In short. A diesel engine in a generator set compresses air until it is hot enough to ignite injected fuel, converts the pressure into torque at the crankshaft, and holds that shaft at a constant 1,500 rpm for a four-pole 50 Hz alternator while the load changes. Its usable output depends on the rating class, the air it can breathe, the heat it can reject and the fuel it is given. Those relationships explain most of what goes wrong with generators.
How compression ignition makes power
A diesel engine has no spark plug. On the intake stroke the piston draws in air alone; on the compression stroke it squeezes that air to a fraction of its volume, typically a ratio between 16 and 18 to 1 on generator engines, which raises its temperature above the self-ignition point of diesel. Near the top of the stroke the injector sprays atomised fuel into that hot air, it ignites within a few degrees of crankshaft rotation, and the expanding gas drives the piston down on the power stroke. The exhaust stroke clears the cylinder and the cycle repeats every two revolutions. Because the engine controls power by the quantity of fuel injected rather than by throttling the air, it draws a full charge of air at every load, which makes it efficient at part load and also leaves unburnt fuel behind when load is very light.
Torque, speed and frequency
Everything the alternator needs from the engine reduces to torque and speed. Power is their product, and the alternator fixes the speed.
P = (2π × N × T) / 60,000 and f = (p × N) / 120
Here P is shaft power in kW, N is speed in revolutions per minute, T is torque in newton-metres, f is electrical frequency in hertz and p is the number of alternator poles. Worked example (illustrative): a four-pole alternator needs 1,500 rpm for 50 Hz, since 4 × 1,500 / 120 = 50. If the set is delivering 100 kW at the shaft, the engine must produce 100 × 60,000 / (2π × 1,500), which is about 637 Nm of torque, continuously, at exactly that speed. When a motor starts, the engine must produce more torque at the same speed; the governor senses the speed falling, increases fuelling, and the engine recovers. The size of that frequency dip and the recovery time depend on the engine’s torque reserve and on how quickly the turbocharger can supply the extra air.
The systems that keep the engine running
| System | What it does | Common failure | How it shows |
|---|---|---|---|
| Fuel | Stores, filters, pressurises and meters diesel to the injectors | Water and sludge in tank, blocked filters, air ingress, worn injectors | Hunting, black smoke, loss of power, failure to start |
| Air intake and turbocharging | Delivers clean air at the mass flow combustion needs | Clogged filter, turbo bearing wear, boost leaks | Black smoke under load, low power, high exhaust temperature |
| Cooling | Rejects about a third of fuel energy through jacket water and radiator | Fouled core, slipping belt, failed thermostat, degraded coolant | High temperature shutdown, especially at full load in summer |
| Lubrication | Separates moving surfaces, carries heat and contaminants away | Old oil, low level, pump wear, blocked filter | Low oil pressure alarm, bearing noise, high consumption |
| Starting and charging | Cranks the engine and keeps the battery ready | Sulphated battery, faulty charger, corroded terminals | Slow or no cranking at the moment of need |
| Exhaust | Removes combustion gas against a limited backpressure | Long or restricted runs, collapsed silencer, leaks | Loss of power, high temperatures, fumes in the room |
Cooling and lubrication are treated together under engine cooling and lubrication, and the air side under turbocharging and aftercooling. Few engine failures begin in the engine’s core; they begin in a neglected supporting system and announce themselves through a symptom that can be read if someone is looking.
Rating classes and what they permit
The same engine carries different ratings depending on its expected use, and the ISO 8528-1 definitions matter when comparing quotations. An emergency standby rating is the maximum output for the duration of a utility failure, with limited annual hours, a typical average load of 70 per cent and no overload capability. A prime rating applies to unlimited hours at variable load, again with an average around 70 per cent, and permits a 10 per cent overload for one hour in twelve. A continuous rating applies to unlimited hours at constant full load with no overload. The standby figure is usually about 10 per cent above the prime figure, so a set sold as 500 kVA standby is roughly a 455 kVA prime machine. Applying a standby-rated set to daily peak shaving or construction-site prime duty shortens its life and voids the basis on which it was rated. The structural choices behind each rating are described under generator engine architecture.
Why diesels dislike light load
A generator engine runs best between about 50 and 80 per cent of its rating. Below roughly 30 per cent for extended periods, cylinder temperatures are too low for complete combustion, unburnt fuel and soot pass the rings into the oil and coat the exhaust, and wet stacking develops: oily deposits in the manifold and turbine, visible as black drips at exhaust joints. Injectors coke, the turbocharger loses efficiency and the engine eventually cannot reach full power when asked. The remedy is operational before it is mechanical: load the set properly during tests, size it against measured demand rather than connected load, and schedule periodic runs at 75 per cent or more. Where light load is unavoidable because a large set protects a small critical load, a load bank or a smaller companion set are the engineering answers.
Indian operating conditions
Three site factors change what an engine delivers in Telangana compared with its datasheet. Ambient temperature above the reference reduces air density and the cooling margin, and sets lose a few per cent of output in May and June. Altitude reduces density further, by roughly 3 to 4 per cent per 300 metres for naturally aspirated engines and less for turbocharged ones with margin, which matters on Deccan plateau sites above 600 metres. Fuel quality decides injector life: CPCB IV+ engines use high-pressure electronic injection and aftertreatment that depend on clean, water-free BS VI diesel, and tanks that collected monsoon water for years without consequence on older mechanical engines will damage the new ones within months. Engine service for all of this, from routine to major, is covered under generator engine service, with the supporting components under generator engine parts.
Related photographs and videos
Common mistakes
- Comparing quotations without noting the rating class; a standby-rated set looks 10 per cent cheaper per kVA than a prime-rated one and is a different product.
- Running a standby set unloaded for its monthly test; wet stacking develops and the set cannot deliver full power in a real outage.
- Ignoring the supporting systems between services; a blocked radiator or a water-laden tank stops a mechanically sound engine.
- Feeding an electronic, CPCB IV+ engine from an old, unmaintained tank; injectors and pumps fail long before their design life.
What to send us
Send the engine nameplate photograph with model and serial number, the set nameplate, the hour reading, the hours run in the last year and the typical load, the site altitude and summer ambient, and a description of any symptom such as smoke, loss of power, high temperature or hard starting. If the fuel tank has not been cleaned in two years, say so. We will advise on service, load correction or further diagnosis. Send your engine details for service or assessment.
Questions engineers ask
Why do generator engines run at 1,500 rpm?
Because a four-pole alternator produces 50 Hz at exactly that speed, and four poles is the practical compromise between alternator size and engine speed for industrial sets. Small portable sets use two-pole alternators at 3,000 rpm, which is cheaper but noisier.
What is the difference between standby and prime rating?
Standby is the maximum output for the duration of a power failure, with limited hours and no overload. Prime permits unlimited hours at variable load with a 10 per cent overload for one hour in twelve. The standby figure is typically about 10 per cent above the prime figure for the same engine.
What is the minimum load a diesel generator should run at?
Manufacturers generally advise keeping sustained load above about 30 per cent of rating and running at 75 per cent or more periodically. Below that, incomplete combustion causes wet stacking and injector coking.
How much does ambient temperature reduce generator output?
It depends on the engine and its reference conditions, but a few per cent of output is typically lost for each 10 °C above the rating reference, and the cooling margin shrinks at the same time. Confirm the derating curve for the specific engine rather than assuming the datasheet applies at 43 °C.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.