Mercedes OM616.918 - A Few Questions

[WStayton]

Hi!  I'm a new member who is getting things together to build an integrated system of solar collectors, solar cells, a battery bank, a wood fired boiler to pick up the slack and a Mercedes OM616.918 4 cylinder engine to drive a 24 kW ST head for supplemental power.

I do have a few questions about the engine/generator setup.

The engine which I have is essentially the same four cylinder diesel of 2404 CC that Mercedes put in their 240D automobile.  However, the copy I have is an ex-marine engine complete with a 1.66:1 reduction gearbox (soon to be removed) and wet exhaust manifold.

The engine is rated at 72 HP at 4400 rpm and 101 ft lbs torque at 2400 rpm.  Beyond these numbers Mercedes doesn't see fit to publish jack-squat, much less anything like torque or horsepower curves vs rpm so that is what I've got for available power information.

I selected the Mercedes engine for a few reasons:  1) Relatively available, both the assembly and spare parts, since something like 250,000 of them have been imported into the US.  2)  has a prechamber and is known to be fairly friendly to WVO/SVO, though I think I would run something like 65% to try to minimize complications  3) it was the largest "small" diesel that's commonly available.  Yea, there were a couple of bigger Nissan engines, the SD-33, and the LD-28 but both of these are 6 cylinder (so more complicated), not nearly as available as the Mercedes especially with respect to aftermarket parts, and the SD-33 only has 5 main bearings.  Anyhoiw, they are not as highly regarded as the Mercedes.  So, after much pencil chewing, market following and nail biting, amd since Mercedes has been making diesel engines since before Mr. Nissan was hammering out stone wheels, I settled on the OM616, for better oir worse!  <grin>

I also have accumulated a Mercedes 4-speed standard transmission that I can use, if warranted.

I intend to run a 24 kW ST generator-head because I figure that is JUST, barely, what is available at 1800 rpm and I would like to use direct drive and avoid the whole belt/pulley mess besides which I don't think the bearings in the engine are beefy enough to stand the side load.  (Spent years working in engine design at Ford Motor Company, though my expertise is really in oxydation catylists).

Of course, I could fudge, and use the transmission but put it in 4th gear for 1:1 and then have the option of shifting into third if needed.

At 2550 I would have about 48 HP available to drive the generator which is about 36 kW, so I would have AMPLE reserve.  However, on the other hand, if I am running a mix of 65% WVO/SVO maybe I would be better off running at nearer max available horsepower??

Also, there are NO efficiency figures published for the trasmission, so I have no idea if I will lose more in transmission friction than I will gain in BSFC by running nearer the peak torque output.  Nanni, who did the marinization of the engine publishes a BSFC of 299 gm/kW-hr which works out to 0.485 lb/hp-hr, for units that most of us are more apt to understand, but they neglect to say at what rpm/BMEP this happens, so I just "guess" that it must be fairly close to the peak Torque rpm and about 85% of available horsepower if the OM616 is anything like most any other diesel engines - couple of big fotnotes there: I am comparing numbers from a 4 cylinder indirect injection engine to a larger 6 cylinder direct injection engine - but I still think the "guesstimation" is fairly accurate, and, since I don't have ANY efficiency numbers for the gearbox it really is a moot point.  And, having looked at the BSFC curves for the OM617 Turbo engine, which they do see fit to publish, the fuel island curves are fairly flat and the peak covers a box about 800 rpm by 40 ft-lbs torque.  A non turbo engine should have more "round: islands but that still leaves something like 600 rpm by, maybe 35 ft-lbs, so its still fairly broad - no real sharp points except at the absolute peak.

So, my question is:  Should I use the Mercedes 4-speed transmission that I have and run it in 3rd gear (3rd=1.42:1) so the engine would then run at about 2550 rpm, slightly above the torque peak and have SOME reserve power beyond what the ST can absorb, or should I just use the input shaft from a transmission as a "jack shaft" into the clutch-like assembly on the engine end and then a Lovejoy connector to hook it directly to the ST???

So, opinions are herby solicited - speak (type?), please!  <grin>

[rcavictim]

My best advice, if 15 kW is enough for your needs would be to direct drive that head through a coupler at 1800 RPM.

[LowGear]

I don't understand why you're discarding the marine final drive. 

Casey

[bschwartz]

I'm CRYIN' over here!!!
I'd love to have that setup to put into a boat!!

