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Small ~2kw Diesel Generator to run on waste oil

sparque

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Mar 31, 2022
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I'm looking for a way to charge my batteries and meet my modest power needs during the meager solar days of winter for cheap or free using waste vegetable oil. I've been eyeing one of these guys:


and a 2kw DC generator like this:


I'd be connecting them via a belt and the output would go into a Growatt SC4880-MPV I have collecting dust through the PV inputs.

As long as the DC generator didn't exceed 250v, there shouldn't be a problem I think??. Does anyone see a reason why this wouldn't work?
 
and if you're wondering about products that are comparable in scale, the closest I've been able to find is this which would be many times more expensive:

 
That 2,000 watt generator is meant for charging a "12 volt" lead acid battery. The spec shows 12 to 14 volt output. Basically a 150 amp automotive alternator. If you are doing a 12 volt system, just connect it directly to the battery bank. Here is a 48 volt alternator. Wow, they are not cheap.
They also have 24 volt units.
IF you are running LFP cells with a proper BMS, the lead acid calibration should be close enough to be safe. But I would also want a battery voltage cut off that will shut down the engine when the voltage is getting close. No need to keep charging from Diesel when the battery is over 90% charged.
 
Ah, good catch. I'm running a 48v LFP system. I meant to link this one:

I guess what I'm asking ultimately is if my MPPT charger received current from this, would it treat it any differently than if it were coming from a solar cell?
 
I am not a turbine guy, but I have read the manuals on several charge controllers, and the ones that work with wind turbines are proud to list it in the manual. The idea behind MPPT is similar for a PM generator vs a solar cell, but it is not the same. If you use a "48 volt" alternator, I would expect it to work pretty well just going straight to your battery bank. This one looks like it could work. Some Midnite Solar charge controllers have specific modes for wind turbines. At the least, I would try using a separate charge controller to see how it behaves. At least if it does blow it up, it does not kill your all in one Growatt.
 
guess what I'm asking ultimately is if my MPPT charger received current from this, would it treat it any differently than if it were coming from a solar cell?
No, it would treat it the same.

But, the issue is whether the engine/generator combo will appreciate being treated like a solar cell!

How a permanent magnet alternator, or a 'regular' alternator with an electromagnetic field controlled by a regulator, would act in this scenario is not the same. But you linked a permanent magnet generator so I'm going with that.

What would happen is that the mppt would see the 'resting' voltage of the alternator output, which would be determined by a balance of two forces: RPM, and load on the output circuit (zero in this case). I believe if you spin up a permanent magnet alternator while the output is open circuit, the voltage will rise to a very high level, possibly enough that i'd be worried about the breakdown voltage of the rectifier diodes. But, seeing as the rectifier is external and easily inspected, this should be easy to rule out in initial testing as a possible issue.

As the mppt starts to allow current to flow and tries to find the 'maximum power point', it would allow more and more current to flow from the alternator into the mppt. Since a permanent magnet is not 'adjustable', the alternator itself will have no reaction to this, but as the current flowing out of it increases, the power requirement for the engine increases. So, the governor on the engine will have to adjust the fuel delivery to try to maintain the 'governor rpm' setpoint, probably ~3600rpm. If the max power the alternator can produce is less than the engine can produce at 3600rpm, the tone of the engine will change as it 'loads up', but it will not really slow down and the mppt will settle at that point and it should all work fine.

But if the max output the alternator can produce is more than what the engine can produce at that rpm, the engine will 'bog down' and lose rpm until the MPPT realizes it has 'gone too far', and starts going back the other way. Depending on how that plays out you could end up in a constantly repeating cycle of engine bogging down and then revving back up, or if the mppt 'loads' too quickly and doesn't 'release' quickly enough, it will simply stall the engine.

So the safest way to attempt this would be to have an alternator whose max possible output is comfortably LESS than the engine's max output, so that the engine could adapt to the load by changing its fueling (and throttle opening, if it were a gas engine) but not actually be pulled substantially down in rpm. Because once you exceed the maximum available power of the engine by ANY amount, it WILL slow down until it stalls, and because the engine's power output drops with rpm, the amount that it 'falls short' increases as its speed drops, which means if the engine starts to bog and the situation is not remedied almost instantly, it will quickly escalate to a stalled engine.

