Question 6 of 7: Discussion – Gas Turbines, Propeller Blade Count, and Engine Pressures

Question 6: Discussion – Gas Turbines, Propeller Blade Count, and Engine Pressures (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Ambient conditions and gas-turbine performance. A gas turbine is an air-breathing constant-volume-flow machine whose power output and efficiency are strongly set by the mass flow (hence density) of the air entering the compressor. Rising ambient temperature lowers air density, so the compressor ingests a smaller air mass for the same volumetric flow; this directly cuts the mass of working fluid available to generate power and also raises the compressor work per unit mass (since compressor work scales with inlet temperature), so both output power and thermal efficiency fall on a hot day – typically 0.5–1% loss in output per °C rise above the ISO rating point (15°C). Rising ambient pressure/altitude has the same effect through density: less power at higher elevation or lower barometric pressure. Higher ambient humidity slightly reduces power (moist air has a lower density than dry air at the same temperature and pressure, and changes the working fluid's specific heat) but the effect is much smaller than the temperature effect. This sensitivity to ambient conditions is exactly why gas-turbine ratings are always quoted at standard ISO conditions (15°C, 101.325 kPa, 60% RH) with correction curves supplied for site conditions, and why marine gas turbines intended for tropical service are commonly de-rated or given water/steam injection to recover lost mass flow.

(b) Propeller blade number and shaft loading. A propeller generates thrust and torque that are not perfectly steady but pulse at blade-passing frequency (shaft rotational frequency × number of blades) because each blade in turn passes through the non-uniform wake behind the hull (the wake is weakest near the keel and strongest close under the stern where the boundary layer has thickened, and is further disturbed by the rudder and any bossings/shaft brackets). With more blades, the same total thrust and torque variation is spread over more, smaller pulses per revolution: the blade-passing frequency rises (so it is easier to keep clear of the shaft's torsional/whirling natural frequencies) and, because more blades average over more of the wake's circumferential variation in a given instant, the peak-to-peak amplitude of both the thrust variation and the resulting vertical bending moment on the shaft is reduced. Conversely a low blade count (3 or 4 blades) concentrates the wake-induced loading into fewer, larger impulses, giving higher-amplitude, lower-frequency thrust and bending fluctuations – more likely to coincide with a shaft-line natural frequency and more prone to vibration and fatigue. This is one reason ships with pronounced stern wake non-uniformity (single-screw, full-form hulls) often favour higher blade counts (5–7) despite the small efficiency penalty.

(c) Mean effective pressure vs mean indicated pressure. The mean indicated pressure (MIP, usually written IMEP) is derived directly from the measured in-cylinder pressure-volume diagram (the "indicator diagram"): it is the constant pressure that, acting over one stroke, would produce the same net indicated work as the actual pressure trace, i.e. $\text{IMEP}=W_{indicated}/V_{swept}$, and it reflects the actual combustion process inside the cylinder before any losses are removed. The mean effective pressure (commonly meaning BMEP, brake mean effective pressure) is instead computed from the power actually delivered at the crankshaft/output flange (measured by a dynamometer or, at sea, inferred from shaft torque), $\text{BMEP}=W_{brake}/V_{swept}$, and is therefore always lower than IMEP because it has friction, pumping and other mechanical losses subtracted out (mechanical efficiency $\eta_m=\text{BMEP}/\text{IMEP}$, typically 85–90% for a well-maintained marine diesel). In short: IMEP describes the thermodynamic process happening inside the cylinder; BMEP describes what actually reaches the output shaft, and the gap between the two is exactly the engine's own friction and auxiliary power.