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Old 07-03-2012, 06:46   #1
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Cooler Engine Intake Air

I would appreciate any comments/guidance regarding the benefit of reducing the engine compartment temperature for a diesel by use of a blower or ventillation. I know that cooler air means better performance due to increased density, but I am not exactly sure how to quantify that. I was also wondering if Yanmar or other manufacturers publish any guidlines on that. There may also be reliability improvements.

Thanks for any info on the above.

Bob
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Old 10-03-2012, 15:04   #2
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Re: Cooler Engine Intake Air

Bob,
Great questions, which I have been pondering myself. Some where on the web is a photos of a diesel fitted with a K&N "performance" air filter, so we are not alone. I was thinking of a fresh air intake for the reasons you stated. I am worried about water intrusion and noise. I could add deck plate with "alternate" air in the engine room for stormy weather, but is there any benefit for all that complication?
The engines I am looking at are in the 15 HP range. a 10% increase (and 10% would be huge) would be 1.5 HP. What are the measuring tolerances of the dynomometer, and what are the normal variations for production engines? Frankly, I never really trusted that a 9.9 HP outboard wasn't really 10 HP, or 9.75. I know that designation is for regulatory reasons. Still can a brake reliably differentiate between 11 and 12 HP?
I guess the other factor is temperature difference between ambient air, and the engine room air. The difference wouldn't be as great as an automobile. So again is the temperature drop worth the time and effort? I'm guessing no, which won't stop me from trying.
Lou
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Old 10-03-2012, 15:39   #3
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Re: Cooler Engine Intake Air

engine manufacturers specify the amount of air needed. it's quite a bit of unobstructed area for fresh air intake actually. Sailboats are often bad about this. A extraction blower and intake are a good idea for other reasons also. Discussed extensively in a recent thread.
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Old 10-03-2012, 19:39   #4
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Re: Cooler Engine Intake Air

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Originally Posted by Cheechako View Post
engine manufacturers specify the amount of air needed. it's quite a bit of unobstructed area for fresh air intake actually. Sailboats are often bad about this. A extraction blower and intake are a good idea for other reasons also. Discussed extensively in a recent thread.

Can you provide a link to the other thread?
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Old 10-03-2012, 19:52   #5
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Re: Cooler Engine Intake Air

The advantage of a cold air intake is that you effectively get more air into the cylinder. The advantage of this is that you can then burn more fuel. It doesn't make a lot of difference during normal operating conditions but it makes a significant difference on what the maximum engine power output can be.

Adding a blower to an engine room may lower the temperature a little bit but you already have a great big air pump which is the engine. Also running a blower will consume power meaning that you will burn more fuel which is the opposite of what you want.

The best way to get the temperature down would be to really open up the engine area and run a dedicated intake hose. Unfortunately, this would make for a very loud engine so running an intake duct directly to the intake is best. This will result in the temperature in the rest of the engine room going up significantly. Performance engines will run a ducted intake and will use plasma sprayed ceramic coating on the exhaust and other hot engine parts.

Regarding testing the horsepower of an engine, you can actually get a very precise measurement. The test stands that I used to work on used a torque meter with an accuracy of .2% of full scale and a magnetic pickup for rpm which are extremely accurate. We would double check this number against an S-beam load cell at a known radius on the dyno. There are some standards for how accurate the measurements need to be and then it really comes down to how tight the tolerances are on critical components in the engine that determine the horsepower.
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Old 10-03-2012, 20:13   #6
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Re: Cooler Engine Intake Air

Come on guys ! all ya got to worry about is that the engine gets enough air to run properly !! it's not a race engine! ya need enough air for the intake and for cooling if your engine runs well and the engine room is not to hot in the tropics you got it licked !! spend your time on REAL projects LOL just my 2 cents
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Old 10-03-2012, 21:06   #7
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Re: Cooler Engine Intake Air

I'm with Bob and Connie on this one, i have been around diesels most of my adult life as a mechanic. and as a diesel is a compression ignition engine(meaning it ignites by raising the air temp inside the cylinder)the warm air is a benefit.
Only if you want performance boost on land would you use a Turbo Intercooler.
For additional air to compartment consider a snorkel air intake.

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Old 11-03-2012, 07:07   #8
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Re: Cooler Engine Intake Air

While subjective comments are of interest, I was really looking for objective data. As an example, if you lower the engine room temperature by 10 degrees, what percent improvement would you see in performance? I am sure such information is out there, but I have not been able to find it. Another factor that I had not considered when I made this post is that restricted air flow creates somewhat of a vacuum in the engine compartment which is detrimental to performance.

