Standby Generator - Diesel or NG?

[SpringHollow]

We have had smaller air conditioners hooked up to our smaller air cooled generators for years with no problems but then our generators have a pretty good surge capability. In my opinion, the Generac you are referring to use an older design and have to be oversized. They are not a brand I would want to be installing for several reasons. However, they are cheap which makes them pretty popular.

For large air conditioners used on larger houses down South, we usually have to go with 30 KW units or even larger.

 

[RobJ]

Sounds like an overload or under powered condition. AC units do cause large draw at startup. I remember the Generacs in the late 70's and 80's. If I recall they were not favorites or some, problematic. I'm sure the new ones are better.

 

[RobJ]

Aircooled engines are less thermally stable so they must be built to lower tollerances to allow for thermal expansion and contraction. Because of this they typically burn more oil. The smaller aircooled generators, if run continously will shut down after a few days on a low oil alarm. I was talking power outages with a tech rep for a generac retailer and he said that is their number one question and repair response during an extended power failure in a region. About day 4 the phone starts ringing off the hook and they attempt to walk the owners thru the procedure to check and top off the engine oil... We have an Onan aircooled Vtwin on a mountain top that is good for about 4 1/2 days before it shuts down on low oil.

Water cooled engines are more thermally stable and can have tighter tollerances. How often do you have to add oil to your auto engine...

I'm not sure where you get your data, or again perhaps a poor design. I worked on air cooled diesels for 6 years. Smallest a 1 cylinder hand crank in a sail boat and the largest a v-12 running a 6" well pump in a rice field. 24/7 for weeks. Like any engine properly designed, under full load a stable temp is obtained and maintained. As far as oil? Well the recommended oil change was every 100 hours, 4-5 days of running at 24/7. Our gen and pump sets had all the standard devices for shutdown, temp, oil pressure, belt breakage, etc. On our engines the temp sensor was on #2(from the flywheel). This was the furthest from the fan and had a cylinder on both sides. I'll have to see if I can find some manuals but the normal operating temp was about 225 degrees f. Not surprisingly about the same as a liquid cooled diesel...why?? Because the parts are made from the same material!! The heat produced is the same right?

AS far as oil consumption in a car/truck, IMO an older engine with more wear will use more oil than a new engine.


Rob

 

[RonMar]

I'm not sure where you get your data, or again perhaps a poor design. I worked on air cooled diesels for 6 years. Smallest a 1 cylinder hand crank in a sail boat and the largest a v-12 running a 6" well pump in a rice field. 24/7 for weeks. Like any engine properly designed, under full load a stable temp is obtained and maintained. As far as oil? Well the recommended oil change was every 100 hours, 4-5 days of running at 24/7. Our gen and pump sets had all the standard devices for shutdown, temp, oil pressure, belt breakage, etc. On our engines the temp sensor was on #2(from the flywheel). This was the furthest from the fan and had a cylinder on both sides. I'll have to see if I can find some manuals but the normal operating temp was about 225 degrees f. Not surprisingly about the same as a liquid cooled diesel...why?? Because the parts are made from the same material!! The heat produced is the same right?

AS far as oil consumption in a car/truck, IMO an older engine with more wear will use more oil than a new engine.


Rob

I have no experience with air cooled Diesels. Because of the higher chamber pressures and additional sealing measures, they probably do not burn as much oil, as a comparable gasoline engine, I would guess that the lube oil contamination is a bit higher than comparable LC engines though(more blowby gasses).

I was refering in general to air cooled gasoline/LPG/LNG engines. Air cooled engines have always burnt more oil than comparable liquid cooled engines and this is perfectly normal.

And I agree that an older LC engine will burn more oil than a newer LC engine, but that older LC engine will still probably burn far less oil than a newer AC gas engine of similar displacement. My two Toyotas have hundreds of thousands of miles on them, and they burn less oil combined than any one of the AC gensets we have maintenance responsibility for. The LC sets we have I don't have to worry about, except getting fuel to in the winter months, but during an extended power outtage, I have to make arrangements to get to the AC sets every few days to get oil into them. One in particular can be a 2000'/3mile snowshoe climb in winter. The older AC engines we used to have, all had 5 gallon make-up oil tanks and float valves that maintained sump oil level for extended runs. We have not yet found suitable make-up systems for our newer sets.

