Effect of increasing fuel on TIT in Turbo-shaft Engines

[Greenvalley285]

So.. in Piston engines, increasing fuel has an effect on cylinder temperatures. It brings them down.

 

But what about, in Turbo-shaft engines. If fuel is increased, then will it lower TIT or TGT (Turbine Gas Temp).

 

I will be particular about T53-L-703 engines. Does increasing fuel (means increasing N1) has an effect of decreasing (a bit) TGT. Now summers are setting in, and we have problems with our engines, as in T53 case, TGT remains in yellow limit (850^o C), with moderate loads, in hover. What may be other remedial measures, besides checking instruments and P3 leakage.

[Rupert]

In gasoline-fueled piston engines one increases the quantity of fuel by increasing the total air-fuel mixture (opening the throttle), or by making the same air-fuel mixture more rich with fuel. Increasing the total air-fuel volume does not decrease cylinder head temperatures, whereas, increasing the richness, or the ratio of fuel to air, does decrease cylinder head temperatures; and, the converse, of course, applies, in that, up to a point, decreasing the ratio of fuel to air increases cylinder head temperatures. This does not transfer to turbine engines.

 

In most turbine engines, the ignited fuel-air mixture creates a flow of hot expanding gases that pass through the second wheel of the N1 turbine, or the hot or power section of the N1 turbine, thus driving the N1 first wheel, or compressor wheel. Theoretically, the engine could dump partial pressure prior to the second N1 wheel, thus enrichening the air-fuel mixture as it ignites just prior to the second wheel. I don't know that any engines do this as a function of the fuel control module, although I do know that many engines make use of a dump valve in order to facilitate acceleration of the first N1 wheel, the compressor.

 

The energy remaining in the flow of hot gasses, after passing through the second wheel of the N1 section, then goes on to drive the power fan, wheel, or N2...in most engines. This remaining energy represents about 25% of the thermal energy of the expanding gasses prior to the second N1 wheel, and means the N1 section uses about 75% of the fuel's energy to simply run the N1 section, or Gas Generator.

 

The N1 section generates gas for the N2 section, and uses 75% of the fuel's energy to do so.

 

The question, then: can the engine dump pressure between the two N1 wheels and thereby increase the richness of the air-fuel mixture and reduce the gas temperatures seen by the second N1 wheel and the N2 wheel?

 

I don't know. I'd ask the engine manufacturer. They know a lot about their engines.

[Eric Hunt]

I think the only time the Fuel Control Unit plays with the fuel/air mix is during start, with the start de-rich operating until the engine is running. After that, it keeps the mix in the stoichiometric ratio, regardless of the air density. No playing with the choke, like in a piston.

 

If your EGT is too high, look at a compressor wash - a good one - and see if you have any bleed air items working. The bleed band should be fully shut above 93% N1, and anyway you can hear it hissing if it is open at all.

Check the labyrinth seals for leakage.

Wash your blades and coat them in (maybe) WD40 or equivalent, reduces the drag.

Lighten the machine if you can.

Fit the flip-flop tail rotor, more efficient than the original Huey. Uses less power, EGT goes down.

[iChris]

But what about, in Turbo-shaft engines. If fuel is increased, then will it lower TIT or TGT (Turbine Gas Temp).

 

I will be particular about T53-L-703 engines. Does increasing fuel (means increasing N1) has an effect of decreasing (a bit) TGT. Now summers are setting in, and we have problems with our engines, as in T53 case, TGT remains in yellow limit (850^o C), with moderate loads, in hover. What may be other remedial measures, besides checking instruments and P3 leakage.

 

Piston engines handle fuel in a relatively casual manner. Turbine engine fuel management is much more formal. In the case of most turboshaft engines used in helicopters, a complex hydro-mechanical fuel control unit manages fuel flow. The goaled of the fuel control is to maintain a stoichiometric air-fuel mixture that provides for the best possible combustor gas velocity and pressure during the combustion process. Stoichiometric just means the air-fuel mixture is at its ideal ratio to allow it to burn completely, leaving no uncombined oxygen nor free carbon, achieving the best velocity and thermal efficiency. See figure below.

