By Bob Witwer, SciTech Board Member & Retiree from Honeywell Aerospace
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 my 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.
In the simplest sense, a gas turbine engine compresses air in the Compressor, mixes that compressed air with fuel and ignites it in the Combustor, and then takes the resulting high-pressure gas and uses the energy in this gas to spin one or more Turbines. 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. 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 ➡️ Turbofan engine
• Connect the shaft to a propeller through a gearbox ➡️ Turboprop engine
• Connect the shaft to anything else (e.g. a helicopter rotor) ➡️ Turboshaft engine
As engines 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.
Honeywell’s HTF7000 is an example of an engine used for a variety of business jet aircraft. As you might expect, it is quite powerful, generating approximately 7,000 lbs. of thrust! 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 that is concentric with the high speed shaft; the high speed shaft is hollow and the low speed shaft rotates separately within the high speed shaft.
A cutaway picture of an actual HTF7000 engine is shown in Figure 1. 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.
Figure 1
Note that there are various other engine architectures than the HTF7000 example described above; here are a couple examples:
• A two-shaft architecture with the high pressure compressor connected to high pressure turbine and the low pressure compressor connected to the low pressure turbine and the driven load (e.g. fan, prop gearbox, etc.)
• Similar to the architecture above but with an additional turbine section – the power turbine, and with three shafts:
– high pressure compressor and turbine on one shaft
– low pressure compressor and turbine on the another shaft
– a power turbine section tied to a third shaft that is connected to the driven load
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. Some of the key design attributes of a gas turbine engine are listed below.
• High Thrust to Engine Weight
• Fuel Efficiency
• Low Emissions (CO2, NOx, CO, Unburned Hydrocarbons)
• Low Cost of Ownership (Acquisition cost and lifecycle maintenance cost)
• Reduced Engine Noise
Note that 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. In addition, when prototypes are built, they are extensively testing and 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 has brief overview of gas turbine engines has left you with two things; a good understanding of the basics, and also a tremendous amount of awe and respect for the experts that design, build, and service this technological wonders that are essential to Aerospace today.