# Do the compressors and turbine rotate at the same speed?

Since the turbine and the compressors are connected to each other via a common shaft, do they both rotate at the same speed. Because I've read that in some jet engines (single spool engines) they don't rotate at the same speed:

Single spool jet engine. These are those jet engines who have one shaft inside them. The compressors and turbines rotate on a same shaft. The speed of compressors and turbines decreases half way down the compressor stages, that is why they can’t match the local airflow. The various turbines and compressors is forced to rotate at same speed which lead to their inefficiencies.

(quora.com)

Is this true?

• quora.com/…. Refer the 1st answer in this link. – Johnson Nov 3 '19 at 13:50
• Thanks, I added the quote I think you are referring to. If you meant something else, please edit again. – Bianfable Nov 3 '19 at 14:07
• I think your quote is talking about airflow speed rather than the spool speed. That's why more efficient engines have two or three spools, or GTF. – JZYL Nov 3 '19 at 14:28
• Thx for the edit Bianfable! Yes, that was exactly what I was referring to. – Johnson Nov 3 '19 at 15:20

Single spool jet engine. These are those jet engines who have one shaft inside them. The compressors and turbines rotate on a same shaft.

Correct. In a single-spool engine, all of the compressor and turbine stages rotate at the same speed because they're attached rigidly to the shaft.

The speed of compressors and turbines decreases half way down the compressor stages, that is why they can’t match the local airflow.

No, the speed of the compressors and turbines doesn't change. It's the airflow that changes, and causes a mismatch.

• English SE could be of assistance – JZYL Nov 4 '19 at 13:18

Your question answers itself. If they have a single shaft and no gear box, then they rotate at the same speed. If there is a gearbox then that gearbox gives you the speed difference.

• Are there turbine engines with a gear box? I've never heard of one (except for geared turbofans, where the fan has a gearbox, or a turboprop, but never between compressor and turbine). – Bianfable Nov 3 '19 at 12:04
• @Bianfable yes. Most large jet engine have three speeds. It improves the efficiency of the flow. Usually you have one slow shaft a medium speed shaft and a high speed shaft, but they all connect compressor and turbines, thus no gearbox. But the engines on the A220 for example have a gearbox for the fan. It is just a design choice of efficiency in flow vs transmission efficiency vs space and heat requirements. – ic_fl2 Oct 5 '20 at 12:43
• "Most large jet engine have three speeds": Only RR Trent engines as far as I know. All others typically have two, not three. "A220 for example have a gearbox for the fan": Yes, I know, it's called a geared turbofan, as I said in my comment. But I have never seen a turbine engine with a gear box between the compressor and turbine, which would give those different speeds. – Bianfable Oct 5 '20 at 12:48