Diesel engine as generator question?

[BAD ORCA]

Do diesel engines that are used as dedicated land based generators run at very high rpm's? What is the difference in use case between two identical diesel engines, one used as a generator and one used as an auxiliary engine?

I'm curious. I hear people say all the time that running a diesel engine at lower rpm's on a sailboat with no load is bad for the engine, but dont diesel generators do this all the time? Why is it bad for a sailboat engine and not bad for the generator? Is it the fact they run for very long lengths of time in one go, or is the electrical producing coil enough of a load to act like an engine in gear? Do they run at very high rpm's? I have heard diesel generators running before and they dont appear to run at 2700+ hundred rpm's, but i may be incorrect on this.

I hear of diesel engines as generators that last decades so it cant be all that bad.

 

[colemj]

If you assume a fixed speed genset and not an inverter variable speed one, then yes, gensets are typically set at close to WOT. One difference is that many of them use engines whose WOT is 1600-1800rpm, so they don't sound like they are going nuts. Others using small high-rev engines might use belts and pulley ratio to bring the engine speed down a bit, but they are still operating close to 3000rpm.

It is also bad to run a genset under light loads. This is the reason larger boats, particularly power boats, have two gensets, where one is a small one and one a large one. They switch depending on loads.

Having said that, many people have generators that aren't in continual use. These last a long time, even when operated non-ideally, simply because they aren't in continuous use, so it takes proportionally longer calendar time to go bad than one operated non-ideally continuously.

I don't think this is as much issue as people make it out to be. Maybe running a 24kW genset continually with a 1kw load might be bad, but operating that same 1kw load on a 5kw genset isn't going to harm it. At least that's our experience with smaller gensets.

As for using a propulsion engine for operating an alternator at low loads, this isn't a problem because one has ample opportunities to run the engine under heavy load every so often to blow out any carbon buildup.

Mark

 

[Maine Sail]

Do diesel engines that are used as dedicated land based generators run at very high rpm's? What is the difference in use case between two identical diesel engines, one used as a generator and one used as an auxiliary engine?

I'm curious. I hear people say all the time that running a diesel engine at lower rpm's on a sailboat with no load is bad for the engine, but dont diesel generators do this all the time? Why is it bad for a sailboat engine and not bad for the generator? Is it the fact they run for very long lengths of time in one go, or is the electrical producing coil enough of a load to act like an engine in gear? Do they run at very high rpm's? I have heard diesel generators running before and they dont appear to run at 2700+ hundred rpm's, but i may be incorrect on this.

I hear of diesel engines as generators that last decades so it cant be all that bad.

Our diesel genset at our off grid property is an 1800 RPM Kubota not 3600 RPM... The engine is specifically designed as a genset engine..

 

[Tally Ho]

A bit off topic, but diesel locomotives (12-16 cylinder) run at under 1,000 rpm’s most of their lives. They turn a very large generator inside the locomotive cabinet, which then provides DC current to the traction motors.

Newer locomotives can generate AC power, and the latest are now burning Hydrogen.



Greg

 

[Foswick]

I can't offer an answer, but I am interested in the question. Specifically, I am unclear of exactly what happens at lower RPMs and why engine designers haven't engineered solutions into their engines.

My (somewhat lacking) understanding is that the problem comes down to mostly (or maybe exclusively) incomplete combustion. In other words, at lower RPMs (or more specifically, lower loads), combustion is incomplete. This, of course, results in things like glazing, carbon build-up, etc. None of this is a problem, per se, other than it decreases engine efficiency. That is not a serious problem until it gets so bad that it starts to interfere with the mechanical operation (like valves sticking, injectors clogging, etc.)

A friend of mine had to replace her engine and was told it was ultimately due to the practice of keeping her RPMs low. What she thought was "taking it easy" on her equipment, was actually the cause of its demise. This is actually how I ever learned this about diesel engines. I would have been inclined to keep my RPMs low too, thinking it would help my engine last longer.

So, my question (that I think is relevant to the OP) is, what specifically causes problems? Is it the RPMs that matter, or only the load? And, if it is the load, then is it really just the temperature that matters, and higher load is the way to get there? And, why is this a problem at all? The issue of incomplete combustion implies that too much fuel is introduced at lower RPMs. But, I doubt that engineers failed to do this basic math. So, my hunch is that the issue has more to do with the fact that no amount of fuel will fully combust under low-load conditions. (Keep in mind, this is a question, not an assertion.)

Then, if the issue is purely (or mostly) just one of temperature, then why not simply design engines with a more robust preheat mechanism designed to run continuously, then automatically turn it on when temperatures are too low for complete combustion? I'm sure there are good reasons.