 

So it is with the T53 fuel control which provides starter fuel for the starting fuel nozzles and schedules fuel flow to the combustion chamber for continued operation. Correct velocity of airflow in the combustor depends on a correct matching of compression ratio, mass airflow, and engine speed, that’s the job of the fuel control.

 

In a hypothetical HOGE case, if you increase collective pitch, the helicopter rotor system will demand more power for a constant N2 and NR of 100%. This causes the fuel control to schedule more fuel that results in an increase in TIT. As the TIT increases, the expansion through the turbine rotors increases. The power developed by the gas producer turbine rotor increases, causing N1 to increase. As N1 increases, the mass airflow through the engine increases. With an increased mass airflow and increased expansion, the power turbine rotor will develop more power.

 

Thus, the result of increased collective pitch is the same N2 and NR RPM, but a higher N1 RPM, a higher TIT., an increased fuel flow, and a higher power output from the engine.

 

The reverse would be true if you decrease collective pitch, the helicopter rotor system will demand less power for a constant N2 and NR of 100%. This causes the fuel control to schedule a reduction in fuel that results in a decrease in TIT. As the TIT decreases, the expansion through the turbine rotors decreases. The power developed by the gas producer turbine rotor decreases, causing N1 to decrease. As N1 decreases, the mass airflow through the engine decreases. With a decreased mass airflow and decreased expansion, the power turbine rotor will develop less power.

 

Consequently, the result of decreased collective pitch is the same N2 and NR RPM, but a lower N1 RPM, a lower TIT., a decreased fuel flow, and a decreased power output from the engine.

 

Although not the exact data from the T53 engine in question, the pressure, temperature, and velocity graph below is a typical example of the temperature and pressure rise occurring in the burner and the acceleration of the expanding gases (gas velocity) from the burner into the turbine section. Note, the temperature peaks in the burner and the gas velocity peaks upon entry into the turbine section.

 

The Lycoming T53-L-703 is an 1,800 SHP, turboshaft engine with a two-stage free-power turbine and two-stage gas-producer turbine. Air compressed by a five-stage axial compressor and single-stage centrifugal compressor. The “L” variant designates a military engine. Likely, being used in an old UH1, AH1 or Bell 205.

 

As stated in Eric Hunts post above, yours is a common problem, low power with high EGT. It is most commonly related to compressor damage, dirty compressor, bleed valve failing to close fully, In the case of the T53, a Bleed band, which is controlled by a slide valve on the fuel control, compressor discharge (P3), and ambient pressure (PA). Additional problems include, excessive air leaks, anti-icing or heat control valve leaks. Normally higher engine inlet temperatures during the summer also amplify the situation.

 

Another important concern is, what have your Engine Power Checks or Power Assurance checks shown? Don’t forget your maintenance manual or the old troubleshooting chronology:

 

1. Verify Problem

2. Isolate Problem

3. Detect Problem

4. Correct Problem

 

NOTE: The T53-L series engines have 22 fuel atomizers or vaporizers (2 per each of the 11 stations). An aircraft accident was linked to these fuel nozzles. See quote below:

 

The combustor was equipped with multiple fuel vaporizers, nearly all of which exhibited significant long-term thermal damage. None of the vaporizers were within the operable damage limits specified by the maintenance guidance.

 

The damaged fuel vaporizers altered the combustor exit temperature profiles, which precipitated the first stage turbine blade failures. Maintenance records indicated that the 600-hour inspection interval of the engine hot section had been exceeded at least twice in the 14 years preceding the accident, and that, at the time of the accident, the engine had exceeded the hot section overhaul interval by about 300 hours. In-service records for the failed components could not be located.

 

See NTSB Report: NTSB Identification: WPR10FA387

 

 

 

Click photo to enlarge

 

Edited April 20, 2016 by iChris

[Wally]

Short answer- no, adding fuel will not cool TGT by enriching mixture. Adding fuel *should* generate higher TGT, N1/NG and thus more power. Something adverse is happening compression side or compressed air leak (usually).