Engine Performance and Efficiency

I would like to discuss the parameters which can/should be followed to help improve the aircrafts engine performance, improving the overall flight. I will be using information which has been acknowledged by me reading: Rolls Royce – The Jet Engine.

Ambient temperature, turbine entry temperature, turbine operating temperature (at various stages), pressure rise through the compressor, airflow, fuel flow, bypass ratio, drag, acceleration, and deceleration: The number of varying conditions that influence an engine’s performance is almost incalculable.

Performance is the thrust or shaft power delivered for a range of given parameters: Fuel Flow, Life, Weight, Emissions, Engine Diameter and Costs. Performance engineering has two pivotal roles: first, it ensures stable engine operation throughout the operational envelope, under all steady state and transient conditions; second, it integrates component technologies so that the product attributes critical to the end user, are optimised for any given application.

Performance is critical to all phases of gas turbine design, development, and operation. It is also a significant part of what a gas turbine manufacture sells and the operator buys. The operating condition where the engine will spend most of its time has traditionally been chosen as the engine design point. For a long-range, civil airliner, this would be its cruise condition, typically 35,000ft, Mach 0.82 to Mach 0.85 on a standard (ISA) day. It is primarily at this operating condition that the engine performance, configuration, and component design are optimised, through the latter two are heavily influenced by more arduous flight conditions.

A number of design point performance parameters can be used to give an initial or first order indication of the engine weight, frontal area, and volume for a given thrust. Also specific fuel consumption is the fuel flow rate divided by the output thrust or power. For long-range, civil aircraft engines, a low specific fuel consumption is critical as the cost of fuel is typically 15 to 25 per cent of aircraft operating costs.

There are number of gas turbine cycle parameters that have a powerful effect on the specific fuel consumption and specific thrust or power. For a turbojet, these are compressor pressure ratio and turbine entry temperature.

Specific thrust improves dramatically with turbine entry temperature, and the optimum pressure ratio is about 8:1 at low turbine entry temperature and 15:1 for high turbine entry temperature. Conversely, specific fuel consumption gets worse as turbine entry temperature is increased but improves as pressure ratios become higher.

The concept designer must, thus make a compromise between achieving the best specific fuel consumption or specific thrust when choosing the cycle parameters. Many other limitations must also be considered including the complexity of engine design resulting from a very high pressure ratio and the mechanical integrity limitations of going to a very high turbine entry temperature. As component efficiencies improve, so do the absolute levels of both specific thrust and specific fuel consumption, but the fundamental shape of the design point diagrams does not change.

Transient performance covers operating regimes where engine parameters are changing with time. Engine operating during transient manoeuvres is often referred to as handling operability. In particular, avoiding engine instabilities such as compressor surge, where the flow in the compressor reveres violently, or combustor weak extinction must be balanced with achieving the engine acceleration and deceleration times required by the application.

Performance parameters vary during a slam acceleration or slam deceleration. During an engine acceleration in response to a step change in throttle demand, the control system increases fuel flow; turbine entry temperature and turbine output power. This higher turbine output power exceeds that required both to drive the compressor and auxiliaries and also to overcome mechanical losses. The excess power is available to accelerate the shaft with the result that airflow, pressures, and temperatures through the engine all increase. This acceleration continues until the steady state condition corresponding to the new throttle setting is reached. The opposite of this process occurs during deceleration.

It is a characteristic of gas turbine engines that the high pressure turbine is usually choked for all operation above idle operation, and, during an acceleration, there is a tension between putting enough over-fuelling to achieve the required acceleration time but not surging the high pressure compressor. It is good to note that retaining engine rotational speeds, temperatures and pressures below mechanical limits means that, the engine cannot operate up to 100 percent referred speed at all flight conditions. The engine control system must be set up to govern, or rate, the engine at key flight conditions so that sufficient thrust is provided but mechanical integrity limits are not exceeded.

In conclusion, it all comes down to the design, size, power, systems, materials, fuels used etc. which affects an aircraft’s engines performance. If we are able to increase the by-pass ratio, use better light-weight/high performance materials, more advance control systems/technology and develop more efficient and effective fuels and lubrications, we are able to increase the efficiency and also input to output performance of an aircraft engine.

It is vital that in the near future, although engines are becoming more efficient, we do look into the mentioned aspects from above and strive more into powerful and efficient engines.