Exhaust capacity question

[fisherman]

Just fitted a new silencer in my dry exhaust stack. Engine is 6litre Ford, exhaust is 2in
steel pipe, as from Mermaid, correct size. A standard silencer from ASAP with a 2in pipe fitting has a capacity of 3.3 cu m/min, so, as my engine shifts 6cu m/min at 2000 revs, I fitted a 3in silencer, capacity up to 7.5 cu m/min, spent £60 extra on adapters. Since the silencer has a 3in pipe running straight through, and presumably so has the 2in, it seems I could have used that, as it is no different from a length of 2in pipe...?? Would have saved me £130 and a lot of running around. Even two smaller silencers in line would have been cheaper.

 

[fisherman]

I bump this up because it has been pointed out to me that, idiot as I am, I assumed the engine shifts its capacity every two revs, not so. Can anyone tell me how many revolutions there are to shift six litres?

 

[TimBennet]

Can anyone tell me how many revolutions there are to shift six litres?

Well it does if it was simply running on the starter motor and acting as an air pump. But to the air intake volume (assuming no turbo) you also add a certain amount of diesel which when combusted turns into a considerable amount of extra gas.

It's also hot, so every is expanded.

 

[VicS]

Well it does if it was simply running on the starter motor and acting as an air pump. But to the air intake volume (assuming no turbo) you also add a certain amount of diesel which when combusted turns into a considerable amount of extra gas.

It's also hot, so every is expanded.

Although the combustion process does produce an increase in volume don't forget that oxygen is also being removed so the increase is not as great as you might at first think.

You can see this from the chemical equation for the combustion process:

4C₁₂H₂₃ + 71O₂ ─► 48CO₂ + 46H₂O​

Also 4/5ths of the air is nitrogen which passes through unchanged.

I think you will therefore find that the biggest increase in volume is due to the increase in temperature. The volume will be proportional to the temperature in °K.

 

[fisherman]

So we assume that the cylinder is still under pressure when the exhaust valve opens, and more than the swept volume is expelled? Still can't work out how much volume is expelled per rev.

 

[VicS]

So we assume that the cylinder is still under pressure when the exhaust valve opens, and more than the swept volume is expelled? Still can't work out how much volume is expelled per rev.

Start with half the cubic capacity, 3 litres, increase this by 20% to represent the absolute maximum possible increase in volume of the exhaust gases at the ambient temperature, based on the chemical equation above but assuming the water produced remains as a vapour ..... say 3.6 litres


now calculate the increased volume due to the increased temperature from V₁/t₁ = V₂/t₂

Where:

V₁ is 3.6
t₁ is ambient temperature in °K
V₂ is the volume of the exhaust gas per revolution
t₂ is the exhaust gas temperature in °K


sorry cannot help with the value for t₂

 

[fisherman]

So your basic premise is 3.6 litres per rev? That would be 7.2cu m/min at 2000 revs.

 

[TimBennet]

Plus temperature expansion:

Use an ambient Temperature of 300°k and I'm sure diesel exhaust gases in a dry stack at full chat run to 1000°K.

= 12 litres.

= 24 m3/min

 

[Mermaid-Jon]

Assuming the you have the naturally aspirated engine which is a 6.22 litre unit, the figures from the Ford technical specification document are as follows;
Exhaust gas flow at full load & speed = 22.6 cu m/min
Expected exhaust gas temp prior to cooling by water cooled manifold = 700 degrees C
Minimum bore of exhaust pipe at manifold outlet = 57mm
Maximum allowable exhaust back pressure = 76mm Hg

I don't know if this will help in your quest to find an answer, but that's what it says in the book!
Best regards
Jon

 

[TimBennet]

Assuming the you have the naturally aspirated engine which is a 6.22 litre unit, the figures from the Ford technical specification document are as follows;
Exhaust gas flow at full load & speed = 22.6 cu m/min
Expected exhaust gas temp prior to cooling by water cooled manifold = 700 degrees C

Well that's not a bad correlation - 700°C is 973°K and 22 1/2 or 24 cu m/min is near enough!

I think you can assume the exhaust volume isn't 7.2 cu m/min

 

[VicS]

Assuming the you have the naturally aspirated engine which is a 6.22 litre unit, the figures from the Ford technical specification document are as follows;
Exhaust gas flow at full load & speed = 22.6 cu m/min
Expected exhaust gas temp prior to cooling by water cooled manifold = 700 degrees C
Minimum bore of exhaust pipe at manifold outlet = 57mm
Maximum allowable exhaust back pressure = 76mm Hg

I don't know if this will help in your quest to find an answer, but that's what it says in the book!
Best regards
Jon

Crums! That figure of 22.6 cu m / min agrees with the theoretical calculation surprisingly closely!

 

[fisherman]

So my silencer with a max of 7.5 m3/min is on the small side. In fact it won't restrict the flow, just maybe a bit noisier than could be achieved.