Caterpillar engine high exhaust back pressure

A place to exchanges questions and ideas of a technical / procedural nature. Go ahead, try to stomp us !
kodiak
Bilge Dweller
Posts: 3
Joined: Sun May 13, 2012 12:40 pm

Caterpillar engine high exhaust back pressure

Postby kodiak » Wed May 16, 2012 1:44 pm

Hello

Please can sone one explain me what could be the reasons for high exhaust back pressure on Caterpillar engine; almost 800 mmWC?

Thank you.

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Wed May 16, 2012 11:54 pm

Welcome to the site.

When measuring the back pressure remember that the Engine Room is always pressurised above the Atmospheric Pressure, so comparing the pressure inside the exhaust pipe with the Engine Room air pressure makes the pressure drop in the Exhaust pipe look better than it really is.


The cause is probably a partially restricted exhaust pipe.

1) If combustion is poor, particularly if the engine is run at low load for prolonged periods of time, and again particularly if it is burning L.O. because of piston ring/ liner problems, heavy deposits of carbon can form on the inside of the exhaust pipe restricting the flow of exhaust gas.


2) Again, if the engine is run for prolonged times at low load, in low ambient temperatures, the exhaust temperature can be below the Dew Point and Acid Rain can form in the exhaust pipe and cause the silencer baffles to collapse.


3) The longer the length of the exhaust pipe, the bigger the diameter it should be to keep the pressure drop within specification, so check the makers installation instructions against the dimensions of your installation.
If you cannot find the installation manual try emailing the service Department they are often helpful with problems like this.

4) Check that the silencer design is approved by the engine maker for that engine, AND CORRECTLY INSTALLED.

I hope this helps, Good luck with the problem solving.

BP
Last edited by Big Pete on Tue May 29, 2012 9:24 pm, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.

User avatar
JK
Enduring Contributor
Posts: 2679
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Caterpillar engine high exhaust back pressure

Postby JK » Thu May 17, 2012 2:39 am

If you have expansion bellows on the exhaust, they have sleeves welded on the inside. I have seen the welds break and the sleeves either relocate to partially block the manifold or get pushed to the turbo inlet.

kodiak
Bilge Dweller
Posts: 3
Joined: Sun May 13, 2012 12:40 pm

Re: Caterpillar engine high exhaust back pressure

Postby kodiak » Thu May 17, 2012 7:28 am

Thank you Big Pete and JK on your answers. Differences between fuel racks on some cylinders are from 3 to 6 divisions (engine is not running). Scavenge air cooler was cleaned and after that this back pressure came to 550 mmWC. What do you think about that?

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Thu May 17, 2012 12:16 pm

Hi Kodiak,

1) Differences in the fuel racks are excessive, most engine makers say they should be within +/- 1.0 mm of the average, bad for the engine but should not really cause excessive back pressure unless maybe it is causing turbocharger surging.

2) Expansion bellows have to be fitted the correct way, usually they have an arrow on them to indicate correct exhaust gas flow. The inner sleeve is a gas tight seal to the bellows (usually wleded) UPSTREAM i.e. inlet end of the bellows piece. If it is assembled the wrong way round exhaust gas and carbon get between the bellows and the inner protctive sleeve and cause the bellows to fail, but again this should not effect the back pressure, just give you an exhaust leak.

3) The fact that there is such a large difference between the fuel rack standouts indicates that the engine has not been correctly maintained. It could be that some one has been playing with the fuel pump deliveries AND INJECTION TIMING in order to balance the exhaust temperatures and Peak Pressures. So I suugest you check the fuel pump timing, this may be causing excessive back pressure if the engine is running inneficiently at full load.
(I sailed on a ship with twin V12 Pielsticks for Main Engines and found that some one had advanced the timing of one engine by 15 degrees, presumably in attempt to lower the exhaust gas temperature, the air coolers were choked and the Turbochargers were worn out. When we checked the other engine( opposite rotation) we found it had been RETARDED 15 degrees. Yes FIFTEEN DEGREES. The World Really is full of idiots.)

4) I would also check the correct fuel rack standout with the engine stopped. Most engines it is + 5mm some + 10 and once I remember -5 but never zero. If the fuel racks are wrong then the fuel shut downs, overload alarms, overload limits and load indicators will not work properly.
A lot of Engineers " assume" that the fuel racks dhould be zero when the engine is stopped, but I have never seen an instruction manual that says so.

