JET ENGINES

In the strict sense all gas turbines are gas generators. Their hot gases are expanded either through a turbine to generate shaft power or through a nozzle to create thrust. Some gas generators expand their hot gases only through a nozzle to produce thrust—these units are easily identify ed as jet engines (or turbojets). Other gas turbines expand some of the hot gas through a nozzle to create thrust and the rest of the gas is expanded through a turbine to drive a fan —these units are called turbofans. When a unit expands virtually all of its hot gases through the turbine driving the compressor and the attached propeller and no thrust is created from the gas exiting the exhaust nozzle—it is called a turboprop. All of the above describe flight engines. However, turboprops have much in common with land and sea based gas turbines. This should not be surprising as in many cases the basic gas turbine is identical for both applications. The engines used in aircraft applications may be either turbojets, turbofans, or turboprops, but they are all commonly referred to as “jet engines.”

The turbojet is the simplest form of gas turbine in that the hot gases generated in the combustion process escape through an exhaust nozzle to produce thrust. While jet propulsion is the most common usage for the turbojet, it has been adapted to direct drying applications, to power a supersonic wind tunnel, and as the energy source in a gas laser. The turbofan (Figure 1) combines the thrust provided by expanding the hot gases through a nozzle (as in the turbojet) with the thrust provided by the fan. In this application the fan acts as a ducted propeller. In recent turbofan designs the turbofan approaches the turboprop in that all the gas energy is converted to shaft power to drive the ducted fan (Figure 2). Turboprops (Figure 3) utilize the gas turbine to generate the shaft power to drive the propeller (there is virtually no thrust from the exhaust). Therefore, the turboprop is not, strictly speaking, a jet engine.

Figure 1. Courtesy of United Technologies Corporation, Pratt & Whitney Aircraft. The JT8D turbofan engine was one of the early “bypass” engines (BPR 1.7:1). The JT8D-200 series produces over 20,000 pounds “take-off” thrust and powers the McDonnell Douglas MD-80. The FT8 is the industrial, aero-derivative, version of this engine.

Figure 2. Courtesy of United Technologies Corporation, Pratt & Whitney Aircraft. The PW4000 turbofan engine is a high “bypass ratio” engine (BPR 5.1:1). This engine produces 68,000 pounds “takeoff” thrust and currently powers the Airbus A330 wide body twin jet aircraft.

Some turboprop engines have made the transition from flight engines to land based applications (Pratt &  Whitney Aircraft Canada PT6/ST6). Indianapolis 500 Race fans may recall the introduction of the Pratt & Whitney type ST6B-62 to that race in 1967. The car, owned by Andy Granatelli and driven by Pernelli Jones, led the race for 171 laps, only to fail a gearbox bearing in the 197th lap1. That car had an air inlet area of 21.9 square inches. Later, the Indianapolis 500 Race Officials modifi ed the rules by restricting the air inlet area to 15.999 square inches or less. A year later race officials further restricted the air inlet area to 12.99 square inches. This effectively eliminated gas turbines from ever racing again. The aero-engines that have been most successful in making the transition from flight applications to land based applications have been the turbojets (Pratt & Whitney Aircraft J75/FT4, General Electric J79/LM1500, and Rolls Royce Avon) and the turbofans (General Electric CF6/LM2500, CF6/

Figure 3. Courtesy of United Technologies Corporation, Pratt & Whitney Canada The PT6/ST6 turboprop engine is a 1,500 shaft horse power unit used primarily in aircraft, Helicopter and marine applications.

LM5000, CF6/LM6000, Rolls Royce RB211, and Pratt & Whitney JT8/FT8). These engines are commonly referred to as aero-derivatives. Of these aero-derivative engines, the LM2500 (shown in 4) has been the most commercially successful. However, not all land based gas turbines were derived from aero engines. Like the steam turbines, these gas turbines have large, heavy, horizontally split cases (hence the designation “ heavy industrial gas turbine”) and operate at lower speeds and higher mass flows than the aero-derivatives (at equivalent horsepower). A number of hybrid gas turbines in the small and intermediate size horsepower range have been developed to incorporate features of the aero-derivative and the heavy industrial gas turbines.

Figure 4. Courtesy of General Electric Company. The LM2500 stationary gas turbine was derived from the CF6 “high bypass ratio” flight engine that powered the C5A, the largest military cargo aircraft built in the USA. The LM2500 is ISO base load rated at 23 megawatts. This cross section of the LM2500 includes the six stage power turbine. This power turbine was derived from the six stage fan-turbine used in a CF6 fl ight engine.