Diesel Exhaust Calculations

[rob2]

Reading through the latest Vetus catalogue, it seems fairly straightforward to calculate the overall capacity of the wet exhaust so as to specify the water separator requirement. However, there must be a set of equations to at least guestimate the backpressure of the proposed system.

My own installation for a 14hp, 600cc twin cylinder has a nominal 40mm ID. It has a Vetus LSS separator and a muffler and I would guess around 2.5m total length. Could the backpressure generated be the reason I can't get the engine above 2/3 revs under load?

The only experiment I can think of would be to by-pass the muffler with hose...

Rob.

 

[earlybird]

I've never heard of installers calculating back pressure as you suggest. It would, in any case, depend on relative heights. Your system, as far as you describe it, seems to be very typical.
I cannot imagine that the restriction on revs. is due to the geometry of the exhaust run.

 

[Tranona]

Reading through the latest Vetus catalogue, it seems fairly straightforward to calculate the overall capacity of the wet exhaust so as to specify the water separator requirement. However, there must be a set of equations to at least guestimate the backpressure of the proposed system.

My own installation for a 14hp, 600cc twin cylinder has a nominal 40mm ID. It has a Vetus LSS separator and a muffler and I would guess around 2.5m total length. Could the backpressure generated be the reason I can't get the engine above 2/3 revs under load?

The only experiment I can think of would be to by-pass the muffler with hose...

Rob.

That is a pretty typical setup and should not cause any back pressure problems - although the muffler is probably redundant unless it is also the swan neck. The Vetus calculations are to ensure the water trap has enough capacity in relation to the overall system volume. Only exhaust related issues that would restrict revs under load are blocked riser or collapsed exhaust hose. Other than that your problem is almost certainly wrong sized prop - too "big" either diameter or pitch.

 

[rob2]

Yes, I keep trying to get at all the bits of the exhaust system, but haven't found anything wrong yet. The thing is, there is a simple relationship between diameter, length and throughput which means that beyond a certain length, the hose will restrict the flow. I did note that the Mitsubishi based engines have a 40mm system and those based on the Kubota have 50mm. I reckon that the Kubota can therefore use a longer exhaust?

Clutching at straws, maybe, but I remember someone having to remove the muffler as it tipped the balance for the whole system.

Rob.

 

[earlybird]

I did note that the Mitsubishi based engines have a 40mm system and those based on the Kubota have 50mm. I reckon that the Kubota can therefore use a longer exhaust?
.
Rob.

My 2 litre 130 bhp diesel-engined car exhaust is probably around 40mm dia. and ~4 metres long. I'm sure that your 40mm boat exhaust can cope with 600 cc and 14 hp. esp. water cooled to reduce the gas volume.

 

[Deleted member 36384]

Yes, I keep trying to get at all the bits of the exhaust system, but haven't found anything wrong yet. The thing is, there is a simple relationship between diameter, length and throughput which means that beyond a certain length, the hose will restrict the flow.

Rob,
There will be a maximum pressure at the exhaust outlet which the designer will have established for the engine.

You can then calculate quite easily the parisitic pressure loss for the maximum gas flowrate. The part of the exhaust that is wet will be a lot more difficult as it's a multiphase flow. However, you can simplify the calculation if you calculate the volume of water in the exhaust and then reduce the hose CSA by that amount of water.
You will need to know roughness values for each section of material. These are available for hoses, plastics and any cast lengths of pipe/ elbow.
You will also need to establish an equivalent length for your system. For each component in the system (elbow, valve) you can establish a length that equals the pressure loss if that component was a straight line.
Add up the equivalent lengths (tables are available) and put this length into the pressure loss equation. Google API Formula for Pressure Relief Valve Sizing. You are looking for the formula that calculates the pressure in the vent line.
It's not difficult but there are a number of variables that need to be established.
I am away from work, so don't have the formula. If you have to shove coolant water uphill the results will not be accurate.

 

[Latestarter1]

Reading through the latest Vetus catalogue, it seems fairly straightforward to calculate the overall capacity of the wet exhaust so as to specify the water separator requirement. However, there must be a set of equations to at least guestimate the backpressure of the proposed system.

My own installation for a 14hp, 600cc twin cylinder has a nominal 40mm ID. It has a Vetus LSS separator and a muffler and I would guess around 2.5m total length. Could the backpressure generated be the reason I can't get the engine above 2/3 revs under load?

The only experiment I can think of would be to by-pass the muffler with hose...

Rob.

Rob,

3 inches of Mercury is pretty much the industry standard for diesel engine exhaust back pressure. Some manufacturers specify a little more and a few Volvo being one used to specify a minimum back pressure on the little 60 Series motor, however this was a white smoke reduction no performance issue.

If we all had dry exhausts life would be good, however with marine wet exhausts the variable volume of water injected makes gas flow calculations pretty worthless. The correct way to test is to drilll and put 1/8 th tapping in the dry part of the elbow. make up male nipple to fit into exhaust elbow and connect to length of high temp silicone hose and connect to a 0-5 Inch Hg Magnaheilic gauge or Fluke meter with pressure adaptor. You can only test engine under loaded sea trial conditions, record readings every 500 rpm.

Whilst my personal view of Vetus exhaust components is absolute and simple junk, however do I think back pressure is your problem.........likey no.

#1 Take a look at your engine spec sheet, should give the high idle rpm. If high idle not on the data sheet find your maximum rated engine speed and add 10%, majority of marine engines have 10% governor droop.

#2 Warm up motor nicely and adavance throttle to maximum on the control lever, no blippig just nice and steady until you are out on the droop, it will do the motor zero harm. Record your rpm.

#3 Disconnect your Morse control cable from the throttle lever and repeat the process, you will very likely find some more revs. Reconnect and adjust your control cable until you get same rpm as operating lever by hand.

Good chance you have now found your 'missing' rpm.