slide1 l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Diesel / Brayton Cycles ASEN 3113 PowerPoint Presentation
Download Presentation
Diesel / Brayton Cycles ASEN 3113

Loading in 2 Seconds...

play fullscreen
1 / 43

Diesel / Brayton Cycles ASEN 3113 - PowerPoint PPT Presentation


  • 792 Views
  • Uploaded on

Diesel / Brayton Cycles ASEN 3113 . Diesel Cycle. Invented by Rudolf Christian Karl Diesel in 1893 First engine was powered by powdered coal Achieved a compression ratio of almost 80 Exploded, almost killed Diesel First working engine completed 1894 - generated 13 hp. Diesel Engine.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Diesel / Brayton Cycles ASEN 3113' - sandra_john


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
diesel cycle
Diesel Cycle
  • Invented by Rudolf Christian Karl Diesel in 1893
  • First engine was powered by powdered coal
  • Achieved a compression ratio of almost 80
  • Exploded, almost killed Diesel
  • First working engine completed 1894 - generated 13 hp
diesel engine
Diesel Engine
  • Also known as Compression Ignition Engine (CI)
  • Can this engine “knock”?
  • Difference from Otto Cycle?
slide4

Thermodynamic Cycles for CI engines

  • In early CI engines the fuel was injected when the piston reached TDC
  • and thus combustion lasted well into the expansion stroke.
  • In modern engines the fuel is injected before TDC (about 15o)

Fuel injection starts

Fuel injection starts

Early CI engine

Modern CI engine

  • The combustion process in the early CI engines is best approximated by
  • a constant pressure heat addition process  Diesel Cycle
  • The combustion process in the modern CI engines is best approximated
  • by a combination of constant volume and constant pressure  Dual Cycle
slide5

Early CI Engine Cycle and the Thermodynamic Diesel Cycle

Fuel injected

at TC

A

I

R

Air

Combustion

Products

Actual

Cycle

Intake

Stroke

Compression

Stroke

Power

Stroke

Exhaust

Stroke

Qin

Qout

Air

Diesel

Cycle

BC

Compression

Process

Const pressure

heat addition

Process

Expansion

Process

Const volume

heat rejection

Process

slide6

Cut-off ratio:

Air-Standard Diesel cycle

Process a b Isentropic compression

Process b  c Constant pressure heat addition

Process c  d Isentropic expansion

Process d  a Constant volume heat rejection

- a=1,b=2,etc…for book

slide7

First Law Analysis of Diesel Cycle

Equations for processes 12, 41 are the same as those presented

for the Otto cycle

23 Constant Pressure Heat Addition

now involves heat and work

Qin

AIR

slide9

Thermal Efficiency

For cold air-standard the above reduces to:

recall,

Note the term in the square bracket is always larger than one so for the

same compression ratio, r, the Diesel cycle has a lower thermal efficiency

than the Otto cycle

So why is a Diesel engine usually more efficient?

slide10

Thermal Efficiency

Typical CI Engines

15 < r < 20

When rc (= v3/v2)1 the Diesel cycle efficiency approaches the

efficiency of the Otto cycle

Higher efficiency is obtained by adding less heat per cycle, Qin,

 run engine at higher speed to get the same power.

slide11

The cut-off ratio is not a natural choice for the independent variable

a more suitable parameter is the heat input, the two are related by:

as Qin 0, rc1

k = 1.3

k = 1.3

- compares performance of engines of the same size

slide12

Modern CI Engine Cycle and the Thermodynamic Dual Cycle

Fuel injected

at 15o before TDC

A

I

R

Air

Combustion

Products

Actual

Cycle

Intake

Stroke

Compression

Stroke

Power

Stroke

Exhaust

Stroke

Qin

Qin

Qout

Air

Dual

Cycle

TC

BC

Const volume

heat addition

Process

Compression

Process

Const pressure

heat addition

Process

Expansion

Process

Const volume

heat rejection

Process

slide13

Dual Cycle

Process 1  2 Isentropic compression

Process 2  2.5 Constant volume heat addition

Process 2.5  3 Constant pressure heat addition

Process 3  4 Isentropic expansion

Process 4  1 Constant volume heat rejection

Qin

3

2.5

3

Qin

2

2.5

4

2

4

1

Qout

1

slide14

Thermal Efficiency

Note, the Otto cycle (rc=1) and the Diesel cycle (a=1) are special cases:

slide15

The use of the Dual cycle requires information about either:

