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Diesel Generators

When considering a design where generators will be used a prime power for remote and small operations (< 1MW) there are several factors that need to be considered.


Technically Generators produce direct current (DC), what you would get from a battery, and Alternators produce alternating current (AC), the same power you get from a power company. While there are design differences in the internal connections of the electrical side of things that make that difference, it is common practice to call the entire system of engine-driven alternators as “generators” or generator-sets (gensets) and we will use that term here. We will use the term “alternator” only do describe design issues that impact the electrical portion of the overall generating system.

DC systems have current flowing in a single constant direction with typically steady voltages and are commonly seen in battery powered systems. AC systems have current directional flow alternating, typically many times a second; the voltage also varies in a sine wave pattern, from zero to a positive peak then back down to zero and down to a negative peak then rising back to zero in each cycle. Since the voltage is constantly varying, the voltage is expressed as RMS (root mean square). RMS is a mathematical calculation of the sine wave voltage to give the DC voltage that would provide equivalent power. Peak RMS voltages are higher than the RMS value by a factor of 1.414 (the square root of 2). So a 120 volt AC system will have peak voltages of about 170 volts, and 220 volt AC system peak voltages of about 308 volts.


Generators are typically sold by kW capacity with a kVA rating also provided. Gensets are typically rated for Prime or continuous duty rating and at higher rating for Standby or limited run hours. It is wise to by the unit based on the lower kVA Prime or continuous duty rating if it is to take to place of utility power and run all day or over 12-14 hours a day.

The kW capacity is controlled by available engine horsepower (work), while the alternator's winding kVA rating (current) is limited due to temperature rise from windings resistance. At higher altitude and hotter inlet air the engine will deliver less horsepower than at sea level and standard temperature. Likewise hotter ambient air will also limit the acceptable temperature of the windings in the alternator thus limiting kVA. Since almost all 3 phase generators are rated with a 0.8 power factor, expect to see typical ratings like 80KW / 100kVA.

While power factor is more complicated than we will deal with here, it is the effect when the current flow of an AC system doesn't match exactly the voltage sine wave. Incandescent lights and resistance heaters have a power factor of 1.0 and induction motors typically have power factors closer to 0.8, lagging while capacitive loads such as UPS input filters tend to have leading power factors. The voltage regulators in the alternators generally have problems responding to leading power factor loads and loads with very low leading power factors can result in loss of voltage stability. Using a generator to feed a normal mix of loads this is not a problem but if you are feeding loads where a UPS kVA rating is over about 50% of the load the generator will be carrying, special attention to getting an engineer’s review would be wise.

At cooler temperatures and for short periods the genset can deliver slightly above its rating. This allows for motor starting loads and brief load transitions. How well it will perform during this overload varies with the ambient air temperature, amount of overload and duration. You are at the edge or outside the envelope here so anything you get be grateful for. Planning under normal conditions not to exceed 80% of the engines rating is wise.

See Prime vs Standby.


The frequency of the output voltage is directly related to the engine speed. Thus with typical 4 pole armatures, a 50Hz machine will run at 1500 RPM and a 60 Hz machine will run at 1800 RPM. Engine speed is controlled by the governor. If you are having frequency and engine speed problems that is where to look, not the alternator or voltage regulator.

So, yes some 60 hertz machines can be dialed back to run at the lower speed, but you will lose some horsepower and torque, so it will not deliver full original KW/kVA ratings at the lower speed. Likewise, some 50Hz machines could be adjusted upward in speed to provide 60Hz, where they would have more available engine horsepower but also might have tendency for the engine to overheat if the radiator system was sized closely at the 50 Hz rating, likewise fuel consumption will increase.

Other than motor loads, many loads such as lighting, resistance heaters (stove, ovens, toasters etc), and many electronic devices are not frequency sensitive, they can work on either 50 or 60 hertz. Check the nameplate of the device to be sure.