All this brings me around to the original comparison - to diesel generators. My understanding is that the point is not to run above a certain RPM, but instead to run closer to the max RPM for your specific engine. (I've always heard the guideline as 70%-80%.) And, if this is really just about increasing the load, and therefore the temperature, then my guess is that diesel generators are just designed so that normal operation keeps them at that load/temperature, even if the RPMs might be lower than what we might expect on other engines. This is my complete speculation - not an answer. Someone will have a real answer.

 

[dlochner]

Are you looking to repower with an old genset engine?

Diesel engines have duty ratings for how long they can run at what speed and load. Often the same engine block will have several different versions with different duty ratings to accommodate different load demands. Running the engine outside of its duty range isn't good for the diesel.

Nigel Calder has done some work on optimal engine rpms for electrical power generation. On most boat there is a sweet spot where increasing the diesel's power output does little to propel the boat faster and below the point where the power curve levels off. His take is running an alternator in this range is the cheapest electricity you can generate, even cheaper than solar. It works when motoring, but not so much sitting on anchor.

The Zeus Regulator has settings to allow electric generation of an auxiliary engine and mimic a generator. As of yet, I don't know any more about it.

 

[Stu Jackson]

Mark and Maine Sail's answers are good ones.

The other important issue is that diesel engines selected to run FOR USE with an electrical generator are sized AS A SYSTEM so that the equipment matches the load for both the propulsion and generating capcities.

 

[dlochner]

So, my question (that I think is relevant to the OP) is, what specifically causes problems?

The simple answer is operating a diesel for long periods of time out side of its design parameters is bad for the diesel. All diesels are not alike, some are designed for constant loads, some variable

 

[Foswick]

Are you looking to repower with an old genset engine?

I took this to be a more general question, like, "If diesel engines shouldn't run at low RPMs, then why can generators do it?"

 

[Foswick]

The simple answer is operating a diesel for long periods of time out side of its design parameters is bad for the diesel.

Right. But it is the definition of "bad for" that I think is key to the OP question.

 

[colemj]

His take is running an alternator in this range is the cheapest electricity you can generate, even cheaper than solar.

Calder is wrong, or hasn't bought solar in the past 10yrs to understand how cheap it is now (almost free at $0.25/W). We have 3kW of solar and a 275A alternator that is rated to produce ~240A hot, but I have it derated to ~200A for robustness. Our solar and its installation gear cost the same as the alternator and its installation gear. The solar produces 13kwh on a sunny day. That is 5hrs of running the alternator to produce the same. The solar costs nothing when producing electricity, but the alternator costs fuel and maintenance. ~6hp is needed to drive a 4000W alternator, so the fuel consumption is ~1/3gph. Where we are, that is ~$1.60/hr to operate. Then oil change and other maintenance every 100-500hrs.

Put another way, every hour running an alternator could buy us another 6W of solar in fuel alone. Running the solar is free, so no potential reciprocal purchase of fuel by doing so. Every 100hr maintenance interval is worth 600W of additional solar, and every 500hr maintenance interval could buy our entire 3kW of solar. Of course, we are out of space for solar, but the point is that running an alternator can never be cheaper than the solar it could buy.

Cloudy days change the equation time length terms, but doesn't make running an alternator any cheaper than solar unless one operates in an area where solar rarely works at all.

To put a finer point on this, we got a tax credit for installing our solar, but not for our alternator.

And solar can be literally free. We spent the summer in a large marina where a lot of boats were doing upgrades. They were buying $150 500W panels to replace their 200-300W panels, and giving the old panels away. There were at least a dozen 200-300W panels given free to anyone who wanted them.

I took this to be a more general question, like, "If diesel engines shouldn't run at low RPMs, then why can generators do it?"

Generators that run at low RPM use engines designed for maximum HP at low RPM. These low RPM engines are also used in boats that can swing a large prop. Higher RPM engine designs are more typical on sailboats because they can't carry the prop sizes needed to take advantage of a low RPM engine.

Mark

 

[BAD ORCA]

If you assume a fixed speed genset and not an inverter variable speed one, then yes, gensets are typically set at close to WOT. One difference is that many of them use engines whose WOT is 1600-1800rpm, so they don't sound like they are going nuts. Others using small high-rev engines might use belts and pulley ratio to bring the engine speed down a bit, but they are still operating close to 3000rpm.

It is also bad to run a genset under light loads. This is the reason larger boats, particularly power boats, have two gensets, where one is a small one and one a large one. They switch depending on loads.

Having said that, many people have generators that aren't in continual use. These last a long time, even when operated non-ideally, simply because they aren't in continuous use, so it takes proportionally longer calendar time to go bad than one operated non-ideally continuously.

