design in the air intake system? Hmmmm...
I'm considering installation of a differential pressure sensor and temperature sensor in the engine room just to see how low the engine room pressure drops below atmospheric when running the engines and generator and even the blowers.
Has anyone else looked at this?

You read the recent article in Power & Motor yacht? I read it and was wondering the same

 

[Nater Potater]

Something the CFM charts don't always make mention of is flow versus pressure drop, so it's not always easy to discern the pipe size needed from those charts. Let's look at some numbers from something familiar:
Using the formula CID X RPM/3456/VE, a pair of Chevy 350's running at 4400 rpm figures to be 891 CFM, then, assuming a volumetric efficiency of 85%, it takes a combined 757 CFM to produce a total of 520 horsepower.
Now, if you look at SBC's from the late Sixties, producing around 350 HP, they had an intake snorkel of around 2-1/2" diameter, or just shy of 5 square inches feeding a 500 CFM Quadrajet. Breaking that down, we could produce around 71 horsepower for every square inch of intake area. With all of that being said, my twins should only need around 7-1/4 square inches of unhindered inlet area to be able to produce their rated horsepower.
Thinking about the flame arrestors on those 350's; they're tiny!

 

[ttmott]

How about a simple M.A.P. sensor ER mounted and wired up to your Maretron display along with that head system vacuum sensor. Sorry but I love this stuff.

A common automotive MAP sensor would be 0 to 5 Volt output which would work but it's pressure range would be too great to get any data. This Dwyer instrument (https://dwyer-inst.com/products/pre...-616kd-differential-pressure-transmitter.html ) which is inexpensive at 0-25 In WC (0 - 0.90 PSID) differential would work great and it is 4-20 ma output compatible with the Maretron CLM100. We would need to add temperature and engine RPM as that data is also required. This would all be good but it's real time viewing; we need some way to record and overlay the data in a spreadsheet. The data needed is - Engine RPM, Engine Room/atmospheric Differential Pressure, Engine Room Temperature, and outside temperature. Throttle position would also be good as that would tell us the engine's behavior at temperature/pressure changes.

 

[Nater Potater]

Here's a unit with a range of 0-10WC that also has your required 4-20ma output. It's for HVAC work, so it already has barb fittings for the sense and reference hoses!