Cooler Engine Intake Air

[Nemo55]

I would have thought negative pressure in the engine spaces would have been more of a safety value, to prevent fumes from escaping into the living quarters. which would require venting the air with a blower.
You had your reasons i would imagine..

Originally Posted by Lagoon4us

True however on all large boats i've built we have always sized ER Fans to give a positive pressure in the space, always get the rush of air outwards upon opening the door if not the fan is U/S.

Cold air venting into the bilge with a vent at the deckhead is the norm, but as you say there are one or four big diesel pumps sucking it out of the space.

Cheers

[Lagoon4us]

Originally Posted by Nemo55

I would have thought negative pressure in the engine spaces would have been more of a safety value, to prevent fumes from escaping into the living quarters. which would require venting the air with a blower.
You had your reasons i would imagine..

Yes barges not passenger style at all. All controls were at the door to shut fans down as needed. VERY NOISY! Cheers

[atoll]

Originally Posted by Lagoon4us

True however on all large boats i've built we have always sized ER Fans to give a positive pressure in the space, always get the rush of air outwards upon opening the door if not the fan is U/S.

Cold air venting into the bilge with a vent at the deckhead is the norm, but as you say there are one or four big diesel pumps sucking it out of the space.

Cheers

i took a new 70 ft moody with a 210 hp up and down the red sea a few years ago,and noticed that the eng room was getting very hot,so reversed the extractor fans to blow into the eng room.

result was eng room much cooler and 2 l/hr improvement in fuel efficientcy!
air temp out out side was 38-40 c during the day.

[Ex-Calif]

Originally Posted by klem

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.

I am sorry but I just have to say your description of the 4 stroke cycle while interesting is pretty far off base.

There is no suction event. There is an intake stroke. The cylinder has a defined volume based on bore and stroke. How much air goes in is a function of this volume and the 14.7 psi static pressure we all live under at sea leavel. The intake losses you describe because of valves is close but the correct statement is, "normally aspirated engine will not achieve 100% volumetric efficiency because of intake losses due to drag.". This is specifically why we have turbocharging so we can increase vlumetric efficiency to 100% and beyond.

Someone else comment about cold air being more oxygen rich is also completely wrong. The "slug of air" has the same mixture of oxygen, nitrogen and all the other elements by volume whether it is cold or hot air. What changes is density.

To enrichen the air with Oxygen there are systems like nitrous oxide that inject Nitrogen/oxygen mix under pressure into the intake. Interestingly early high power radial recips used water injection because as we all know water is H + 2O and the water provided oxygen and actually cooled the intake as well.

So turbo charged or not turbocharged cold air is more dense and for a given volume will provide more power when mixed with appropriate quantities of diesel or gaslonie.

Turbo chargers BTW have intercoolers becausse the act of compressing the air also heats it and cooler air is better. They also compress the air getting a bigger mass of air for volume.

In any case the volume of air stays the same for a given bore and stroke.

Bottom line for many good reasons stated <50 hp engines probably wont benefit from intake cooling and engine bay cooling. Over that I think it is worth investigating. However its not compartment temp that is the worry it is intake air temp.

OP asked for a formula for power loss. Air density vs. temperature is well known and any physics text have the formula. Mine is at home or I would check. I am not sure power density function is linear but probably close would be my guess. 10% less dense air probably means 10% less horsepower. Your engine spec probably calls out hp based on 15degc day and standard 14.7psi baro pressure at sea level. There is also a density function for humidity but it is a prelatively smaller factor.

Hint - altitude (baro pressure) has a bigger impact on density than temperature. Good thing we don't motor sail much in denver...

[chala]

To be adequate a supply fan must be able to supply the air required by the engine plus the air required to ventilate the engine room. If to small it will over rev and may even disintegrate. An exhaust fan needs only to be able to ventilate the engine room. Remote switching off, of the fan, is required in case of fire.

Originally Posted by atoll

noticed that the eng room was getting very hot,so reversed the extractor fans to blow into the eng room.

Obviously the air supply inlet was to small.

[belizesailor]

Originally Posted by lillia28

... Some where on the web is a photos of a diesel fitted with a K&N "performance" air filter....

I fitted my Volvo MD2020's with K&N air filters. Not for performance reasons, but for maintenance reasons. Big improvement over the standard Volvo filter (just a foam sock basically for which they charge a ridiculous price and which starts to dry rot in about 6 months). No noticeable increase in performance, but a big improvement in maintenance (but then again these are 20HP engines -- so to be noticeable the increase would have to be pretty dramatic).

