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Gen-sets represent an increasingly important market place for diesel and gas engines, with a demanding customer base. Developing world economies and recently more common power outages are fuelling the massive demand for standby power. Customers continually expect more from these products in terms of power density, load acceptance, emissions, fuel consumption, noise, cost of ownership. The customer needs a tailored product.

What is a gen-set and how does it differ to the industrial engine on which most are based? The following list sets out to explain many of the basic terms applicable to gen-set design, development and ownership.

Electro pack – A fixed speed diesel engine with ratings to suit a gen-set application. Comes complete with radiator, cooling group and fan.

Electro unit – An Electro pack without radiator, cooling group and fan. Suitable for individually installed Combined Heat & Power setups.

IOPU – Industrial Open Power Unit. These are multi speed non vehicle power units. The are normally sold with radiator, cooling group and fan, and typically share ratings from their off highway derivatives. Typical applications include pumps and compressors.

Operating Speed – Gen-sets are normally governed to fixed speed running. 1500 rpm to produce 50Hz electrical supply for European market and 1800 rpm to produce 60 Hz for US market. 60Hz supply can be achieved at 1200Hz with some alternator sets- this is uncommon.

kWe – Kilowatts electrical is a measure of electrical power produced by a gen-set. 60Hz generator sets are usually marketed in terms of kWe.

kVa – Kilovolt amps is a measure of electrical power produced by a genset. 50Hz gen-sets are usually marketed in terms of kVa. As gensets produce an alternating current P=VI doesn’t hold true. Voltage and current follow sinusoidal wave forms with a phase shift due to the reactance (generated by inductance & capacitance) of the load on the alternator, and hence a power factor is used. Industry assumes a 100% resistive load for which a 0.8 power factor is used. This relates kWe to kVa by the following:

kWe = kVa x 0.8

Fuel Coolers – Gen-sets are normally fitted into a frame, which holds a small fuel or “day tank” for limited time running. If the gen-set operates in elevated ambient temperatures, or the engine has a high fuel spill ratio, the temperature of the fuel will often be controlled by a small fuel cooler (air-to-fuel) mounted on the cooling group. The cooler prevents rises in “day tank” temperatures preventing fuel injector damage.

Fans – All Electro units are fitted complete with fans to provide cooling to the radiator and charge cooler if fitted. Two versions of fans are normally offered for gen-sets and IOPU’s, pushers (which blow air through the radiator) and pullers (which pull air through the radiator). The customer is able to specify the type most relevant to their application/installation. The type of fan used will affect the ambient air temperature the bulk of the engine will see, and may have consequences on the engines rating and performance.

Fan Power (Fp) – Depending on the size of the gen-set, the power required to drive the fan will vary between 10% for smaller gen-sets (<10L) and 5% for larger gen-sets ( />10L).

Alternator Efficiency (ha) – The alternator on the gen-set converts the mechanical energy delivered by the engine into electrical energy, and has an associated efficiency. Typically alternators have an efficiency of 0.95 (95%).

kWm – Gen-sets are marketed in terms of the electrical power which they produce. However engine manufacturers are more interested in the mechanical power which their engine needs to deliver to the alternator to provide the quoted electrical power. This includes fan powers and alternator efficiency:

kWe = (kWm -Fp) x ha or kWe = kWm x 0.90 x 0.95 (<10L engine)
kWe = (kWm -Fp) x ha or kWe = kWm x 0.95 x 0.95(>10L engine)

Emissions- Genset emissions are complicated and specific to the country in which they operate. Generally requirements are less demanding than other off highway equipment, but are often driven by marketing rather than legislative needs. Legislative limits are complicated, determined by introduction date, engine powers and power rating. The three most important limits are listed below with links to sites where full documentation can be found.

European Limits (Directive 97/98/EC)
Commonly referred to as EU Stage ‘X’ Emissions

North American Limits (Federal Regulations 40 CFR Part 89)
Commonly referred to as EPA Tier ‘X’ Emissions



Ambient/Altitude Clearance – Gen-sets are operated in global environments, with extreme ambient and altitude operating environments. Running at higher ambient temperatures adds additional loads on the cooling system, and at elevated altitudes the inlet system struggles to deliver sufficient air for combustion with the lower air density/pressure. Gen-sets are expected to run at altitudes up to 4000m and ambient temperatures of 55 ?C, which may require derate. Clearance is defined as the margin on the altitude/ambient performance limiting parameters (such as coolant and exhaust temperature) when tested at standard operating conditions (sea level 25 ?C). From the Ambient/Altitude clearance, curves are developed to assist application engineers in sizing appropriate derates for extreme operating conditions.

Power Rating - Gen-sets are sold at three main power ratings determined by their application. Power ratings are defined by ISO 8528-1. Generic power ratings are summarised below:

Rating Summary

Governing – Gen-sets are fixed speed applications with governors developed to maintain the desired running speed within careful limits. This is particularly important as electrical equipment powered by the genset may be damaged by supply outside of the normal 50/60Hz limits. Gen-set governing is detailed by ISO 8528.

Load Acceptance – Gen-sets are often used for standby/emergency power, where they will be expected to start-up, run up to running speed and then accept a large % of maximum electrical load. Load acceptance is measured in terms of a % frequency dip and a recovery time, and are defined by ISO 8528-5 and NFPA 99/110. Additional requirements are customer driven demanding typically 80% of the prime rating within 10 seconds of start-up, within ISO 8528-5 limits. Engine load acceptance has been demonstrated as a linear function of trapped mass.

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