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A Cummins diesel generator of 500kVA in a tourist resort in Egypt.

A diesel generator is the combination of a diesel engine with an electrical generator (often called an alternator) to generate electric energy. Diesel generating sets are used in places without connection to the power grid or as emergency power-supply if the grid fails. Small portable diesel generators range from about 1 kVA to 10 kVA may be used as power supplies on construction sites, or as auxiliary power for vehicles such as mobile homes.


Diesel generator set

The packaged combination of a diesel engine, a generator and various ancillary devices such as base, canopy, sound attenuation, control systems, circuit breakers, jacket water heaters, starting systems etc, is referred to as a generating set or a gen set for short.

While the larger industrial generators can range from 8 to 30 kVA for homes, small shops & offices up to 2,000 kVA used for large office complexes, factories. A 2,000 kVA set can be housed in a 40ft ISO container and be fully packaged and portable. Sizes up to about 5 MW are used for small power stations and these may use from one to 20 units. In these larger sizes the engine and generator are brought to site separately and assembled along with ancillary equipment

Diesel generators, sometimes as small as 250 kVA are widely used not only for emergency power, but also many have a secondary function of feeding power to utility grids either during peak periods, or periods when there is a shortage of large power generators.

Diesel generator on an oil tanker

Ships often also employ diesel generators, sometimes not only to provide auxiliary power for lights, fans, and winches, etc. but also indirectly for main propulsion. With electric propulsion the generators can be placed in a convenient position, to allow more cargo to be carried. Electric drives for ships were developed prior to WW I. Electric drives were specified in many warships built during WW II because manufacturing capacity for large reduction gears was in short supply, compared to capacity for manufacture of electrical equipment. [1] Such a diesel-electric arrangement is also used in some very large land vehicles.

Generating sets are selected based on the load they are intended to supply power for, taking into account the type of load, ie emergency or for continuous power, and the size of the load, and size of any motors to be started which is normally the critical parameter.

Power plants - electrical "Island" mode

One or more diesel generators operating without a connection to an electrical grid are referred to as operating in "Island Mode". In island mode, several parallel generators provides the advantages of redundancy and better efficiency at part loads. An islanded power plant intended for primary power source of an isolated community ("Prime Power") will often have at least three diesel generators, any two of which are rated to carry the required load. Groups of up to 20 are not uncommon.

Generators can be electrically connected together through the process of synchronization. Synchronization involves matching voltage, frequency and phase before connecting the generator to a live bus-bar. Failure to synchronize before connection could cause a high current short-circuit or wear and tear on the generator and/or its switchgear. The synchronization process can be done automatically by an auto-synchronizer module. The auto-synchronizer will read the voltage, frequency and phase parameters from the generator and bus-bar voltages, while regulating the speed through the engine governor or ECU (Engine Control Module). Typical manufacturers are DSE, ComAp, GAC, Woodward and Heinzman who dominate this market

Load can be shared among parallel running generators through load sharing. Load sharing can be achieved by using droop speed control controlled by the frequency at the generator, while it constantly adjusts the engine fuel control to shift load to and from the remaining power sources. A diesel generator will take more load when the fuel supply to its combustion system is increased, while load is released if fuel supply is decreased.

Supporting main utility grids

In addition to their well known role as power supplies during power failures, diesel generator sets also routinely support main power grids worldwide in two distinct ways:


Peak Saving

Maximum demand tariffs in many areas encourage the use of diesels to come on at times of maximum demand. In Europe this is typically on winter weekdays around tea time (3 pm), whereas in the United States this is often in the summer to meet the air conditioning load.

Grid support

Emergency standby diesel generators such as those used in hospitals, water plant etc, are, as a secondary function, widely used in the US and the UK to support the respective national grids at times for a variety of reasons. In the UK for example, some 0.5 GWe of diesels are routinely used to support the National Grid, whose peak load is about 60 GW. These are sets in the size range 200 kW to 2 MW. This usually occurs during say the sudden loss of a large conventional plant of say 660 MW, or a sudden unexpected rise in power demand eroding the normal spinning reserve available.[2]

This is extremely beneficial for both parties - the diesels have already been purchased for other reasons; but to be reliable need to be fully load tested. Grid paralleling is a convenient way of doing this.

