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GENERATOR FUEL - WHAT GENERATOR FUEL IS BEST?

There are many advantages and disadvantages to different types of fuel. Nearly all Generators use either gasoline, diesel, natural gas or propane. The following information will hopefully answer any questions or concerns you may have concerning different fuel sources.  Some general features of the generator itself influence purchase decision. Where possible we indicted hardware and environmental differences in generator set types. You should consider all the factors shown.  POSITIVE,  NEGATIVE,  GENERATOR FACTORS, are shown by the color of the features.
Gasoline:
  • Advantages:
    • Common fuel source - easily obtained
    • Increases portability of smaller generators

 

  • Disadvantages:
    • Highly flammable
    • Short shelf life of fuel (approximately 12 months)
    • Storing large quantities of fuel is hazardous
    • May not be available during power outages
    • Somewhat expensive fuel, check your local prices
    • Inefficient
Diesel:
  • Advantages:
    • Least flammable fuel source
    • Fuel easily obtained (fuel is easier to obtain during a disaster because it is a necessary fuel for the military, trucking industry, and farming operations)
    • On site fuel delivery available
    • Engine life for liquid-cooled 1800 RPM engines can approach 20,000 hours if properly serviced depending on the application and environment.
    • High speed 3600 RPM diesel engines normally have a 10,000 to 15,000 hour life expectancy with proper maintenance and service under most conditions
    • Less expensive to operate. The general rule of thumb for fuel consumption is 7% of the rated generator output (Example: 20 kW x 7% = 1.4 gallon per hour at full load).
    • Designed for off-road applications and can operate on dyed or farm/construction diesel fuel which is sold without the road tax and thus is considerably cheaper to purchase.
    • Engines designed to work under a load for long periods of time and perform better when worked hard rather than operated under light loads.
    • Can operate in sub-artic conditions with fuel additive.
    • Equipment is competitively priced for a comparative sized water-cooled gaseous models with the same features.
    • In high use situations overall long term cost of operation is much lower than gaseous GenSets.

                                 Diesel Fuel Use Chart

  • Disadvantages:
    • 18-24 month shelf life, without additives
    • Installing large storage tanks raises cost of system
    • May not be available during power outages.
    • Diesel fuel storage must be considered relative to required run time in your geographical area. If you live in hurricane country you might need a large fuel tank due to the high possibility of extended power outages
    • Engine noise is higher on a diesel compared to a gaseous engine. Use of a properly designed enclosure and sound attenuation system is more critical on a diesel engine system.
    • Subject to "wet stacking" or over fueling if run for long periods of time with ultra light loads (less than 40% of the rated output). "Wet Stacking" causes the engine to smoke and run rough because the injectors become carbonized. Running a heavy load will usually clean up the over-fuel condition and allow the engine to perform normally. Diesel engines operate better and are more fuel efficient when loaded (70-80% is optimum).
    • In sensitive emission areas in some states diesel engines are prohibited from operating over a prescribed number of hours per year to help reduce pollution levels
    • Requires clean moisture free fuel and a bit more maintenance than a comparable gaseous unit;
    • Some cities and counties require the generator on-board fuel tanks to be double-wall containment type which can increase the cost of the generator system.
    • Typically heavier and require more planning to load and unload than a lightweight gaseous GenSet.
Bio-Diesel:
  • Advantages:   (Same as Diesel see above)
    • Least flammable fuel source
    • Easily obtained
    • On site fuel delivery available


    • Bio-Diesel is a cleaner-burning diesel replacement fuel made from natural, renewable sources such as new and used vegetable oils and animal fats. Like petroleum diesel, Bio-Diesel operates in compression-ignition engines. Blends of up to 20% Bio-Diesel (mixed with petroleum diesel fuels) can be used in nearly all diesel equipment and are compatible with most storage and distribution equipment. These low level blends (20% and less) don't require any engine modifications and can provide the same payload capacity as diesel. Using Bio-Diesel in a conventional diesel engine substantially reduces emissions.
  • Disadvantages: (Same as Diesel see above)
    • 18-24 month shelf life, without additives
    • Installing large storage tanks raises cost of system
    • May not be available during power outages
    • Obtaining oils and other fuels.
    • Mixing and maintaining proper percentage of oils/fuel
Emulsified Diesel:
  • Advantages:  (Same as Diesel see above)
    • Least flammable fuel source
    • Easily obtained
    • On site fuel delivery available