I don't think this is as much issue as people make it out to be. Maybe running a 24kW genset continually with a 1kw load might be bad, but operating that same 1kw load on a 5kw genset isn't going to harm it. At least that's our experience with smaller gensets.

As for using a propulsion engine for operating an alternator at low loads, this isn't a problem because one has ample opportunities to run the engine under heavy load every so often to blow out any carbon buildup.

Mark

Interesting, this brings up a question I have been thinking about for some time now.

At some point in the coming few years I plan to re-power my sailboat with a BETA engine. My intention is to sail in low latitudes mostly, so alternator power generation will be a big contributing source of electrical, probably the main source. I can not fit a generator so the diesel engine will do double duty.

I wonder if it would be better to get a slightly smaller engine and work it harder while generating electricity and just when motoring?

 

[BAD ORCA]

A bit off topic, but diesel locomotives (12-16 cylinder) run at under 1,000 rpm’s most of their lives. They turn a very large generator inside the locomotive cabinet, which then provides DC current to the traction motors.

Newer locomotives can generate AC power, and the latest are now burning Hydrogen.



Greg

Yeah was going to buy one of those, but i couldnt get it to fit under the dodger

 

[BAD ORCA]

I can't offer an answer, but I am interested in the question. Specifically, I am unclear of exactly what happens at lower RPMs and why engine designers haven't engineered solutions into their engines.

My (somewhat lacking) understanding is that the problem comes down to mostly (or maybe exclusively) incomplete combustion. In other words, at lower RPMs (or more specifically, lower loads), combustion is incomplete. This, of course, results in things like glazing, carbon build-up, etc. None of this is a problem, per se, other than it decreases engine efficiency. That is not a serious problem until it gets so bad that it starts to interfere with the mechanical operation (like valves sticking, injectors clogging, etc.)

A friend of mine had to replace her engine and was told it was ultimately due to the practice of keeping her RPMs low. What she thought was "taking it easy" on her equipment, was actually the cause of its demise. This is actually how I ever learned this about diesel engines. I would have been inclined to keep my RPMs low too, thinking it would help my engine last longer.

So, my question (that I think is relevant to the OP) is, what specifically causes problems? Is it the RPMs that matter, or only the load? And, if it is the load, then is it really just the temperature that matters, and higher load is the way to get there? And, why is this a problem at all? The issue of incomplete combustion implies that too much fuel is introduced at lower RPMs. But, I doubt that engineers failed to do this basic math. So, my hunch is that the issue has more to do with the fact that no amount of fuel will fully combust under low-load conditions. (Keep in mind, this is a question, not an assertion.)

Then, if the issue is purely (or mostly) just one of temperature, then why not simply design engines with a more robust preheat mechanism designed to run continuously, then automatically turn it on when temperatures are too low for complete combustion? I'm sure there are good reasons.

All this brings me around to the original comparison - to diesel generators. My understanding is that the point is not to run above a certain RPM, but instead to run closer to the max RPM for your specific engine. (I've always heard the guideline as 70%-80%.) And, if this is really just about increasing the load, and therefore the temperature, then my guess is that diesel generators are just designed so that normal operation keeps them at that load/temperature, even if the RPMs might be lower than what we might expect on other engines. This is my complete speculation - not an answer. Someone will have a real answer.

Yeah I wonder the same. I dont really understand the details of where all the bad comes from. Might be time to take Nigel Calder's boat-how-to diesel course.

 

[BAD ORCA]

Are you looking to repower with an old genset engine?

Diesel engines have duty ratings for how long they can run at what speed and load. Often the same engine block will have several different versions with different duty ratings to accommodate different load demands. Running the engine outside of its duty range isn't good for the diesel.

Nigel Calder has done some work on optimal engine rpms for electrical power generation. On most boat there is a sweet spot where increasing the diesel's power output does little to propel the boat faster and below the point where the power curve levels off. His take is running an alternator in this range is the cheapest electricity you can generate, even cheaper than solar. It works when motoring, but not so much sitting on anchor.

The Zeus Regulator has settings to allow electric generation of an auxiliary engine and mimic a generator. As of yet, I don't know any more about it.

No, more to try and figure out and get some insight into the topic. I need to start considering re-powering my boat in the coming next few years and am trying to determine if a slightly underpowered engine might be a better choice so that it gets worked harder than a bigger/more horse powered engine. Have been thinking about the topic and the genset question came to mind because I have seen identical engines sold as both.

 

[BAD ORCA]

I took this to be a more general question, like, "If diesel engines shouldn't run at low RPMs, then why can generators do it?"

Exactly.

