Advantages and Disadvantages: Triple Spool vs Double Spool Engines
 

Hi guys,

Now I know there are some threads on this, and I have searched/read them all but there are a lot of contradictory answers.

As far as I know the advantages of a triple spool engine are:

- Turbines run closer to optimum speeds
- More efficient
- Easier to start

Advantages of double spool engines are:

- Lighter
- Simpler design
- Lower manufacturing cost


These seem to be the only factors that are commonly pointed out. Seeing as double spools are still made today, surely there are more to go in each of the engine categories?

I look forward to some feedback.

Cheers

There would be more triple spool engines had RR not patented it! :)

Ah huh...I see!

That makes sense now.

I believe triple spools costs more per shop visit due the extra stages....:8

Chapter 1 – 3 Spool vs 2 Spool engine
Advantages of a 3 spool engine:
• More flexible due to aerodynamic matching at part load
• Lower inertia of rotating components
• Easier to start as only one spool needs to be turned by the starter
• Allows for higher ratios of fan air flow to engine flow. This allows for increased thrust without a corresponding increase in jet velocity and reduction in propulsive efficiency leading to high SFC.
Note: The older turbo jet engine accelerates a small mass of air to a very high velocity to achieve a given amount of thrust. To obtain the same amount of thrust from a turbofan, it takes a much larger mass of air and does not need to accelerate it to as high of a velocity, thus giving a lower SFC.

Easier starting might have been the theory but the early RB211 series needed bigger starters with more starter airflow, more cranking and took longer to reach idle than its 2 spool competitors.

Only one spool is mechanically driven on 2 spool engines also.

For modern high bypass commercial engines with large diameter fans and high pressure ratios, a three spool configuration is worth the added cost and complexity over a conventional two spool in terms of efficiency.

As turbofan engines get smaller, packaging three shafts can get difficult. The innermost shaft coupling the fan and LP turbine has the smallest diameter, longest length, and transfers the most torque.

The best solution for large commercial applications presently seems to be two spools plus a geared fan, like Pratt's GTF. Stepping the shaft speed down between the LPC/LPT spool and fan allows each to be more highly optimized. The diameter/speeds of the LPC/LPT are no longer compromised to accommodate large fans, which also allows fewer stages.

http://i0.wp.com/airinsight.com/wp-c...ble1.jpg?w=792

Rolls-Royce plans to combine their three spool configuration with a geared fan for their future Ultrafan Engine. This slide shows a comparison between their advanced three spool engine and their Ultrafan engine with a geared fan. Notice how much smaller the diameter of the LPT section is on the Ultrafan engine.

At current fuel prices, are any of the more sophisticated and hence more costly engine designs still economically valid?
Do we not rather have the makings of a glut of aircraft as the economics of somewhat older types such as the A330s and B777s are very competitive when fuel costs are $1/gal.

GE's UDF (of 30 years ago) very cleverly captured the benefit of higher bypass ratio, lower LPT rpm and NO shafting or gearbox worries. :ok:

But the UDF was developed when the world saw no end to increasing fuel cost; when this paradigm was upset, the market for the UDF vanished. Today, we may be seeing the reprise of this. :}

yotty:
3-shaft (or 4, or. . .) engines exist in the industrial world. The extra shafts may reside in a different building, with huge ducts and heat exchangers completing the systems. Of course there are fewer weight or bulk constraints!

I wonder if we have run to the end of this thread?

If weight is not a problem and cost is not a problem, how far can we go in number of compressor stages, turbine stages and length of combusters before we run out of efficiency vs stability in a Brayton cycle propulsion engine with today's materials?

Seems to me we will max out with a prop fan limited by heat loss vs length without regard to aircraft speed

For large commercial aircraft engines fuel efficiency will always be a primary concern. History has shown that the long term trend is for fuel costs to increase. More fuel efficient engines reduce the economic impact of fuel price fluctuations on the operator. Another thing to consider are the much tighter emissions regulations being implemented for CO2, NOx, etc.

Improving commercial aircraft turbine engine efficiency can be achieved with higher bypass ratios, higher cycle pressure ratios, higher cycle temperatures, lower mechanical & thermal losses, and improved controls. The higher cost from using more complex engine systems to get better efficiency is almost always a good bargain for large commercial aircraft engines.

As barit1 noted, open rotor/propfan designs are likely the ultimate solution for high bypass ratios. They can involve some additional complexity, especially with counter-rotating configurations. The approach used by GE on their UDF design eliminated the gearbox by coupling the fan blades directly to the OD of LPT stages. But the tradeoff was a more complicated LPT design in terms of stages, sealing, fan blade pitch control linkage, etc. and more compromise between the turbine blade and fan blade geometries.

There are existing large three stage turboprop/propfan engines, such as the Rolls-Royce TP400-D6 and the Progess D-27.