How does the Onan generator compare to the 7.4L engine to charge a 300 AH Lifepo4 battery?

[DonTom]

Since I will not be using my genny for the rest of the trip, I would like to know its advantages and disadvantages when it is used to charge the battery and nothing else, compared to using the 7.4L engine at idle.

Some things I already realize:

The genny runs at 3,600 RPMs to get 30 amps of charging current at 60% SOC via the converter in this RV. My 7.4L engine runs at 550 RPMs to get the same 30 amps at 60% SOC via the DC2DCC I added.

So what are the differences between running the modern water-cooled rock stable 7.4L MPFI engine for a couple of hours @ 550 RPMs to running the anciently designed Onan carb genny at 3,600 RPMs?

Is there any reason why I could not let my 7.4L engine idle for a couple of hours? Does it damage anything to do such?

Is there a big difference in gasoline consumption?

I am thinking about some more boondocking--at Gunsight Wash, which I have never been to before and is not far from here. I do not want to run my genny anymore on this trip with its extra thin oil problem, discussed here.

-Don- Tucson, AZ

 

[Isaac-1]

I suspect you are going to see considerably more fuel consumption running the 7.4L, but other than that it will probably be ok, if not a long term solution. The Onan KY 4000 is rated at at .4 gph at 1/4 load (1000 watts), 30 amps at 12V is less than 400 watts. Online sources, mostly for the marine boat engine version of the 7.4L Vortec suggest fuel consumption at idle between 1.5 - 2 gallons per hour, so 3 to 4 times more gas consumed idling the engine to recharge the batteries.

 

[Henry J Fate]

RPM of the chassis engine comes into play. I do not know what the engine alternator output is at idle but it will be considerably less than the maximum output.

Converter charger will be a more dependable and stable method of charging and should provide the specified current continously.

Fuel consumption will be considerably less running the generator. How much less is a good question. 50% less would not surprise me.

Most important is the current generator status. It seems to be a better choice to run the chassis engine and not risk damage to the generator.

You could change the generator oil while on your trip. Maybe you will get good results if attention is spent on what it is doing when you fire it up with new oil. A better option is to burn some extra fuel with the chassis engine until you are able to begin service on the generator.

 

[DonTom]

so 3 to 4 times more gas consumed idling the engine to recharge the batteries.

I would not have expected that much of a difference considering it is only 550 RPMs and the genny runs @ 6.5 times that speed and is not fuel injected, etc.

-Don- Tucson, AZ

 

[DonTom]

Converter charger will be a more dependable and stable method of charging and should provide the specified current continously.

My Victron App shows either of them to be a steady 30-amps of charging. RPMs at 550 and above make no difference to that 30 amps (around 400 watts).

You could change the generator oil while on your trip.

Yeah, it is less than 2 quarts even when overfilled, and no oil filter to deal with. Wouldn't be difficult to do. But I will probably wait until I get back home and also do a valve adjustment. Besides that, if I stay at Gunsight Wash for less than around four days I will not need to recharge the 300-AH battery anyway. And I have a couple of hundred watts worth of solar I can set up.

I am mainly just curious about my options, mainly in case I decide to stay there longer.

-Don- Tucson, AZ

 

[Henry J Fate]

My Victron App shows either of them to be a steady 30-amps of charging. RPMs at 550 and above make no difference to that 30 amps (around 400 watts).


Yeah, it is less than 2 quarts even when overfilled, and no oil filter to deal with. Wouldn't be difficult to do. But I will probably wait until I get back home and also do a valve adjustment. Besides that, if I stay at Gunsight Wash for less than around four days I will not need to recharge the 300-AH battery anyway. And I have a couple of hundred watts worth of solar I can set up.

I am mainly just curious about my options, mainly in case I decide to stay there longer.

-Don- Tucson, AZ

I would expect a voltage variance with the alternator and RPM. Maybe it is insignificant but any voltage variance will effect battery absorption. Might be a good exercise to check the charge rate from an idle to higher RPMs.

Hope everything works out.

 

[DonTom]

Might be a good exercise to check the charge rate from an idle to higher RPMs.

