About Diesel Generators

Diesel generators have been available since the 1800’s and began with the contribution of two renowned inventors, Michael Faraday who produced the first induction generator and Rudolph Diesel who produced the first diesel engine. Both technologies have evolved since then to become the comparatively light weight, compact and quiet running packages that we have today.

Diesel generators are considered to be a robust and reliable source of electrical energy that can be used all day and every day or left dormant for prolonged periods then needed to respond immediately without intervention. Their usable life will vary and is usually based on hours of operation and, if serviced correctly in accordance with the manufacturer’s recommendations, will range from 3,000 hours for small high speed 3,000rpm machines to nearly indefinitely for large low speed 750rpm machines.

The most common medium size 1,500 rpm machines are expected to have a usable life from 14,000 to 24,000 hours before overhauls but then will continue to be usable until they become uneconomical to overhaul. 

The composition of a diesel generator is made up of two major components, a diesel fuelled drive engine and an induction alternator. These two components are usually hard coupled with the rotational part of the alternator directly attached to the engine flywheel. Other ancillary components that complete the package include, engine starting battery, engine cooling system, engine exhaust system, generator controller, power output circuit breaker and electrical power outlet.

The diesel engine will require periodic servicing with service intervals ranging from 250 to 500 hours of operation for small to medium size diesel generators.

Early Diesel Engine

Early Induction Alternator

Disciplined service with high quality lubrication and genuine replacement filters and components will result in the best possible life. Servicing is generally conducted by the operator and should be as detailed within the operator’s handbook and is generally limited to the replacement of the crankcase oil and oil filter, and sometimes air and fuel filter. Other components that will need less frequent attention include fan belt and coolant, for radiator cooled engines, and starting battery. If the diesel generator is left dormant for extended periods, the start battery should be kept fully charged through three monthly checks and recharging or continuous maintenance charging. High quality AGM start batteries have a life of up to 5 years providing they are charged correctly.   

There are many types of diesel fuelled engines and many types of induction alternators and each diesel generator manufacturer will offer their choice of these two components based either on availability, cost, efficiencies, durability, market acceptance and their association with suppliers. The cost of these two major components will significantly influence the cost of the diesel generator and generally this cost is a good representation of quality. There were around 4,000 new diesel generators delivered in Australia in 2022 and this number has increased at a steady rate over the years and is expected to continue. Today, most packaged diesel generators are fully imported, however there are some local manufacturers such as Eniquest who specialise in Australian built superior quality diesel generators for those applications that demand the best probable life before failure. Unlike motor vehicles, most of us will not purchase many diesel generators and will have little understanding of value for money without extensive research. All diesel generators sold in Australia should comply with Australian Standard AS 60034.22, and AS/NZS 3010 where applicable.

Diesel generators have been the choice for providing electric power in remote and mobile situations where mains grid power is not available. These situations are widespread and include residential, commercial operations, mining, communications, agriculture pumping and irrigation, service vehicles, marine and Defence. They are also used as backup for mains grid power in the event of its failure. These situations include critical equipment in industry and commercial, hospitals, high rise buildings, communication towers, shopping centres, transport infrastructure, and sporting lighting. 

When selecting a diesel generator consideration must be given to the output voltage and the electrical power required. The output voltage must be the same as the equipment being powered and can be either three phase 230/400 Volts or single phase 230 volts 50Hz. Single phase 230 Volt equipment can be used on three phase diesel generators provided that each phase is reasonably balanced with other mixes of single phase or three phase equipment. Electrical power available is generally rated in kilovolt-amps or kVA at 0.8 power factor, but kVA isn’t a widely used term outside of the generator industry and can be confusing when determining the size required. Electrical equipment is generally rated in kilowatts or Amps. 

