The gas turbine engine is unarguably one of the most complicated pieces of high tech machinery in the world. However, like many other deeply complicated and technical topics, the basics of gas turbine engine operation can be grasped by most people with a modest technical background.
I need to confess right up front that I am not a deep engine technologist, but I have had the extreme good fortune to work with a number of such technologists at Honeywell’s Engine facility in Phoenix, Arizona, and they have taught me more than I would have thought possible. It is due to their effort and patience that I am able to share the basics of how a gas turbine engine operates with you.
What Happens at the Core: Suck, Squeeze, Bang and Blow
In the simplest sense, a gas turbine engine compresses air in the Compressor (Suck and Squeeze), mixes that compressed air with fuel and ignites it in the Combustor (Bang), and then takes the resulting high pressure gas and uses the energy in this gas to spin one or more Turbines (Blow).
The turbines are attached to a shaft that rotates as the turbines spin. This shaft can then be used to do work. Unlike an automobile engine, which only generates periodic “explosions” in its cylinders, a gas turbine engine maintains a standing flame in its Combustor.
One of the work tasks that the turbine-driven shaft does is to spin the compressor once the engine is running. However, since we add a great deal of energy to the system by burning fuel, the turbine-driven shaft has excess energy even after driving the compressor, and it is this excess energy that allows the gas turbine engine to do a variety of useful work tasks, as shown below. These tasks are usually characterized by what we connect to the spinning shaft. Below are the typical uses for this spinning shaft:
Connect the shaft to a big fan to generate thrust a Turbofan engine
Connect the shaft to a propeller through a gearbox a Turboprop engine
Connect the shaft to anything else (e.g. a helicopter rotor) a Turboshaft engine
Driving the Load
As engine designers attempt to generate greater levels of power, they often require multiple stages of compressors and turbines.
When this happens, it is often more efficient to spin the lower pressure compressor and turbine components at lower speed than the higher pressure components.
Pressure in the compressor increases from left to right, and since we are extracting pressure energy from the ignited gas as we progress through the turbine section, the pressure drops in the turbine section as we go from left to right.
How a Honeywell Turbine Engine Works
The diagram above illustrates Honeywell’s HTF7000 engine, an engine which generates approximately 7,000 lbs. of thrust for a variety of business jet aircraft.
In this engine there are two rotating shafts; the first is a high speed shaft connecting the compressor with the high pressure turbine stages. Note that the compressor is doughnut shaped, so the shaft goes through the middle of the compressor. The low pressure turbine stages are connected to the thrust-generating fan via a low speed shaft. This is concentric with the high speed shaft, which is hollow. The low speed shaft rotates separately within the high speed shaft.
Shown above is a cutaway picture of an actual HTF7000 engine. The fan is at the extreme left in the figure but is not visible. The compressor section is blue, and the hot sections of the engine (combustor and turbine sections) are orange.
The design of a gas turbine engine starts with understanding the power requirements the engine must deliver; i.e. how much work the engine needs to do on the driven load. Then, as with all engineering, the magic lies in trading off various engine attributes to achieve the optimal design. Key design attributes of a gas turbine engine are:
High Thrust to Engine Weight
Low Emissions (CO2, NOx, CO, UHC)
Low Cost of Ownership (Acquisition cost and lifecycle maintenance cost)
Reduced Engine Noise
This is a notional list; the full list of key design attributes will vary based on the engine application and customer requirements.
So, those are the basics. The last item I want to mention is the importance of aerodynamic modeling to modern engine design. Understanding the airflow patterns, speeds, pressures, and temperatures at all points in the engine is essential to an efficient design.
With today’s computer modeling tools, we are able to do very accurate modeling before we ever “cut metal” to build prototype engine components. This greatly reduces the cost and cycle time for engine development. Prototypes are extensively tested when built; the resulting data is compared to the engine aerodynamic models. When there are differences, we can use the actual test data to update the modeling tools, thereby making future modeling more accurate.
I hope this brief overview of gas turbine engines has left you with two things: 1) A good understanding of the basics, and 2) A tremendous amount of awe and respect for the experts that design, build, and service this technological wonders that are essential to Aerospace today.