In this way the UK National Grid can call on about 2 GW of plant which is up and running in parallel as quickly as two minutes in some cases. This is far quicker than a base load power station which can take 12 hours from cold, and faster than a gas turbine, which can take several minutes. Whilst diesels are very expensive in fuel terms, they are only used a few hundred hours per year in this duty, and their availability can prevent the need for base load station running inefficiently at part load continuously. The diesel fuel used is fuel that would have been used in testing anyway. See Control of the National Grid, National Grid Reserve Service[3][4]

A similar system operates in France known as EJP, where at times of grid extremis special tariffs can mobilize at least 5 GW of diesel generating sets to become available.In this case, the diesels prime function is to feed power into the grid. [5]

During normal operation in synchronization with the electricity net powerplants are governed with a five percent droop speed control. This means the full load speed is 100% and the no load speed is 105%. This is required for the stable operation of the net without hunting and dropouts of powerplants. Normally the changes in speed are minor . Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all powerplants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication. [6]

Cost of generating electricity

See: Relative cost of electricity generated by different sources

Typical operating costs

Fuel consumption is the major portion of diesel plant owning and operating cost for power applications, whereas capital cost is the primary concern for backup generators. Specific consumption varies, but a modern diesel plant will consume between 0.28 and 0.4 litres[7][8] of fuel per kilowatt hour at the generator terminals.

However diesel engines can operate on a variety of different fuels, depending on configuration, though the eponymous diesel fuel derived from crude oil is most common. The engines can work with the full spectrum of crude oil distillates, from natural gas, alcohols, gasoline, wood gas to the fuel oils from diesel oil to residual fuels.[9] This is implemented by introducing gas with the intake air and using a small amount of diesel fuel for ignition. Conversion to 100% diesel fuel operation can be achieveved instantaneously.[10]

  • Fuel cost 11p - 16p/kWh (using red diesel at 40p/litre)
  • lifetime engine maintenance about is 0.5p/kWh - 1.0/kWh[4]

Typical costs of conversion to paralleling for grid operation

To be able to operate in parallel with the mains certain modifications are necessary which include the following:

  • Approx. £3k to fit a PLC to the set
  • Paralleling and synchronising gear and G59 equipment (this allows grid connection) Approx £5k
  • Tidying up set (noise, larger fuel tank) Approx another £5k
  • So for a 1MW set…£13/kW
  • 50 kW…maybe £260/kW

This capital cost of £13/kW - £260/kW is low compared to combined cycle gas turbines that cost £350/kW.[4]

Generator Sizing and Ratings


Generators must be capable of delivering the power required for the hours per year anticipated by the designer to allow reliable operation and prevent damage. Typically a given set can deliver more power for fewer hours per year, or less power continuously. That is a standby set is only expected to give its peak output for a few hours per year, whereas a continuously running set, would be expected to give a somewhat lower output, but literally continuously, and both to have reasonable maintenance and reliability.

To meet the above criteria manufactures give each set a rating based on internationally agreed definitions.

These standard rating definitions are designed to allow correct machine selection and valid comparisons between manufacturers to prevent them from misstating the performance of their machines, and to guide designers.

Generator Rating Definitions [11]

Standby Rating based on Applicable for supplying emergency power for the duration of normal power interruption. No sustained overload capability is available for this rating. (Equivalent to Fuel Stop Power in accordance with ISO3046, AS2789, DIN6271 and BS5514). Nominally rated.

Typical application - emergency power plant in hospitals, offices, factories etc. Not connected to grid.

Prime (Unlimited Running Time) Rating based on: Applicable for supplying power in lieu of commercially purchased power. Prime power is the maximum power available at a variable load for an unlimited number of hours. A 10% overload capability is available for limited time. (Equivalent to Prime Power in accordance with ISO8528 and Overload Power in accordance with ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models.

Typical application - where the generator is the sole source of power for say a remote mining or construction site, fairground, festival etc.

