Improving airflow to engine

[IanD]

If the problem is a hot alternator, blowing cold air from outside directly onto the alternator is the best solution.

 

If the problem is temperature around the engine generally, blowing or sucking will make very little difference, what it more important is *where* the blowing or sucking is done to/from -- you want to either blow cool air into the hottest region or suck hot air out from there.

 

There have been many similar debates in other apparently unrelated areas (but actually very similar, thermodynamically speaking) like PC case cooling. Measurements showed what I said above -- location (and amount) of blow/suck is much more important than which one it is.

 

As far as blocking up the fan intake is concerned  (the PC equivalent is dust) a sucker may be less likely to get bunged up than a blower...

Edited August 25, 2021 by IanD

[Tony Brooks]

10 minutes ago, IanD said:

As far as blocking up the fan intake is concerned  (the PC equivalent is dust) a sucker may be less likely to get bunged up than a blower...

 but may be less efficient because it will have to work against whatever depression the engine intake itself creates in the engine bay.

[IanD]

16 minutes ago, Tony Brooks said:

 but may be less efficient because it will have to work against whatever depression the engine intake itself creates in the engine bay.

True, but depends on the fan characteristics. Centrifugal blowers aren't much affected by back-pressure unless it's very high, which is unlikely to be the case for an engine bay with any reasonable intake area. Axial fans like PCs use are more affected, they don't like working against significant pressure. Yes I've spent far too much time looking at this for cooling electronic systems...

[Machpoint005]

The same principles apply when getting cooling and combustion air into a large diesel generator enclosure.

I don't dispute Loddon's findings of a temperature 7° lower in the engine 'ole, but I doubt that it is significant in this context. The 2-litre turbodiesel in my car runs very happily at 90°C (yes, I realise this is the internal engine temperature).  

 

 

[blackrose]

2 hours ago, Tony Brooks said:

 but may be less efficient because it will have to work against whatever depression the engine intake itself creates in the engine bay.

 

Blow or suck, that's the question. But if there are two vents appox 12" x 3" , one on on either side of the engine space, would a fan blowing hot air out really be working against the draw of the engine's air intake? Surely whatever volume of hot air is blown out must be replaced by air drawn through the vent on the opposite side?

Edited August 25, 2021 by blackrose

[Tony Brooks]

Just now, blackrose said:

 

Blow or suck, that's the question. But if there are two vents appox 12" x 3" , one on on either side of the engine space would a fan blowing hot air out really be working against the draw of the engine's air intake? Surely whatever volume of hot air is blown out must be replaced by air drawn through the vent on the opposite side?

 

True, apart from the restriction caused by vents. Jen pointed out the volumes of air the engine will draw into the engine room. I suspect that volume wise, blow is likely to produce slightly more air flow than suck.

[IanD]

9 minutes ago, Tony Brooks said:

 

True, apart from the restriction caused by vents. Jen pointed out the volumes of air the engine will draw into the engine room. I suspect that volume wise, blow is likely to produce slightly more air flow than suck.

It's the *additional* air throughput due to the fan that matters, not the volume through it which will undoubtedly be higher when blowing. If the fan is blowing in the same direction as the inlet air then the *extra* amount of air pushed in depends on the airflow, vent area and fan characteristic curve (airflow vs. pressure). If the fan is sucking out then the *extra* amount of air pulled out depends on the airflow, vent area and fan characteristic curve.

 

Either could win, or there could be no difference, the only way to be sure is to try it -- suspicion is like common sense, sometimes it's right, sometimes it's wrong 😉

Edited August 25, 2021 by IanD

[David Williams]

Silly question,I know.Does your engine have a temperature gauge?It makes me wonder if you have a problem as engines are built to run with thermostat fitted.A cold running engine can cause rapid wear I believe.Just a thought.

[OldGoat]

Methinks 'we are missing a trick' by just discussing the engine block / bulk temoerature, but threre are other bits and bobs in the engine hole that should be cooler. For example - batteries, equipment cables etc.

Soo my engine bay has a pair of girt big computer fans that blow cooler air over everything (my engine is covered by a wooden box.

Blow rather than suck, because it's easier to control a fan's out put that its input.

[blackrose]

33 minutes ago, David Williams said:

Silly question,I know.Does your engine have a temperature gauge?It makes me wonder if you have a problem as engines are built to run with thermostat fitted.A cold running engine can cause rapid wear I believe.Just a thought.

 

Yes it does. It's not overheating if that's what you were thinking.

[blackrose]

10 minutes ago, OldGoat said:

Blow rather than suck, because it's easier to control a fan's out put that its input.

 

Blow hot air out rather that suck cold air in? 

 

But as someone said earlier, cold air is denser so it might be more efficient to suck cold air in. 

 

I'll probably just decide based on what's the easiest way to fit it. If it's blowing hot air out it needs to be on the exhaust side of the boat.

[David Williams]

That's good then,am just wondering why the thermostat removal in the first place.The enginebuilders must have thought it necessary.Nice you have the temperature gauge as so many just have a red light when the engine is red hot.Did you fit it or is your boat of an age where they did things properly?

