Turboprop engines are designed with a specific integrated speed reduction gear to drive an airplane propeller whereas the turboshafts do not include a specific speed reduction gear and are generally used to power helicopters. Both are light weight and somewhat interchangeable.
There are two new turboprop engines now under development, one by GE and the other by P & W C, to power future high wing, high tail commuter planes. Each one is designed to carry 70 to 90 passengers for routes of 200 to 400 miles. Both GE and Pratt’s studies show that there is a growing market for such planes and new more advanced engines. Airplane manufactures agree but are slow to commit new plane development at this time. GE and Pratt Canada, however, are moving forward with the all new engines in the 6000 to 8000 HP size range for such planes. GE seems to be ahead as of now. These engines, when in production, can be used for industrial applications for compressor drivers, ship propulsion and electric power, and possibly a new railroad electric locomotive. Natural gas fuel is the key.
The new GE 38 turboshaft engine
The new GE 38 turboshaft has completed component tests and is being run as a full unit. The engine will have a five-stage axial flow compressor followed by a last-stage centrifugal. It will have a pressure ratio of 18.6. An axial flow three-stage turbine will be incorporated. The latest high TITs and materials, such as single crystal TBC blading, as used in GE’s new large fan engine, GEnx, will be applied. The target is for the new turboshaft to have a 20 percent lower fuel burn and a 30 percent lower maintenance cost. Its cousin, the turboprop GEx38, will have a speed reducing gear to drive a new Hamilton-Standard 8-bladed propeller to lower the sound level by limiting the prop tip speed below sonic velocity. The GE unit will benefit from the military funding in the development of the GE 38 designated for the new US Marine Corps Sikorsky CH-53K Super Stallion helicopter. Tests show that the targets are being met.
GE is actively courting the commuter airplane manufacturers for a launch order for the turboprop version which GE says can be run in 36 months and available in 48 months.
P&W C’s new turboprop engine
Pratt Canada is currently testing component parts of its new Next Generation Regional Turboprop (NGRT) engine. It will also have a multi-stage axial flow compressor followed by a centrifugal last stage and an axial flow turbine. The engine is aimed to have a 20 percent higher efficiency and a 30 percent lower maintenance cost. New 3D computer design programs for centrifugal compressors coupled with new milling processes have made improvements in the new P&W C centrifugal compressors possible. The German company MKU, a partner in the engine development program, is being used to run extensive compressor test on the new centrifugal to map all operating conditions.
Pratt Canada is, like GE, trying to get a launch order from one of the regional airplane manufacturers. Pratt Canada has extensive favorable turboprop engine experience for a variety of small airplanes such as the one used for the Kingair for corporate use.
In the next part of this series, the author explains how the first IG-100 engine was designed, built and operated.
Ivan G. Rice, past chairman of the South Texas Section of ASME (1974 - 75) and the ASME Gas Turbine Division (now IGTI) (1975 - 76), has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection. He is a Life Fellow Member of ASME and Life Member of NSPE/TSPE.