The voltage regulator controls output voltage. Engine speed and operation would only impact voltage if the engine is at the end of its performance range and can't hold any more power OR if a large load is suddenly applied to the engine. This block loading (adding or removing a large say >=25% fraction of the generator's rating) will briefly impact both speed (frequency) and voltage output but in general the engine should recover within about 3-5 seconds. Try to specify permanent magnet excited alternators, if they are available, other types can, when out of service for some time, (maintenance issues) have to have the field windings 'flashed' to get the unit generating voltage again. While this is a fairly simple procedure, it is easy for the procedure to be forgotten or trained personnel to leave before it is needed again. Thus, getting written manufacturers troubleshooting and basic maintenance procedures should be a part of the purchase if at all possible.

Some 277/480 VAC units could be dialed down at the voltage regulator to provide 230/416 or possibly 220/400 VAC power for overseas systems. If the voltage regulator won't adjust that low possibly a replacement one can be obtained that will allow for the lower voltage.

3 Phase alternators are available in 6 lead and 12 lead winding arrangements, the leads are typically marked T1- T12. The 12 lead machines can be connected to provide two or more voltage levels across the three phases (120/208 or 277/480VAC). 6 lead machines have the two ends of each of the three windings available to be connected as a delta or star (WYE) arrangement. Be sure the connection is made as needed to match the electrical system design you are working with. The delta arrangement has a single voltage between any two phase connections and such will give only a single voltage, 120, 208, 240, 400, 480. While a star or WYE connected arrangement ties one end of each phase winding to a common point and then grounds them. This provides two voltage ranges, with the phase to ground voltage 120 in a 120/208 system, 220 in a 220/400 system and 277 in a 277/480 system. While the phase to phase voltage gives the higher level in each of those pairings. If the machine is reconnectable there will be a diagram in the owner’s manual and often a wiring diagram on the alternator cover. Reconnecting to get a different voltage arrangement may require changes in the sensing leads landed on the voltage regulator, watch closely if you have to make a reconnection.

One vs. Three Phase AC

A single phase unit can be connected to a 3 phase panel wiring it to each hot phase, but of course it will not support true 3 phase loads, still all those single phase loads can be powered. The KW and kVA ratings will still apply since the three connections each add load. It is possible to use a 3 phase unit and wire to a single phase panel but the windings in the alternator will be heating unevenly and you need to reduce the loading to 1/3 of kVA rating if you only connect 1 phase. If you connect 2 legs to a US standard 120/240 panel you can use about 2/3 of the kVA rating of the unit.


Engine generators in smaller sizes are generally available in gasoline from under 1KW to about 150KW, with natural gas (propane) and diesel units available in the whole range of 20-1000+ KW. In most majority world settings, diesel is the best choice for the following reasons. Diesel is less flammable (listed as combustible) than gasoline. Liquid fuels are categorized based on flash point, the temperature where the fuel gives off enough vapor to ignite. Combustible fuels (kerosene, D-2, Jet A etc) have flashpoints above 100F, while flammable fuels gasoline, alcohol have flashpoints under 100F (~37.7C). Thus combustible fuels are safer to transport and store.

Diesel fuel should not be stored or piped with galvanized iron pipe. Black iron pipe is fine and the interior is preserved by the oily nature of the fuel. Diesel tends to strip the zinc galvanizing off the pipe and it causes problems for the engine injectors.

In general, diesel fuel stores pretty well, as least compared with gasoline, and so it is more widely available in the more rural areas. Exceptions to this would be areas served by small boats with gasoline outboard engines. Diesel engines are also generally considered longer lived and to require less maintenance since they do not have points, spark plugs and electric ignition systems.

While natural gas and propane are also scarce in the more rural areas due to transportation issues, gaseous fueled engines also have slightly slower responses to varying loads, and require spark ignition systems. There are some dual fueled engine that use some diesel fuel as an ignition source and mix the intake air with gaseous fuel for the rest of the power requirements but these are generally larger than the sizes reviewed here.