5) I was on one ship where we had Paxman RPH V gen sets, they had a single fuel pump assy. We had a problem with one suddenly giving a very high exhaust temperature, only a single Pyroemeter on the Turbo. Checked everything on the engine including spill timing but couldn't find a cause so we did a full strip down, overhaul and rebuild. Problem still there. So in desperation we changed the fuel injection pump and everything was fine. I suspect the pump shaft had failed in the middle of the pump, so we still got the correct spill timing on the unit we checked.

Happy hunting!
BP
Last edited by Big Pete on Tue May 29, 2012 9:28 pm, edited 2 times in total.
It is always better to ask a stupid question than to do a stupid thing.

kodiak
Bilge Dweller
Posts: 3
Joined: Sun May 13, 2012 12:40 pm

Re: Caterpillar engine high exhaust back pressure

Postby kodiak » Fri May 18, 2012 12:35 pm

Thank you Big Pete. Very useful post for me. One of my first opinions was that turbocharger is worn out. What is the mechanism of excesive back pressure if fuel pump timing is not good? Engine/Turbocharger isnt using exhast gas energy as it should?

User avatar
JK
Enduring Contributor
Posts: 2679
Joined: Tue Sep 04, 2007 2:29 am
Currently located: East Coast, Canada

Re: Caterpillar engine high exhaust back pressure

Postby JK » Fri May 18, 2012 2:05 pm

If the turbo was worn out, I would expect to see an excessive LO consumption and extremely high exhaust temperatures.

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Sat May 19, 2012 12:36 am

The makers set the fuel injection timing to the optimum, so that combustion starts fractionally before TDC.

If the the fuel injection timing is advanced, combustion starts earlier, and the the pressure is rising due to combustion AT THE SAME TIME AS THE PISTON IS STILL COMING UP ON THE COMPRESSION STROKE.
Some people misguidedly believe this will lower the exhaust temperature, but that would only happen if the quantity of fuel injected stayed the same.
If some of the energy in the fuel is being wasted opposing the rotation of the engine(i.e. burning during the compression stroke) then extra fuel has to be burnt during the power stroke to keep the engine RPM/Power constant.
If the timing is early, extra fuel is being burnt during both the compression and power stroke to give the same power output, and the engine will be burning far more fuel with higher peak temperatures and exhaust temperatures and a higher volume of exhaust gas being produced than should be for the power output. There will be more energy in the exhaust gas so TC revs will be higher and so will charge air pressure.

If the timing is retarded, combustion starts later, when the piston has already started to travel down the power stroke. Pressure that is available at TDC is expanded, doing work on the piston, until the exhaust valves open and is therefore used most efficiently. Less of the available energy will be used from fuel burnt later in the stroke, so the conversion of Chemical energy to power is less efficient requiring more fuel to do the same work.
In this case peak pressures won't be higher, but exhaust temperatures will be much higher and again there will be more energy available in the exhaust gas so TC revs and charge air pressure will be up.

The theoretical combustion cycle that I was taught when doing my tickets was the "Dual Cycle", initial combustion at "Constant Volume" i.e when the piston is instantaneously stopped at TDC, followed by combustion at "constant Pressure" i.e sufficient fuel is injected to maintain the peak pressure until the desired power outpur/RPM is reached.

The Peak pressure is limited by the mechanical strength of the machinery, and combustion at Constant Volume would only deliver a tiny amount of power, in a practical engine we have to use combustion at constant pressure to "fatten" the power card and produce useful amounts of power, and give good power to weight ratios. In the compromise we accept slightly lower thermal efficiencies.

When I sailed on the Orient Express they had just fitted a blender for the Gen sets instead of running on MDO, the Third decided that running on Blended FO would delay ignition and decided on his own initiative to advance the injection timing on all the gen sets, without telling anyone. As a result we had very high exhaust temperatures. Once we overhauled all the injectors and put the timing backl to the maker's value the engines ran fine on Blended FO.

