Tach install on an Onan generator??

[SargeW]

Here is a curveball for the tech guys.  I have a Onan Quiet Diesel generator in my 06' Itasca MH.  It runs great, but there is one issue I would like to address if it were possible.  The generator has it's one hour meter installed in the One Place panel in the coach.  But being the slightly obsessed guy that I am, I would like to know what RPM the generator is turning while it is going about making it's power.  The higher the RPM, the higher the fuel consumption. 

I have a neat little RPM/hour meter that I bought at West Marine that is mounted on my 10 HP Nissan outboard.  It connects to the #1 spark plug and reads out the RPM while it is running, and the hours while it is off.  However, the write up on it on the West Marine site indicates that it is designed to work with gas motors of one or two cylinders.  The Onan is a 3 cylinder diesel.  Any ideas on how to pull this off?  An old Kubota 3 cylinder tractor I used to drive umpteen years ago had a tach, so I know that is is possible.  What do you think guys? ???

Sarge

 

[Luca1369]

Hey Sarge, have you got a link to that meter?  Or a name?  I'm interested in one for my 2-cyl. Onan.

Thanks.

Steve

 

[Luca1369]

Sarge,

    Ignore my last post, I found the item.

Thanks anyway...

Steve

 

[Mark R.]

Steve just buy his , sounds like the tach is designed to take spark field readings using a transducer, may be tough to try to adapt it. Most diesel tachs I am familiar with use the alternator as a pulse supplier for the tack.

 

[dvsmith86]

Just wondering... isn't the frequency of the output dependent on the RPM of the generator?  If so, perhaps a frequency meter would work as a tach.

 

[Mark R.]

Just buy a tach for a diesel RV , one wire to the small alternator and one ground wire and you have a tach.

 

[bross]

I think you'll find that the generator runs at a constant speed it will only vary a few RPM's . The speed has to be constant to make 60 cycle electricity.

 

[hikerdogs]

If you really "need" to know the engine speed try one of these:

http://www.tinytach.com/tinytach/diesel.php

At $95.00 they're not exactly cheap, but they will give you the information you're looking for

 

[SargeW]

Thanks Hikerdog, that is exactly what I was looking for.  The West Marine tach was $50, so the extra $ to get what I was looking for is not an issue.  I would rather spend a few extra $ and have it right the first time.  Great info guys!! Thanks............. ;D

 

[SargeW]

Bill, that is one of the things that I am concerned with.  My Onan at half throttle burns .49 GPH.  At full thottle it is rated at .96 GPH.  The top end for an Onan 7500 is 3600 RPM.  I would be able to monitor the RPM for the load that I am using at the time. 

Sarge

 

[Alaskansnowbirds]

Just wondering... isn't the frequency of the output dependent on the RPM of the generator?  If so, perhaps a frequency meter would work as a tach.

An Onan Quite Diesel is an inverter generator. The inverter regulates the voltage and frequency so the engine only runs as fast as it needs to for the load.

 

[Mike and Sherrie in VA]

Forgive me if I am being ignorant, but if the genset needs to run at a higher RPM to cover the load demand, what good is a tach going to do?  If you throttle back then you're preventing the genset from doing what it "needs" to be doing. 

Of course Sarge did say he was being slightly obsessed, but I thnk maybe I am just missing the point in the first place.

 

[Chet18013]

If it was me, I'd opt for a amp meter on each leg of the generator output. This will tell you the load on the generator and give you an idea how hard it is working. In fact our Beaver has a continuous display of the generator amps. I am much more comfortable reading this than I would be if all it showed was RPMs.

Chet18013

 

[Gary RV_Wizard]

As has been said, the Onan QD is an inverter type generator and runs at variable speed depending on the load. Knowing the RPMs doesn't help, since it is dictated by the amp load and the only way you can change it is to reduce the laod. As Chet says, if you want to monitor performance and load, use an AC ammeter (which may well already be part of the coach's  power management display panel).

 

[hikerdogs]

It sounds like Sarge wants to use the tachometer to monitor the speed of the engine so he can calculate the fuel consumption.  It also appears that he intends to vary the load to minimize consumption.  It doesn't sound like he has any intention of trying to manually manipulate the engine speed.

The generator does have an ammeter built into the One Place monitoring center.  If his is anything like ours it's not in the most convenient thing to read unless you like standing in the hall way.

 

[Gary RV_Wizard]

The point is that you can monitor fuel consumption quite accurately via the amps. The Onan 7.5 QD puts out 62 amps max, so 50% load is 31 amps and Onan gives a fuel consumption for that load.  RPMs won't tell you that until you experiment and build a table of RPMs vs load, which means RPMS vs Amps anyway. So why not just use amps in the first place?

But hey, Sarge wants a tach on his gen set and already stated he is just a bit anal about it, so he certainly can have one if he wants it!

 

[SargeW]

You are all a little bit right.  But mostly it's about spit balling fuel usage to estimate the overall MPG with the rig.  I keep an eye on all that stuff, that way if something changes I know that there may be a problem somewhere. 

 

[Chet18013]

SargeW,

Check your generators hour meter. Generally these meters register engine hours at a specific RPM, not the total hours the engine has run--regardless of the RPM. If you use the fuel consumption @ the RPMS the hour meter is calibrated for, you will get a good average fuel consumption of your generator. I have been recording my generator hours every time I fill up for years now. I have been using .48 gal/hr as the ave. fuel use and then deduct genhrs run* 0.48 from my fuel volume at the fillup and use the resultant fuel volume to calculate my MPG for the engine. If you do this a few times, then purposely make a couple of fillups where the generator has not been used, you can verify the generator fuel consumption per hours registered on the hour meter.

Chet18013

 

[SCVJeff]

Could be wrong, but it occurs to me that you have all you need to do what you want already. You just need to graph it.

You're gonna need several loads for this but you ought to be able to look at the house ammeter, add load until the genset up-shifts, mark it down, and move on the the next level by adding load. Once done graph it, laminate it, and plaster it on the wall..