Air / Fuel ratio control 1

[FireLover]

I work in the automotive industry, specifically on automotive spark ignition (SI) engines. On automotive SI engines, the air / fuel ratio is typically controlled to the stoichiometric value for reduced exhaust emissions using a three way catalyst (due to heavy goverment legislation by EPA, CARB in the U.S.), fuel economy, and performance. The charge is pre-mixed. Air / fuel ratio control is acomplished by monitoring lambda with an exhaust gas oxygen (EGO) sensor and controlling fuel flow with closed-loop control on the EGO sensor.

I am curious as to how the air / fuel ratio is controlled and / or monitored on turbine engines (power gen or air craft propulsion). Based on my limited knowledge, I believe the flame in a turbine engine can be a diffusion flame or pre-mixed, is this correct? What is the range of lambda values used and why?

 

[FireLover]

I forgot... I would also like to know if any emissions control devices are used, such as a catalyst.

 

[gujun]

As I remember, the air supply of the turbin is not same as the diesel engine. Some primary air is mixed with the fuel and secondly air is supplied to supporting the fuel combustion along the flame flow.
By the way adding catalyst is realy a method of control emission, however how to control combustion process is also an aspect. Since the NOx, SOx production depending on the temperature and pressure inter the combustion cylinder.

 

[evelrod]

Under the heading of MORE rambling thoughts---
In the power generation ( CoGen) units I have worked on in the Los Angeles area ( refinery and cogen units in Wilmington and Vernon) the use of a very LARGE catalyst tower is used to clean up the exhaust to meet AQMD standards or better. The single turbin unit in Vernon cleans the air to a point where the exhaust is MUCH CLEANER than the intake air!!! The downside is maintenance. The catalyst packs must be periodically scrubbed and reinstalled---labor and time consuming, working 24 hours a day it takes about 3 days of down time each.


Rod

 

[radomir]

Sir FireLover believe it or not gas turbines operate with air three times in excess. The reason why, is because it is a cooling agent too, particularly for hot section parts.

I think that maximum turbine entering temperature (TET, at least for aircraft engines), the modern design turbine vanes and blades (made of "superalloys", hollow, internally cooled, investment castings unidirectionaly solidified or even single cristal) can stand is about 1400 deg.C. It applies to turbine blades in particular because of a "creep" fenomenon and intention to maintain certain reliability and reasonably long life.

That low temperature is maintained by a terciar air entering a combustion chamber, diluting combustion products and reducing TET. Ser Gujun is right about the first two portions which are approximately the quantity of air required for stoichiometric mixture and the rest (2/3) is just a coolant. Nevertheless exhaust gases temperature (EGT) is above 700 deg.C (what a waste). That is why those gas turbine power plants are usually cogeneration type.

Even more than tripple quantity of air pases through a compressor (mainly axial), than a portion is bleed to different parts of engine for cooling (hot section) and oil sealing (bearings) purposes. The remaining passes through a diffuser (reducing velocity and increasing pressure) and than enters combustion chamber which is, in a case of a turbojet (bypas or turbofan), usually annual type (something like axially streched bagel). The chamber design (openings) provides that required primary air is supplied around fuel nozzles for initial mixing and flame initiating; secondary air is entering through the remaining thoroidal chambre section and maybe initial cylindrical section holes (from both sides inner and outer) while the terciar is completely inroduced through the holes on cilindrical sections (inner and outer). Expansion is through turbines and exhaust is through exhaust pipe.

The thermodynamic cycle is very similar to Diesel (isobaric combustion) and all four "strokes" are simultaneously and continually happening in different sections of a gas turbine.