      This is diesel that is mixed with a small percentage of water and an agent that keeps the water and diesel mixed. By adding the water to the diesel a smaller amount emissions are created when the fuel is burned.
  • Disadvantages: (Same as Diesel see above)
    • 18-24 month shelf life, without additives
    • Installing large storage tanks raises cost of system
    • May not be available during power outages
    • Obtaining oils and other fuels
    • Mixing and maintaining proper percentage of water/fuel
       

Propane:*

*See propane notes below.

  • Advantages:
    • Long shelf life
    • Clean burning
    • Easily stored in both large tanks or in smaller 5 - 10 gallon cylinders
    • Obtainable during power outages - gas stations may be unable to pump fuel during an area wide outage
    • Home delivery available for larger tanks
    • Quieter engine noise level
    • More emission compliant
    • Gaseous engines do not have a problem with "wet stacking like diesels
    • Less expensive units with air-cooled engines are budget priced.
    • Engine life for liquid-cooled 1800 RPM engines can approach 5,000 to 6,000 hours on industrial quality gaseous GenSets
  • Disadvantages:
    • Pressurized cylinder of flammable gas
    • Fuel system is more complicated (increased possibility of failure)
    • Larger tanks are not aesthetically pleasing (unsightly)
    • Fuel system plumbing results in higher installation cost
    • Somewhat expensive fuel, check your local prices
    • Propane can become very dangerous if lines are broken.
    • Propane begins to derate around -20 degrees above zero
    • Initial cost of generator is somewhat higher, 15 to 20% especially in sizes larger than 30 kW.
    • More expensive to operate by as much as 3-times the fuel consumption compared to diesels;
    • Shorter life expectancy by a factor or 10 to 1for air-cooled models and 3 to 1 for water-cooled models compared to diesel powered GenSets
    • Smaller air-cooled gaseous engines are less expensive than comparable diesels but have a short life expectancy as low as 500-hours depending on engine make and use
    • Shorter life than diesel engines

Natural Gas:

  • Advantages:
    • Unlimited fuel source - refueling not necessary
    • Clean burning
    • More available during power outage.
    • Quieter engine noise level
    • More emission compliant
    • More convenient fuel source (natural gas)
    • Gaseous engines do not have a problem with "wet stacking like diesels
    • less expensive units with air-cooled engines are budget priced.

      Natural gas is a mixture of hydrocarbons (mainly methane (CH4)) and is produced either from gas wells or in conjunction with crude oil production. Because of the gaseous nature of this fuel, it must be stored onboard a vehicle in either a compressed gaseous state (CNG) or more commonly as liquefied state (LNG).
  • Disadvantages:
    • May be unavailable during natural disasters (earthquakes, etc)
    • Lower power output (30% less BTU's per unit than gasoline.
    • Fuel system plumbing results in higher installation cost.
    • Fuel not available in many areas.  
    • Natural gas (NG) begins to derate at +20 degrees above zero.
    • Initial cost of generator is somewhat higher, 15 to 20% especially in sizes larger than 30 kW.
    • More expensive to operate by as much as 3-times the fuel consumption compared to diesels;
    • Shorter life expectancy by a factor or 10 to 1for air-cooled models and 3 to 1 for water-cooled models compared to diesel powered GenSets
    • Smaller air-cooled gaseous engines are less expensive than comparable diesels but have a short life expectancy as low as 500-hours depending on engine make and use.
    • Hurricanes and earthquakes can disrupt the flow of natural gas lines with up-rooted trees
    • Natural Gas can become very dangerous if lines are broken.
 