I have. No difference. Alternators put out quite a bit of current at idle these days. But I am fairly certain if the idle was below the 550 RPM spec the charge current would be a lot less.

Besides, my DC2DCC will only allow 30 amps max to the battery and since I get that at idle, it is not expected to increase at higher RPMs.

-Don- Tucson, AZ

 

[DonTom]

The Onan KY 4000 is rated at at .4 gph at 1/4 load (1000 watts),

I have reasons to believe my genny was much worse than that at 7k' Flagstaff, AZ. Perhaps it will help when I adjust the valves and now I am considering putting on a genuine Onan carb (as you suggested a while back) to see if it helps at 7,000' elevation.

In Flagstaff, I filled up this 50-gallon tank, and the gas gauge was pinned. A few days later, with no driving, the gauge was at around 90% after running the genny for about 1.5 hours per day for three days. I could have driven more than 100 miles before the gauge would go down that far. Perhaps a lot of that gasoline ended up in the genny oil!

-Don- Tucson, AZ

 

[Lou Schneider]

The Ford 7.3L gas engine has documented issues running at idle for extended periods, the valve tappets riding on the camshaft don't get adequate lubrication and wear out prematurely. I don't know if the Chevy 7.4L is the same.

I couldn't find exact figures but either engine will use about the same amount of fuel. An idling V8 uses about 2/3 gallon per hour, an Onan gas generator uses about the same under a light load.

 

[DonTom]

The Ford 7.3L gas engine has documented issues running at idle for extended periods,

I wonder how long an "extended period" is, and if it helps to rev up the engine every five minutes or so for a few seconds and then let it idle again. And I also wonder what RPM is high enough to prevent such a problem for an "extended period". I know my 1971 BMW motorcycle has an issue with lubrication when below 1,000 RPM, but the fix is simple. Set the idle to 1K RPM or slightly above.

An idling V8 uses about 2/3 gallon per hour, the gas generator uses about the same under a light load.

That is a lot closer to what I expected than message number 2 here "3 to 4 times more gas consumed idling" and 1.5 GPH. Perhaps something with the boat engine design? Perhaps it means with the outdrive running at idle in water? Water is a rather heavy load to an outdrive. That is why boats eat up a lot of gas per hour of operation.

However, it's unlikely your alternator will produce much current at idle. I had to rev mine up to around 1000 RPM to see full voltage out of it.

I checked mine when I was in Flagstaff to compare to my converter on my Victron. I did wait for the idle to drop, but I did not check the tach. I am going by memory (which isn't all that great in my old age)that the idle spec in this rig is 550 RPMs. I will check again later. But the idle speed is set by a chip in this RV. I just checked on the web, and it should idle between 575 and 750 RPMs. I have an added tach on this Y2k Chevy Express Van so I will check the warm idle speed soon. But whatever it is, I get the full 30 amps out of my DC2DCC at warm idle.

I had to rev mine up to around 1000 RPM to see full voltage out of it.

What year, make and engine size is yours?

-Don- Tucson, AZ

 

[Henry J Fate]

I get the full 30 amps out of my DC2DCC at warm idle.

A check of the voltage at idle and at higher RPMs would be interesting.

It may not make much, if any difference. Just curios how it all works. All these little details become part of the equation when determining fuel efficiency between chassis engine and generator for battery charging. The generator is a fairly solid number. A varying speed alternator may be fluctuating at different speeds. The chassis engine will use more fuel because of its physical mass but the chassis engine alternator is also doing lots more than just charging the house batteries.

These are all things to keep in mind but in your current situation, you will be fine. You definitely do not want to damage a $3500 generator over 20, or so, gallons of fuel.

 

[Mark_K5LXP]

Being a 2000 it should be running OBDII. With an OBD reader you can see real time mass airflow values and from there learn grams of fuel used per second.

Mark B.
Albuquerque, NM

 

[DonTom]

Being a 2000 it should be running OBDII. With an OBD reader you can see real time mass airflow values and from there learn grams of fuel used per second.

Mark B.
Albuquerque, NM

I have a Scangauge 2 connected to this RV at all times and it shows such things, but the genny is not OBD2 and I wouldn't know how to compare.