I even have a 1980 240d.
If you end up not using any of the marine stuff, I might be interested in purchasing it.

I'm with RCA on direct drive.  With WVO, load on the engine is important.  How much you pull from the ST head to load the engine vs. it's potential output at any given speed, is in my opinion a critical factor to consider.

[WStayton]

Maybe a couple of modifications/refinements/exositions are in order.

My estimate of my max load when using the generator to provide battery charging when solar panels are not doing the job is 18-20 kW.  My fear is that, since everything is figured with a blunt pencil on the back of an envelope from the standpoint of what I'm really going to need, before its built, that I will get it done and need the full 24 kW that have available because of a multitude of factors including not knowing exactly what is going to be requied to charge the batteries until I HAVE them in place and am using them.  I'm pretty sure that no automotive engine will run at 100% of available power for long - not even a Mercedes!  Hence my trepidation!   Since I do not to run the engine-generator 24/7, in fact I'm hoping for something more like 6 hrs twice a week in the short, poor photovoltaic times.  That will provide a max of half of the make-up heat that I require, for domestic heating, when the solar water heater can not keep up with my needs.  The rest will have to come from a wood-fired boiler.

  About the marine reducation gear - If I left that in place, I would be running the engine at nearly 3000 rpm's, for a 1800 rpm genertor input,  which is far above the max BSFC point of the engine.  Since I will only be able to absorb a max of 24 kW from the engine, I would then be running it at about 41 kW available so it would be running at 58% of available HP - and I not sure that is enough to keep from fouling injector when I'm running on 65% WVO, and I'd prefer to not find out the hard way!!!  Actually, since the marine gear has feet on it that support the back end of the engine, I plan on cutting it away to leave the feet but to also leave access to its clutch-like apparatus.

  My thinking is that I should probably put the transmission in the drive line and then use it in 4th gear which is 1:1, unless I NEED more HP, and then shift it into 3rd to allow the engine to run at 2600 rpm instead of 1800 and then be able to run at 100% of generator output without damaging the engine but still be using enough power to keep things warm enough for the Vegetable oil.  I would sacrafice the efficiency of the transmission even if I don't need it, but I should think that in 4th gear at 1:1 it should be at least 95% efficient - in 3rd, notsomuch, maybe 90%.  My thoughts are that I will be willing to pay 5% to have the capability to get more power if needed.   Also, in 3rd gear it won't really be as bad as all of the loss from running in 3rd since the engine will be running nearer to peak BSFC, so I'l probably get 2% or 3% of it back, for a net loss of maybe 7% or 8% when running in 3rd gear, compared to running with adirect connection - no transmission, and 5% worse than direct drive when running in 4th gear.

Anyhow, that is my thinking, so far!  Anybody see any big holes in it???

Regardz,

Wayne Stayton

[WStayton]

Side note about the "marine" stuff on the engine:

  It has:

1)  An exhaust heat exchanger that permits some cooling water to go through the water jacket and then it has a heat exchanger included in it to allow you to reject that heat to cooling water.  I'm going to adapt that to recover some oif the exhaust heat for domestic heating.

2)  A water cooled transmission cooler that cools the lube oil in the reduction gear.  Since I am not going to use the reduction-gear part of the reduction gear, this is available for use to cool the to-be-used road=type transmission, if I can figure out some way to circulate the transmission oil through it, other wise it is surplus.

3)  A water cooled engine oil heat exchanger that I am planning on leaving in place and recovering the waste heat in the oil for domestic heating use.

4)  A reduction gear on the back of the engine.  This unit is a Hurth model HBW-220 which provides gear reduction of 1.66:1 and has the feet on it that are the rear support for the engine.  The feet are such a neat design and execution, that I am planning on using them, after some judicious hacksawing, to support the back of the engine.  The rest of the unit is surplus to my needs, but after I get done with the hacksaw (power?) it won't be very valuable except for somebody who need spare parts!

Otherwise it is exactly the same engine used in the road applications - well, not exactly.  The road engines underwent a minor redesign in 1977 that reduced the displacement by 5 cc, from 2404 to 2399, and improved the breathing/head-design such that they got 8 more horsepower at 200 rpm higher speed.  The marine engine has the "old" displacement of 2404 cc and the new head design allowing it to have the rating of 72 HP at 4400 rpm, so it isn't really either engine, it sort of a hybrid of the two.

The pieces/parts that I have indicated I have no need for are available for anybody who want them for the charge of shipping them to you, or you can come and pick them up, but I'm not ready to start the teardown just yet.