[blackrose]

1 minute ago, David Williams said:

That's good then,am just wondering why the thermostat removal in the first place.The enginebuilders must have thought it necessary.Nice you have the temperature gauge as so many just have a red light when the engine is red hot.Did you fit it or is your boat of an age where they did things properly?

 

I didn't remove the thermostat?

 

My boat came fitted with a temp gauge.

[David Williams]

Apologies it was another member on this post .London.Showing my ignorance now,what IS a lumpy water boat please?

[Col_T]

A boat designed to be used on lumpy water - the sea, for instance.

[GUMPY]

1 hour ago, David Williams said:

Apologies it was another member on this post .London.Showing my ignorance now,what IS a lumpy water boat please?

No you jumped to conclusions. I said thermostat with reference to the one that controlled the snail blower not the engine one. I hadn't mentioned the engine at all.

The blower is there to keep stuff other than the engine cooler, batteries, alternator etc.

Edited August 25, 2021 by Loddon

[OldGoat]

Lumpy water is a rejoinder to those who insist that the only way to boat is "Lee rail under". Anything else is considered as no proper boating.

Methinks any sort of boating is 'proper'.

Better to go out when ever you want and not determined by the tide or weather....

[David Williams]

Hi both,well thanks for the lesson.Seems like I still got a lot to learn.

[Momac]

23 hours ago, blackrose said:

 

To improve cool airflow to the engine.

 

 

So I should install it to blow the hot air out?

 

I think that would probably also have the benefit of fewer leaves, insects and debris being sucked in.

I think the engine itself  is quite capable of drawing in as much air as it needs and a  little 12 fan makes no difference.

If the combustion air supply is not sufficient a larger vent would be a better investment.

But if the fan is to remove fumes from the engine bay, especially before starting the engine or  when the engine is stopped and hot  I do begin to understand the thinking 

 

[1st ade]

14 hours ago, David Williams said:

Hi both,well thanks for the lesson.Seems like I still got a lot to learn.

We all have...

 

I've been a boater for 58 years (apparently my first boat holiday was at three months!) and I'm still learning!

[Col_T]

With apologies for hijacking the thread, what are the symptoms of a diesel running with inadequate air supply?

 

My boat, trad stern, has a Beta 1505 and the engine bay has no obvious air intakes, though there are gaps around the edges of the flooring over the top of the engine / around the alternator. At tickover and upto about 1500, 1600 rpm it sounds fine, anything much above that, 1800-ish, a kind if 'tinkling' noise can be heard. I know petrol engines experience 'pinking', and that is ignition related, though I don't actually understand what it is, and wonder whether diesels suffer anything similar?

[Tony Brooks]

2 minutes ago, Col_T said:

With apologies for hijacking the thread, what are the symptoms of a diesel running with inadequate air supply?

 

My boat, trad stern, has a Beta 1505 and the engine bay has no obvious air intakes, though there are gaps around the edges of the flooring over the top of the engine / around the alternator. At tickover and upto about 1500, 1600 rpm it sounds fine, anything much above that, 1800-ish, a kind if 'tinkling' noise can be heard. I know petrol engines experience 'pinking', and that is ignition related, though I don't actually understand what it is, and wonder whether diesels suffer anything similar?

 

Pinking is when the petrol/air charge explodes instead of burning uncontrollably. The difference is the speed of flame travel across the cylinder. It is caused by over hot parts of the combustion chamber, glowing carbon, or excess pressure caused by igniting the mixture too early while the piston is still going up. Low octane fuel that has a lower resistance to exploding will make pinking worse.  The pinging sound is the result of the shockwaves of the explosion hitting the metal parts of the engine.

 

None of this can happen in a diesel because the fuel is sprayed in over a period of time. The nearest thing is diesel knock, where at the start of injection the fuel being injected takes a few microseconds to actually catch fire. Once it has caught fire the build up of fuel causes a pressure wave but the noise it makes is far deeper than pinking. Diesel knock, unlike pinking, is unlikely to damage the engine parts. It is made worse by fuel with a lower cetane rating.

 

The noise you are hearing is unlikely to be diesel knock and is more likely to be a something loose or worn.

 

As far as the symptoms of a lack of air in a diesel are concerned, the only ones are a lack of power and black exhaust smoke but as someone said above the fact that diesel made in the last 50 to 60 years always draw in far more air than they need to burn all the fuel it is very unlikely that your engine will suffer a lack of air at canal  and typical river speeds.

[Col_T]

Many thanks, Tony.

 

Much as I expected, though nice to hear from someone that understands these things!

[Chewbacka]

Extract or blow in?  Depends on where the vents are.  So if extracting from high up, the fan will extract mainly hot air as hot air rises.  If you blow in cold air low down, and excess air escapes on the opposite side low down air vent, then a blanket of lighter hot air will sit above the air flow.  So best is to extract from a high level and allow cold air in ideally ducted onto things like alternators to cool them.