Thanks,
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Old 11-03-2012, 07:43   #9
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Re: Cooler Engine Intake Air

cooler intake air temperatures are better for diesel performance. That's a fact. BUT, on a little one in a sailboat, it's a waste of time trying to do anything about it. If you have a big detroit 2 stroke in a motor yacht then maybe but not on a sailboat. 2 strokes all have blowers at least on them and some even have a turbo pre feeder before that trying to force all the air they can into the cylinders. Colder air is more dense so you can force more in with a setup like that. If you are naturally aspirated or have a tiny little turbo, you aren't going to help much. If any.
Also, And I watch this on my heavily modified truck, the colder the intake air the hotter your exhaust temperature when you are running it hard. The general accepted temp for pistons melting is 1400 to 1500 degrees measured at the manifold. My alarm goes off at 1200 quite often when i have cold ambient air temps. It runs like a scalded dog though. When it's hot out it almost never goes off. Some people run water injection to help control this but its more than I want to mess with. Since boats are under constant and often more load than a truck, I would be worried about this.
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Old 11-03-2012, 07:59   #10
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Re: Cooler Engine Intake Air

Cummins requires that the engine space temperature not exceed ambient air temperature by not a whole lot. But unless your engine space is ridiculously hot, I would not worry about it. You do need some ventilation down there in order to remove the waste heat. Powered or passive depends on the size of the vents. For a small diesel, two 4 inch vents are usually adequate. Turn your dorades in opposite directions. Put a thermometer with a remote down there and see what temps you are getting. You probably do not want it going above 120F or so.
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Old 11-03-2012, 08:15   #11
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Re: Cooler Engine Intake Air

one thing i've noticed on many production yachts that have engine room blowers is that often they are set up to extract air from the engine room!

this seems counter productive,since the engine it self is a large extractor fan!!
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Old 11-03-2012, 08:25   #12
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Re: Cooler Engine Intake Air

Klem,
Thanks for the correction regarding the accuracy of the brakes, I never would have guessed they could be that accurate. Still with production variation etc, a .2% increase in a 15 HP engine isn't going to blow your hair back.
Rabend's original about specific measurements of intake air temp vs HP produced is still unanswered. At my end of the scale (15HP) the game is not worth the candle as an additional .5 or even 1 HP will not be measured in the seat of my pants, and unlikely to be reflected in real world calculations of liters per hour fuel consumption. I suspect you could gain more real world performance increase by reducing the weight of stuff on board and brushing the bottom more frequently. During the cold winter this type of speculation is fun, and can be informative. I did at one point consider a charlie noble as a fresh air duct to the diesel, but decided the noise would be too annoying. Using a better air filter may reduce maintenance, but I do not see how lowering intake charge temp will.
Even at that once you are at hull speed, the additional hp is "wasted" so you have to throttle back to a possibly less efficient rpm. I am sure that is a non technical explanation, but my experience has been that you push the throttle forward and you go faster. Eventually you push the throttle forward and you don't go faster but the engine makes more noise and uses more fuel. Then you pull the throttle back a bit.

Hey Bob, seriously you never considered nitro burning funny sailboats? Admittedly hard to do burnouts, but the boats are already made out of fiberglass!

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Old 11-03-2012, 08:32   #13
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Re: Cooler Engine Intake Air

Don't forget that the engine compartment of a car or truck is fundamentally different from a boat. The car /truck has hot air from the radiator usually blowing into the compartment, significantly raising the air temp under the hood.
Lou
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Old 11-03-2012, 09:04   #14
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Re: Cooler Engine Intake Air

If you have a properly pitched propeller, you will not be able to notice the difference if you get a bit of extra horsepower available because you will never put a big enough load on the engine to use it. Unless you have a controllable pitch propeller, the benefit that you would get would be in increased efficiency.

The techincal explanation of what is going on has to do with the flow of gases in and out of the cylinder. During the suction event on a naturally aspirated engine, there will always be a little bit of negative pressure in the cylinder because there is a pressure drop across the inlet valves, head, intake manifold, air filter, etc. With hotter, less dense air, the pressure drop across the inlet will be greater meaning that the engine has to work a tiny bit harder. The compression event starts at a lower pressure but a higher temperature so it reaches approximately the same temperature and anyways, timing is determined by the injection event. During the combustion/expansion event, there is less oxygen available for the fuel to burn which is only a problem if there is too little. Also, this event starts at a slightly lower pressure. During the exhaust event, there is less massflow to pass through the exhaust valves which is good but it is usually at a higher temperature making it less dense which is bad. As you can see, there are several conflicting factors but generally speaking, cooler is better. With cooler air, you will be able to burn more fuel meaning you will have a higher power output and also your efficiency will go up a tiny bit.

Turbocharging is a way of getting more air into the cylinder so that more fuel can be burned. In this case, having cold air is very important which means that these engines must have an intercooler to cool the intake air before it goes into the cylinder.

If your goal is to save fuel, much bigger savings can be made by doing things like optimizing the prop, keeping the bottom clean, slowing down when under power, only using the engine when you need to. If your goal is to make more power, a good start would be a cold air intake and upping the fuel slightly but really, you need a bigger engine or a turbo.

An example of a device where intake air temperature is very important to efficiency is an air compressor. An air compressors job is to make air at a given pressure and volume flowrate. The denser the air going in, the higher the flowrate will be relative to the power to run the unit meaning that the efficiency will go up dramatically.
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Old 11-03-2012, 10:40   #15
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Re: Cooler Engine Intake Air

Sorry, I forgot to include in my last post that it is hard to put numbers on all of this because it is so engine specific. The valve design, head design and chamber geometry play a large role in how all of this plays out. I have played around with cold air intakes a little but never really took any data for efficiency on them. I have not noticed a significant change in efficiency with one but I have been able to significantly increase the horsepower by being able to add more fuel.
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