Edit: I just now noticed the "GPH" scale. I didn't realize it showed "Gallons Per Hour" until now. So now I can compare to the Onan genny's 0.4 GPH at idle!

When I do such, I will post the info here.

-Don- Tucson, AZ

 

[DonTom]

You definitely do not want to damage a $3500 generator over 20, or so, gallons of fuel.

Yes, but I would rather do that than ruin this 7.4 L engine by letting it idle too long. But I wonder how long "too long" would be and if stepping on the gas pedal every few minutes to increase the RPMs for a few seconds would solve the problem--if this "Ford 7.3L problem" also exists on the Y2K GM 7.4L.

-Don- Tucson, AZ

 

[Henry J Fate]

Yes, but I would rather do that than ruin this 7.4 L engine by letting it idle too long. But I wonder how long "too long" would be and if stepping on the gas pedal every few minutes to increase the RPMs for a few seconds would solve the problem--if this "Ford 7.3L problem" also exists on the Y2K GM 7.4L.

-Don- Tucson, AZ

You will need to research the characteristics of those idling failures. Lots of variables there and I think you will find that a couple hours a day will be fine on a temp basis. I would suspect the bulk of the failures is from far more than anything you will ask, given current circumstances.

Throttling it up during each cycle is an option and of course only running it for as long as you need is also important. This should be done for any chassis engine idling for extended periods.

 

[NY_Dutch]

For

You will need to research the characteristics of those idling failures. Lots of variables there and I think you will find that a couple hours a day will be fine on a temp basis. I would suspect the bulk of the failures is from far more than anything you will ask, given current circumstances.

Throttling it up during each cycle is an option and of course only running it for as long as you need is also important. This should be done for any chassis engine idling for extended periods.

For an older Class C I owned years ago, I had a "high idle stick" I wedged between the accelerator and the edge of the dashboard that I used for charging on dry sites. I guess it didn't do any harm, the old girl is still in service.

 

[DonTom]

mostly for the marine boat engine version of the 7.4L Vortec suggest fuel consumption at idle between 1.5 - 2 gallons per hour,

I now have the info. on my 7.4 L engine WRT the GPH:

According to my Scangauge 2, the 7.4 L GM MPFI engine in this Y2K RV (while charging house battery via my DC2DCC at a low rate--near full charge.)

700 RPM (true warm idle says my tach): 0.67 GPH.
1K RPM:1.04 GPH
2K RPM:1.94 GPH
3K RPM:3.05 GPH

Looks like not much advantage to running the Onan Genny to charge my house batteries--or for the use of much anything else as I normally use my high-power inverter with the MW oven, hair drier, etc.

Anyway, I have everything covered in so many ways out here boondocked in Gunsight Wash, near Why AZ.

I have 200 watts worth of solar outside plus a little from my old roof solar. My 300 AH Lifep04 battery was fully charged to start with when I got here, but got used a little before the readings were taken.

I can charge my electric motorcycle in Ajo at near full capacity of any of their four J-1772 plugs (These charge stations are capable of 7KW, I charge my Zero DS at around 6.5 KW), 15 miles from here (opposite side of the town, or it would have been 13 miles), which is as far as I will ever need to go while here. I was up there today, my bike was fully charged by the time I purchased my hand oil pump at Olsen's Market Place which is combined with an ACE Hardware store.

I found two empty quart bottles in the trash in Tucson at the Dollar General store several days ago, to put the old oil in (no, I am not the type to dump the oil in the desert here!). I will pump out the genny oil tomorrow into these bottles. That way I will be sure not to spill any of the old oil.

20 cents per KWH is less than half the price for a home charge in Auburn (the average is 42 cents per KWH, higher at peak times). But Reno is only 16 cents per KWH for a home charge at any time).

Here I am charging up at Olsen's Market Place in Ajo, AZ today.

I won't go broke over it:



I would have done the oil change at Gilbert Ray if I had a hand oil pump. I didn't take the time to go over Gate's Pass to buy one as I wanted to hike those few last days.

I will change the genny oil tomorrow.

-Don- Why, AZ (boondocked exactly here).

