Caterpillar engine high exhaust back pressure

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Re: Caterpillar engine high exhaust back pressure

Postby jimmys » Sun Nov 11, 2012 9:28 am

Its best we put the units of 800mmWG into a unit we can understand and for this purpose we can assume 28 inches of water head is eqaul to 1psi. We are talking about a pressure in the duct that is one tenth of atmosphere. Very difficult to measure this in a boosted engine room. Certainly nothing will burst. A human being can blow 28 inches of water out the manometer without effort.
Its been stated the exhaust pipes have laminar flow inside, this is not correct and only Naval Architects think that. All along the wall of the exhaust pipe the flow is turbulent and this area is called the boundary layer. The thickness of the Boundary Layer is variable and depends on the condition of the wall of the duct. Normal dry soot allows a reasonable thickness of boundary layer and an acceptable laminar flow. Oils/acids in the exhaust duct due to viscosity can cause a large increase in the thickness of the boundary layer and a serious reduction in the laminar flow. This type of turbulence causes high exhaust temperature and increased back pressure.
Check the crankcase pressure in your engine, look at the crankcase vent. Problems in the crankcase in a medium speed, cause oil carrying through the scrapers. Check the lub oil purifier, put off the auto blow and see how often it needs cleaning.


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Big Pete
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Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Mon Nov 12, 2012 12:34 am

Hi again Pengze,

You have had a busy weekend posting questions.

1) & 2) You should have records from the Test Bed and Sea Trials of what the Exhaust back Pressure was originally, and the maximum permitted back pressure should be detailed in the Instruction Manuals, along with the diameters of exhaust pipes required and the additional diameter required as the length and number of bends in the exhaust pipe increase. If not email the maker's Service Engineers and they will tell you what it should be for your specific engine. There will inevitably be a small increase due to fouling, over time, as Jimmy says, but this should be small, and the original Sea Trials reading should be well below that in order to allow for later fouling. I would guess within the good old Engineers allowance of 10% for everything!

3) Fouling of the TURBINE causes it to develop more power, running at higher speed. and delivering more scavenge pressure while the efficiency and output of the engine will be reduced. Any Basic Marine Engineering Text Book will confirm this. As I said before fouling reduces the area between the blades through which the gas flows, therefore, the gas velocity has to increas in order for the exhaust gas to get away from the engine. This causes the pressure between the Engine and Turbocharger to rise, reducing the engine power output. At the same time the increase in gas velocity gives it more Kinetic Energy and Momentum which the turbo Charger converts into mechanical power increasing the TC RPM and hence the scavenge air pressure.

4) The Manometer at the Turbo Charger inlet is usuaally connected across the intake filter and is there to tell you when the air filter needs to be cleaned. It doesnt tell you anything about the efficiency of the T/C only that a dirty filter is restricting the air flow.

It is always better to ask a stupid question than to do a stupid thing.

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Re: Caterpillar engine high exhaust back pressure

Postby jimmys » Mon Nov 12, 2012 5:38 am

Check the boost/scavenge air pressure and temperature in the scavenge trunk. Look back in old logbooks or makers data and compare.
The blower and turbine are on the same shaft so they have the same power requirements so only an increase in speed causes the increase in power as long as torque remains constant. ie. steady running.

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Re: Caterpillar engine high exhaust back pressure

Postby JollyJack » Mon Nov 12, 2012 7:50 pm

A fouled turbine will also cause the turbocharger to surge, or "bark", a very noticeable condition, especially when increasing engine speed. When this happens, waterwashing the turbine is the answer, if you can do that to a Cat. Knowing Cats as I do, it probably isn't possible and you'll have to buy a new turbocharger. Cats are, after all, designed to sell spare parts.
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Re: Caterpillar engine high exhaust back pressure

Postby JK » Tue Nov 13, 2012 4:23 am

The thing about Cats is you do exactly what they say, exactly when you are suppose to do it and you will have little issue with them.
And yes, getting Cat in to do the work is the most cost efficient IMO, though most disagree.

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