On the ship with the Pielstick Engines, (Leona 3, ex Lumiere) we limped from Lagos to Malta Drydock and all the cylinder relief valves had to be overhauled for Class survey and the yard reported that one engines valves were all choked solid with carbon. When we refitted all the valves to the engines and ran them up we found all the relief valves on one engne were lifting with the engine clutched out and running at idling RPM. When we checked the timing that was the engine with advanced fuel injection. There are lots of Engineers out there that shouldn't be let loose without a minder, and the problem appears to be getting worse.
That is why I am a great believer in this site as it gives an opportunity for those who want to learn to get a full picture, from people who know what they are talking about.

I hope all this helps you solve the problem.

BP
Last edited by Big Pete on Tue May 29, 2012 9:32 pm, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.

jimmys
Officer of the Watch
Posts: 127
Joined: Tue Nov 16, 2010 4:09 am
Currently located: Glasgow Scotland

Re: Caterpillar engine high exhaust back pressure

Postby jimmys » Wed May 23, 2012 11:14 am

When we time an engine we look for the commencement of injection, this is directly tied up to fuel pump Lift in the older engines. It will be around two millimeters of lift. The timing of injection can be around 20 degrees before top dead centre compression stroke in a four stroke. The injection takes a finite time it is not instantanious. Advanced injection increases peak pressure. The time taken to inject is easily calculated depending on load. The engine is positioned using a vernier protractor on the crankshaft and the fuel pump is positioned using shims. If it is a common pump we use the spill, it is not accurate. The makers provide these figures.
A large bore ten degrees before to ten after. Not so crucial here. Never use marks on flywheels if there is any doubt or problems. Always first principles. The principle of dwell, how long an injection takes is most important in a computer controlled engine.

regards

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Tue May 29, 2012 9:51 pm

I have just re read this thread and noticed a made a couple of careless mistakes in my previous posts. Apologies to all and I have now corrected them.
I don't think I fully explained Kodiak's last question about the link between fuel timing and exhaust pressure, so I will expand my answer: -
Wrong timing will reduce the efficiency of the engine, increasing the fuel consumption, it will also increase the energy in the exhaust, increasing Turbocharger revs, boost pressure and air consumption. Therefore mass flow of exhaust will be greater and at a higher temperature. Consequently the Volume flow of exhaust gas will be much greater.
The pressure drop across the length of the exhaust pipe will be proportional to the volume flow at low Reynold's numbers, i.e. laminar flow.
The makers will specify the volume flow and pressure drop for their engine and when the Naval Architect designs the installation he will calculate the length of the exhaust piping and the number of bends and either calculate, or derive from standard tables, the minimum exhaust pipe diameter required to keep the exhaust pressure within limits. Obviously the smaller the pipe diameter the cheaper the pipe and insulation are to buy and fit, and the less space is required.
If there is very little margin between the capacity of the exhaust pipe and the designed flow of exhaust gas, the increase in flow will increase the Reynold's Number of the flow, the laminar flow will break down and become turbulent. At this point additional volumes of exhaust gas will start to cause exponential rises in the back pressure.
I hope that explains the mechanism better.
Good luck Kodiak.
It is always better to ask a stupid question than to do a stupid thing.

Pengze
Mechanic
Posts: 60
Joined: Tue Sep 23, 2008 8:57 pm

Re: Caterpillar engine high exhaust back pressure

Postby Pengze » Wed Jul 11, 2012 8:48 pm

Very good advice by BP. However I am curious to know whether there is an allowance or margin to advance or retard , let say 1 or 2 degs deviation from the maker's shop test records. And would this affect and result in high exhaust temperatures and fuel consumption ?

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Thu Jul 12, 2012 3:17 am

I have never heard that the makers have any allowance at all. The timing should be set exactly according to the workshop trials data if that is available. Sometimes there are slight differences between cylinders when they are set up by the makers. If this data is not available then the timing should be set according to the workshop manual. The difference between the two sources might only 0.5 degrees on a couple of units, but it is always preferable to set everything up according to the Test Bed Trials.