So if you take a 2.2kw engine and hook up a >2.5kw alternator to it (considering efficiencies it is probably over 2.5kw on the input side), and then an mppt to the 2.5kw alternator, it might stall the engine. If it doesn't, it is likely to go through an irritating cycle of bogging the engine down and then letting it rev back up every time it tries to recheck the max power point. If that's not hurting anything and it doesn't bother you it's technically fine, but i don't see that scenario resulting in the engine just happily purring along at a steady rpm and load.


And now the part i have no idea about... 3-phase ac rectified through some diodes is still pretty wavy. If you feed that to an mppt, is it going to actually react to the constant small changes in output and be 'seeking' mppt constantly? And if so, is that harmful? I don't know what amount of waviness you can feed into an mppt without causing issues.
 
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No, it would treat it the same.

But, the issue is whether the engine/generator combo will appreciate being treated like a solar cell!

How a permanent magnet alternator, or a 'regular' alternator with an electromagnetic field controlled by a regulator, would act in this scenario is not the same. But you linked a permanent magnet generator so I'm going with that.

What would happen is that the mppt would see the 'resting' voltage of the alternator output, which would be determined by a balance of two forces: RPM, and load on the output circuit (zero in this case). I believe if you spin up a permanent magnet alternator while the output is open circuit, the voltage will rise to a very high level, possibly enough that i'd be worried about the breakdown voltage of the rectifier diodes. But, seeing as the rectifier is external and easily inspected, this should be easy to rule out in initial testing as a possible issue.

As the mppt starts to allow current to flow and tries to find the 'maximum power point', it would allow more and more current to flow from the alternator into the mppt. Since a permanent magnet is not 'adjustable', the alternator itself will have no reaction to this, but as the current flowing out of it increases, the power requirement for the engine increases. So, the governor on the engine will have to adjust the fuel delivery to try to maintain the 'governor rpm' setpoint, probably ~3600rpm. If the max power the alternator can produce is less than the engine can produce at 3600rpm, the tone of the engine will change as it 'loads up', but it will not really slow down and the mppt will settle at that point and it should all work fine.

But if the max output the alternator can produce is more than what the engine can produce at that rpm, the engine will 'bog down' and lose rpm until the MPPT realizes it has 'gone too far', and starts going back the other way. Depending on how that plays out you could end up in a constantly repeating cycle of engine bogging down and then revving back up, or if the mppt 'loads' too quickly and doesn't 'release' quickly enough, it will simply stall the engine.

So the safest way to attempt this would be to have an alternator whose max possible output is comfortably LESS than the engine's max output, so that the engine could adapt to the load by changing its fueling (and throttle opening, if it were a gas engine) but not actually be pulled substantially down in rpm. Because once you exceed the maximum available power of the engine by ANY amount, it WILL slow down until it stalls, and because the engine's power output drops with rpm, the amount that it 'falls short' increases as its speed drops, which means if the engine starts to bog and the situation is not remedied almost instantly, it will quickly escalate to a stalled engine.

So if you take a 2.2kw engine and hook up a >2.5kw alternator to it (considering efficiencies it is probably over 2.5kw on the input side), and then an mppt to the 2.5kw alternator, it might stall the engine. If it doesn't, it is likely to go through an irritating cycle of bogging the engine down and then letting it rev back up every time it tries to recheck the max power point. If that's not hurting anything and it doesn't bother you it's technically fine, but i don't see that scenario resulting in the engine just happily purring along at a steady rpm and load.


And now the part i have no idea about... 3-phase ac rectified through some diodes is still pretty wavy. If you feed that to an mppt, is it going to actually react to the constant small changes in output and be 'seeking' mppt constantly? And if so, is that harmful? I don't know what amount of waviness you can feed into an mppt without causing issues.
This is very informative! Thank you for your input. I really appreciate it.
 
