BMW Rolls-Royce Power Plant for the Boeing 717


he BR715 engine has been selected to power the 717-200, the newest model in the Boeing fleet. The engine is a member of the BR700 family, which has a thrust range of 14,000 to 23,000 lb, and is produced by BMW Rolls-Royce GmbH, a joint venture company founded in 1990 by BMW and Rolls-Royce. Derived from a number of successful Rolls-Royce power plants, the engine is designed specifically to meet the rigors of high-cycle operation that the 717 will encounter.

owered by BMW Rolls-Royce engines, the Boeing 717 completed its first flight in September 1998. Because the 100-seat twinjet will be used for short routes and will experience a high number of takeoffs and landings every day, its systems--including its power plant--must withstand the added severity of high-cycle operations. The BR715 engine was designed to accommodate this and other operator requirements through the following characteristics:

  1. Proven design and operation.

  2. Minimal environmental impact.

  3. Efficient and reliable performance.

  4. Ease of maintenance.

Proven Design and Operation
The BR715 is a twin-spool engine designed to provide thrust from 18,500 to 21,000 lb. The high- pressure turbine drives the high-pressure compressor, and the low-pressure turbine drives the fan and boosters. All members of the BR700 family use the same engine core design. The BR715 comprises a 10-stage axial flow compressor, an annular combustor, and a two-stage high-pressure turbine. The compressor is derived aerodynamically and mechanically from the Rolls-Royce-designed high-pressure compressor used in the V2500 engine produced by International Aero Engines, and it enables the engine to be highly efficient by providing a high-pressure ratio. Four stages of variable guide vanes, coupled with four handling bleeds, all scheduled by the engine electronic controller, ensure large surge margins.

Twenty air-spray burners feed fuel into the combustion chamber. The walls of the combustion chamber are cooled by airflow through a series of machined Z-rings.

A two-stage high-pressure turbine minimizes loading on each of the stages to achieve the high efficiencies of the engine. Single-crystal blade material offers improved durability. Shrouds at the blade tips provide performance retention capability, preventing a rapid deterioration of engine fuel burn and turbine gas temperature margins in service.

The low-pressure system uses a three-stage low-pressure turbine that drives a 58-in fan and two booster stages. The fan has 24 solid-titanium wide-chord snubberless blades. Installed in a high-bypass arrangement, these blades minimize noise levels while providing high performance. The position of the core inlet is located to minimize foreign object damage, and centrifugal force is used to move foreign objects away from the core inlet. The design allows the fan blades to be replaced with the engine installed.

The fan case is made of a solid ARMCO steel for fan-blade containment. The intermediate casing, made of titanium, provides the mounting for outlet guide vanes and engine section stators that diffuse and straighten the flow at entry into the bypass duct and core. Two booster stages, machined from solid titanium forgings, form integrated blade/disks called "blisks."

The three-stage low-pressure turbine that drives the fan and the boosters uses high-lift blading for improved efficiency. These turbine blades are shrouded, eliminating the need for active turbine-case cooling to maintain performance. Blade, disk, and casing materials have thermally matched expansion properties to provide the correct clearances between blades and casings without complicated, active clearance-control systems.

A titanium structural bypass duct is connected to the intermediate casing at the front and to the mount ring at its rear. The stiffness provided by the casing further improves performance retention by protecting the core from maneuver loads. It also eliminates the need for the complicated opening devices found on engines with core cowls.

The nacelle, built by BFGoodrich, comprises an inlet cowl, fan cowl doors, and an apron assembly. The primary material for the nacelle is aluminum, which is less costly to repair and easier to maintain than composites. Systems inside the nacelle provide fire detection and extinguishing, ventilation and pressure relief, and intake anti-icing capabilities. The thrust reverser uses a set of linked pivot doors positioned behind the exhaust mixer to reverse both the fan and core streams. Deflection plates are used on the doors to tune the exhaust plumes to achieve greater reverser efficiency, as well as to provide better airplane stability and control during reverser operation. This simple design fits in the nacelle envelope to produce low drag and minimal leakage when the reverser is stowed.

