Specific fuel consumption:

The specific fuel consumption, SFC, is a very essential parameter in describing the engine’s performance. Usually an engine with low SFC is considered to be an efficient engine. The SFC was one of the main criteria the engine choice for the ALBA 430 was based upon. When comparing the SFC for the LET 420 at maximum takeoff rating, 0.383 kg/kW.hr to the SFC of the newly installed engine on the ALBA 430 for the same engine rating, 0.309 kg/kW.hr, it is observed that the SFC for newly installed engine is noticeably lower than its forerunner, a reduction of approximately 20% in SFC is experienced which implies that new engine is less fuel consuming and hence more efficient and more sustainable.

The SFC for the new engine at different ratings,

Rating SHP [kW] SFC [kg/kw.hr]MAX. TAKE-OFF 1062 0.309

ALERNATE TAKE-OFF 918 0.316

MAX. CONTINOUS 910 0.317MAX CLIMB

CRUISE 956 0.335

SFC is optimized for maximum range and endurance, for a turboprop engine. SFC decreases with altitude and maximum range is achieved at service ceiling. Most turboprops will achieve maximum endurance at less than 10,000 feet.

The SFC is affected by various factors such as; engine power setting, operating altitude and compressor fouling and turbine wear.

1- Engine power setting:

The fuel flow increases at a higher engine setting but the SFC decreases slightly. This happens as the engine is approaching its design point and is consistent. Note that for turboprop engine to be efficient they have to be operated at maximum rpm.

2- Operating altitude:

As the altitude increases the temperature drops and density is decreased which results in lower SFC at higher altitudes.

The minimum specific fuel consumption of the turboprop engine is normally available in the altitude range of 25,000 feet to the tropopause with Mach numbers below 0.7.

Turboprop engines are more efficient when air temperature is cold at high altitudes.

3- Compressor fouling and turbine wear:

In case of compressor fouling, the power output and fuel flow decrease due to frictional effects and hence resulting in an increase in SFC.

For the turbine wear, the fuel flow stays constant while the power output experiences a reduction.

Therefore, the engine fuel efficiency can be improved by avoiding compressor fouling conditions and ensuring turbine wear is minimized.