I'm looking for a way to charge my batteries and meet my modest power needs during the meager solar days of winter for cheap or free using waste vegetable oil. I've been eyeing one of these guys:


and a 2kw DC generator like this:


I'd be connecting them via a belt and the output would go into a Growatt SC4880-MPV I have collecting dust through the PV inputs.

As long as the DC generator didn't exceed 250v, there shouldn't be a problem I think??. Does anyone see a reason why this wouldn't work?
I have no idea on how waste veggie oil affects the life of a small engine, but for many diesel cars, it’s not good. I’ve seen the inside of an engine that was running pure filtered vegetable waste oil and it basically just gunked it up bad. I had considered the idea myself until I saw this mess. The mechanic who showed me this said that it would kill an engine in about 20,000 to 60,000 miles.

I’m sure there are Diesel engines better designed for the muck of waste oil than the Volkswagens this mechanic dealt with, but I’d want to know more before potentially sacrificing an expensive engine. Unless you’re handy with engine rebuilds.
 
Well, the engine in question is $200-odd bucks. It is quite possible you end up with just as much money in filtration equipment as the engine itself cost, but i would still pursue this idea considering the low risks.

I have a small Kubota B7100 'parts tractor' for my good b6100 tractor, that has a rebuildable (i can get it to start..barely) 3cyl 750cc diesel on it i have considered trying to run waste oil through to act as a dc generator..
 
If you want your engine whatever you choose to last well beyond rated hours then belt drive the generator unit so that at a lower RPM like 1,800-2,000 you get the 3,600rpm the generator is probably looking for. Running a 9hp yanmar diesel since 1992 with 6,000hrs+ driving a Dayton 8KW generator. If you can repurpose a 48v generator from a micro hydro like Kansas wind power sells…they are very efficient.
 
I really appreciate the input. I have made some purchases and have started experimenting. Here's what I cobbled together.

1668132065700.png

I went with a chain and centrifugal clutch because it's what I'm familiar dealing with on go-karts. I ran it for about 10 minutes earlier without a load with no issues. The voltage scaled with RPMs as expected. Next step is to connect it to the MPPT charger and a battery to see how it all behaves
 
I would be worried about the clutch slipping at the lower rpm of the diesel engine. You should not have a problem just running another fixed sprocket on the engine. The extra spinning mass should not be an issue for starting, and there won't be much load until the revs come up.
 
Definitely agree about the centrifugal clutch. I think most of them are set up to not fully grab until something like 2500 RPM. Im sure the engine is stock governed at 3600 but if you want to run it under load at a lower rpm be aware of this issue.

Anyway, great progress!! This is exciting to me because i have about 250 gal of waste oil, and id have more if i wanted it.. i might pursue this at some point!
 
I would be worried about the clutch slipping at the lower rpm of the diesel engine. You should not have a problem just running another fixed sprocket on the engine. The extra spinning mass should not be an issue for starting, and there won't be much load until the revs come up.
Definitely agree about the centrifugal clutch. I think most of them are set up to not fully grab until something like 2500 RPM. Im sure the engine is stock governed at 3600 but if you want to run it under load at a lower rpm be aware of this issue.

Anyway, great progress!! This is exciting to me because i have about 250 gal of waste oil, and id have more if i wanted it.. i might pursue this at some point!

So fun thing about the alternator that I didn't realize until I had it in my hands is that there's a built-in gearing ratio of 1:6. This engine is by itself a literal pain to start so I couldn't imagine adding that additional resistance. I'll probably wire up the charge controller to my bank and give it a whirl some time next week. I'll let you all know how it goes. Might end up abandoning the DC charging and just stick a 3kw Winco generator head on it.
 
Wait, a 1:6 reduction?? Or overdrive? Making sure i understand this right.

Does the engine have a compression release mechanism to make it easier to crank?
 
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I have no idea on how waste veggie oil affects the life of a small engine, but for many diesel cars, it’s not good. I’ve seen the inside of an engine that was running pure filtered vegetable waste oil and it basically just gunked it up bad. I had considered the idea myself until I saw this mess. The mechanic who showed me this said that it would kill an engine in about 20,000 to 60,000 miles.