SUMMARY OF FUEL FACTORS

FACTOR GASOLINE DIESEL & MIXES NATURAL GAS* VAPOR PROPANE* LIQUID PROPANE*
ENGINE
COST
EXCELLENT
(many low-cost GenSets on market)
VARIES
(higher cost in small sizes)
VARIES
(low cost in small sizes)
VARIES
(low cost in small sizes)
VARIES
(low cost in small sizes)
FUEL SYSTEM INSTALLATION & STORAGE COST VARIES
(low cost in small sizes)
VARIES
(low cost in small sizes)
EXCELLENT
(if gas service already available at site)
MEDIUM
(if adequately sized tank already at site)
MEDIUM
(if adequately sized tank already at site)
FIRE & PERSONNEL
SAFETY
POOR
(highly flammable, vapors poisonous)
EXCELLENT
(high flash point)
MEDIUM
(rare leak risk)
MEDIUM
(rare leak or tank explosion risk)
MEDIUM
(rare leak or tank explosion risk)
ENVIRONMENTAL
IMPACTS
POOR
(spill risk, exhaust not clean)
POOR
(spill risk, exhaust not clean)
EXCELLENT
(clean burning)
EXCELLENT
(clean burning)
EXCELLENT
(clean burning)
FUEL
AVAILABILITY
MEDIUM
(easy to purchase)
MEDIUM
(must be delivered & stored)
EXCELLENT
(storage not required, supply rarely lost)
MEDIUM
(must be delivered & stored
MEDIUM
(must be delivered & stored
COLD
STARTING & OPERATION
POOR
(forms gum deposits)
MEDIUM
(hard starting at cold temperatures)
EXCELLENT MEDIUM
(tank must be large and full for vaporization)
EXCELLENT
(no tank vaporization issue)
ENGINE
LIFE/WEAR
POOR/ MEDIUM
(depends on engine type)
EXCELLENT MEDIUM MEDIUM MEDIUM
*See propane notes below.

Gaseous fuels such as natural gas, vapor propane and liquid propane are the most common choice for small automatic standby generators. Propane engines are economical to build and these fuels provide good starting reliability and are in common use. These fuels are available everywhere.  When purchasing a gaseous fueled GenSet you must remember the following:

Specify which kind of fuel you will be using when placing the order. If there is a mistake it may not be possible to convert a GenSet in the field between natural gas/vapor propane and liquid propane. It usually is possible to convert between natural gas and vapor propane however.

If you order a GenSet for vapor propane it is sometimes set up at the factory for natural gas and must be converted in the field to vapor propane. The installation manual will describe the steps for this simple conversion which usually involves connecting a fuel line to another port or changing an orifice and a mixture setting.  If you fail to make this conversion the GenSet will not run properly and may not start at all.

*A vapor propane system draws the fuel from the top of the tank usually through a pressure regulator at the tank. The liquid in the
lower part of the tank must be able to absorb sufficient heat from the tank surroundings for vaporization to take place. Therefore, it is important that the tank has enough exposed surface area for this heat transfer. There can be a problem of insufficient fuel flow in very cold weather or if the tank is less than half full or is too small. In practice this only is an issue in the far northern areas of the USA.

*A liquid propane system draws the liquid from the bottom of the tank and small high pressure tubing is used to carry it to the GenSet.  The GenSet is then equipped with a special device to vaporize the fuel before combustion.  This eliminates the low temperature vaporization concerns at the tank in cold climates.  However it may complicate using propane for other appliances since it is being supplied in liquid form to the point of use.

Fuel Preference by Geography and General Use

Place Use Preference Avoid or Reasons
Pacific Time Zone Residential Propane, Diesel Avoid Natural Gas due to earthquakes
  Ranch Diesel, Propane
  Industrial Diesel, Propane, NG
Mountain Time Zone Residential Propane, Diesel Propane preferred in mountain areas.

Diesel preferred on ranches and farms for duel use.

