Bjorn’s Corner: Geared turbofans

By Bjorn Fehrm

January 13, 2017, ©. Leeham Co: The time has come to go through the reasons why some turbofan engines are designed with a gearbox between the fan and the low pressure shaft.

The principle design is shown in Figure 1. It’s a graphical representation of a geared turbofan from the engine analysis software GasTurb.

Figure 1. GasTurb principal representation of a geared turbofan. Source: GasTurb.

The base idea is to have the low pressure spool of the engine to run at a considerably higher RPM than the fan. Read more

Bjorn’s Corner: Turbofan developments in 2017

By Bjorn Fehrm

January 06, 2017, ©. Leeham Co: Before we finish of our series on airliner turbofan technology, let’s spend this Corner on what will happen on the airliner engine front during 2017.

While there is no totally new engine that comes into the market during 2017 there are a number of new variants of existing engine families that will be introduced.

Figure 1. GasTurb principal representation of a three shaft turbofan like our reference Rolls-Royce Trent XWB. Source: GasTurb.

 

If we start with the engines for regional/single aisle aircraft and then climb the thrust scale, we will cover the engines in climbing thrust class.

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Bjorn’s Corner; Turbofan engine challenges, Part 7

By Bjorn Fehrm

December 16, 2016, ©. Leeham Co: After the turbine comes the engine’s exhaust system. This is where the thrust characteristics of the engine are formed. It is also the environment that defines the back pressure for the fan and turbines. It’s therefore more high-tech than one thinks.

For the very high bypass airliner engines of tomorrow, the common fixed bypass exhaust of today (Station 18 in Figure 1) will not be acceptable. Variable exhaust areas will have to be introduced.

Figure 1. GasTurb principal representation of a three shaft turbofan like our reference Rolls-Royce Trent XWB. Source: GasTurb.

On engines that function in high supersonic speed, it gets really complex. Not only is the exhaust area variable, it must have a dual variation exhaust, a so-called Con-Di nozzle.

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Bjorn’s Corner; Turbofan engine challenges; Part 6

By Bjorn Fehrm

December 08, 2016, ©. Leeham Co: We have now come to the turbine in our trip through a modern turbofan.  The turbines make up the rear of the engine, before the propelling nozzle.

The turbines are the workhorses in the engine. They take the energy released by the fuel in the combustion chamber and convert it to shaft hp to drive the fan or compressors.

Figure 1. GasTurb principal representation of a three-shaft turbofan like our reference Rolls-Royce Trent XWB. Source: GasTurb.

The hotter they can operate, the better. They can then generate more hp on a smaller size turbine. The temperature of the gas entering the high pressure compressor is one of the key parameters of a gas turbine. It dictates the power efficiency of the core and how much work it can perform to drive the fan and the compressors. Read more

Bjorn’s Corner: Turbofan engine challenges, Part 5

By Bjorn Fehrm

December 02, 2016, ©. Leeham Co: We will now look at the combustor area in our series on modern turbofan engines. There is a lot of activity in this area, as it sets the level of pollution for the air transportation industry for some important combustion products.

We will also finish off the compressor part of our series by looking at the bleeding of cooling air for the engine and for servicing the aircraft with air conditioning and deicing air.

trent-xwb-model-stations_

Figure 1. GasTurb principal representation of a three-shaft turbofan like our reference Rolls-Royce Trent XWB. Source: GasTurb.

The amount of air which is tapped from compressor stages for cooling and other purposes can exceed 20% of the core flow (some of the flow paths are shown in Figure 1).  At that level, it has a marked influence on the performance of the engine. Read more

Bjorn’s Corner: Turbofan engine challenges, Part 4

 

By Bjorn Fehrm

By Bjorn Fehrm

November 18, 2016, ©. Leeham Co: In our series on modern turbofan airliner engines, we will now go deeper into the compressor part. Last week, we covered the fundamentals of compressors. As compressors and turbines use the same principles, we also covered the fundamental working principles of turbines.

We also described that compressors are temperamental parts, which can protest to wrong handling with violent “burps” (burst stalls with the combustion gases going out the front of the engine) or end up in a rotating stall where it simply stops working.

gasturb-graph

Figure 1. Stylistic cross section of a two shaft turbofan with both axial and radial compressor. Source: GasTurb.

