Diesel generator consumption formula

Question

I am working on a project to monitor a power meter parameters like voltage, frequency, current and so on and this monitoring for both AC and DC. For DC we are using Cummins diesel generator, now I need to calculate the generator running hours based on the load.

For example, I have a generator with 10 kVA capacity which consumes the diesel as follows:

On 1/4 load the generator will take 1.5 ltr/hour

On 1/2 load the generator will take 2 ltr/hour

On 3/4 load the generator will take 3 ltr/hour

On Full load, the generator will take 5 ltr/hour

How to calculate these 1/4, 1/2, 3/4 and full load from the power meter parameters? And how can I calculate the diesel consumed by the generator using the same parameters?

Answer 1

For the best estimate of fuel consumption, you should use the power in kilowatts (kW), not apparent power in kVA. Most DGs have three-phase output for which power is equal to voltage X current X square root of 3 X power factor. Since it is difficult to measure power factor (phase difference between voltage and current) you should use a three-phase wattmeter.

The DG manufacturer may assume that the average load power factor is 0.8 or higher. You could assume that pf is 1.0 and say that power = voltage X current, but that would make your fuel consumption estimate high. You could measure the line to neutral voltage and the current for each phase and then add the three power calculations to estimate power.

Say that you have a 10 kVA generator that has a 3-phase, 400 volt output that has connection points to provide three single phase 230 volt supplies. Each supply would be rated 3.3 kVA and be capable of 3300/230 = 14.3 amps. If you connect an assortment of lighting and heating appliances that draw 8, 11 and 13 amps, you would have 8 X 230 + 11 X 230 + 13 X 230 = 7360 watts or 7.36 kW. That would be about 3/4 load, so the fuel consumption would be about 3 liters per hour.

Answer 2

A very simple means of calculation will be as follows.

• 1 litre of fuel oil can release about 10 kWh of energy. (Wikipedia's Energy density page says 10.7 kWh/L.
• You state that your generator consumes 5 L/h at 10 kVA. Let's assume that 1 kVA = 1 kW for the moment. That means that 1 litre of fuel is giving out only 2 kWh of electrical energy. The energy conversion is only 20%! This may be reasonable for a small generator as efficiency can increase with scale.

In summary:

• 1 L/h --> 2 kWh
• 1 kWh --> 0.5 L/h

Now let's compare with your figures with our simple rule of thumb above.

Load     kVA     Manufacturer  Rule of thumb
1/4       2.5    1.5 L/h       0.5 x 2.5 = 1.25 L/h
1/2       5.0    2.0 L/h       0.5 x 5.0 = 2.50 L/h
3/4       7.5    3.0 L/h       0.5 x 7.5 = 3.75 L/h
4/4      10.0    5.0 L/h       0.5 x 10  = 5.00 L/h

If you plot these on a chart the rule of thumb will give you a straight line whereas your manufacturer's figures will form a curve with fuel consumption being disproportionately higher at low load due to further inefficiencies.

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I would be inclined to dip the tank and take a meter reading at the start of operations and again at before the tank requires a refill and work out an average fuel consumption per kWh.

If an exact measurement is required then add an in-line fuel meter. Check your electrical energy meter to see if it has a pulse input that you could use as many have for gas / water monitoring.