Gas engine idling 2

[stason]

Hello,

I am performing some simulations with CAT gas engines (3800 kW).

I wonder if there is any time limitation to the idling mode of a gas engine. So, let's say, the gas engine is running and the generator that it is driving is connected to the grid, but is not injecting any power. I.e. the gas engine only must provide the power needed to drive the generator and cover its losses.

I've heard that gas engines should be loaded to 50% of their capacity 15 minutes after their start. Does that include the idling?

Thank you in advance!
Best regards,
stason

 

[dicer]

The LPG fork lifts spend lots of time with engines unloaded.
I don't see why an engine that is broken in would need to be loaded to 50% capacity, as even in the automotive applications loading maybe in the %20 to 30% area, and not always continuous. Besides when a reciprocating engine is running it is loaded even at idle.

 

[ivymike]

Gas engines can suffer from carbon deposits on the pistons and valves and increased oil consumption as a result of extended operation at light load. There are guidelines for the G3616 (assume that's what you have based on fuel and power stated) which your dealer can share with you. A particular characteristic of light load on these (turbocharged) engines is that manifold pressure is lower than atmospheric pressure (it's normally higher).

I didn't find the guidelines on the website. The guidelines are along the lines of "every x hours you have to get above 70% load for at least 2 hours." x, the time in between high load periods, will depend on how light the load in between is. If you're running between 30% and 50% load, I think they recommend going up to 70% for 2 hours after 2 hours of light-load operation. Again, your dealer can look that up for you if you don't have the operating manual.

 

[catserveng]

Based on the rating these are likely G3600 series engines, correct? Extended periods of light load operation cause significant problems, especially on these engines.

At low loads excessive deposits start to form, these will negatively affect the combustion probes and will cause plugging and premature failure of the multi-torch spark plugs. The general recommendation for genset engines is to try and maintain at least 45% load, also note that below 50% load emissions control is diffcult and the unit will not likely be in compliance. Also cylinder deposits raise compression pressure and you lose detonation margin, so after extended low load run periods you may have detonation shutdowns.

These engines were designed to operate long term at or near rated power, their controls and turbo match give best performance in the 70-100% power range.

I have many years experience with these engines, running them at low loads will only cause you (and the engines) problems.

Curious as to why you would parallel a unit and leave it at no or low load? I can't think any any modern engine that would be a good idea to do.

The general recommendation for these engines is to parallel them and warm up at 25% load until lube oil temp is above at least 165F, then ramp them up to rated load. With the high compression (12:1) engines mostly used in Europe there is control strategy to monitor the difference between jacket water and lube oil temp. The piston clearences are close and to big a difference in JW vs LO temp can cause some problems.

Hope that helps,

 

[stason]

Thanks a lot for all your inputs.

As you, ivymike and catserveng, conrrectly recognized, I am running the G3600 series engines. There are several in fact, G3616, G3612 but also a bigger one G16CM34.

The reason I would let them run in idle for a while is because I am running an islanded grid and want to synchronize all the gensets on the electrical side before I start ramping them up. Because once I do, it might get hard to reach the right sync conditions due to frequency fluctuations.

So I think we are talking about a period of several minutes and as I read from your comments it should not be a problem if I load them completely afterwards.

So, catserveng, how fast do you think 165F can be reached?

Thanks.
Regards

 

[stason]

And another question: does a ramp rate of 10% / minute seem to be a realistic value for the engines of this kind?
Or is it rather a minimum / maximum theoretical ramp rate?

 

[catserveng]

How fast an engine heats up depends on lots of things, you'll find if you have a large cooling system with poor control, low ambient conditions, and an engine at no load rated speed it could take quite some time to get the oil and water up to temp. Are you going to install jacket water and lube oil heaters? What are your site ambient conditions? You may also find that paralleling a group of large gas engines with little or no load can make for some interesting real and reactive power sharing issues.

You would likely be better off if you could bring on a unit, get some load on it, then add units and load in steps if possible. If you're worried about paralleling, you will find you'll have better luck on these engines using slip frequency synchronizing rather than phase match, so you may want to make sure what ever controls you're using have that capability.

As to your ramp rate, you will find a near mid load instability point you will want to drive thru, can't tell you for sure where it is because your rating, load dynamics and emissions settings all affect the actual area it occurs, but it is usually best to warm at a lower load then get the load up to about 60% fairly quickly. Most of my customers have to be above 55% load in less than 15 minutes due to air quality regulations, to get into compliance.

Hope that helps,

Mike L.

 

[stason]

Hi Mike,

thank you for the input.

I just looked up and discovered that the engines are actually kept pre-warmed all the time by district heat. But I don't know what that means in terms of temperature. My guess would be around 80 degrees C, that is 176F. So that should make it possible to ramp the engine directly after the sync procedure.

Concerning the active/reactive power issues, it is certainly worth investigating in real life rather than in simulations.

Regards

 

[patprimmer]

There is more to warming an engine up than coolant temp.

The temperature gradients from a running engine and the effects on internal clearances should be considered. These will normalise much quicker though if the coolant and preferably also the oil is preheated.

Regards
Pat
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[bcs5274]

x2,
run coolant and oil preheat and lube, recommend kim hotstart units which circulate and prelube so that all engine surfaces are at the set temp and the engine has oil psi before starting, can be set up to post lube after shutdown also