Back Up Power Generator: Understanding Fuels–Part II

by Guest Author Ted Blanchard

Generator Fuels

What is the best type of fuel for your generator?

Different types of generators are designed to run on fuels that may include, gasoline, propane, natural gas and diesel.  Don’t get one that runs on electricity though (that’s a little joke, folks). For the sake of brevity, I won’t go into water-powered or gasified wood-powered generators. Battery started engines are just fine and make the use much more enjoyable.  Some generators can be modified to run on something other than their original design fuel, and some can selectively run on more than one type of fuel.  These are not very common and potentially add complexity to the system that could translate to shorter life span or undesirable operation, but properly configured tri-fuel (propane, natural gas and gasoline) generators provide flexibility and can make sense in some circumstances. The dry fuel “carburetor” does not add much complexity but, as with all things, the KISS principle applies.

I’m going to generalize here and will likely incur the wrath of several readers by slighting their favorite form of fuel.  Too bad.  I’m writing and you’re reading it (at least as far as this point) so give me some additional time and I may manage to lessen your anger.

Propane or Natural Gas

Gaseous fuels– propane and natural gas (Nat Gas) – have distinct advantages, not the least of which is an incredible shelf life.  A tank that is kept free of leaks should hold your propane or Nat Gas supply in a usable state for longer than you live.  On the down side, plumbing can be tricky and if you do develop a leak, your first indication may be when you house is “remodeled” by the force of the explosion as the leaked gas reaches an ignition source.  Propane is heavier than air and can creep along the ground until it finds a spark or flame.  Nat Gas is lighter than air and so tends to waft up and away from the tank.  Small comfort, however, if it encounters a spark from a chimney or a static discharge from anything that it finds nearby, even many hundreds of feet away.  Natural Gas that is distributed in the gaseous state is considered to be part of the grid, with the associated vulnerabilities.  All that said, gaseous fuels as a generator supply are still a good choice, if proper care is used in selecting the placement of the tank and in running the plumbing to the generator. Gaseous fuels have a decent specific energy and so is generally a cost effective fuel, but this will depend on your location and how much the rates change between seasons.  Here in North Idaho, propane was a delightful $1.40 per gallon this summer, but climbed very rapidly to $2.05 so far this winter and will probably go up even more several times in the coming months. Generators that use gaseous fuels are often very quiet compared to gas or diesel powered systems and that is a distinct advantage if you want to run your generator overnight.  Availability in normal and abnormal times must also be considered.  Will your propane supplier keep refilling your tank if we have a TEOTWAWKI event?  If not, do you have the means to go get some at the supplier – assuming they are still selling it?

Liquid Fuels

Gasoline and diesel each have their own good and bad points.  We’ll start with gasoline.  Typically affordable and available almost anywhere, we drive our gas-powered cars without giving too much thought to where we will fill the tank when it approaches empty.  We will simply stop at a station that is convenient and sells at a price we are willing to pay.  Several grades may be available, but generators typically do not require anything better than the lowest grade, with an octane level of 87i.

Gasoline does not store well for long periods of time.  Since it contains several very volatile compounds that separate out of the solution and evaporate given half a chance, fuel that you count on for emergencies should be rotated regularly and even treated with a stabilizer such as “Sta-Bil.”  I’m not recommending them over other providers or products but they have become something of a “household” name.  I do not like the fact that they charge what I consider to be exorbitant prices for what is, chemically, an inexpensive formula.  Nevertheless, properly treated, sealed and stored gas can exhibit a useful shelf life of up to two years. Beyond that you are slowly transitioning to gunk.  Engines do not run well on gunk. To get the best possible shelf life, keep three things in mind.  (1) Store the fuel in an air- and liquid-tight container with as little air in the container as possible (to prevent moisture condensation), remembering that gas expands and contracts significantly when the temperature changes so don’t chance bursting a thin-walled container by leaving zero air space.  (2) Keep the fuel away from light, which can cause heat-induced expansion and also speeds up the chemical decomposition process, and (3) make sure the gas is not subject to wild temperature swings in either direction, which also speed up the decomposition process and may overcome the integrity of your storage container.  Anecdotal stories of shelf life longer than 2 years may be true.  I wouldn’t count on it, in fact 2 years is really pushing it in my estimation.  That’s why you should rotate your emergency supply every year if possible.  Use the stabilized one-year-old gas in your car, boat or other engines.  Treat and store the next batch.  Make sure to put the date on an attached slip of paper.  Remember that as gasoline ages its octane rating decays so it might make sense to store a higher octane fuel. Some will argue that properly stabilized low octane gas is sufficient and that may well be the case.