For instance, electric motors are rated in kW which relates to the mechanical power available from its drive shaft. When sizing a diesel generator for this piece of equipment, the losses of the electric motor will need to be added to its mechanical rating then converted to kVA. Its losses are measured by efficiency and if its rating is 10kW with an efficiency of 82% the actual power required to drive the electric motor is 10kW ÷ 0.82% efficient = 12.2kW. This can then be converted to kVA, for example, 12.2kW ÷ 0.8 power factor = 15.25kVA from the diesel generator. Alternatively, the diesel generator rating can be converted to kW by simply multiplying its kVA by 0.8 power factor. Available Amps from a diesel generator can also be calculated from its kVA and is different for three phase and single phase machines. 

This calculation is fairly complex, and an approximation can be determined by simply using a multiple of 1.4 for three phase and 4.1 for single phase. For example, 10kVA three phase machine will provide 10kVA x 1.4 = 14 Amps per phase and a 10kVA single phase machine will provide 10kVA x 4.1 = 41 Amps. It should be noted that this current is available for 0.8 power factor equipment such as an electric motor but must be modified for other equipment such as heating and lighting with power factors closer to 1.0 or unity. To modify the available current available form a diesel generator at unity power factor, multiply by 0.8, for example, 14 Amps x 0.8 power factor = 11.2 Amps at unity power factor.

Diesel generators are generally rated for maximum continuous power at specific ambient temperatures and altitudes. Deration will be required for higher ambient temperatures and altitudes and sometimes humidity. It is good practice to leave some headroom for changing conditions and varying demand from the connected electrical equipment. However, if the selection is too conservative where average electrical load is below 60% there is a danger of early engine failure due to wet stacking and glazing.

Wet stacking is a condition in diesel engines where unburnt fuel passes into the exhaust system and is evident by a tar like substance oozing from its manifold, piping, or turbo charger. For efficient combustion, diesel engines should run at not less than 60% of its rated power. Poor combustion can also produce hard caron particles within the combustion chamber that are extremely abrasive. As these particles are collected at the piston rings, scraping the honing on the cylinder bore will lead to bore polishing and loss of compression.

DC Diesel generators

There are also diesel generators that provide DC electrical power to support battery of solar systems. These are usually only rated in kW with a two wire +ve and -ve outlet. It is a little easier to understand the conversion from kW to Amps as there is no three phase or power factor to consider. The calculation is simply kW x 1,000 ÷ Voltage, for example, a 10kW 28 Volt DC diesel generator will provide 10kVA x 1,000 ÷ 28 Volts = 357 Amps. DC diesel generators will come in voltages to suit battery charging. It should be noted that the charge voltage needs to be higher than the battery voltage to force a charge, for example, a 24 Volt battery will require 28 to 29 Volts and a 48 Volt batterie will require 56 to 58 Volts. 

When used for battery charging, the sizing of DC diesel generators is based on the time required to charge a battery and the recommended maximum charge current for a particular battery. The amount of power available from a DC diesel generator is generally limited by the diesel engine fuel stop setting and when this limit is reached the diesel engine slows resulting in a drop in output voltage. This voltage drop will limit the output Amps and prevent the diesel generator from over loading. For a 28 Volt 800 Amp/hour battery with a recommended charge rate of no more than C5, or Capacity ÷ 5, or 800 Amp/hours ÷ 5 = 160 Amps, the suitable sized DC diesel generator would be, 160 Amps x 28 Volts ÷ 1,000 = 4.5kW, to charge the battery in the shortest possible time. Smaller DC diesel generator can be used but would require a longer run time to charge the battery. It is important that DC diesel generator is not over sized for a battery as they may provide a charge rate higher than the maximum recommended, resulting in reduced battery life.

DC diesel generators should only be used to provide a bulk charge to the battery and never a float charge. As the battery is charged its voltage will increase and will creep closer to the DC diesel generator’s output voltage to a point where the DC diesel generator becomes lightly loaded and operate inefficiently.

There are many other applications for DC diesel generators such as powering mobile LED flood lighting, operating DC drives and operating electromagnets. In these cases the same rules for AC diesel generators should apply for suitable sizing.