I'm planning on tearing the whole thing down, mic-ing the cylinders/pistons and grinding the valves and replacing the rings, at least, before I put it in service - it all depends on what I find at teardown.  Maybe boring and honing and oversized pistons will be warranted since it has accumulated 2400 hrs in marine service (should be the same as about 124,00 miles) - I figure that it SHOULD be capable of operating for 10,000 hours total before it needs new liners, etc, with new pistons at least once along in there somewhere.  Mercedes has an engine that they say went 1,000,000 km in service, which would be about 20,000 hours + / -, and never had the head off.  Obviously ALL of them aren't going to do that well, but the are VERY durable engines!

[rcavictim]

If you haven't yet purchased a 24k ST head might I strongly suggest getting a 24k STC 12 wire 3-phase head with AVR.  I bought a pair of these last year from Tom Osborne and discovered that these produce a very clean output sinewave, unlike the highly distorted one that is part and parcel with all the single phase harmonically excited heads.  At these power levels you really ought to be in 3-phase anyhow for better efficiency of your battery charger etc.  That is the route I've gone.  I have one on a Changfa 1115 engine for 3-phase power in the shop and the other one is installed in my wind turbine still under construction.

Another consideration,  a 3-phase head will present a smoother mechanical load to your prime mover than a single phase head.  I fear that you may beat the crap out of that transmission if you decide you must include it, IF you have a large capacity single phase head hanging off the back of it.  That was part of my suggestion to nix the transmission idea.

[mike90045]

My estimate of my max load when using the generator to provide battery charging when solar panels are not doing the job is 18-20 kW. 

18-20KW of power, in to what size of battery bank ??

48v bank would be 384A (52V charging)  YoW !  that's going to bubble the electrolyte for sure!

[Tom Reed]

Mike has a good point, how much pv and how many AH of battery do you plan to install?

[WStayton]

Ok, let's address the questions:

rcavictim:

   Is the distortion of the waveform enough that it causes real problems with anything?  I'm not the "It has to be beautiful" type so if the distortion of the waveform is only cosmetic and doesn't really interfere with anything, I'm inclined to ignore it, especially in that going to three phase introduces some problems if I stay hooked to the grid.  I won't be able to use a simple transfer switch to switch the whole electical distribution box from line to generator.  And three phase in rural upstate New York is about as common as 10 carat diamonds - like nonexistent, so I would definately have a somewhat more complicated set up to be able to switch back and forth. On the other hand, if there are real benefits, other than cosmetic, to be had, I am willing to (reluctantly) get onboard!

   About the hammering from four cylinder engine and four pole generator:  I have hoped that the elastic "block" in the middle of the Lovejoy coupler would help with this problem.  I have been wondering if there is anything to gain by fiddling with the phase of the generator in relation to the crankshft phase of the engine to try and not have a pulse of power on one end against a pulse of the generator hitting on of the maxes of one of the poles at the same time.  The coupler does let you have three (or four, depending on coupler) differnet positions of the two relative to each other.  In this vein, I have wonder how you can figure out where in the cycle the peaks are coming from the generator - on the engine end they are pretty well defind, more or less corresponding to TDC or slightly after.

mike90045, and Tom:

  How big a set up:  I sort of planed around Schott ASE145W modules since they, like me, are relatively cheap!  I can get a pallet of 20 of the for $2,842 which seemed like a bargain to me.  They are 145 watt, 33.6 Voc, 24.8 Vmpp.

  My plan is to use these to feed sixteen Surett S-600 batteries of 500 amp-hour capacity at 6 volts each.

  The S-600 will feed two Outback model VFX-3648, inverter's whch are 48 volt units, so each of them would have to feed off of eight S-600 batteries.

   With the S-600 batteries, i have 16 * 450 = 7,200 amp hours.  If they are run down to 50% capacity before charging, I will need 3,600 amp hours to recharge them.  If recharge efficiency is 80%, I will need to generate 4,235 amp-hours.  If I get them recharged in six hours, I will need 705 amps in charging which is equal to 4235 watts which equals 6 HP, + / -.  Allowing for losses in making the 120V AC to drive the charger, I THINK I will need something like 10 kW for charging.  I figure another 10 kW for loads not associated with the battery charger, such as water pumping, domestic use, electricity while charging and not on inverter, etc., etc. and that was how I got the 20 kW that I THOUGHT I would need.  20 kW is the WHOLE load, not just the charging load - I mis-spoke, sorry!

