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Pratt & Whitney PW100 Turboprop
The Pratt & Whitney PW100 series of engines are the forefront in modern, state of the art regional and commuter turboprop engines. With its three spool design, easy to maintain modular construction, and high power rating, the engine is a logical choice for medium to large turboprops including the Bombardier Q400, DeHavilland Dash 8 and Embraer EMB 120. Some consider the PW100 to be the replacement to the venerable Pratt & Whitney PT6, but in reality the PW100 picks up where the PT6 Large version left off in terms of power, fuel economy, and reliability. The PW100 series consists of a number of variations. There is no actual PW100 engine; the engines start with the PW118 engine and end at the PW127J. The engines are essentially the same, with, for the most part, a steady increase in power output, as well as slight variations in engine output speed and in the proportion of mechanical shaft horsepower vs. thrust produced. In other words, each turboprop is rated in equivalent shaft horsepower, (eshp) which is a combination of the actual mechanical horsepower provided at the output shaft combined with the amount of horsepower available as a conversion of the thrust that is produced at the tailpipe. The proportion varies, but is typically in the range of 80% power produced by the propeller, 20% produced by the tailpipe.
The PW100 engine is completely modular in its construction; that is, it is made up of a number of interchangeable modules that can easily be removed and replaced in the event that there is a problem with the engine. The modules consist of the turbomachine, the power turbine assembly, the inlet housing, and the output reduction gearbox. The compact turbomachine consists of the twin spool gas generator and the accessory gearbox. The power turbine connects to the rear of the turbomachine and features a two stage power turbine which drives a shaft that runs forward up the center of the turbomachine shafts. The inlet housing mounts to the front of the turbomachine and provides the space for air to be drawn into the compressor and the support for the output reduction gearbox. The output reduction gearbox mounts to the front of the inlet housing and takes the high speed input from the power turbine shaft and converts it to a high torque, low rpm output taken off the propeller flange at the front of the gearbox.
Atmospheric air is drawn in through the engine nacelle behind the propeller into a passive particle separator, which is part of the nacelle. Clean intake air is drawn upward into the downward facing scroll type engine inlet. Air is drawn into the turbomachine by the single stage, centrifugal compressor. The air is accelerated outward by the compressor and fed into numerous curved diffuser ducts which smoothly deposit the airflow to the face of the single stage, centrigugal, high pressure compressor. The high pressure compressor raises the pressure to a design pressure ratio of nearly 15:1 on some of the later models. The high pressure compressor feeds the airflow to a diffuser which converts the dynamic pressure into static pressure, as it enters the annular, reverse flow combustor. The compressed air enters the inner combustion liner where it is mixed with jet fuel and ignited. The resultant gas is expanded through the high pressure nozzle to impinge upon the single stage axial high pressure turbine, which drives the high pressure compressor and the accessory gearbox. The gas is then further expanded through the low pressure nozzle to drive the low pressure turbine, which drives the low pressure compressor. Finally, the gas is expanded through the two stage power turbine to drive a concentric shaft up to the front of the engine, which drives the output reduction gearbox. The exhaust is then directed out of the short, axial flow, fixed area exhaust outlet to provide close to 2,000 lbs. of thrust on some models. The output reduction gearbox reduces power turbine speed down to a usable 1,200 or 1,300 rpm, to drive a four bladed, constant speed propeller. Accessories include a generator, oil pumps, fuel pumps, hydraulic pumps, and a FADEC fuel control.
The twin spool compressor offers many advantages over a similar single spool compressor. By allowing the two compressors to run at different speeds, the compressors can be optimized a wide range of airflows. This allows for a higher design pressure ratio, much better part power efficiency, and very rapid engine response. High pressure ratios and high turbine inlet temperatures allow for very low specific fuel consumption, and advanced cooling techniques and state of the art materials allow for long time between overhaul periods.
There are a couple of other variants of the PW100 that are worth mentioning. The PW150 engine is a high power development of the PW100; it is very similar in overall design and dimensions to the PW100, except that the low pressure compressor is a single stage axial followed by a single stage centrifugal. Pressure ratio is a higher 18:1, and the engine produces power in the 5000 eshp class, which makes it a suitable replacement for the Allison T56 or an alternative to the Rolls Royce AE1107C. There are also turboshaft versions of the PW100, where the output reduction gearbox and inlet are removed and replaced with a screened bellmouth inlet and a carrier bearing. The engine output is at power turbine speed. The engine could be an alternative to the CT7 (T700) turboshaft in medium helicopters, although so far it hasn't been used in this application. There is a marine variant of this turboshaft available for modern miltary surface effect ships, however. These models are referred to as the ST18M. Power output is approximately 3,200 shaft horsepower.
Pratt & Whitney PW127G Turboprop
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