 

[DonTom]

Here is the long version since I have nothing else to do as I am now charging up my batteries (I have a battery maintainer wired to the engine battery).

I changed the genny oil, it was very easy to pump it all out, and very little was left in as far as I could tell. A much neater way to do an Onan genny oil change. I didn't lose a drop of oil in the process.

So then tried to start the genny, It wouldn't start with many tries, so I hope I didn't screw up the new oil too.

I then removed the spark plug. Lots of garbage on it, not just gasoline, carbon, or whatever too, shorting it out. So then I put in a new spark plug. Started it at what sounded like around 6,000 RPMs or so so it shut down by itself (I adjusted this at Flagstaff, so it would run at the 7,000' elevation without dying--at least when the load was very light).

Today, here, I adjusted it blindly at the screw and got it to run well enough to not die so I could read the frequency. I adjusted it to 62 hz/ 122 VAC without a load. I then plugged it in and now all is close perfect, I did a big load test, stuff that would NOT run at Flagstaff from the genny:

Under the load as shown below, 117 VAC, 58 to 59 hz:

At the same time, I am charging my electric motorcycle from the OBC which is 1,400 watts AC input regardless of AC voltage going in as long as it is from 90 to 250 VAC.

I also ran the refrigerator on AC (330) watts and plugged in the Hot Rodd for the water heater (400 watts) and the Converter at 382 watts. The grand total load right now is 2,512 watts. No way could I run all this at Flagstaff at 7K elevation. The elevation where I am now at Gunsight Wash is 1,781' about the same as in Auburn where this genny doesn't give me any trouble.

BTW, I have a 30-amp Lifep04 converter in this Y2K RV. If I also run the engine it goes from 382 watts to 588 watts of charging. I assume I would get even more if my SOC% was lower. I was at just below 90% SOC on the house battery when I did this testing.

I will look at the oil again after I get done with the genny, to make sure I didn't cause more problems when it would not start.

If I had a new valve cover gasket with me, I would have done the valve job today right here at Gunsight Wash.

One question, can a valve adjustment make a bigger difference at higher elevations? I assume so, at least if the valve adjustment is too tight and lowering the compression by the valve not fully seating. Perhaps that will cause more problems at a higher elevation than here at 1,781' elevation. But I am only guessing. I will most likely also put on a new Onan Carb. And then soon another trip to Flagstaff for a test!

-Don- Why, AZ

 

[Mark_K5LXP]

If your valve lash is set tight to the point of the valves not closing the net result is burned valves and seats and general undesired running characteristics (think exhaust gas backfed into the carb). It's not a valid or desirable method of compression control/release. The 2 mil lash at cold is as tight as I'd go, better to be too loose than too tight. What's the lift of this cam, 200-300 mils? A few mils shy of that by extra loose rockers isn't going to make the engine run any different. The issue with excessive lash is impact damage to rockers, lifters and valve tip damage. With a once every 450 hour service interval there isn't a lot of wear there to overcome.

Mark B.
Albuquerque, NM

 

[DonTom]

If your valve lash is set tight to the point of the valves not closing the net result is burned valves and seats and general undesired running characteristics (think exhaust gas backfed into the carb). It's not a valid or desirable method of compression control/release. The 2 mil lash at cold is as tight as I'd go, better to be too loose than too tight. What's the lift of this cam, 200-300 mils? A few mils shy of that by extra loose rockers isn't going to make the engine run any different. The issue with excessive lash is impact damage to rockers, lifters and valve tip damage. With a once every 450 hour service interval there isn't a lot of wear there to overcome.

Mark B.
Albuquerque, NM

Yeah, just like my 1971 BMW motorcycle, too much clearance could make some noise, but normally will not do any damage, unlike when too tight.

I have no excuse for never checking the valve clearance on this genny. I cannot say I ever thought about doing it until recently.

Perhaps because it runs very well until 7K feet and then gives all types of performance issues. It's running great here at Gunsight Wash, after adjusting things back, which is around the same elevation as Auburn where I usually work and adjust my genny frequency and such, but never thought about the valve adjustment.

I sure wish Onan would get up to at least a 1990 design!

-Don- Why, AZ