The Impact of Generator Set Underloading

by Visitor bjabeck ‎03-16-2015 06:49 AM - edited ‎03-16-2015 06:52 AM

CAT50309 Brian Jabeck banner.jpg


System health and reliability are critical to backup and prime power solutions for any installation—from mission critical data centers to neighborhood grocery stores. While power systems vary in operation, application and load profile, they’re all designed to provide reliable power and maximized efficiency. The generator set is a key piece of the power system that requires special attention for long-term system reliability, availability and uptime. To achieve these, it’s important to understand system operation, load profile and required maintenance. This blog will focus on generator set operation in low-load scenarios, and what can result if they are used outside of specific parameters.


First, it is important to understand that generator sets are designed to run with load. This may seem trivial, but loading a generator set properly is essential to availability and a long engine life. Manufacturer service intervals and projected component life are based on operation in load ranges to deliver an ideal mix of product performance, power density and long-term operational life.


Incorrect generator set operation can result in reduced output, component damage, reduced lifecycles, and unscheduled downtime. The ideal operation targets of each generator set will depend on the application and rating.


Underloading Diesel Generator Sets


Generally speaking, standby- and prime-rated diesel generator sets are designed to operate between 50 and 85 percent load, while continuous-rated diesel generator sets optimize between 70 and 100 percent load. Operating diesel generator sets at loads less than 30 percent for extended periods can impact uptime and engine life.


The most prevalent consequence of underloading is exhaust manifold slobber, or wet stacking, which is the black oily liquid that can leak from the exhaust joints when the engine does not reach minimum temperatures and pressures. Visible engine slobber doesn’t necessarily indicate a problem, but it signals possible underloading concerns, low ambient temperatures or jacket water temperatures that are too low. Additionally, long periods of light loading can lead to deposit build-up behind the piston rings or inside the cylinders, which can cause power loss, poor performance, accelerated wear and in extreme cases, cylinder liner polishing.


Underloading Gas Generator Sets


Natural gas and biogas generator sets, independent of application and rating, are typically designed for operation between 60 and 100 percent load. Without enough cylinder pressure to maintain oil control at low loads, gas engines can develop ash deposits, a reduced detonation margin and damaged engine components. Similar to diesel generator sets, deposit build-up on valves, spark plugs and behind piston rings can occur—which may cause cylinder liner polishing, power loss, poor performance and accelerated component wear—ultimately increasing the likelihood of higher maintenance costs and downtime.


Managing Low Load


If maintained properly, diesel and gas generator sets can operate at light loads for long periods without harmful effects. After underloading, the generator sets should run at increased load to raise the cylinder pressure and temperature, which will clean the deposits from the combustion chamber. Regular low load operation requires a more aggressive maintenance plan to reduce excessive component wear and may require engine modification.


Time Limits for low load operation of natural gas generator sets.jpg

Diesel engines should run at a minimum of 30 percent load for about 30 minutes for every four hours of light-load operation, and operators should measure exhaust temperature prior to the turbo for analysis. Natural gas engines are more sensitive to low loads, so there are specific underloading time limits required to maintain optimal performance, see the chart above. After the time limit has expired, the engine should run at a minimum of 70 percent load for at least two hours.


Power system underloading impacts individual components as well as overall performance, so it should not be taken lightly. While the simple solution is to operate generator sets at a load that meets design requirements, the reality is that system needs sometimes change. This makes underloading common in the realm of power generation—especially in standby applications. However, the effects of underloading can be minimized with a thorough operation and maintenance plan to preserve system health and avoid extra costs down the road.


  • We’d like to hear from you. Tell us how generator set underloading has impacted your operation.
  • What strategies have you implemented to ensure optimal performance at low loads?
  • In your experience, how has underloading diesel generator sets differed from natural gas-fired engines?
  • What can Caterpillar do to improve system design and help reduce the impact of underloading for your application?


For more information, visit or contact your local Cat dealer.

by Visitor Bamgles2
on ‎03-19-2015 04:45 AM

By underloading the generator it will affect the  service of the generator and electrical system.

by Super Contributor
on ‎03-22-2015 02:37 PM

In general, at least based on my experience, diesel engines tend to be much more robust in dealing with underloading issues. In general you can run the diesels  underloaded for a fairly long time period, and as long as you periodically run the engine at a higher load to "clean it out" you can minimize the affects of low load operation. Multiple factors do come into play, like original age and design of the engine. Older designs with rectangular piston rings, valve stem seals and lower fuel injection pressures tend to be much more tolerant of long periods of low load operation than newer design engines with keystone rings and high fuel injection pressures. Turbochargers are also negatively affected by low load operation, mostly due to uneven buildup of deposits causing imbalance and changes to the turbo's response.