Base Load (Continuous) Rating based on: Applicable for supplying power continuously to a constant load up to the full output rating for unlimited hours. No sustained overload capability is available for this rating. Consult authorized distributor for rating. (Equivalent to Continuous Power in accordance with ISO8528, ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models

Typical application - a generator running a continuous unvarying load, or paralleled with the mains and continuously feeding power at the maximum permissible level 8760 hours per year. This also applies to sets used for peak shaving /grid support even though this may only occur for say 200 hour per year.

As an example if in a particular set the Standby Rating were 1000 kW, then a Prime Power rating might be 850 kW, and the Continuous Rating 800 kW. However these ratings vary according to manufacturer and should be taken from the manufacturer's data sheet.

Often a set might be given all three ratings stamped on the data plate, but sometimes it may have only a standby rating, or only a prime rating.


Typically however it is the size of the maximum load that has to be connected and the acceptable maximum voltage drop which determines the set size, not the ratings themselves. If the set is required to start motors, then the set will have to be at least 3 times the largest motor, which is normally started first. This means it will be unlikely to operate at anywhere near the ratings of the chosen set.

Manufactures have sophisticated software that enables the correct choice of set for any given load combination.

Correct Generator Installation

To ensure correct functioning, reliability and low maintenance costs generators must be installed correctly. To this end manufacturers provide detailed installation guidelines [12][13] covering such things as:

  • Sizing and selection
  • Electrical factors
  • Cooling
  • Ventilation
  • Fuel storage
  • Noise
  • Exhaust
  • Starting systems

These are frequently ignored causing problems for users

Diesel engine damage due to misapplication or misuse of generating set

Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing (occasionally referred to as bore glazing or piling) and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines. Ideally, diesel engines should be run at least 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings, which causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the now poorly-sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.

Unburnt fuel then leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.

The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.

Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities and neighbours.

Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.

The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines.

For emergency only sets which are islanded, the emergency load is often only about 1/4 of the sets standby rating, this apparent over size being necessitated to be able to meet starting loads and minimising starting voltage drop. Hence the available load is not usually enough for load testing and again engine damage will result if this us used as the weekly or monthly load test. This situation can be dealt with by hiring in a load bank for regular testing, or installing a permanent load bank. Both these options cost money in terms of engine wear and fuel use but are better than the alternative of under loading the engine. For remote locations a Salt water rheostat can be readilly constructed.

Often the best solution in these cases will be to convert the set to parallel running and feed power into the grid, if available, once a month on load test, and or enrolling the set in utility Reserve Service type schemes, thereby gaining revenue from the fuel burnt. [14][15][16]

See also


  1. ^
  2. ^ Commercial Opportunities for Back-Up Generation and Load Reduction via National Grid, the National Electricity Transmission System Operator (NETSO) for England, Scotland, Wales and Offshore.
  3. ^ Paper given at Open University conference covering use of diesel generators to assist dealing with intermittency of renewable energy " Open University Diesel Generators for Load Management"
  4. ^ a b c Andrews, David (June 2007). Emergency diesel standby generators of Wessex Water potential contribution to dealing with renewable energy sources intermittency & variability. 11. The Institution of Diesel and Gas Turbine Engineers. 
  5. ^ Demand_side_management Energy demand side reduction
  6. ^ Speed Droop and Power Generation. Application Note 01302. 2. Woodward. Speed
  7. ^
  8. ^
  9. ^,%20wartsila.pdf | title=Dual-fuel-electric LNG carrie==
  10. ^ "Man Diesel Se - Press->Press & Trade Press Releases->Trade Press Releases ->Stationary Power->Medium-Speed". 2008-11-19. Retrieved 2009-05-11. 
  11. ^
  12. ^
  13. ^
  14. ^ "How to Turn Standby Generation Into Profit-Making Assets".  Wayne Boakes, generation manager, Wessex Water, Claverton Energy Group Conference, Bath, oct 24th 2008
  15. ^ "Breaking in a Diesel Engine".  Jay Chlebowski, retrieved December 22, 2009
  16. ^ "Bore glazing in diesel engines".  Cox Engineering, retrieved December 22, 2009


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