The jet and turbo fan engines that power most modern aircraft are extraordinarily complex and highly developed pieces of engineering For an aircraft to fly cheaply and safely requires an engine that is light in weight powerful in thrust cheap in maintenance. and so reliable that it will hardly ever fail in service. All of this must be achieved under a wide range of hostile operating conditions. Aircraft have to take off in -40ºC of frost and +40ºC of desert heat They have to fly high up where the air is very thin as well as they do at ground level. They have to be able to withstand the injection of one or more large birds at high speed. If they fail they must fail safely.
In terms of reliability and compact power a car engine is hardly comparable. One jet engine turbine blade that would fit into a cigarette packet will produce more power than a typical car engine. A jet engine can travel three million miles before a major overhaul in comparison to about one hundred thousand miles for a car engine.
The final design of a jet engine involves the bringing together of many different engineering disciplines. Materials engineers develop new materials that can withstand the high temperature and loads within the engine and stress engineers perform complex calculations to predict the performance of shafts discs and blades that make up the engine. Aerodynamicists predict the flow patterns of the air passing though the engine so that losses can be minimised and thrust maximised. Heat transfer engineers design the cooling system for the hot parts of the engine using cooling air bled from the cold part. Combustion engineers design combustion chambers so that a stable efficient flame is held in the combustion can.
All of these diverse engineering disciplines are taught in a typical mechanical engineering degree programme. This point cannot be made too strongly because people very often do not know what mechanical engineers do. It is fairly obvious what lawyers, architects and medical doctors do but mechanical engineering is, in comparison, a hidden profession. Part of the reason for this is that mechanical engineers do so many things. Many mechanical engineers move swiftly into management positions where only a small fraction of their time is spent on technical matters, so you will also find mechanical undergraduates studying trade unions, law, management, accountancy and economics.
Mechanical engineering is then a very broad subject and within it there are many specialisations. Just like the medlc who specialises in ears, noses and throats, the engineer may specialise in, for instance, any of the subjects I listed when describing some of the engineering elements of a jet engine.
I specialise in what is broadly called thermofluids: that is I claim to know a little about aerodynamics, fluid mechanics, heat transfer and thermodynamics. I also know something about many other engineering disciplines, but actually not enough to make a big fuss about it. I've specialised in thermofluids simply because it interests me and the more I study it the more it interests me.
At UL my time is divided between teaching and research. I teach some basic and some advanced courses in thermofluids to the mechanical and aeronautical engineers. My research time at the moment is mostly taken up with investigating the thermofluids of the jet engine.
I and three other co-workers in the Department of Mechanical and Production Engineering are involved in developing instrumentation systems for two large scale engine test facilities that will be built in Belgium and Germany over the next three years. Just as engine designers co operate, so do research workers. In this project there are some 15 partners, engine manufacturers and Universities, from across Europe co-operating to take a set of measurements of velocities, pressures, Mach numbers and heat transfer rates that have never been taken before. Our aim is to help those engineering design teams that I described earlier.
Much of the design of an aircraft engine is performed on a computer terminal using a vast array of different programmes to predict how both the individual components and the whole engine system will perform. If the designer is to believe the results from these programmes they must be first tested against experimental evidence. This evidence must be gathered under conditions as close to those found in the engine as possible This is where the difficulty lies. Engines are very hostile places to put instrumentation. We cannot instrument a real englne well enough, so instead we instrument a model. That is the purpose of my research project.
It is great work. I travel and work in a team; the problems are complex, taxing, sometimes highly matematical and sometimes highly practical. It is also very important that they are solved as people's lives depend on it. For those readers who hold a particular prejudice, I never, ever get my hands dirty.
If you have an interest in either mechanical or aeronautical engineering try to find out some more about it, and if it still interests you and you do well enough at school we will be very happy to have you study it with us at the University of Limerick.
Dr. Mark Davies is a lecturer in Mechanical Engineering at UL. He is involved in aero engine research.
