Engine Performance Analysis

Engine Performance Analysis

© 2015 SIM University. All rights reserved.

• The basics of engine performance analysis

• The parameters and tools used in engine

performance analysis


Introduction

• Parametric cycle analysis:

– Independently selected input:

�Flight condition M0, T0, P0

�Design choices �c, Tt4, ec, et, etc.

– Determines �� = 1 −�

� ��

�

�� − 1 (a variable)

– Performance output: F/ṁ0, S, η, etc.

– Performance is determined at specific design flight conditions,

values and limits

– Input design or reference point


Introduction

• Consider performance of engine built with selected design point

• When installed, its performance varies

– flight conditions

– throttle settings

– limited by engine control system

• When input is changed, component design changes (not possible

with engine once built)


Limitations of Parametric Cycle Analysis

• Looks at how specific engine perform at conditions other than

design point (Off-design analysis)

• Engine built with selected design point

– Selected compressor pressure ratio �c

– corresponding turbine temperature ratio �� (essentially constant)

• �c is dependent on the throttle setting (Tt4) and flight condition (M0

and T0)

• Off-design analysis:

– Changing through the throttle and indirectly by changing flight

conditions

– Performance other than their design point




Variable Parametric

cycle

Engine

performance

Flight condition (M0, T0, and P0) Independent Independent

Compressor pressure ratio �c

Independent Dependent

(on engine speed

and flight

conditions)

Main burner exit temperature Tt4 Independent Independent

(throttle input)

Turbine temperature ratio �� Dependent

(on flight

conditions, �c

,

and Tt4 )

Constant

Table 7-1 Comparison of Analysis Variables

Source: Page 438

“Elements of Propulsion: Gas Turbines and Rockets” by Jack D. Mattingly


• Engine design point or reference point

– Specific set of input values that basic engine is designed to:

�Flight condition M0, T0, P0

�Design choices πc, Tt4, ��, ec, et, etc.

– Resulting F/ṁ0, TSFC are valid only for the given engine cycle

and values of Tt4, M0, T0, etc.

– Normal design point – Sea-level Static (SLS)


Design (Reference) Point

Low Pressure Spool (N1)

High Pressure Spool (N2)

© 2015 SIM University. All rights reserved.Source: Page 439


Twin-Spool Gas Turbine Engine

Subscript H denotes HP spool and L denotes LP spool

Table 7-2 Additional Temperature and Pressure Relationships



Twin-Spool Gas Turbine Engine

• Fixed-Area Turbine (FAT) conventional turbo machine - choked

flow at

– Turbine inlet nozzles (HP turbine and LP turbine)

– Exhaust nozzle (core engine and fan duct, if separate)

• Unchanged from reference values:

– �� , ��, ��, �� , �� , ��, �� , ��, �� ,��

• Turbine cooling and leakage effects are neglected

• No power is removed from the turbine to drive accessories

• Constant �� and �� , and�� and �� that do not vary with power

setting

• The term 1 + � is considered as constant


Reference Values and Assumptions

• Impossible to test and record all operating conditions

• Dimensional analysis identifies correlating parameters that allow

engine performance to be analysed under different conditions

• Dimensionless pressure and temperature represented by δ and θ

• Corrected mass flow rate at engine station i

where i is the station number


Dimensional Analysis



Table 7-3 Corrected Parameters

Corrected Parameters

Figure 7-2 Compressor Performance Map

• Steady operation above the surge line is

impossible

• Entering the region even momentarily is

dangerous to the gas turbine engine.



Compressor and Fan Performance Map

Compressor stall may take one of two forms:

• High angle of attack (positive incidence) blade stall

– A front stage problem at low speeds

• Low AOA (negative incidence stall)

– Windmilling of blades

– Affects the rear stages at high speed

• Leads to blade vibration which can induce rapid destruction


Watch video on Compressor Stall

Compressor Stall

• Caused by excessive demand for compressor pressure rise

– Beyond blades’ handling capability

• Results in:

– Instantaneous breakdown of flow through the engine

– High pressure air in the combustion system is expelled forward

through the compressor

– ‘Loud’ bang’

– Resultant loss of engine thrust


Watch video on Compressor Surge

Compressor Surge

• Bleed valves to release air from

middle stages

• Multi-spool compressors

– Optimum speeds for different

spools

• Variable stator vanes, including the

inlet guide vanes to guide the airflow

at a desired angle

• Proper fuel scheduling to operate

within Compressor Performance Map

– Limiting excessive pressure ratio

during acceleration


Source: Page 29, “The Jet Engine” (1986) by

Rolls Royce plc,

Compressor Stall / Surge Prevention

Figure 7-3 Combustor Efficiency



Combustor Performance Map

Figure 7-4 Combustor Efficiency

Burner Temperature Rise

Burner inlet pressure



Combustor Performance Map

Figure 7-5 Turbine Flow Map

To achieve high

power to weight

ratio, flow

entering 1st stage

rotor is normally

supersonic. Thus,

sonic conditions

are present at the

minimum

passage area of

the inlet guide

vanes



Turbine Performance Map

Figure 7-6 Turbine Efficiency Map

Turbine efficiency

assumed as constant

during performance

analysis



Turbine Performance Map


Performance Analysis Variables for Single-Spool Turbojet Engine

Source: Page 465


Single-Spool Turbojet w/o Afterburner

© 2015 SIM University. All rights reserved.Source: Page 467 - 469






Adapted from R-1, Example 8.4









Altitude = 20 kft

Mach number = 0.8

• As thrust is reduced from 100%

(reading graph from right to

left),

o ηP increases

o ηT decreases

o ηO increases to optimum at

40% power setting

o TSFC decreases until about

40% of maximum thrust, then

increases

• Effect due to increase in ηP

dominating the reduction of ηT

at throttle settings down till

40%.


Turbojet Performance at Partial Throttle

Source: Page 476



• The basics of engine performance analysis

• The need for design point and dimensional

analysis

• Plot and critique component performance maps


Summary

• Review the component performance maps of a

compressor and turbine:

– Explain the compressor stall/surge line.

– Discuss the rationale that the flow at the

turbine inlet nozzle is assumed to be choked.


Reflection Question

Documents

Engine Performance Analysis