  Ranch Diesel, Propane
  Industrial Diesel, NG, Propane
Central Time Zone Residential NG, Propane, Diesel Natural Gas very dependable in these time zones
  Ranch Diesel, Propane
  Industrial Diesel, NG, Propane
Eastern Time Zone Residential NG, Propane, Diesel
  Ranch Diesel, Propane
  Industrial Diesel, NG, Propane

Six Classes of Fuel Oil

Fuel oil in the United States is classified into six classes, according to its boiling temperature, composition and purpose. The boiling point, ranging from 175 to 600 °C, and carbon chain length, 20 to 70 atoms, of the fuel increases with number. Viscosity also increases with fuel oil number and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases. No. 1 fuel oil, No. 2 fuel oil and No. 3 fuel oil are referred to as distillate fuel oils, diesel fuel oils, light fuel oils, gasoil or just distillate. For example, No. 2 fuel oil, No. 2 distillate and No. 2 diesel fuel oil are almost the same thing. Diesel is different in that it also has a cetane number limit which describes the ignition quality of the fuel. Distillate fuel oils are distilled from crude oil. Gas oil refers to the process of distillation. The oil is heated, becomes a gas and then condenses. It differentiates distillates from residual oil (RFO). No. 1 is similar to kerosene and is the fraction that boils off right after gasoline. No. 2 diesel is the diesel that trucks and some cars run on, leading to the name "road diesel". Heating oil is the same as No. 2 Diesel without dye. No. 3 fuel oil is a distillate fuel oil and is sometimes used for large trucks and power generation but it is rare. No. 4 fuel oil is usually a blend of distillate and residual fuel oils, such as No. 2 and 6, however, sometimes it is just a heavy distillate. No. 4 may be classified as diesel, distillate or residual fuel oil. No. 5 fuel oil and No. 6 fuel oil are called residual fuel oils (RFO) or heavy fuel oils. As far more No. 6 than No. 5 is produced, the terms heavy fuel oil and residual fuel oil are sometimes used as synonyms for No. 6. They are what remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation. No. 5 fuel oil is a mixture of No. 6 (about 75-80%) with No. 2. No. 6 may also contain a small amount of No. 2 to get it to meet specifications. Residual fuel oils are sometimes called light when they have been mixed with distillate fuel oil, while distillate fuel oils are called heavy when they have been mixed with residual fuel oil. Heavy gas oil, for example, is a distillate that contains residual fuel oil. The ready availability of very heavy grades of fuel oil is often due to the success of catalytic cracking of fuel to release more valuable fractions and leave heavy residue.

The US nomenclature is used in most of the world. In the United Kingdom the classes comprise 6 commonly used fuels using alphabetical designations, from Class C1 (kerosene) to Class G (heavy fuel oil). There is a Class H designation which is not yet in general use. The characteristics of these oils are specified in British Standard BS2869:1998 - soon to be updated to BS2869:2006.

Table of Fuel Oils

Name Alias Alias Type Chain Length
No. 1 fuel oil No. 1 distillate No. 1 diesel fuel Distillate 9-16
No. 2 fuel oil No. 2 distillate No. 2 diesel fuel or heating oil Distillate 10-20
No. 3 fuel oil No. 3 distillate No. 3 diesel fuel Distillate  
No. 4 fuel oil No. 4 distillate No. 4 residual fuel oil Distillate/Residual 12-70
No. 5 fuel oil No. 5 residual fuel oil Heavy fuel oil Residual 12-70
No. 6 fuel oil No. 6 residual fuel oil Heavy fuel oil Residual 20-70

 

Marine Classification for Fuel Oils
MGO (Marine gasoil) Roughly equivalent to No. 2 fuel oil, made from distillate only
MDO (Marine diesel oil) A blend of gasoil and heavy fuel oil
LFO (Light fuel oil) A blend of gasoil and heavy fuel oil with very little gasoil than marine diesel oil
IFO (Intermediate fuel oil) A blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil
MFO (Medium fuel oil) A blend of gasoil and heavy fuel oil, with less gasoil than intermediate fuel oil
HFO (Heavy fuel oil) Pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil

 

For additional information see:

Fuel Energy Content and Conversions

Generator Uses and Diesel Types

 Tables, Conversions and Formulas

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