A diesel genset's fuel use depends mainly on how much power it delivers and how efficiently its engine converts fuel. Ask the builder for specific fuel consumption at several loads, then convert it to litres per hour for your own load profile.
Fuel figures in brochures are often best-case numbers. A test record from your actual sets is worth more.
The two units you will see
Specific fuel consumption, in grams per kilowatt-hour (g/kWh), measures engine efficiency. Lower is better. Engine makers publish it at different loads, usually from their own test bed under reference conditions.
Litres per hour (L/h) is what your customer pays for. To convert, multiply power in kW by g/kWh to get grams per hour, then divide by diesel density. Diesel density is typically around 0.83 to 0.85 kg per litre.
An illustrative example
These numbers are for illustration only, not a promise for any engine. Suppose a set delivers 80 kW and the engine uses 230 g/kWh at that load. It burns 80 × 230 = 18,400 g, or 18.4 kg, per hour. At a density of 0.84 kg per litre, that is about 22 litres per hour.
Remember that the engine must also cover alternator losses. If the brochure gives g/kWh at the engine shaft, the fuel per kWh of electricity delivered is somewhat higher. Ask the builder whether its figure is per kWh of engine output or per kWh of electrical output.
To estimate a day's fuel, repeat the sum for each load your customer runs and multiply by the hours at that load. Add the results for a daily total.
Why part load matters
Diesel engines are most efficient near their upper load range. At light loads the specific fuel consumption rises, so each unit of electricity costs more fuel. A set that runs at 25% load most of the day is oversized and wastes fuel.
Long periods at very light load also cause wet stacking, where unburnt fuel and oil build up in the exhaust. Sizing the set to the real load, as covered in genset kVA ranges, solves both problems.
Field conditions push fuel use above test figures. These factors are the most common:
- High site temperature and altitude, which reduce engine efficiency.
- Dirty air filters and worn injectors, which waste fuel and add smoke.
- Poor-quality or contaminated diesel.
- Unbalanced loads across the three phases.
- Frequent starts and long idling between loads.
What to ask for
- Specific fuel consumption at 25%, 50%, 75% and 100% load, stated on a clear basis.
- The reference conditions and fuel used for those figures.
- Fuel readings recorded during the genset load test on your sets.
- Fuel tank size and the running time it gives at your expected load.
Next step
We record fuel use during witnessed load tests so you can check brochure claims. To compare sets on running cost, book a free sourcing review.