BMC 1.5 gas flow/back pressure

[sueanddaren]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

 

Daren

[pwl]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

 

Daren

 

Call Calcutt Boats. They do a lot of work with BMCs

 

P

[RobinJ]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

Not sure, with it being an old engine, whether there are any figures?

Normally an adsorbtion type as it reduces back pressure, if your looking to reduce noise and go for a better silencer, you may have to increase the capacity?

You always get crankcase fumes sucked into the inlet which will coke up, two much back pressure will only make this worse especially as the engine wears or if its under heavy load.

[sueanddaren]

Call Calcutt Boats. They do a lot of work with BMCsP

Tried Calcutt but although I spoke to three very nice people they didn't know.Daren

Not sure, with it being an old engine, whether there are any figures?Normally an adsorbtion type as it reduces back pressure, if your looking to reduce noise and go for a better silencer, you may have to increase the capacity?You always get crankcase fumes sucked into the inlet which will coke up, two much back pressure will only make this worse especially as the engine wears or if its under heavy load.

Spoke to Servais who make the silencers that most chandlers sell, but without the gas flow rate and back pressure figures they can't say if any of the 3 types they sell (medium absor, Residential, High atten) are OK.Daren

[Yoda]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

 

Daren

 

I presume that you mean a diesel engine. Gas flow rate is quite small, use any figures for a 1.5 engine or there abouts. Back pressure should be low as the need to scavenge the cylinders is high. On petrol engines you need a bit to stop the fuel passing through due to valve overlap.

[sueanddaren]

I presume that you mean a diesel engine. Gas flow rate is quite small, use any figures for a 1.5 engine or there abouts. Back pressure should be low as the need to scavenge the cylinders is high. On petrol engines you need a bit to stop the fuel passing through due to valve overlap.

Yoda

 

Could you be a bit more specific? Swept volume X ?????

 

Daren

[Yoda]

Yoda

 

Could you be a bit more specific? Swept volume X ?????

 

Daren

 

I didn't mention swept volume.

 

The swept volume is the capacity of the engine, bore x stroke, in this case I think 1.5 or 1500 cc was mentioned. The compression ratio off the top of my head is about 22:1 which probably gives about 25 hp. I know nothing about this engine except it was probably a truck engine.

 

Go to a motor factors and they will have the info you want.

[Alistair]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

 

Daren

 

Why do you need to know?

 

Why not just ask for a silencer for a BMC 1.5?

 

<sorry if this seems obvious, it's meant to be serious tho>

[pwl]

There are owners guides and tech manuals for the 1.5 and 1.8 BMC here http://www.calcuttboats.com/engines.html which may help. Worth downloading anyway.

 

P

[RLWP]

Why do you need to know?

 

Why not just ask for a silencer for a BMC 1.5?

 

I would also like to understand why you have asked the question. Most narrowboat equipment is specified on the "that will do" principle rather than being carefully matched to any particular performance criteria, with perhaps the exception of some of the electrical kit. I would have thought that any silencer in a chandlers would do for your engine. If you are concerned buy a bigger one.

 

You may also find more constraints from the space that you have to put it in and from the fittings that are available to pipe it up than from the engine.

 

Apologies if you are an expert in exhaust design and gas flow, and if you are, I would be interested in learning more.

 

Richard

 

Edited - Why can't I spell??

Edited March 7, 2008 by RLWP

[bottle]

This may be totally stupid but would the local car exhaust specialist be able to help.

 

They usually have a reference book that they check.

[sueanddaren]

I would also like to understand why you have asked the question. Most narrowboat equipment is specified on the "that will do" principle rather than being carefully matched to any particular performance criteria, with perhaps the exception of some of the electrical kit. I would have thought that any silencer in a chandlers would do for your engine. If you are concerned buy a bigger one.

 

You may also find more constraints from the space that you have to put it in and from the fittings that are available to pipe it up than from the engine.

 

Apologies if you are an expert in exhaust design and gas flow, and if you are, I would be interested in learning more.

 

Richard

 

Edited - Why can't I spell??

 

Sorry Im no expert in exhaust design, but a quick look on the internet tells me that there are different types of silencer and some are better than others, at silencing. So far I have learnt that a silencer designed for a car would not be as good on a boat, as a car silencer is at the back and thus by the time the gases have reached it they will be laminer not turbulent. Thus on boats or other aplications were the distance from the engine is small the silencer should have a multiple pass section to smooth the gas flow. This reduces back pressure which is not a good thing to have on a diesel, petrol engines arnt as critical (scavenging).

 

Yes I am very constrained by space thus I can't fit a hospital silencer, so I am trying to get the best silencer I can in the space I have. None of the manuals help and I was hoping that someone would know the factor/ relationship between swept volume and exhausted volume on a diesel, ignoring turbo types they must all be about the same. From this I can calcutate the gas flow rate and for any choosen silencer from a good manufacturer I can look up the back pressure. As long as this is less that the engine will accept Im OK.

 

Fittings arn't an issue, again the internet will get you just about anything you want.

 

Daren

[Tony Brooks]

Sorry Im no expert in exhaust design, but a quick look on the internet tells me that there are different types of silencer and some are better than others, at silencing. So far I have learnt that a silencer designed for a car would not be as good on a boat, as a car silencer is at the back and thus by the time the gases have reached it they will be laminer not turbulent. Thus on boats or other aplications were the distance from the engine is small the silencer should have a multiple pass section to smooth the gas flow. This reduces back pressure which is not a good thing to have on a diesel, petrol engines arnt as critical (scavenging).

