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Old 02-10-2013, 06:03 PM   #1
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The Price of Quiet

I come from an engineering and computer programming background, and that makes me look into many technical details that others find boring. This may just be one of those cases.

Iíve been around a lot of mobile diesel generators for a long time with boats and motorhomes. I have always thought that Onan is the gold standard in RV diesel generators. Then when Onanís quiet series came out I was even more impressed.

My current motor home came with a PowwrTech 7-CD generator (7-CD), and I was somewhat concerned that it wouldnít measure up to an Onan. After talking with them and reading their very complete Userís Manual, my worries went away. The Userís Manual contains most everything an old engineer would want to know, including a fuel consumption table, and I created an Excel spread sheet for it to get an idea about how much fuel it will burn under various loads.

Then a little while ago a thread came up about carrying a small portable (Honda) generator for those light loads like a TV and such. The argument was that it burned less fuel with those light loads than the big diesel gen set in the motor home. With the sparse data from the Honda 2000 series, I was able to demonstrate that my 7KW CD-7 burns about half as much fuel as a Honda 2000 with a 100 watt load.

I said that the CD-7 has a Kubota engine, and someone noted that the Onan Quiet (QD-7500) series also uses Kubota engines. Since I could download the QD-7500 Userís Manual and I already had the CD-7 Userís Manual, this old engineer just had to look to find the devil in the detail, and to this engineer, it is interesting and revealing. I wish I could show you the complete spreadsheet, but the forum doesnít allow any table format I could use, so Iíll just take some of the things from that spreadsheet.

One bit of data missing from the CD-7 data is the noise specification. The QD-7500 produced 71 DBa at full load and 66 DBA at 50% load. You might not be able to visualize just what those numbers mean, but most will appreciate that the Onan Quiet series is really quiet. The CD-7 produces more noise than the QD-7500, but it is still pretty quiet.

Load Ratings:
All of the generator data from the generator manufacturers that I have seen show the correct data in their manuals, but the sales department, sometimes, generate names that donít tell the whole story.

The QD-7500 manual says that 7,500 watts is for intermittent use only and full load should be 90% of that. That makes it really a 6,750 watt generator and all the fuel comparisons use that figure.

The CD-7 load ratings are net ratings and the 7,000 watt figure is its full load rating.

How Did Onan make it Quiet?
The manual doesnít say, but someone said, and I think it is so, that these generators use inverters to produce AC current. Now to complete that story, the QD-7500 must first generate DC current and then invert it to 120AC. Both the DC generator and the AC inverter have inefficiencies and are present no matter what the load. Those inefficiencies take fuel, as weíll see in a minute. By doing it this way, the RPM can be reduced at partial load and reduce the noise.

The CD-7 is a constant speed of 1800 RPM and the fuel consumption is determined by the load.

Load and fuel consumption:
In normal operation, diesel engine fuel consumption is almost proportional to the load. That is evident in the CD-7 fuel table. Weíll compare the fuel consumption of
the CD-7 and QD-7500 shortly. Keep in mind that the QD-7500 is doing things in a different way from the CD-7.

Fuel Consumption Table
The first clue about fuel efficiencies came from the fact that the CD-7 uses a 10.5 hp engine and the QD-7500 uses a 16.6 hp engine.

For those of you not familiar with the scientific numbering notation for very large and very small numbers, hold on to your hat. Iíll try to help you through the issue.

The very best way to compare fuel consumption between two different systems is to generate a number for each system that describes the basic fuel consumption for each system. That number is Gallons Per Hour (GPH) per watt. This is a very small number, but is the basis for all the fuel comparisons.

At Full load
GPH per Watt:
CD-7: 9.14E-05 or .0000914
QD-7500 1.42E-04 or .000142

That all means that the CD-7 burns 35.71% less fuel at full load as the QD-7500.

The extra load because of the way the QD-7500 reduces the noise also makes it less efficient at half load. In this condition, the CD-7 is 44.9% more efferent than the QD-7500.

Total Fuel Burned:
In a typical load situation with the following:
2 AC units, TV, 100 amp battery charger, microwave, and ref with some assumed running factors, the fuel consumption of the two units look like this;

Gallons ------
4 Hrs. 1.11--------1.87
6 Hrs. 1.66--------2.80
8 Hrs. 2.21--------3.73
12 Hrs. 3.32-------5.60

These are two very top notch units, and this report isnít about discrediting either one. It just shows that there is a fuel cost in making a diesel generator produce less noise. It is rare that we have a choice about which generator is in our rig. This is just to let you know that if you have a Quiet Onan generator, youíll burn more fuel for that fine quality.

Wil Andrews
2007 Newmar Kountry Star DP
Cummins ISL, Cummins E-Brake
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Old 02-10-2013, 06:11 PM   #2
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Old 02-10-2013, 06:28 PM   #3
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We have an 8 Kw Power Tech in our DSDP. One thing about the PT is that the 8 Kw unit is fixed speed @ 1800 rpm. The Onan's are usually, but not always variable speed, up to 3600 rpm. The sound of the PT doesn't change much under load where the Onan does get louder as the load increases.
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Old 02-10-2013, 07:02 PM   #4
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Inverter generators are variable speed, traditional generators need to be at a constant speed since the speed determines the frequency, How Onan made my generator compete with a Honda Inverter is a study in sound containment and a few other technologies.

Since all the parts on the generator turn at the same speed at all times (Even if it's a variable speed, at any given instant the speed of each part is directly related to the others) in theory it should be possible to phase two noise making parts 180 degrees out of phase, epically on a constant speed device, two identical noise sources 180 degrees out of phase = silence.
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Old 02-10-2013, 09:41 PM   #5
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I've had a Kipor apart (similar to the little hondas) and my Onan apart, as well as looking at several different generations of Onans at a friend's generator shop. The quietness really comes from very careful control of vibration of all the various parts and ducting the cooling air, since the noise moves with that air. If stuff on the gennie gets loose, it gets loud.

The loudest thing on my onan, besides the exhaust, is the little facet interrupter pump. You can hear it clicking over the exhaust.
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Old 02-13-2013, 06:55 PM   #6
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I'm not an engineer but I can tell you from months of dry camping that my Onan 7500 quiet Diesel burns between .5 and 1 gal of fuel per hour depending on load.

My Honda eu2000i burns about 1 gallon every 4.5 to 6 hours, depending on load.

The Honda has paid for itself.

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