So in general, diesel is the preferred fuel for this application. Be careful since much overseas diesel fuel is very high in sulfur content. If the local fuel is high sulfur make sure to check with the manufacturer and get an engine with proper valves and cylinder sleeves rated for use with high sulfur fuel. This will save a lot of maintenance issues and expense. If you have an engine designed for use with the newer Ultra low sulfur (>15ppm post 2010 U.S. EPA certified engines), it can be damaged or even fail if run on higher sulfur content fuels that may be available in developing countries. Also be aware that “marine” diesel may be extremely corrosive to exhaust systems. Engines in the 2004-2010 range in the US can be used with fuel up to 500 ppm, pre-2002 engines should handle almost any nations standard diesel fuel with up to 5000 ppm.

Many diesel engines are rated by their manufacturers to run on alternate fuels. Some fuels have slighter lower energy per volume and so reduce the generator KW capacity, these rarely require more than 5-10% derate. The key issue is often the fuel pump needing a certain level of lubricity. Telephone companies in the US often operate diesel engine generators on K-1 kerosene and D-1 (lighter weight or winter diesel) since they are more stable in storage. Most diesel engine vendors also have recommendations on use of so called bio-diesel fuels, while long term storage of bio fuels is generally not recommended many engine vendors have approved these fuels for operation with little if any KW derating. Always check with the engine manufacturer for their recommendations. Likewise many diesel operators blend used and filtered motor oil back into diesel fuel at ratios under 10% of used motor oil with satisfactory results.

Bio fuels are not recommended for longer term storage (.>~45 days) since they tend to degrade and or grow microbial bugs. If bio fuels are to be used be sure there is no water contamination as any water (even in standard diesel fuel) increases fuel microbial growth that can lead to filter plugging and fuel deterioration. High storage temperatures are also very bad for bio-diesel blends. Higher ratio blends of bio-diesel also can have greater than 12% derate for horsepower and increased fuel quantity consumption values above 15%. If biodiesel fuels are to be used, seriously consider use of fuel stabilizers. Biodiesel blends may also tend to clean the tanks of any accumulations of sludge or other sediments and can result in more frequent filter plugging than straight diesel.

If you are located in extremely cold climates be aware that diesel fuel has a paraffin point or cloud point, a temperature where small crystals of waxy particulate condense out of the fuel. To avoid plugging the fuel filter or even having the fuel gel in the tank, the fuel must be kept above the paraffin point temperature. Your local fuel supplier should be able to provide you info on the cloud point of their winter fuel. Be aware that fuel blends can change summer to winter, so take that info account.

Underground storage of often recommended since it reduces the temperature swings of aboveground tanks and so reduces moisture condensation that mixes into the fuel and tends to promote microbial growth. A nearly full tank has less air space and so less moisture condensation problems, but a large tank reduces fuel turnover so there is a greater chance of the fuel having some deterioration due to longer storage times. It is best to try to use the tank down to very low level before refueling, as just mixing a little fresh fuel into a near full tank gives a longer average life of the fuel being stored and so a higher risk of deterioration. Environmental concerns recommend enclosing the diesel storage tank with a secondary containment able to prevent release of the fuel into watercourses if the tank leaks or is spilled. Since water is heavier than diesel fuel, be sure and pump any water or sediment laden fuel off the bottom of the tank at least yearly. You can use a rigid small diameter pipe to get the water or fuel off the bottom of the tank, if there is no sump drain. For the same reason, make sure the fuel pickup piping is installed at least a couple of inches (5cm) or so off the bottom of the tank, so any water isn’t picked up and run to the fuel injectors as they can be damaged by this. On larger tanks the pickup point can be 4-6“ off the bottom of cylindrical tanks.

Fuel Consumption

At least for planning and budgetary reasons, the designer needs to be aware of fuel consumption requirements. Until a specific generator is selected and purchased, how do you estimate fuel consumption and thus fuel storage requirements? Rule of thumb - For each 10KW of load being operated, figure a diesel generator will use about 1 GPH (~4 LPH) of diesel fuel. This gets somewhat better with larger engines fully loaded and a little worse with smaller engines or engines under small partial load, but gives you a starting point for planning purposes.