 

[Skyco]

Referring to air cooled diesels- Rob sez-

Like any engine properly designed, under full load a stable temp is obtained and maintained.

I have no question with a steady load the temp will stabilize. But- assuming there is no temperature controlled air flow system, it seems like there would be a wide variance in what that temp would be, in zero deg weather it would run much cooler than in 100 degree weather, correct?

Don't know how much difference it makes in the real world but I'd opt for liquid cooling if given the choice

 

[coobie]

I am thinking about installing a 40kw packaged standby generator but can't decide between diesel and NG. I don't expect it to run a lot - we haven't had a power outage in the 10 years we have lived here. But if we did it would be a disaster. If I thought it would get a lot of use I would definitely go diesel. My guess is that almost all of the run time will be in the weekly automatically scheduled test. The fuel oil tank for the furnace for my garage is very close to where it will be installed so I could use it to run a diesel. But NG is a cheaper and cleaner fuel and should be easier starting. It will take some piping changes to run extend a NG line to the generator but not a big item. It looks like a NG unit is a couple thousand dollars less than a diesel unit. Any suggestions?

First off I would go with the NG & have someone do a load study,maybe your local power company could assit you in that.Second I think that 40KW is over kill IMHO.I have worked as a electric lineman for 30 plus years and I would say all you would need is 25KW.Our 25 kw tranformers sometimes run 6-8 houses in some citys we supply power to.

 

[techman]

... all you would need is 25KW.

That is a good statement. I have a 12 KW diesel and I can run my 2 heat pumps (starting 1 at a time), lights, TV and the computer. No hot water heater, dryer or range, but I could run any of them individually after shutting down 1 heat pump.

15 or 20 KW would be life as normal.

paul

 

[SpringHollow]

One really needs to calculate the loads that will be running. One house needed 60 KW (it was 5500 sqft in Louisianna so huge air conditioners) but they also had a 400 Amp service from the electric company.

You need to know the starting amperage of your largest load. A large air conditioner is usually the largest starting load although some of the new ones are not bad. If you look at the face plate of your air conditioner, there will be a LRA rating (Locked Rotor Amps). Take that and add all of your other loads that will be hooked up to whatever panel is run by your generator. Then add a safety/expansion factor. That will be a good estimate of what you need. In my case, I can run the whole house including two small air conditioners, well, sump pump, etc. with a 12 kw generator as long as I do not wire in the electric stove or electric clothes dryer.

Some people say they will not run appliance X that is hooked into the standby panel (which could be the panel for the whole house depending upon how they hook it up) when there is no power, for example, the electric stove I mentioned above. But with an automatic transfer switch, you might have the appliance on when the power goes out. The generator switches quickly so you will not have time to turn it off before the generator switches over. So, in my example, I can not have the eelctric stove wired into the generator panel. However, if i want, I can wire a second stove outlet to the generator panel and manually move the stove plug from the normal panel to the standby (generator) panel during a power outage where I can manually turn off some other breakers first if necessary.

Probably being as clear as mud!

Ken

 

[Highbeam]

The automatic transfer panel complicates things significantly. Manual transfer panels allow the user to switch off large loads like water heaters, stoves, heat pumps, etc. Before those large appliances overload the genset.

If you choose to use one of those automatic transfer panels then I agree that you need to have enough generator capacity for everything. That is not ideal and I wouldn't recommend it.

 

[SpringHollow]

Where I live, my pipes could freeze from a power failure before i could get home to manually start the generator so an automatic transfer switch is necessary. My tropical fish would die as well. But it does mean you have to size the generator for the loads you need to run on the generator. If you install a sub panel (took 2 hours to install the subpanel and transfer the wires from the main panel to it - ~16 circuits), it greatly simplifies things.

Ken