 

Yes I am very constrained by space thus I can't fit a hospital silencer, so I am trying to get the best silencer I can in the space I have. None of the manuals help and I was hoping that someone would know the factor/ relationship between swept volume and exhausted volume on a diesel, ignoring turbo types they must all be about the same. From this I can calcutate the gas flow rate and for any choosen silencer from a good manufacturer I can look up the back pressure. As long as this is less that the engine will accept Im OK.

 

Fittings arn't an issue, again the internet will get you just about anything you want.

 

Daren

 

 

Not quiet as easy as one may suppose because the gas will be hot whilst the swept volume will be for cool air.

 

The 1.5 (unless re-bored) has a swept volume of about 1498cc

 

The volumetric efficiency is hard to quantify because it will alter with type of air cleaner, engine speed, port design and absolute pressure in the engine area.

The 1.5 has siamesed inlet ports and one siamesed exhaust port so it will not have a particularly good volumetric efficiency- blind guess 85 to 90%

 

@ 1000 rpm the theoretical AIR (not exhaust gas) flow in the exhaust at AMBIENT temperature will be about 1498 x 500 power strokes = 74900 cc per minute or 74 litres. To this you must add the products of combustion from the fuel, but as the injection per cycle is likely to less than a pinhead's worth this will not be a massive quantity.

 

Combustion temperates can reach about 1000C, but it will be less in the exhaust so if we say 500C this gives us a temperature rise of about 480c (from 20C ambient). Unfortunately none of my text books lists the coefficient of volumetric expansion of air and a quick google did not show anything I could work with to get further. Perhaps one of our physicists can help.

 

My Bukh DV36 with a similar swept volume also gives no data on back pressure or gas flow.

 

Even if the silencer did create a degree of excess back pressure it will be proportional to engine speed, so for inland work you may never notice it and unless you get things very exceptionally wrong I doubt you will notice any loss of power. I am sure the silencer manufacturers will quote an attenuation figure - choose the largest in the size of silencer you can fit. I think you may be able to get a composite unit containing both baffled and absorption sections.

 

Also contact TW marine who specialise in quietening boats, they have a range of silencers and talk good sense.

[LEO]

Try AMC diesels somewhere in Blackpool (they advertise widely) very helpful when I had a BMC !.5 - Lovely engines, now swapped for a Gardner, equally as nice but gets through the fuel a bit.

I had a wet silencer on my BMC and it was as quiet as a mouse when running.

Best of Luck

Mie.

[Big COL]

Any one know the gas flow rate and back pressure for a BMC 1.5? Im trying to get the correct size silencer.

 

Daren

 

 

Are we looking for a silencing quality or performance. I have yet to see a F1 narrow boat.

[sueanddaren]

Not quiet as easy as one may suppose because the gas will be hot whilst the swept volume will be for cool air.

 

The 1.5 (unless re-bored) has a swept volume of about 1498cc

 

The volumetric efficiency is hard to quantify because it will alter with type of air cleaner, engine speed, port design and absolute pressure in the engine area.

The 1.5 has siamesed inlet ports and one siamesed exhaust port so it will not have a particularly good volumetric efficiency- blind guess 85 to 90%

 

@ 1000 rpm the theoretical AIR (not exhaust gas) flow in the exhaust at AMBIENT temperature will be about 1498 x 500 power strokes = 74900 cc per minute or 74 litres. To this you must add the products of combustion from the fuel, but as the injection per cycle is likely to less than a pinhead's worth this will not be a massive quantity.

 

Combustion temperates can reach about 1000C, but it will be less in the exhaust so if we say 500C this gives us a temperature rise of about 480c (from 20C ambient). Unfortunately none of my text books lists the coefficient of volumetric expansion of air and a quick google did not show anything I could work with to get further. Perhaps one of our physicists can help.

 

My Bukh DV36 with a similar swept volume also gives no data on back pressure or gas flow.

 

Even if the silencer did create a degree of excess back pressure it will be proportional to engine speed, so for inland work you may never notice it and unless you get things very exceptionally wrong I doubt you will notice any loss of power. I am sure the silencer manufacturers will quote an attenuation figure - choose the largest in the size of silencer you can fit. I think you may be able to get a composite unit containing both baffled and absorption sections.

 

Also contact TW marine who specialise in quietening boats, they have a range of silencers and talk good sense.

 

Thanks again Tony

 

If your 500C is correct I can apply Charles's gas law this states that for every 1 deg C rise in temp the volume increases by 1/273 (http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0031050.html). So I have now calculated the flow rate and applying the graphs on the servais web site I can see that I can easily use one of there SN116 SE (1 1/4) silencers even at 3000rpm. That means I don't have to use the 1 1/2 size which is 50 mm longer and was going to be a bug*er to fit.

 

Daren

[Tony Brooks]

Thanks again Tony

 

If your 500C is correct I can apply Charles's gas law this states that for every 1 deg C rise in temp the volume increases by 1/273 (http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0031050.html). So I have now calculated the flow rate and applying the graphs on the servais web site I can see that I can easily use one of there SN116 SE (1 1/4) silencers even at 3000rpm. That means I don't have to use the 1 1/2 size which is 50 mm longer and was going to be a bug*er to fit.

 

Daren

 

 

Daren

 

My DV36 which has a similar capacity, a slightly lower power and a slightly higher torque gives an exhaust gas temperature of 550C at maximum power. It goes on to state that it is about 100C lower than "corresponding engines".

 

My graph shows 300C at 100rpm, 400C @1500, 450C @2000 and just under 500C @ 2500rpm. This must be at full power, so at reduced power at the same speed it should be lower.

 

I think I read somewhere, but I am bl owed if I can find it now, that Bukh recommend a 1.75 inch exhaust, however I am sure mine is only 1.5" without any apparent ill effects - It romps away from a boat with a BMC 1.8!

 

Please remember the volumetric efficiency is an educated guess and not fact.