Gas engines usually see the negative impact of low load operation much sooner in the service cycle, mainly due to spark plug fouling, that causes hard starting, rough runing and poor acceleration. Once a plug is fouled it almost always requires the plug to be removed and cleaned. A run at higher load typically won't "cure" a fouled plug.  Also deposit formation due to low load operation can lead to an increase in compression ratio, and with that increasing the risk of detonation. On engines without detonation protection system this can lead to mechanical damage, on engines with protections it will lead to unwanted shutdowns.


Underloading affects the operation of the prime mover, it causes problems with starting, ramping to rated speed, ability to respond to transient conditions, and stability.  It will also causes "slobbering" and deposits of incompletely burned fuel and oil in the exhaust systems that can lead to excessive smoke and in some cases, exhaust fires.


I don't think underloading has a negative affect on the generator end or the electrical system as the electrical side of the system is more prone to problems due to overloading and heat.


Exhaust aftertreatment if installed also can have many problems with extended periods of operation below recommended minimum loads due to fouling of catalysts, inability to regenerate partculate traps, visible emissions (smoke) and out of compliance on permitted exhaust gas constituents.


My two cents worth, MikeL

by Visitor Progen904
on ‎03-26-2015 04:39 PM

How is it possible to have an EPA rated engine at any Tier level that produces excessive exhaust emissions at any load level???

by Super Contributor
on ‎03-26-2015 11:15 PM

By operating it outside of "standard conditions". Things such as a plugged air cleaner, excessive exhaust restriction, wrong grade/type of fuel, incorrect air inlet temperture. when you look at the TMI exhaust emissions data at the end you see a lot of definitions that define the normal operating characterisitics that are required to maintain emissions compliance.


A tier rated engine still does not have any form of active feedback controls (such as  CEMS system) and while it does has more extensive protections to limit deviation from desired emissions levels, they can still operate unders certain conditions out of compliance.

by Visitor Progen904
on ‎03-27-2015 07:56 AM

So then the question may be ammended to read - how is it possible to have a properly installed and maintained EPA rated engine at any Tier level that produces excessive exhaust emissions at any load level ???

by Visitor bjabeck
on ‎03-30-2015 01:47 PM

Progen - This is a very good question and one we receive often and there are multiple scenarios where emissions may be measured above EPA published limits.


1) Mfr's  that provide stationary engines certified  to EPA emissions standards report emissions based on 5 engine operating loads and speeds and  results are reported as  weighted average  test cycle emissions.  Weighted average means  that some of the test points count more than others in the average. Further, the weighted cycle means that  an individual test condition may be above the EPA limits, but the weighted average of all 5 test points must be below the EPA emissions limits.

2) Measurement variability and or different test methods can result in emissions levels that differ from those provided by the manufacturer.

3) Local site conditions which include ambient temperature, humidity, barometric pressure can also result in emissions above the EPA published emissions limits.

4) The engine may be operating off the test cycle which may allow slightly higher emissions than the standard yet remain capped by EPA.  These are called NTE or not-to exceed emissions levels.

5) The equipment may be experiencing a component problem due to age, lack of maintenance or manufacturing defect which can be resolved by contacting the appropriate dealer to diagnose and provide a mechanical repair.


Below is the information that dealers  provide with  our U.S. EPA Tier 4 certified emissions data.
Emissions data measurement procedures are consistent with those described in EPA CFR 40 Part 89, Subpart D & E and ISO8178-1 for measuring HC, CO, PM, NOx. Data shown is based on steady state operating conditions of 77°F , 28.42 in Hg  and number 2 diesel fuel with 35° API and LHV of 18,390 btu/lb. The nominal emissions data shown is subject to instrumentation, measurement, facility and engine to engine variations. Emissions data is based on 100% load and thus cannot be used to compare to EPA regulations which use values based on a weighted cycle. Emissions values are tailpipe out with aftertreatment installed. Values shown as zero may be greater than zero but were below the detection level of the equipment used at the time of measurement.