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

Pengze
Mechanic
Posts: 60
Joined: Tue Sep 23, 2008 8:57 pm

Re: Caterpillar engine high exhaust back pressure

Postby Pengze » Sat Nov 10, 2012 3:55 am

Dear Sirs,
Would like yr valuable advice & clarify on the following details?
1. If the exhaust back pressure is high, will this also reflect & indicate on the exhaust temperture showing high (after the turbocharger)?
2. If we do not have the U-Tube or other mechanism to record exhaust back pressure, what are the other ideas to find out the exhaust back pressure?
3. If we operate on high exhaust back pressure, this will cause the turbocharger to break down, right?
4. If the propeller shaft is entangled with ropes / fishing nets, will this also cause high exhaust back pressure & temperature?
5. If the exhaust back pressure is high, will the expansion bellow above the turbocharger result in failure / damage ?

Thank you for yr feedback.

User avatar
Big Pete
Engineering Mentor
Posts: 790
Joined: Fri Apr 24, 2009 11:18 pm
Currently located: Solihull, England

Re: Caterpillar engine high exhaust back pressure

Postby Big Pete » Sun Nov 11, 2012 1:56 am

Hi Pengze,

1) If the exhaust back pressure is high, then the engine will be running less efficiently because the power out put will depend on the difference between the peak pressure and the pressure in the exhaust manifold. Efficient scavenging also depends on the difference in pressure between the charge air manifold and exhaust manifold during the valve overlap period. High back pressure = low differential pressure = poor scavenging. I would expect that the exhaust temperatures after the cylinder and before and after the turbocharger would all be higher. You will find the fuel consumption is high and power output low.

2) You can easily make a U tube manometer, it is only a piece of plastic tube preferably transparent, but if it is sufficiently translucent to see the liquid it will do. The diameter is unimportant, anything from a couple of mm to 25 mm will do the job, you just need a tape measure or ruler to read the vertical difference between the water level in the two arms of the "U" tube. Usually there are plenty of blank plugs or thermometer pockets that you can use to connect into the exhaust pipes.
It is a simple fabrication job to make an adaptor from whater tube you have to the threaded holes in the exhaust. You need a long enough adaptor to protect the plastic tube from the heat or it will melt!

3)The high exhaust pressure should not effect the operation of the turbocharger.The Turbine is driven by the momentum and kinetic energy in the exhaust gas, this depends on the pressure drop across the turbocharger not the absolute pressure.
I would expect the high back pressure to be caused by a blockage after the turbocharger. If the back pressure is caused by fouling inside the turbocharger this would cause very high turbocharger RPM and scavenge pressure. (Because the passges that the gas flows through will become smaller, this will require a bigger differential pressure to force a fixed volume of gas through the turbocharger at a higher velocity. More velocity means more momentum and kinetic energy in the gas so the turbine produces more power.

4)If the propeller is entangled with ropes or fishing nets the load on the engine will increase but the fuel racks are limited, normally to 10% overload so the exhaust pressures and temperatures shouldnt be any higher than they were on the 110% load trials. In my experience of this happening, fortunatly both times were on twin screw CPP ships, the engine slowed down and went into high exhaust temperature alarm and we were unable to put any pitch onm the engine so we had to shut it down and procede to Port on one engine.

5)There is a massive reserve of strength in the exhaust pipes, silencers and bellows it is very unlikely that the high exhaust pressure would cause any damage to these, we are only talking about a few mm of water head!

I hope this helps.
BP
It is always better to ask a stupid question than to do a stupid thing.

Pengze
Mechanic
Posts: 60
Joined: Tue Sep 23, 2008 8:57 pm

Re: Caterpillar engine high exhaust back pressure

Postby Pengze » Sun Nov 11, 2012 3:35 am

Dear Sir,
Once again , thank you for yr valuable info.
In addition to that, would appreciate to clarify the following points:-
1. By looking /reading at the vertical difference in height of the water column in the U-tube, how can we know / identify that the exhaust back pressure is high / abnormal ?
2. What should be the normal reading of the vertical difference in height of the water column in the U-tube for exhaust gas ?
3. My understanding of the fouling of the nozzle ring & turbine blades of the turbocharger would affect the TC efficiency (hence slower RPM) and also a decrease in scavenge air pressure, could you pls elaborate on this?
4. On another matter, what is the purpose of the U-tube at the turbocharger blower air inlet side ? What does this tells us, related to the efficiency of the TC ?

Thanks for yr precious time.


Return to “The Workshop”

Who is online

Users browsing this forum: No registered users and 1 guest