How do engine manufacturers derive varying amounts of thrust from a single engine model?

Question

What changes do manufacturers make to an engine to vary the amount of thrust it produces for different applications? Take the GE CF34 for example where there is a 10780lb difference between the thrust produced by the smallest CF34-3 and CF34-10E.

Answer 1

The CF34-3and CF34-10E are quite different engines. The -10 produces more thrust for a number of reasons:

It has a higher mass flow. The CF34-3 has a 44 inch fan, while the -10E has a 54 inch fan. So, the mass flow will be higher in the -10E, and thrust is related to mass flow.

The bypass ratio is different, the -3 is 6.2 while the -10E is 5.4. This is also due to the different fan diameters, and will cause a difference in thrust (even if the mass flow was the same.)

They are significantly different engines. The -3 weighs 700kg, while the -10E weighs 1700 kg. So, it's a lot more than just a tweak of the blades, or a change in fuel scheduling, neither of which will change the weight. With such a weight difference, you would actually expect the -10E to be about twice as powerful, so a question would actually exist if it wasn't considerably higher than the -3.

The turbine temperatures are also different, due to the large difference in technology levels of each engine. The -3 was certified in 1995, in comparison to the -10E in 2013.The EASA type certificate EASA for the -10E shows the max allowed EGT at takeoff is 983 Celsius. The -3 max EGT for takeoff is only 871 Celsius, in comparison.

With regards to overhaul intervals, this info is generally proprietary, as it's commercially sensitive. So, you won't find much info. Suffice to say, airlines don't make money if the engines need to be overhauled often, so later versions with better high temperature material capability are likely to have longer overhaul intervals, than earlier generation engines made in a time when shorter periods was the norm, and acceptable then, but not now.

Answer 2

When originally developed, an engine design will have a "thermodynamic limit" that sets the maximum energy you can pull out of the basic core configuration. Because the initial development is super conservative, when first developed there will be ample thermodynamic margin in the design, allowing more energy to be passed through it with various detail changes and improvements on subsequent versions, as the design is proven in service. For example, compressor blade improvements or the addition of compressor stages will allow you to dump more fuel into the burner can, letting you drive a bigger fan and increase the thrust rating.

There are negative side effects. At some point as you pull more and more power you start to bump up against the physical limits of the core and as you do, you start significantly eating into the engine's service temperature margins (the difference between the normal operating ITT and the maximum ITT among other things), to get the thrust ratings you want, which means lower and lower time between overhauls.

The -3 versions of the CF-34 are quite under stressed and these run quite a long time between overhauls. As you move up the dash numbers the average hours between overhauls gets shorter and shorter.

On the CF34 particularly, there is a another major difference between the -3s and later ones, because the compressor inlet behind the fan is "flush" to the inner cone of the fan's exhaust duct to make the engine more FOD resistant (a goal of the A-10 program where the engine was originally born). On later dash numbers, to improve the windmilling ability and operating efficiency, GE altered the compressor inlet to get a bit more ram effect. This had the side effect of engine pulling in a lot more grit, which erodes the compressor blades a lot faster and shortens engine life (using flex thrust helps with this a lot).