Diesel genset with black smoke and high fuel consumption
A rice mill runs a three-phase diesel generating set rated 25 kVA, 415 V, 1500 rpm, directly coupled to an alternator of about 90 per cent efficiency. During a four-hour evening shift the set carried an average electrical load of 15 kW at 0.8 power factor. The operator recorded 22.4 litres of diesel consumed in that shift and complained of black smoke, a hot exhaust manifold and a hunting note whenever the husk mill motor starts. The engine air cleaner is an oil-bath type last serviced 400 hours ago, the injectors were changed 800 hours ago, and the fuel comes from a local dealer. Diesel density is 0.84 kg/L and calorific value 42.5 MJ/kg; the manufacturer's specific fuel consumption is 245 g/kWh at rated load. The owner asks whether the engine needs an overhaul or whether the problem lies in the fuel or the load.
1. Calculate the specific fuel consumption and thermal efficiency from the recorded data, making the correct allowance for the alternator, and state whether the engine is over-fuelling.
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Fuel rate = 22.4/4 = 5.6 L/h → 5.6 × 0.84 = 4.704 kg/h. Fuel energy = 4.704 × 42,500 = 199,900 kJ/h = 55.5 kW.
Naive calculation on electrical output: bsfc = 4,704/15 = 314 g/kWh, efficiency = 15/55.5 = 27.0%.
Correction for the alternator: the 15 kW is electrical. At ~90% alternator efficiency the engine shaft output is about 15/0.90 = 16.7 kW, so engine bsfc = 4,704/16.7 = 282 g/kWh and engine efficiency = 3,600/(0.282 × 42,500) = 30.0%.
Against 245 g/kWh at rated load, and perhaps 255–265 g/kWh expected at 60% load, the engine is running 7–10% over-fuelling - a real but not catastrophic excess. This is not enough on its own to justify an overhaul. The complaint is about smoke and hunting, so fuel and load causes must be examined before wear is blamed.
2. Diagnose the black smoke at 60 per cent load and the hunting when the mill motor starts. Is an overhaul the right response?
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Black smoke at 60% load means unburnt carbon - too much fuel for the air available. Work through, cheapest first:
- air cleaner (oil bath overfilled or sludge-blocked; 400 h is overdue for cleaning);
- collapsed or restricted intake pipe;
- injector dribble or wrong pop pressure (800 h since replacement);
- retarded injection timing;
- blocked fuel filter or water in fuel.
Hunting on motor start is a separate problem. A 15 kW squirrel-cage motor started direct-on-line draws 5–7 times full-load current, roughly 160–200 A. The set is 25 kVA, about 20 kW at 0.8 pf, so starting that motor DOL is like adding 50–60 kVA of load. The engine lugs, the governor opens the fuel rack fully (black smoke), and recovery produces the hunting note.
Verdict: do not overhaul. Fit a star-delta or soft starter on the husk mill motor, service the air cleaner, check governor droop and fuel-rack freedom, and re-test. Overhauling would not correct a starting-current problem and would waste the shutdown.
3. What fuel-quality checks would you insist on, and what is wrong with blending kerosene to stretch the diesel?
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Checks: cetane number (≥45 for HSD; low cetane gives long ignition delay, white smoke and harsh running); sulphur (BS-VI HSD is 10 ppm, high sulphur adds wear and sulphate deposits); density and distillation range (heavy ends smoke and coke the nozzles); water and sediment, checked by drawing a tank-bottom sample into a clear jar; and filter plugging point where nights are cold. Keep a fuel log with supplier, batch and density so a bad consignment can be traced.
Kerosene blending is harmful and unlawful: it lowers lubricity and viscosity, so the injection pump plungers and nozzles wear; it reduces cetane and raises ignition delay, worsening the knocking and smoke; it delivers less energy per litre, so the operator burns more, not less; it lowers the flash point, creating a fire risk in storage; and blending without excise permission is illegal.
Better measures: fit a water separator, change the fuel filter on schedule, and clean the tank - a dirty tank defeats any fuel specification.