Gasoline vapors explode. That’s why your car, boat, and generator engines work.  Improper storage of gas can result in leaks that pose a significant fire and/or explosion hazard.  Gasoline composition is important.  Automobile gasoline (every grade) is increasingly treated with ethanol, up to 10% (and up to 85% in the case of ethanol multi-fuel).  This have some dubious benefits to the environment and cost of production, but it has one very clear drawback, which is one of the reasons why I tend to lean toward diesel generators.  Ethanol attracts moisture.  The technical term is hygroscopic, if anyone cares.  Ethanol literally sucks water vapor out of the air.  This water may or may not be visible in a tank used to store your generator’s fuel supply, but unless very specific steps are taken to reduce or eliminate the water before it reaches your generator you can end up with both short term and long term problems. Water will alter your consumption rate, usually for the worse.  When water is absorbed into gasoline it can form corrosive compounds that damage engine components.  Water left sitting in fuel lines, carburetor bowls, injection systems and so forth can make the engine hard to start, maybe even impossible, depending on how long the unit sat with old fuel in it. Leaks in hard fuel lines are a common result of trapped water sitting for long periods.

Now we’ll cover diesel fuel.  Also available in several “grades,” diesel is formulated for variable speed engines operating at normal temperatures, cold temps, and also for near-constant speed engines.  Trying to start an engine with diesel formulated for normal temps when the engine and surrounding air is very cold can be a frustrating experience. Road diesel gels at cold temps and may not be easily pumped through the system, so low temp formulas are warranted in colder regions.  All on-highway diesel fuel in the US is now Ultra-Low Sulfur diesel (ULSD) <15 ppm and there may be issues with older engines not engineered for ULSD. For a complete description of the various diesel grades and their positions in the spectrum of refined petroleum products, please see this piece.

Diesel exhaust is typically white and dense when the engine is cold and turns almost clear as it gets warm, though this can shift to grey or even black if the engine is under very heavy loads.  Good quality diesel generators are designed to deliver their rated power without reaching this engine loading point, but some people do not like the smell of diesel fumes at all, and it’s a definite consideration when choosing your generator and fuel combination.

Diesel has a significantly longer shelf life than gasoline, with 8-, 10-, and even 12-year old fuel still working satisfactorily.  To get the best results, depending on your location and engine type, you may need to treat the diesel with some combination of a cetane booster, an anti-gel product and/or a biocide.  Seems a fair number of algae and microbe species like to eat diesel fuel and they can foul it beyond recall if not kept in check.  An obvious sign that you have an algae or microbe problem is that the fuel becomes extremely murky, or even black. The biocide keeps the bacteria from attacking your fuel. Mix it in well with a paddle or stir stick.  Warmer climates are more prone to infestations and cold climates may not require any biocide treatment at all.  Higher speed diesels (e.g. 3600 RPM vs 1800 RPM) operate more effectively with higher cetane number fuels.  Anti-gel additive helps keep your fuel flowing well in colder temps and keeps the diesel stabilized.  Diesel is nowhere near as bad as gasoline in this respect, but it is still wise to prevent problems that you can easily avoid.  Diesel is much safer than gasoline since the vapors will not explode or even burn at normal air pressure. You can throw a lit match into a coffee can with an inch of diesel in the bottom and the match will be quenched immediately.  As a result, the long-term storage of diesel fuels is simpler and safer than with gasoline.  Don’t be careless, however, as almost any materials lying around will burn much better if they have diesel on or in them.  Diesel still likes the same handling as gasoline: air and liquid-proof containers, dark storage areas (this also inhibits microbe growth) and near constant temps.  Underground storage is ideal, but not essential. While diesel does not attract moisture nearly as much as ethanol-treated gasoline, diesel engines hate water in the fuel and may not run at all if badly contaminated.  A good quality water-block filter, in line between the tank and the engine, solves this easily.  Keeping an emergency supply tank full is always a good idea since water vapor-bearing air can’t condense out and contaminate the fuel.

To be continued…

Back Up Power Generators Part I

Back Up Power Generators Part III