The SI Unit of Specific Fuel Consumption

SFC = fuel mass flow rate / power output

SFC = (kg/s) / W = kg / (W·s) = kg/J

#Why kg/J is impractical, and what is used instead

The SI unit produces awkward numbers. A good diesel engine has an SFC of around 0.00000006 kg/J — six followed by eight leading zeros, which nobody can read at a glance.

Engineering practice therefore uses grams per kilowatt-hour (g/kWh):

1 g/kWh = 1 x 10^-6 kg / 3.6 x 10^6 J = 2.78 x 10^-13 kg/J

Typical values become readable:

Engine type	Typical BSFC (g/kWh)
Large marine two-stroke diesel	155–175
Modern automotive diesel	200–250
Modern petrol engine	250–350
Petrol engine at part load	350–500
Small two-stroke petrol	400–600
Gas turbine (simple cycle)	250–350

Older texts and US practice may use lb/(hp·h); 1 lb/(hp·h) ≈ 608 g/kWh.

#Brake, indicated and other variants

The measurement point matters:

• Brake specific fuel consumption (BSFC) — measured against brake power at the output shaft, i.e. useful work delivered. This is the standard comparative figure.
• Indicated specific fuel consumption (ISFC) — measured against indicated power developed inside the cylinder, before friction and pumping losses. Always lower than BSFC.

The difference between them is the mechanical loss in the engine.

#The link to efficiency

SFC and thermal efficiency are two views of the same quantity:

thermal efficiency = 1 / (SFC x calorific value)

Worked example. A diesel engine with BSFC = 200 g/kWh, fuel calorific value 43 MJ/kg:

Fuel energy per kWh = 0.200 kg x 43,000 kJ/kg = 8,600 kJ
Output per kWh      = 3,600 kJ
Efficiency          = 3,600 / 8,600 = 41.9%

So a BSFC of 200 g/kWh corresponds to roughly 42% thermal efficiency — about right for a good modern diesel.

This relationship is why SFC is the more useful workshop number: it is measured directly from fuel mass and shaft power, without needing to assume anything about the fuel.

#Jet engines: thrust specific fuel consumption

An aircraft engine in cruise produces thrust, not shaft power, so the denominator changes:

TSFC = fuel mass flow rate / thrust

Dimensionally kg/(N·s) in SI, usually quoted as kg/(N·h) or, in US practice, lb/(lbf·h).

Engine	Approx. cruise TSFC
Early turbojet	0.10–0.12 kg/(N·h)
Low-bypass turbofan	0.075–0.085 kg/(N·h)
Modern high-bypass turbofan	0.055–0.065 kg/(N·h)

The roughly 45% improvement from early turbojets to modern turbofans is most of the reason air travel became affordable.

#Why the metric matters

Comparing across sizes. A ship engine burns thousands of times more fuel per hour than a car engine and is more efficient. Only a specific measure reveals that.

Operating cost. For an airline or shipping line, fuel dominates operating cost, so a few percent in SFC decides fleet purchases.

Design and tuning. Engineers map BSFC across the whole speed-load range. The resulting contour map — the "BSFC island" — shows where an engine is most efficient, and hybrid powertrains are controlled specifically to keep the engine inside that island.

#Related but different measures

Do not confuse SFC with these:

• Fuel economy — distance per volume (km/L, mpg). A vehicle measure, not an engine measure.
• Fuel consumption rate — volume or mass per hour (L/h). Says nothing about output.
• Heat rate — for power stations, energy input per unit electrical output (kJ/kWh or BTU/kWh).

SFC is the engine-level measure that removes both size and fuel type from the comparison.

The related property for diesel fuel quality is cetane number, and for the fuel itself, calorific value — both appear in the efficiency formula.

#The compact answer

Specific fuel consumption is the mass of fuel consumed per unit of energy produced. Its SI unit is kg/J, equivalently kg/(W·s). In practice it is expressed in g/kWh for engines, and as thrust specific fuel consumption in kg/(N·h) for jet engines. A lower value indicates a more efficient engine.