About the CFM 56 jet engine - Corrigedum | Read a Revolution

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About the CFM 56 jet engine

About the CFM 56 jet engine

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The CFM56 is a high-bypass turbofan engine (most of the air accelerated by the fan bypasses the core of the engine and is exhausted out of the fan case) with several variants having bypass ratios ranging from 5:1 to 6:1, generating 18,500 to 34,000 lbf (80 kN to 150 kN) of thrust. The variants share a common design, but the details differ. The CFM56 is a two-shaft (or two-spool) engine, meaning that there are two rotating shafts, one high-pressure and one low-pressure. Each is powered by its own turbine section (the high-pressure and low-pressure turbines, respectively). The fan and booster (low-pressure compressor) evolved over the different iterations of the engine, as did the compressor, combustor and turbine sections.

Combustor
Most variants of the CFM56 feature a single-annular combustor. An annular combustor is a continuous ring where fuel is injected into the airflow and ignited, raising the pressure and temperature of the flow. This contrasts with a can combustor, where each combustion chamber is separate, and a canannular combustor which is a hybrid of the two. Fuel injection is regulated by a Hydromechanical Unit (HMU), built by Honeywell. The HMU regulates the amount of fuel delivered to the engine by means of an electrohydraulic servo valve that, in turn, drives a fuel metering valve, that provides information to the full authority digital engine controller (FADEC). In 1989, CFMI began work on a new, double-annular combustor. Instead of having just one combustion zone, the double-annular combustor has a second combustion zone that is used at high thrust levels. This design lowers the emissions of both nitrogen oxides (NOx) and carbon dioxide (CO2). The first CFM56 engine with the double-annular combustor entered service in 1995, and the combustor is used on CFM56-5B and CFM56-7B variants with the suffix "/2" on their nameplates. GE started developing and testing a new type of combustor called the Twin Annular Premixing Swirler combustor, or "TAPS", during the Tech 56 program.This design is similar to the doubleannular combustor in that it has two combustion zones; this combustor "swirls" the flow, creating an ideal fuel–air mixture. This difference allows the combustor to generate much less NOx than other combustors. Tests on a CFM56-7B engine demonstrated an improvement of 46% over single-annular combustors and 22% over double-annular combustors.The analytical tools developed for TAPS have also been used to improve other combustors, notably the singleannular combustors in some CFM56-5B and -7B engines.

Compressor

The high-pressure compressor (HPC), that was at the center of the original export controversy, features nine stages in all variants of the CFM56. The compressor stages have been developed from GE's "GE1/9 core" (namely a single-turbine, ninecompressor stage design) which was designed in a compact core rotor. The small span of the compressor radius meant that the entire engine could be lighter and smaller, as the accessory units in the system (bearings, oiling systems) could be merged to the main fueling system running on aviation fuel. As design evolved HPC design improved through better airfoil design. As part of the Tech-56 improvement program CFMI has tested the new CFM-56 model with six-stage high-pressure compressor stages (discs that make up the compressor system) that was designed to deliver same pressure ratios (pressure gain 30) similar to the old nine-stages compressor design. The new one was not fully replacing the old one, but it offered an upgrade in HPC, thanks to improved blade dynamics, as a part of their "Tech Insertion" management plan from 2007.

Exhaust
CFMI tested both a mixed and unmixed exhaust design at the beginning of development; most variants of the engine have an unmixed exhaust nozzle.Only the high-power CFM56-5C, designed for the Airbus A340, has a mixed-flow exhaust nozzle. GE and Snecma also tested the effectiveness of chevrons on reducing jet noise. After examining configurations in the wind tunnel, CFMI chose to flight-test chevrons built into the core exhaust nozzle. The chevrons reduced jet noise by 1.3 perceived loudness decibels during takeoff conditions, and are now offered as an option with the CFM56 for the Airbus A321.  

Fan and booster 
The CFM56 features a single-stage fan, and most variants have a three-stage booster on the low-pressure shaft, with four stages in the -5B and -5C variants. The booster is also commonly called the "low-pressure compressor" (LPC) as it sits on the lowpressure shaft and compresses the flow initially before it reaches the high-pressure compressor. The original CFM56-2 variant featured 44 tip-shrouded fan blades, although the number of fan blades was reduced in later variants as wide-chord blade technology developed, down to 22 blades in the latest variant, the CFM56-7. The CFM56 fan features dovetailed fan blades which allows them to be replaced without removing the entire engine, and GE/Snecma claim that the CFM56 was the first engine to have that capability. This attachment method is useful for circumstances where only a few fan blades need to be repaired or replaced, such as following bird strikes. The fan diameter varies with the different models of the CFM56, and that change has a direct impact on the engine performance. For example, the low-pressure shaft rotates at the same speed for both the CFM56-2 and the CFM56-3 models; the fan diameter is smaller on the -3, which lowers the tip speed of the fan blades. The lower speed allows the fan blades to operate more efficiently (5.5% more in this case), which increases the overall fuel efficiency of the engine (improving specific fuel consumption nearly 3%).

Reverse thrust
The CFM56 is designed to support several reverse thrust systems which help slow and stop the aircraft after landing. The variants built for the Boeing 737, the CFM56-3 and the CFM56-7, use a cascade type of thrust reverser. This type of thrust reverse consists of sleeves that slide back to expose mesh-like cascades and blocker doors that block the bypass air flow. The blocked bypass air is forced through the cascades, reducing the thrust of the engine and slowing the aircraft down. The CFM56 also supports pivoting-door type thrust reversers. This type is used on the CFM56-5 engines that power many Airbus aircraft. They work by actuating a door that pivots down into the bypass duct, both blocking the bypass air and deflecting the flow outward, creating the reverse thrust.

Turbine 
All variants of the CFM56 feature a single-stage high-pressure turbine (HPT). In some variants, the HPT blades are "grown" from a single crystal superalloy, giving them high strength and creep resistance. The low-pressure turbine (LPT) features four stages in most variants of the engine, but the CFM56-5C has a five-stage LPT. This change was implemented to drive the larger fan on this variant. Improvements to the turbine section were examined during the Tech56 program, and one development was an aerodynamically optimized low-pressure turbine blade design, which would have used 20% fewer blades for the whole low-pressure turbine, saving weight. Some of those Tech56 improvements made their way into the Tech Insertion package, where the turbine section was updated.The turbine section was updated again in the "Evolution" upgrade. The high-pressure turbine stages in the CFM56 are internally cooled by air from the highpressure compressor. The air passes through internal channels in each blade and ejects at the leading and trailing edges.


Source: Wikipedia and CFM international documents

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