I’m sure there are Diesel engines better designed for the muck of waste oil than the Volkswagens this mechanic dealt with, but I’d want to know more before potentially sacrificing an expensive engine. Unless you’re handy with engine rebuilds.
I'm a little late to the party. Several friends and I have run 7.3 Power Stroke diesels on WVO with several 100k miles between us. The problem you are describing is referred to as coking. To avoid this problem the engine needs to start/warm up (140F+) on diesel before switching over to WVO. On shut down it needs to run on diesel long enough to purge the lines. If you don't you get incomplete burn and the WVO will trash the engine. Additionally, we all run the WVO through a preheater to get it as hot/thin as possible before it is injected into the cylinder. Another problem area is ensuring water content is reduced below about 500ppm. If not, it will eat injectors. In the suggested use case with the generator, I just don't see it being worth the investment.
 
Wait, a 1:6 reduction?? Or overdrive? Making sure i understand this right.

Does the engine have a compression release mechanism to make it easier to crank?
It's very difficult to turn by hand so I'm guessing it's a multiplier? I'm not too familiar with mechanical gearing. My guess is it has a planetary gear system that converts 1 turn to 6 internally.

It does have a compression release for easier starting, I'm just not sure even with that if it'll be possible with it tied directly to the geared inverter. I ordered another sprocket to at least try it out

I'm a little late to the party. Several friends and I have run 7.3 Power Stroke diesels on WVO with several 100k miles between us. The problem you are describing is referred to as coking. To avoid this problem the engine needs to start/warm up (140F+) on diesel before switching over to WVO. On shut down it needs to run on diesel long enough to purge the lines. If you don't you get incomplete burn and the WVO will trash the engine. Additionally, we all run the WVO through a preheater to get it as hot/thin as possible before it is injected into the cylinder. Another problem area is ensuring water content is reduced below about 500ppm. If not, it will eat injectors. In the suggested use case with the generator, I just don't see it being worth the investment.

I'm planning on processing the oil via transeserfication.
 
Keep in mind that the diesel engine you picked is not a low speed engine. It will NOT last as long as a good slow speed engine, like the small tractor engines for example. Essentially it will last as long as a lawn mower engine. The only advantage to it over a gasoline engine is that the fuel is easier to keep for long periods.
 
Eh.. I'm sure some bad ones are mixed in with the bunch but i haven't had a cheap chinese small engine fail for 'unavoidable' reasons.

One thing I have tried to force myself to do on any new small engine of 'very low price!' is change the oil multiple times within the first hour or so. The first time i did it was on a $170 inverter generator (which lasted for a couple years, and im happy with that). Put clean oil in it, run it 5-10 minutes, oil comes out dirty. Put more clean oil, run 5mins, oil comes out dirty. Do that until it comes out clean. Then put GOOD oil in it. A whole whopping <$10 quart of top shelf stuff.

When a small engine only holds 7oz to maybe a quart of oil and weighs <30lbs and draining the oil doesn't require you laying on the ground, it costs you very little financially and physically to do the above routine. Mostly just 1 hour of your time.
 
I've done a bit of research on something like this, because I generate a good bit of waste engine oil, and would like a way to get some use out of it, as well as make fewer trips to the very inconvenient recycling center.

What I found was that a dedicated lifepo4 optimized alternator would be the best bet, but $$$$$$$$$$$$$. Closest work around I found for my system was a 12v-24v lifepo4 charger, meant for the RV or vanlife crowd to charge their 24v lifepo4 house batteries from the vehicle's 12v electrical system. Any random alternator I have laying around, belted to a cheap Chinese or Indian diesel, any random car battery I have laying around, and that 12v-24v lifepo4 charger.


If my batteries needed topped up and I was out of waste oil, I could even burn diesel in it, in a pinch!

I also have my Honda 2kw inverter genny, and could just plug one of the AIO's into it to charge batteries. Doesn't take care of my waste oil issue.
 
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