Please let us know if you have any other questions.

by Visitor Progen904
on ‎03-31-2015 10:20 AM


Thank you for the summary on EPA limits and certification. I hope that we may continue this blog for many more discussions. Does Caterpillar have records of any documented cases where underloading caused eventual failure of the engine to power the connected load? These would be cases where adding additional connected load solved the problem without any mechanical or electrical repairs. For instance consider a single generator set with a 100kW rating. The connected load when it was installed was 25kW steady state with an occasional large motor start that might add 5kW for ten seconds. This is a typical scenario where the generator set was sized to meet voltage dip minimums during motor starting. If the voltage dip issue is laid aside for simplicity, any problems created by underloading would have to render inoperable the 75% reserve (unused capacity) of the engine. Some documented cases of this actually occuring would be very interesting.

by visit their website
on ‎05-12-2015 02:48 PM

One of key considerations for generators is ability to refuel your generators easily.  Often a generator is placed in underground parking lots in buildings with nearly impossible to refill.  Great at event of emergency not so good when you have to refuel after 8 hours?

by Visitor kz
on ‎05-19-2015 02:17 PM

This is more of a question and a shout for advice than a comment. Please note * Spares are a major issue*

Engines running at 1200RPM.

I am on a Jack up with 5 x 3516B gen sets. I am new here but I have noticed that we frquently have 3 cats running on light loads, around 30%. There is evidence of slobbering and we have sooty smoke from our exhaust system.

There have been regular injector issues but the replacement parts although stamped with Cat may not be OEM. Recent Cut out tests have come back with several of the injectors as NOT OK even after being changed for new, I have no other info on this.

I recently changed a head where the exhaust valve was stuck about 10mm inside the head body which was coked up and when I inspected the Cylinder bore there was a build up of laquer and signs of detonation distress on the crown. The rocker arm hadalso sheared in the process.

These engines have all done around the 22K mark with no overhauls.

Is there any way back from this? Would it make any difference at this stage if I implemented a policy whereby the load share was decreased i.e. convinced the guys to run 2 engines at higher loads (50/60%) instead of 3.


Any pointers welcome.

by 3512B G
on ‎08-18-2015 03:44 PM

This article matches my experiences, though diesel engines do not like light loads, it leads to crankcase dilution and slobbering or wet stacking.
Sometimes, however, the load profile makes light running unavoidable.

As to oil change frequency: some of the computer controlled engines set their own oil change interval based on an algorithm which allows extended intervals when the engine is less than fully loaded.
Due to the possibility of crankcase dilution at light loads it may not be wise to extend change intervals on very lightly loaded sets.

by Ganpati Kumbhar
on ‎03-29-2016 12:40 AM

We have 250kVA CAT DG set. Year of mfg-2007, Running hours @ 4500hrs.


Since 5 years it runs for average 3 hrs/month as backup/testing.


2 days back, even at 40% load (150A), its temp increased and speed hunting started and DG tripped on high temp.


it shoul withstand to minimum load of 250Amp and Max.300Amp in summer (at ambient temp 40deg C)


.. Radiator is clean.

..Diesel Tank is clean.

..Filters and Lub oil are new.


What could be the reason? Please guide.


Thanks and regards

Ganpati Kumbhar

by Ganpati Kumbhar
on ‎03-29-2016 12:50 AM

We have 250kVA CAT DG set. Year of mfg-2007, Running hours @ 4500hrs.


Since 5 years it runs for average 3 hrs/month as backup/testing.


2 days back, even at 40% load (150A), its temp increased and speed hunting started and DG tripped on high temp.


it shoul withstand to minimum load of 250Amp and Max.300Amp in summer (at ambient temp 40deg C)


.. Radiator is clean.

..Diesel Tank is clean.

..Filters and Lub oil are new.


**During Radiator cleaning we found fine metal dust (burr). what is the reason for metal dust(burr) generation in radoator?


What could be the reason for rise in temp? Please guide.


Thanks and regards

Ganpati Kumbhar

by kargaramin
on ‎05-03-2016 05:44 AM

hello guys


i am an chief electrician in the onshore rig

we have four number of CAT 3512B in engine room. 

this is my question

you know we dont have an stable or fix load in oil and gas progects . specialy in the exploration.i ask about the table on top of this page. time limit   Corresponding  to load percent % is this correct ? 

i read the possibelly problems and they seems right friends.

