Engine inwards air requirments

[Guest]

Sort of related to the Perkins thread but seperate as it's just interesting.

 

Say you have a 160hp Perkins V8-510. Goes well but seems short on getting to the right revs. Leaving prop matching out of it, which I suspect is a big part of this specific issue as is the gauge knot being calibrated correctly, what about air the motor can breath. This boat has a 100mm odd exhaust and when running is pumping a lot of air out of it. But the inwards vents I can find are relatively tiny and the engine room does seem quite well insulated hence relatively air tight. I just can see where the motor is getting a lot of it's air from.

 

If said motor, and I'd assume it probably applies to all of them, isn't getting enuff air would that have an effect on the revs the motor will get too?

 

What other possible downsides to a air supply shortage could there be?

[Jon 360]

If It's starving for air then there will be lots of black smoke as the governor pours in more fuel as it tries to deliver the revs your asking for.

 

Try running it with the engine room door open and see if it goes any better, but I would think an engine of that size should have a blower of some sort

The other thing is it will run hot if its running in a vacuum

 

To work out air volume requirements its just cc x rpm will give you the amount of air its drinking per minute

You should have the ability to deliver 1.5 to 2 times the max requirement this will allow some cooling

[Guest]

Thanks D.

She blows low level back smoke on start up until she gets warm (5 min max) then that swaps to wisps of steam. Certainly no 'laying smoke' seen.

 

Found a blower switch but the vent it's next to would probably be only 60 square cm's which to my totally untrained eye just doesn't seem anything like enuff and noting like the space to get the volume she looks to be pumping out. It's all a bit weird really.

 

It runs quite warm but knot 'hot' unless you push her into a place that just sound big time shite so at 'nice sounding' revs she runs quite warm, the gauges say 70-75 degrees but I'm knot 100% sure they are correct but they might be.

 

The buggers put the batteries down in there and I'm bloody sure that's knot good for them.

 

I think I'm coming back to a poorly matched prop and/or pitched prop again.

 

One thing going for her though is she's bloody cheap to run. At nice sounding revs we get 6-6.5kts and at those revs she will make Aussie on a tank full of gas.

 

The more I get into her the more she would be 100% at home in 60kts in the middle of an ocean (which she is having been there done that together) but knot nearly as flash for H Gulf summer cruising. Maybe the prop is sized/pitched to cross oceans rather than for a Akl to Kawau run.

[Jon 360]

Temp sounds about right

Have you tried the High idle test yet ? that will tell you if your proped correctly

 

Full revs in gear should be no more than 10% down on full revs out of gear

If your gauge isn't correct then it makes it a bit harder but you should be able to convert this to what it should rev out to

Sounds like it may be under proped so then it will rev the same which isn't good as not enough load

[Guest]

Wheels is heading North and bringing his grey matter on things big diesel which is far better than mine so guess who I'm taking away for a weekend boat?

 

He has a dodacky we can use to check actual revs against gauge reading revs. It may come down to a simple dodgy gauge and KM's lack of being able to ear-gauge actual revs. But I do think the prop needs work.

 

Maybe a simple case of a problem being found or think it's found by one of the most dangerous things known to man, an enthusiastic amateur, in this case me I can usually get them going and keep them going if at sea and pushed but the intricacies are beyond my standard scope.

 

But ask the questions and even if knot right for the specific indecent I still learn which is enjoyable. Learnt a lot of handy stuff in the last few weeks which I'm sure will come in handy sometime. Another month or so and I'll be able to pop over and service your motor........ if you're brave and/or drunk enuff

 

Keep educating the dummy, often we need it, I sure do

Thanks again D.

[wheels 543]

I'm now in a motel room. Nice!

A sure sign that any engine room is starving from getting air, is to check the carpet (if it has any) around any hatch to the room.That it providing the hatch is not air tight of course. If there is a ring of black around that edge, then the engine room is under a vacuum and the air is coming from where ever it can get in.

KM, here is an idea to also follow some lines on. My boat has a Pilot house. At the helm station, there is a cabinet that the wheel attaches to and the instruments are in. this cabinet is sealed from floor to the instrument panel. But there is a gap between the panel and cabinet that you can not see unless you look for it. Inside the cabinet and the floor, is a dorade type arrangement. So if the Pilot house get a big greeny inside, then water should not be able to gain access to the Engine room below. But that is where the enigne gets it's air from. The fuel breathers also come up through that cabinet and all the wiring to the engine room runs down through it. Scuppers in the Pilot house floor take care of the water if it does come in.

Both air intake and exhaust outlet diameters, coupled with length, are critical to engine performance, and often over looked in designs. Even just a bend can make an enormous difference to performance and when you introduce water for cooling, a lift box, a muffler and soon, then further restrictions are created and it all can make a big difference to Hp.