  The only probelm with this is that it has so many ifs/maybes leaning on each other that if they all stacked up in the right (read:wrong!) direction, I could easily need the whole 24 kW available.  Of course, things seldom all stack up in one direction - when I was working at Ford we had one engine, the 460-4v used in Lincolns, that if everything stacked up in one direction, the crankshaft would touch the bottom of the cylinder bores!!!   As far as I know, that NEVER happened - though the design guys did take a lot of flack for letting that one out the door!

  Anyinput for things that I have overlooked or just plain misfigured/malfigured?

[rcavictim]

If you are going to all this rather huge cash outlay and physical effort and have not decided to get off the grid then why are you worried about paying a bit extra for the generator head?  If you stay on grid you don't need any of this equipment.  If you have done the research you say you have then you know that there is no way you can make electricity cheaper than the grid can provide.

Frankly I don't want to hear you complaining about having to pay a bit extra for your generator head while saying you are going to spend twenty grand on Rolls Surette batteries that you don't need.

BTW, you have evaluated the capacity of your proposed battery bank incorrectly.  You said, " With the S-600 batteries, i have 16 * 450 = 7,200 amp hours. "  IF you were making a 6 volt system then yes you would have 7200 Ah.  Your system as I understand it will be 48 volts. That means you have two sets of eight batteries in parallel.  That is 900 amp hours at 48 volts. Assuming 80% charging efficiency it will take 1250 Ah to charge them.  A 15 kW generator can charge those fully in 4 hours.

To me the system you have been discribing sounds like a completely off grid system.  If that is the case then you don't have to worry about having a more efficient 3-phase gen head not being compatible with the grid.  You can get a 3-phase battery charger and you can also wire a single phase house to 3-phase.  3-phase motors are way cheaper than single phase and also more efficient. If you want to stay on grid and be compatible then the 12 wire 3-phase STC head can be wired to produce conventional single phase electricity.

The harmonic distortion in the output of the ST heads can drive computer UPS systems bonkers.  It can also cause some devices with xfmers in them sing acoustically.

[Tom Reed]

RCA is right on the AH calc for the batts. Max charge rate on FLA batts is c/10 which is 90 amps or only about 5.4 kw! I have the Outback inverters you mention, while they are good inverters unfortunately the built in chargers to not include generator support. These inverters will back off on the charge rate as the load demand increases which is good. Better is an inverter with generator support so that when the load demands more than the rated generator output the inverters kick in and sync with the generator output to deal with the increased load. This would allow you to size your diesel generator to run efficiently at 75-80% load and still start a huge load. Check out the xantrex xw inverters.

We're 3 years off-grid, 21 Sharp 167 watt panels, 1055 AH Hawker battery bank and 2 Outback vfx 3648 inverters. For generator we're using a Listeroid 6/1 ST5 and getting 3.125 kw out of it and even with the lack of generator support we function pretty well.

[rcavictim]

RCA is right on the AH calc for the batts. Max charge rate on FLA batts is c/10 which is 90 amps or only about 5.4 kw!

Tom,

I did not realize that for FLA you have to charge at such a miniscule level.  Is this for just this particular brand and model FLA?  I was always under the impression you can really charge hard, like approaching 1C during the bulk charge phase of FLA.  IIRC you can do this with at least some of the AGM's.  At such charging rates battery temperature montitoring would be very important.

[Lloyd]

Just for the record.

Any good quality deep cycle FLA bat bank, can be bulk charged up to 25% of the amp-hr rating of the bank up to 75 to 80% state of charge, as long as you have a way to temp compensate the charge, after this point the charge cycle converts to constant voltage typically 14.4(29 volts) volts temp compensated, until the 100% charge has been made.

My guess is that you would be smarter to get two  24 volt isolated Leese Neville alternators, and run them in series controlled by a Balmar 6-24 max charge plus regulator. That includes a temp compensation for the bat banks and temp compensation for the alternators.

Lloyd

[rcavictim]

My guess is that you would be smarter to get two  24 volt isolated Leese Neville alternators, and run them in series controlled by a Balmar 6-24 max charge plus regulator. That includes a temp compensation for the bat banks and temp compensation for the alternators.

Lloyd

There ought to be a way to reprogram that Leese Neville alternator controller to operate into a 48 volt regieme.  If there is I need one.