by dhanaseely
on ‎05-07-2016 03:01 AM

What is the size of DC generator fo support two number of 275KVA UPS

by dhanasalfa
on ‎05-07-2016 03:04 AM

What is the size of DC generator to support 275 KVA UPS

by Bunny
on ‎06-28-2016 02:18 PM

Hello All,

We have recently overhauled our Cat engine generator 3406C,replaced F.O. Injection pump assembly,F.O. transfer pump,cyl.head and injectors. After that overhauling we tested run the Aux. engine and it looks fine. tested for black out test to make that the standby generator will automatically run and supply power to the board. and the issue now is that newly overhauled Aux. generator cannot hold on 50% kw load the RPM was dropped and fluctuated. And when shared parallel to the other generator the kw loads fluctuates but this time the RPM stable .  Tried to replaced a new AVR but the same issue. All F.O. filters replaced. Fuel lines cleaned. But no develoment. With normal parameters and no dark smoke.Any ideas or advise?


by New member Oluwatoosin
on ‎07-03-2016 07:03 AM

For how long can a prime generator (CAT) run after its due for service?

by Visitor bjabeck
‎07-21-2016 09:16 AM - edited ‎07-21-2016 09:17 AM


There is not a fixed answer to your question above. A prime rated unit does not have a limitation on run hours, so from a rating perspective, there is no limit. However, as you indicate, the unit will have to be serviced. The time a unit can operate will be affected by the product, the application, maintenance schedule, and local permits and regulations. The best solution is to work with your local Cat Dealer to review your current product and application and create a specific maintenance plan for your site.

by Tony Hoevenaars
on ‎07-22-2016 12:51 PM

I'd like to get your comments on how non-linear loading, such as variable speed drives, adds to the problem of running under lightly loaded conditions.  All generator manufacturers recommend that a generator be oversized by 2x - 2.5x when supplying harmonic generating, non-linear loads. This is primarily due to the additional losses that the harmonic currents introduce and the high harmonic voltage distortion that results from these harmonic currents passing through the subtransient reactance of the generator. However by oversizing that much, it makes it very difficult to run in any condition other than lightly loaded. Do you typically recommend that harmonic mitigation equipment be added so that the generator would no longer need to be oversized?

Also, it might be a bit late but I'd like to comment on KZ's earlier comment on May 19, 2015.  The likely reason that 3 generators are run when 2 would suffice is that Drill Rigs typically have very high concentrations of non-linear loads (DC Drive and/or AC Drives). In order to keep voltage distortion within reasonable levels, the generators must be lightly loaded. By adding harmonic mitigation, the system could be run on less generators at heavier loading levels.  This would not only protect the generators but would substantially reduce fuel consumption and emissions since the generators would be more efficient at the higher loading levels.  

by Super Contributor
on ‎08-10-2016 08:58 PM

One possible solution is to install oversized tail ends, if you look at the KATO oil field tails ends for example, when you look at their kVA rating, the amount of iron and copper , the size of the generator leads and the design of teh excitation system, you find that it really is a very much oversized generator, the Baylor (now NOV) are similar. Depending on what type of load you're driving you may find that this is a viable solution by utilitzing a properly sized prime mover and a generator end sized for the electrical requirements of the load.


While harmonic mitigation equipment may also be a solution, properly applying it, at least in my experience, requires some careful thought and if system loads change may not be effective any more.


I was recently on a diesel electric research vessel that had 0.7 pf 125% rated generator ends installed, in this particular application it provided the best solution for the available space and operating conditions. 


As with any problem, there are usually multiple solutions, in general, generator suppliers like to provide "standard" packages to meet customer needs. Having to offer over sized tail ends also requires possible changes in base design, torsional couplings and other possible changes.  All of which come at a cost.



by New member thoevenaars
on ‎08-19-2016 11:25 AM


You might find it much easier now to apply harmonic filters when you're dealing with the right filters.  We have a case study on an application we recently did where application of a passive harmonic filter provided a payback of only 6 weeks based on fuel savings alone.  The customer was also able to downsize their generator substantially. You can find it at the following link:  


by To rodante s hucom
on ‎03-07-2017 03:42 PM
on ‎06-03-2017 10:10 AM

What will happen if the the DG set is switched off when the DG Set is taking the load of Power Plant(SMPS) and Split AC Units . Pleases comment

by Vipin kumar
on ‎09-12-2017 12:41 AM
I want to know the effects of unbalance loading on DG set engine.
by New member sitamenterprise
on ‎01-09-2019 02:03 AM - last edited on ‎01-09-2019 06:45 AM by Administrator

Nice information but How long its run? and what is the best procedure?






Help us grow the Caterpillar Community: Invite a Friend

Meet Our Bloggers