Diesel genset smoking black at part load
A four-cylinder, four-stroke diesel genset drives a garment factory load. Bore 100 mm, stroke 115 mm, compression ratio 17.5, rated output 60 kW at 1500 rpm, specific fuel consumption 230 g/kWh, calorific value of the diesel 42 MJ/kg. The set runs at a site where ambient temperature reaches 42 °C, and the air filter element has not been changed for 2,000 hours. Operators report black smoke at about 70% load and an exhaust manifold temperature 40 °C above the commissioning record. A local mechanic has already opened up the fuel rack to "recover the lost power". Over a two-hour test at 50 kW on the resistive load bank, the set consumes 26 kg of fuel. The engine is naturally aspirated. The workshop holds a spare air filter element, a nozzle tester and a contact pyrometer, but no dynamometer beyond the 50 kW load bank.
1. Calculate the total swept volume, the brake mean effective pressure at the rated output, and the rated brake thermal efficiency.
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Swept volume per cylinder: V_s = πd²L/4 = π × (0.100)² × 0.115/4 = 9.03 × 10⁻⁴ m³ = 0.903 L.
Total for four cylinders: 4 × 0.903 = 3.61 L. (Compression ratio 17.5 gives a clearance volume per cylinder of 0.903/16.5 = 0.0547 L, useful for later checks but not needed here.)
BMEP at rated output (four-stroke): P = BMEP × V_s,total × N/(2 × 60), so BMEP = 2 × 60 × P/(V_s,total × N) = 2 × 60 × 60,000/(3.61 × 10⁻³ × 1500) = 7,200,000/5.415 = 1.33 × 10⁶ Pa ≈ 13.3 bar.
A naturally aspirated diesel at 13.3 bar BMEP is already highly rated - near the practical ceiling without turbocharging. That matters for the diagnosis: there is very little air margin to trade away.
Rated brake thermal efficiency: bsfc 230 g/kWh = 0.230 kg/kWh. Fuel energy per kWh = 0.230 × 42 = 9.66 MJ. Since 1 kWh = 3.6 MJ, η_bth = 3.6/9.66 = 0.373, i.e. 37.3%.
This is a normal modern diesel figure and gives a reliable benchmark for judging the field test in the next part.
2. From the 50 kW test data, compute the actual brake thermal efficiency and specific fuel consumption, compare with the rated figures, and state what the gap tells you.
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Test figures: Fuel rate = 26 kg/2 h = 13.0 kg/h.
bsfc = 13.0/50 = 0.260 kg/kWh = 260 g/kWh (rated 230 g/kWh, so 13% worse).
Fuel energy rate = 13.0 × 42 = 546 MJ/h = 546,000/3600 = 151.7 kW.
η_bth = 50/151.7 = 0.330, i.e. 33.0% against a rated 37.3%.
Interpretation. A 4.3-percentage-point drop in brake thermal efficiency at only 50 kW - about 83% of rating - is significant but not catastrophic, and it is consistent with an air-side problem rather than a fuel-side one:
- The efficiency loss is roughly 13% in fuel per kWh, which is the order expected from 10–15% loss of trapped air mass. Air density falls from about 1.18 kg/m³ at 27 °C to 1.12 kg/m³ at 42 °C (about 5%), and a loaded 2,000-hour filter element can add another 5–10% pressure drop.
- Because the engine is naturally aspirated and already at 13.3 bar BMEP, it has no reserve: the fuel rack has been opened, but no extra oxygen exists to burn that fuel. Extra fuel without extra air produces exactly the observed black smoke (unburnt carbon) and the 40 °C rise in exhaust manifold temperature (afterburning in the manifold).
- The measured efficiency loss confirms the set is over-fuelled relative to air, not that the injection pump has simply "lost power".
Note also that the test was at 50 kW, where bsfc is usually slightly better than at full load; measuring worse than the rated full-load figure strengthens the diagnosis.
3. Give the order in which you would investigate and correct the problem, justify why opening the fuel rack is the wrong first move, and state what should not be done without a dynamometer.
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Order of work - cheapest and most probable first:
- Air filter element. Fit the spare element and record the restriction before and after. This is a 10-minute, zero-risk action on a 2,000-hour element at a dusty site.
- Inlet and ambient. Check for recirculation of hot air from the radiator, blocked louvres and duct restrictions. At 42 °C ambient, ensure the radiator and charge-air path are clean; confirm the set is not drawing its own exhaust.
- Injection nozzles. Pull the injectors and test on the nozzle tester for opening pressure, spray pattern, dribble and leakage. A dribbling nozzle gives black smoke and high exhaust temperature independently of the rack setting.
- Fuel rack and pump setting. Only after the air side and nozzles are proven good should the rack setting be checked against the maker's marks; restore it to the original setting, since the mechanic moved it without instrumentation.
- Load verification. Re-run the 50 kW test and re-compute bsfc (should approach 230–240 g/kWh) and exhaust temperature before declaring the set healthy.
Why opening the rack is wrong: the smoke and the temperature rise are symptoms of too much fuel for the available air, so adding fuel attacks the symptom and worsens both the smoke and the thermal load on pistons, valves and the manifold. It also destroys the reference setting, so later diagnosis loses its baseline. Power "lost" at high ambient is an air-density effect, and it is recovered by restoring airflow and, if genuinely necessary, by derating the set - not by over-fuelling.
Do not attempt to re-set the rack or the pump phasing by ear or by smoke colour without a dynamometer; a load bank limited to 50 kW cannot verify full-load delivery, so the pump should go to a calibrated injection test bench, or the set should be re-commissioned with a proper load test. Until then, log exhaust temperature and smoke opacity as the acceptance indicators.