Many larger capacity diesel engines have fuel return lines. Excess fuel is pumped by the fuel pump to the injector pump and circulates in the fuel rail, cooling the injectors, the fuel that isn't needed at the existing engine loading is returned via the fuel return line. Some manufacturers have optional fuel coolers, small radiators for this fuel to pass thru before it is returned to the fuel tank. For planning purposed only the fuel consumed by the engine per hour is needed, but be aware that the fuel returned to the tank will tend to warm the fuel up and thin it out somewhat.

Fuel Security

Security of your fuel supply is a critical item. Loss by theft is all too common and a source of unneeded expense. For critical applications (hospitals etc.) having a separate concealed reserve can be helpful not only for emergencies requiring more fuel consumption than normal but as a bridge to cover delays in fuel delivery, or in emergencies due to theft. Locking and frequent inventory of fuel storage by principals to verify that (even otherwise trusted) staff is faithfully managing the expensive fuel supply is a wise management control item. While this seems harsh, such measures can also serve to reduce temptations to divert what seems to be such a plentiful and valuable liquid commodity. Initial planning for fuel storage should take these measures into consideration.

Radiator Cooled units

The rule of thumb for radiator cooled units says you need at a clear open inlet area least 1 ½ to 2 times the area of the radiator. Since many units in majority world installations don't have enclosed engine rooms or ducted air exhaust and inlet, this may not impact your design. But it is wise to make arrangements so the air flow from the radiator is not easily recycled into the inlet side of the radiator. The effect of such re-circulated air is to reduce engine cooling capacity and if the hotter air is entrained into the diesel engine inlet filters, the reduced density gives the effect of higher altitude operation and reduces available engine horsepower and thus KW. Thus pay attention to prevailing wind directions, radiator discharge into prevailing winds should be avoided or else use scoops or diverters to direct off radiator air upwards or sideways to allow normal airflow to help dissipate radiator exhaust hot air plume.

If you have use for large quantities of hot water (hospitals) consider design of a heat exchanger to use waste engine heat to provide domestic hot water.


Remember the noise factor when designing placement of a diesel generator. Will it need to run at nighttime? While voltage drop issues force the unit to be located close to higher amp draw uses, look at orientation of the unit to minimize noise impact to sensitive areas. Pointing the engine exhaust and radiator fan away from those areas is a basic starting point. Scoops to divert radiator exhaust upwards can make significant noise reduction, as can baffling of air inlets around the sides of the generator.

Operating an Engine genset at reduced loads

While diesel engines can and do operate in wide range of loads, be aware that prolonged use under about 30% of rated loading can result in “wet stacking” where the engine tends to ooze a black tarry viscous liquid that is a mix of unburned diesel fuel, soot and carbon particles. This can cause the engine problems over time and is cured by running the engine for several hours at higher loads (50-75% of rating), before the problem gets too serious. Wet stacking can reduce the ability of a generator to supply it full rated load and cause other maintenance issues, so don't oversize a generator if the future load growth is several years off. Better to buy a more closely right sized unit now and trade when the load exceeds 90% or so of rated capacity. This problem is common with the older 2 cycle designs from Detroit diesel.

Operational Issues

While operational costs will depend on the local staff skills of the facility, distance from a service provider and cost of parts for that brand of unit in the country where it is located. Preference should be given to using manufacturers with good support in the region or at least the country. Importing the “finest” make in the world into an area where parts are simply hard to get, or unobtainable will result in an out of service unit all too soon. Air freight and import duties for specially imported parts will cost more than similar parts that are imported in greater quantity due to wider use of that make and model of engine.

Consider specifying a fuel/water separator unit, and provide for spare fuel, oil and air filters. Try to get manufacture’s training for the local service staff to at least be able to change oil and all filters. The recommended service intervals will depend on how clean the air is at the generator, wind blown dust and dirt will shorten service intervals and increase costs as air filters will have to be replace more often and oil changes made more frequently. Labor costs will of course vary so investigate those costs and make sure the owner is aware of them.

ge/diesel-generators.txt · Last modified: 2018/05/28 22:29 by