Turbines, on the other hand, are your robust companions. Aerodynamically they just work, albeit more or less efficiently dependent on what one asks them to do (mechanically it can be very different; we recently saw a turbine disintegrate with large consequences on an American Airlines Boeing 767 in Chicago). More on the turbines later.

In the GasTurb cross section of a two shaft turbofan in Figure 1, the engine has both an axial and a radial compressor. We will consider why engine designers combine these two for certain engine types. Read more

Bjorn’s Corner: Turbofan engine challenges, Part 3

By Bjorn Fehrm

By Bjorn Fehrm

November 12, 2016, ©. Leeham Co: In our trip through a modern turbofan airliner engine and its technologies, we looked last week at the engine intake and the fan. We now continue with the compressor parts.

As compressors and turbines use the same principles (but in opposing ways), we will look at these principles this week and how their roles in the engine create their special characteristics.

trent-xwb-model-stations_

Figure 1. Stylistic cross section of a three-shaft turbofan with section numbers. Source: GasTurb.

As before, to make things concrete, we use a GasTurb simulation of a Rolls-Royce Trent XWB 84k engine to look at practical data when needed. As before, I have no specific knowledge about the engine and will not use any data outside what is public information.

The GasTurb cross section of a three-shaft turbofan is shown in Figure 1. We will examine the sections between station numbers (22) and (3) and (4) and (5) in the general discussion of compressors and turbines. We will then look at some data for common compressors. Read more

Bjorn’s corner; Turbofan engine challenges; Part 2

By Bjorn Fehrm

By Bjorn Fehrm

November 04, 2016, ©. Leeham Co: We will now start to go through a modern turbofan airliner engine and look at the technologies which are used and what are their technical challenges. We will start today with the engine intake and the fan.

To make things concrete, we will use a GasTurb simulation of a Rolls-Royce Trent XWB 84k engine. This will provide us with realistic example data for the different parts of the engine. I want to stress that all values are assumed as typical for such an engine. I have no specific knowledge of the Trent XWB and will not use any data outside what is public information.

Figure 1. Stilistic cross section of a three-shaft turbofan with section numbers. Source: GasTurb.

The GasTurb cross section of a three-shaft turbofan is shown in Figure 1. We will use the station numbers in the figure to navigate the engine and the data from the simulation to understand modern airline engines and their typical data. Read more

Bjorn’s Corner: Turbofan engine challenges, Part 1

By Bjorn Fehrm

By Bjorn Fehrm

October 28, 2016, ©. Leeham Co: Before we go into the details on the innards of airliner turbofans, we will look at some basics. We do that so that everyone is on the same page.

A turbofan engine generates thrust by pumping air out the back of the engine. This air has a higher speed than surrounding air. Air is actually quite heavy: it weighs 1.2kg per m3 at sea level. By kicking out air at an overspeed in relation to the aircraft, thrust is generated.

In a modern turbofan, the kicking gets done by the fan to 80-90% in the modern By Pass Ratio (BPR) 8-10 engines. A single aisle engine generating 10 tonnes of thrust throws around 350kg of air per second backwards at close to sound speed in a take-off situation. To drive the fan to do that, there is a lot of shaft horse-power needed, around 30,000hp.

Figure 1. Work cycle for jet engine/turbofan core compared to car engine. Source: Rolls-Royce book “The Jet Engine.”

These hp are generated by the core. The thermodynamic cycle to generate all these hp in a jet engine or turbofan core (we call both a gas turbine) is like the one in a normal car engine, Figure 1, with the difference that it is a continuous cycle.

We will now go through this cycle in steps. Read more

Bjorn’s Corner: The Engine challenge

 

By Bjorn Fehrm

By Bjorn Fehrm

October 21, 2016, ©. Leeham Co: In our Corners on East bloc aeronautical industries, we could see that the hardest part to master in a new civilian airliner is the engine.

Both new airliners from Russia and China (Irkut MC-21 and COMAC C919) start their lives with Western engines.

Why is this so? What are the challenges that make engines harder to create than aircraft?

leap-1c-engine

LEAP-1C which will be standard engine on COMAC C919. Source: COMAC.

We will spend several Corners on the main reasons that airliner engines are harder to do than aircraft. Read more