 

[kloudie1]

Some diesel notes: Diesels, unlike gasoline engines operate "unthrottled" , meaning that they are pumping their full displacement of air constantly..The governor adds only enough fuel to make the engine speed satisfy the governor set point which might be the "throttle" on your pedestal.. What happens at low power settings is that there is very little fuel injected and because the engine is pumping a lot more air than is necessary, for combustion, the combustion temperature is low and the exhaust temperature is very low which makes a condensate of the water of combustion and the unburned heavy distillate in the diesel fuel and the soot particles (and of course salt from the seawater). That is the stuff that clogs the exhaust ell and glazes the cylinder bore and sometimes sticks the rings. (also why diesel exhaust has MUCH less toxic carbon monoxide in it) Running at higher power levels (not necessarily RPM) really heats up the combustion and exhaust gas temperatures and results in a lot less condensate. For a propeller loaded diesel, horsepower is a cube function on RPM (roughly) so doubling the prop speed (not engine speed), requires 9 times more horsepower.. That makes it easy to load the engine enough to heat up the combustion gasses enough to stop the schmutz deposition. I don't know what that load is, but if the boat has the correct pitch prop, that usually is around the 70-90% RPM range that you hear about.
The engine overloads when the amount of air that it can pump is not enough to burn the fuel that the governor is adding in an attempt to get the rpm to the set point. This also will hurt the engine because the unburned fuel will wash the cylinder bores and dilute the lube oil. ..

 

[BAD ORCA]

Right. But it is the definition of "bad for" that I think is key to the OP question.

Yes, what does it do exactly to the engine?

Some diesel notes: Diesels, unlike gasoline engines operate "unthrottled" , meaning that they are pumping their full displacement of air constantly..The governor adds only enough fuel to make the engine speed satisfy the governor set point which might be the "throttle" on your pedestal.. What happens at low power settings is that there is very little fuel injected and because the engine is pumping a lot more air than is necessary, for combustion, the combustion temperature is low and the exhaust temperature is very low which makes a condensate of the water of combustion and the unburned heavy distillate in the diesel fuel and the soot particles (and of course salt from the seawater). That is the stuff that clogs the exhaust ell and glazes the cylinder bore and sometimes sticks the rings. (also why diesel exhaust has MUCH less toxic carbon monoxide in it) Running at higher power levels (not necessarily RPM) really heats up the combustion and exhaust gas temperatures and results in a lot less condensate. For a propeller loaded diesel, horsepower is a cube function on RPM (roughly) so doubling the prop speed (not engine speed), requires 9 times more horsepower.. That makes it easy to load the engine enough to heat up the combustion gasses enough to stop the schmutz deposition. I don't know what that load is, but if the boat has the correct pitch prop, that usually is around the 70-90% RPM range that you hear about.
The engine overloads when the amount of air that it can pump is not enough to burn the fuel that the governor is adding in an attempt to get the rpm to the set point. This also will hurt the engine because the unburned fuel will wash the cylinder bores and dilute the lube oil. ..

Very interesting, thanks for elaborating.

 

[dlochner]

Calder is wrong, or hasn't bought solar in the past 10yrs to understand how cheap it is now (almost free at $0.25/W). We have 3kW of solar and a 275A alternator that is rated to produce ~240A hot, but I have it derated to ~200A for robustness. Our solar and its installation gear cost the same as the alternator and its installation gear. The solar produces 13kwh on a sunny day. That is 5hrs of running the alternator to produce the same. The solar costs nothing when producing electricity, but the alternator costs fuel and maintenance. ~6hp is needed to drive a 4000W alternator, so the fuel consumption is ~1/3gph. Where we are, that is ~$1.60/hr to operate. Then oil change and other maintenance every 100-500hrs.

You missed Calder's point. This is not for running the diesel as a generator, but maximizing electrical production when the engine is running for propulsion. When the diesel is being used for propulsion the cost for a slight increase in fuel consumption to generate electricity at an optimal point on the power curve is minimal. It may increase the fuel consumption by a quart an hour to drive a 24v 200a alternator. (Note, numbers here are arbitrary and for an example, not necessarily real world data.) That would be 4800 KW for about a $1 in diesel.

If you attend one of his electrical seminars he goes over this in some detail. Since it wasn't relevant to me, I listened and didn't bother with specific details. He does market an electrical generation system that capitalizes on this characteristic.

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[SV Temenos]

John Harries at Attainable Adventure Cruising has a great article on this. You need to buy a subscription to read it but it is well worth it. It is not expensive. I found this link on a similar thread on this site and have really enjoyed all the information I have gotten from it. I highly recommend it. How To Stop Killing Your Engine With Kindness