Minimal Environmental Impact
A number of different design features allow the BR715 to meet the current ICAO stage 3 noise requirements with substantial margins (fig. 1). For example, a 16-lobe mixer fitted to the rear of the engine mixes the core and bypass flows to achieve a low exhaust velocity at the nacelle exhaust nozzle and reduce sideline noise. The nacelle has noise attenuation throughout, and the number of blades and the spacing of the rotors and stators minimize turbomachinery interactions.

The annular combustor keeps emissions well below the levels currently required and proposed by ICAO (fig. 2) . It accomplishes this by providing a lean-burn primary zone followed by quick-quench dilution to reduce formations of nitrogen oxides. This also reduces the amount of other pollutants, such as carbon monoxide and unburned hydrocarbons (UHC).

Efficient and Reliable Performance
The specific fuel consumption of the BR715 is significantly lower than that of other engines in its thrust class (fig. 3). In addition, BMW Rolls-Royce has ensured that the in-service deterioration rate of specific fuel consumption of the BR700 family is extremely slow. Key elements in the effort include a stiff engine, provided by the structural bypass duct, and robust high-pressure turbine provided with two shrouded stages. The deterioration rate of specific fuel consumption using BR710 data is illustrated in figure 4.

Full-authority digital engine control (FADEC) provides engine control while operating in conjunction with other onboard systems. The electronic engine controller (EEC) is a key element in the FADEC. It controls the engine in response to flight-crew demands, associated system demands, and engine limitations. The EEC has two channels, each of which can fully control the operation of the engine.

Ease of Maintenance
The modular design of the BR715, as well as other ease-of-maintenance features, is largely the result of operator suggestions. Input from potential operators influenced component placement and accessibility. Industry advisory team comments also resulted in the elimination of safety wire on routinely serviced components to reduce maintenance crew work load. In addition, the modular design offers quick interchangeability of components for easy maintenance (fig. 5).

Borescope access, provided through panels in the structural bypass duct, allows for visual inspection of the compressor, combustor, and turbine. Along with analysis of chip-detector deposits, oil condition, engine temperature, and vibration monitoring, this access permits operators to maintain the engine "on condition."

It also delivers such benefits as less rework, the need for corrective action only in accordance with predetermined maintenance schedules, and the elimination of unnecessary component replacement during troubleshooting.

Summary
The BR715 engine, the product of a BMW and Rolls-Royce joint venture company, was designed to help ensure high dispatch reliability for operators of the Boeing 717 twinjet. Its environmental impact exceeds ICAO requirements for noise emissions by substantial margins, and the engine maintenance characteristics help ensure low in-service costs for operators. The BR715 engine received certification ahead of schedule by both the Joint Aviation Authorities in August 1998 and the U.S. Federal Aviation Administration in September 1998.



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Powering the Future

The BMW Rolls-Royce BmbH Company
The formation of BMW Rolls-Royce GmbH in 1990 brought together two well-known companies. BMW was a pioneer in airplane engine design and manufacture from the earliest days of manned flight until the end of World War II, while Rolls-Royce is a recognized leader in the modern gas turbine industry. The joint venture was established specifically to create the BR700 family of engines, which are built at a dedicated facility in Dahlewitz, south of Berlin, Germany.

The Dahlewitz facility is the focal point for all customer support activities. Teams at this location manage all in-service issues by offering training programs, spare parts provisioning, and resolution of in-service problems. The existing network of Rolls-Royce field service representatives based with operators coordinates with the Dahlewitz team to provide 24-hour, seven-day-a-week service. BMW Rolls-Royce GmbH provides a comprehensive suite of technical publications with a full revision service, a wide range of warranty services, and fleet hour agreements tailored to support individual operators.


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Graham Hopkins
Chief Engineer
BR715
BMW Rolls-Royce

Rob McGovern
Senior Manager
717 Power Plant
Boeing Long Beach
Division

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