  • the fractions of constant volume and constant pressure heat addition
  • (common assumption is to equally split the heat addition), or
  • ii) maximum pressure P3.
  • Transformation of rc and a into more natural variables yields

For the same initial conditions P1, V1 and the same compression ratio:

For the same initial conditions P1, V1 and the same peak pressure P3

(actual design limitation in engines):

brayton cycle
Brayton Cycle
  • Introduced by George Brayton (an American) in 1872
  • Used separate expansion and compression cylinder
  • Constant Combustion process
other applications of brayton cycle
Other applications of Brayton cycle
  • Power generation - use gas turbines to generate electricity…very efficient
  • Marine applications in large ships
  • Automobile racing - late 1960s Indy 500 STP sponsored cars
brayton cycle22
Brayton Cycle
  • 1 to 2--isentropic compression
  • 2 to 3--constant pressure heat addition (replaces combustion process)
  • 3 to 4--isentropic expansion in the turbine
  • 4 to 1--constant pressure heat rejection to return air to original state
brayton cycle analysis
Brayton cycle analysis
  • Because the Brayton cycle operates between two constant pressure lines, or isobars, the pressure ratio is important.
  • The pressure ratio is not a compression ratio.

Efficiency:

Net work:

brayton cycle analysis24
Brayton cycle analysis

1 to 2 (isentropic compression in compressor), apply first law

**When analyzing the cycle, we know that the compressor work is in (negative). It is standard convention to just drop the negative sign and deal with it later:

brayton cycle analysis25
Brayton cycle analysis

2 to 3 (constant pressure heat addition - treated as a heat exchanger)

3 to 4 (isentropic expansion in turbine)

brayton cycle analysis26
Brayton cycle analysis

4 to 1 (constant pressure heat rejection)

We know this is heat transfer out of the system and therefore negative. In book, they’ll give it a positive sign and then subtract it when necessary.

brayton cycle analysis27
Brayton cycle analysis

net work:

Substituting:

brayton cycle analysis28
Brayton cycle analysis

Thermal efficiency:

slide29

Brayton cycle analysis

assume cold air conditions and manipulate the efficiency expression:

slide30

Brayton cycle analysis

Using the isentropic relationships,

Define:

slide31

Brayton cycle analysis

Then we can relate the temperature ratios to the pressure ratio:

Plug back into the efficiency expression and simplify:

slide33

Brayton cycle analysis

An important quantity for Brayton cycles is the Back Work Ratio (BWR).

example problem
EXAMPLE PROBLEM
  • The pressure ratio of an air standard Brayton cycle is 4.5 and the inlet conditions to the compressor are 100 kPa and 27C. The turbine is limited to a temperature of 827C and mass flow is 5 kg/s. Determine
      • a) the thermal efficiency
      • b) the net power output in kW
      • c) the BWR
  • Assume constant specific heats.
start analysis
Start analysis

Let’s get the efficiency:

From problem statement, we know rp = 4.5

net power output
Net power output:

Net Power:

Substituting for work terms:

Applying constant specific heats:

need to get t 2 and t 4
Need to get T2 and T4

Use isentropic relationships:

T1 and T3 are known along with the pressure ratios:

T2:

T4:

Net power is then:

back work ratio
Back Work Ratio

Applying constant specific heats:

brayton cycle41
Brayton Cycle
  • In theory, as the pressure ratio goes up, the efficiency rises. The limiting factor is frequently the turbine inlet temperature.
  • The turbine inlet temp is restricted to about 1,700 K or 2,600 F.
  • Consider a fixed turbine inlet temp., T3
brayton cycle42
Brayton Cycle
  • Irreversibilities
    • Compressor and turbine frictional effects - cause increase in entropy
    • Also friction causes pressure drops through heat exchangers
    • Stray heat transfers in components
    • Increase in entropy has most significance
  • wc = h2 – h1 for the ideal cycle, which was isentropic
  • wt = h3 – h4 for the ideal isentropic cycle
brayton cycle43
Brayton Cycle
  • In order to deal with irreversibilities, we need to write the values of h2 and h4 as h2,s and h4,s.
  • Then