KM, I am pretty sure I have the manual for that engine here, so might do some homework on it and see what the recommended in and outs are for it. Although I have to say, 100mm for exhaust on a engine of that size is a touch on the small side. It does depend on length however.

[Guest]

With Petrol engines a good rule of thumb is that 100cu/meters/minute of air = 350HP at 100% volumetric efficiency (which is nearly impossible without turbo/supercharging) and average air temp of around 15deg c and at sea level.

 

Not sure what the deal with diesel is but if you use that number your 160HP uses around 45cu/m/minute at full power or say 20cu/m/min at cruise. That is a lot of gas to flow and as wheels points out length and bends in tubes makes a big difference.

 

Its not 100% but it gives you an idea of the volumes you need to flow. My Volvo V70 R (2.5L, 5 cyl, 20 valve, turbocharged and massively intercooled) has factory twin 3inch exhausts for its 300hp and you should hear it suck when you boot it. Even inside the car you can hear the air being sucked in.

 

Its often not thought out in the design stages....with more priority on sealing up the engine room to keep it quiet.

 

So do you think the air inlets are going to let in 20cub/m/min or 1 cubic meter every 3 seconds at cruise? and will the exhaust get it out given the gas is now hot and therefore a greater volume and less dense?

 

EE

[Guest]

...and if you take the swept volume of the Perkins V8-510 as 8.4L, assume 100% volumetric efficiency then you get 8.4L x (say)2000rpm - 16800L/m or 16.8 cu/m/min so not far off the rough 20 cu/m/min from the petrol calculation based on HP output. Diesels are more efficient - hence the slightly lower volume.

[Guest]

Its not 100% but it gives you an idea of the volumes you need to flow. My Volvo V70 R (2.5L, 5 cyl, 20 valve, turbocharged and massively intercooled) has factory twin 3inch exhausts for its 300hp and you should hear it suck when you boot it. Even inside the car you can here the air being sucked in.

Shite, I thought you were talking about your boat there for a sec and started to wonder how it still managed to float. Glad you mentioned cars as I see Volvo and only think motors :)

 

So do you think the air inlets are going to let in 20cub/m/min or 1 cubic meter every 3 seconds at cruise? and will the exhaust get it out given the gas is now hot and therefore a greater volume and less dense?

No place I can see where that volume can get in but as Wheels mentioned there could be cunning places stashed I don't know about.

 

The boat was launched mid 70's and is very 70's in it's thinking re offshore distance cruising, which she does very well and even if knot at the pace of current vessels her size but then she would out survive most of those with ease if it all went badly tits-up weather wise. Enuff wire holding up the rigs to outfit 3 maybe 4 modern boats

[Farrari 4]

One of the quick and simple tests they talk about is to open the engine compartment door while underway. If you feel a vacuum then you know there is not enough ventilation. Easy snuff done.

[wheels 543]

So more info on the engine from the manual.

The engine requires 10.5 cubic m of air/min (365 cu ft/min).

Exhaust back pressure is not suposed to be above 75mm/Hg (3"Hg)

Idle is supposed to be 625RPM

[Guest]

Once warm the gauge sits at 350-400revs and she sounds fine and you can engage gear no worries. She will hold ground and even inch forward into 20-25kt headwinds at that indicated rpm.

 

No idea on back pressure but the exhaust has no problem blowing the ocean away if the outlet goes below water level, which it often does when heeling or in decent waves.

[wheels 543]

When it comes to sailboat engines, there is always some form of compromise. From Budget, to what you can fit and where. When it comes to a true motor sailor (equally as good in either mode) then the engine installation does need slightly more consideration than often give.

Exhaust is often the most underconsidered part of any sailboat installation. Mainly because the exhaust hosing is stupid expensive and that cost is exponential with size increases. And it is a pig to work with, also becoming exponential with size increases. And often the information given for an engine will usually give a recommended size of hose, but what they sometimes don't tell you is what happenss as length increases and if there are any bends. Bends are one of the biggest area's of restriction and often not discussed much in installation information. Even the difference between a dry and wet exhaust has a big difference. A wet exhaust needs an automatic extra 1'' daimeter increase over a dry exhaust for instance.

The part that many never consider however, is a bend. One 90deg bend (wet exhaust) is like adding another 7m of exhaust hose.

[Guest]

another aspect re marginal/ restricted engine room air flow is when the engine is shut off (even after running it down) the engine room temp increases , sometimes dramatically due to latent heat in engine block/s transmission/s etc etc not being removed internally via water cooling and can really play havoc with aircooled refrigeration condensing units amongst other things that live in the engine room and like/need to be cooled

[Guest]

What other possible downsides to a air supply shortage could there be?

 

It may have been said already but an air starved engine has an accelerated build up of carbon inside of it therefore robbing it of power and shortening the engines life. Kinda like cholestorol in the arteries....