[Lagoon4us]

Originally Posted by atoll

i took a new 70 ft moody with a 210 hp up and down the red sea a few years ago,and noticed that the eng room was getting very hot,so reversed the extractor fans to blow into the eng room.

result was eng room much cooler and 2 l/hr improvement in fuel efficientcy!
air temp out out side was 38-40 c during the day.

LMAO i've seen a large passenger cat with the same problem here in CQ the engineer strung gauges above the engines and off they went on the midday run. Result = plastic gauges melted engineer flat on floor out to it!!!

Totally agree push the air to the bilges and let the engines and natural ventilation remove it.

Diesels love cold air and cool fuel = grunt no matter the size...

Cheers

[rabend]

I appreciate the subjective opinions and comments. Does anyone have objective data or a reference to objective data that quantitatively defines performance of a diesel engine versus intake air temperature, even if it is for a specific engine and may not apply to all engines? A curve/graph would be helpful.

Thanks

[DeepFrz]

Positive pressure of the engine rooms works okay on larger boats where the builder takes the effort to seal off the engine space from the living space. On a small boat you are going to get the smell of diesel, perhaps fumes from the batteries and hot air pushed into the living space. It is best on a small boat to have exhaust fans with enough vent area to supply the cooler fresh air. IMHO.

[Nicholson58]

Originally Posted by rabend

I appreciate the subjective opinions and comments. Does anyone have objective data or a reference to objective data that quantitatively defines performance of a diesel engine versus intake air temperature, even if it is for a specific engine and may not apply to all engines? A curve/graph would be helpful.

Thanks

Nothing subjective about it. I'm a registered professional PE and studied engine mechanics in college. I now design engine and vehicle dynomometers that are installed in most of the auto makers labs and assembly plants world-wide. Colder air is more dense. Denser air permits greater amounts of oxygen to be inhaled by the engine with each stroke. More oxygen is more available power. We see and measure this all the time. It is why a turbo charger boosts power. It compresses the air to a higher density so that more oxygen is available for combustion with greater quantities of fuel.

[rabend]

Originally Posted by Nicholson58

Nothing subjective about it. I'm a registered professional PE and studied engine mechanics in college. I now design engine and vehicle dynomometers that are installed in most of the auto makers labs and assembly plants world-wide. Colder air is more dense. Denser air permits greater amounts of oxygen to be inhaled by the engine with each stroke. More oxygen is more available power. We see and measure this all the time. It is why a turbo charger boosts power. It compresses the air to a higher density so that more oxygen is available for combustion with greater quantities of fuel.

Thanks for your comment. By coincidence, I am also a registered PE. What I mean by objective, is objective data. An exemplar would be: Intake air temperature rises 20 degrees; horsepower and/or efficiency decrease by 2%. I am not sure if the latter is a linear function, so a graph might be helpful. That kind of information may not be available though.

This has been an interesting discussion containing aspects I had not fully considered.

Thanks,

[Hud3]

A side benefit of a cooler engine room is that your alternator will perform marginally better.

[Wotname]

Originally Posted by Nemo55

Obviously cooler air is more oxygen rich, as is air with high moisture content.
............

Originally Posted by Ex-Calif

............ Interestingly early high power radial recips used water injection because as we all know water is H + 2O and the water provided oxygen and actually cooled the intake as well.............

Old thread I know; pretty sure you are both wrong re water.

There is no way that the engine can break H2O into its components of H & O therefore the O content does not provide additional oxygen. Even if it could be broken into H & O, the H component would then be a fuel which would recombine (ie burn) with the O. The energy released in burning the H & O would be less than the energy required to break it apart in the first place - unless of course you guys are making the first peptual motion machine

What the water (liquid water, not water vapour) does do however is to increase the compression ratio as you are introducing an incompressible liquid into the cylinder. The cooling effect is as you have stated, a good thing.

Water vapour being a gas and thus compressible has the reverse effect, it decreases the amount of O2 available as it takes takes up space during the intake cycle that would be better taken up by air.

Hence high humidity (water vapour) is bad while fog (water as liquid drops) is good.

[Roy M]

I use two four inch blowers in my engine (Yanmar 3GM 30hp) room. One sucks fresh air from the cockpit locker and blows it directly onto the alternator via vent hose. The other exhausts heat from the top of the engine room. The power supply for the fans runs via a relay connected to the Halon fire extinguisher to shut down ventilation in case of an extinguisher discharge.

[Hud3]

I had a large-case alternator (210 A) installed on my Yanmar 4JH3(B)E. Installing a 4" ducted exhaust fan that pulled cooler air from the bilges directly over the alternator measurably increased it's output.