THERMAL ENGINEERING
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THERMAL ENGINEERING (ME 2301 ). M.R.SWAMINATHAN Assistant Professor Department of Mechanical Engineering Anna University Chennai Chennai-25. DIESEL CYCLE .

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Thermal engineering me 2301

THERMAL ENGINEERING(ME 2301 )

M.R.SWAMINATHAN

Assistant Professor

Department of Mechanical Engineering

Anna University Chennai

Chennai-25


Thermal engineering me 2301

DIESEL CYCLE

The diesel cycle is the ideal cycle for CI (Compression-Ignition) reciprocating engines. The CI engine first proposed by Rudolph Diesel in the 1890s, is very similar to the SI engine, differing mainly in the method of initiating combustion.


Thermal engineering me 2301

In diesel engines, ONLY air is compressed during the compression stroke, eliminating the possibility of auto-ignition.

Diesel engines can be designed to operate at much higher compression ratios, typically between 12 and 24.


Thermal engineering me 2301

The fuel injection process in diesel engines starts when the piston approaches TDC and continues during the first part of the power stroke.

Therefore, the combustion process in these engines takes place over a longer interval.


Thermal engineering me 2301

Because of this longer duration, the combustion process in the ideal Diesel cycle is approximated as a constant-pressure heat-addition process.

This is the ONLY process where the

Otto and the Diesel cycles differ.


Thermal engineering me 2301

Ideal Cycle for CI Engines


Thermal engineering me 2301

Thermal efficiency of Ideal Diesel Cycle

Under the cold-air-standard assumptions, the efficiency of a Diesel cycle differs from the efficiency of Otto cycle by the quantity in the brackets.


Thermal engineering me 2301

The quantity in the brackets is always greater than 1. Therefore, hth,Otto > hth, Dieselwhen both cycles operate on the same compression ratio.

Also the cuttoff ratio, rc decreases, the efficiency of the Diesel cycle increases.


Thermal engineering me 2301

DUAL / LIMITED PRESSURE CYCLE


Thermal engineering me 2301

BRAYTON CYCLE – GAS TURBINE

The open gas-turbine cycle can be modeled as a closed cycle, as shown in the figure below, by utilizing the air-standard assumptions


Thermal engineering me 2301

BRAYTON CYCLE - PROCESSES

12 Isentropic compression (in a compressor)

23Constant pressure heat addition

34Isentropic expansion (in a turbine)

41Constant pressure heat rejection


Thermal engineering me 2301

The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand.


Valve timing

VALVE TIMING

  • Why valve Timing?

  • Is it significant?

    Yes

    Burning exact quantity of Air/Fuel mixture at right time and place produces more power output

    Moreover more economy and lesser emissions


Valve timing diagram actual

VALVE TIMING DIAGRAM- actual


I c engines

I.C.ENGINES

External Combustion Engine

A device where the working fluid is obtained indirectly

e.g Steam or Locomotive boiler

Coal is first burnt and then the heat energy obtained from coal is utilised to transform water to steam which is the working fluid.

So the name External combustion engine just to differentiate from an I.C. engine


Classification of i c engines

CLASSIFICATION OF I.C ENGINES

Internal combustion engines are classified according to

  • Fuel used – D/P/G

  • Cycle of operation - O/Di/Du

  • Ignition system – SI / CI

  • Number of strokes – 2S / 4S

  • Type of cylinder arrangement – In / V/ H

  • Type of cooling – Air/Water


Thermal engineering me 2301

CLASSIFICATION OF I.C ENGINES

  • Speed of engine – L / M / H

  • Type of Lubrication

  • No. of cylinders – Single / multi


Components of i c engine

COMPONENTS OF I.C ENGINE

  • Cylinder Block

    • Part of engine frame that contains cylinders in which piston moves

    • Supports liners & head

  • Cylinder Head

    • Serves to admit, confine, and release fuel/air

    • Cover to cylinder block

    • Supports valve train

  • Crankcase

    • Engine frame section that houses the crankshaft


Components of i c engine1

COMPONENTS OF I.C ENGINE

  • Piston

    • Acted on by combustion gases

    • Lightweight but strong/durable

  • Piston Rings

    • Transfer heat from piston to cylinder

    • Seal cylinder & distribute lube oil

  • Piston Pin

    • Pivot point connecting piston to connecting rod

  • Connecting Rod

    • Connects piston & crankshaft

    • Reciprocating (rotating motion)


Components of i c engine2

COMPONENTS OF I.C ENGINE

  • Crankshaft

    • Combines work done by each piston

    • Drives camshafts, generator, pumps, etc.

  • Flywheel

    • Absorbs and releases kinetic energy of piston strokes (smoothes rotation of crankshaft)

  • Valves

    • Intake: open to admit air to cylinder (with fuel in Otto cycle)

    • Exhaust: open to allow gases to be rejected

  • Camshaft & Cams

    • Used to time the addition of intake and exhaust valves

    • Operates valves via pushrods & rocker arms


I c engine terminology

I.C ENGINE TERMINOLOGY

  • Bore is the diameter measurement of the cylinders in a piston engine

  • Stroke is a single traverse of the cylinder by the piston (from TDC to BDC)

    • 1 revolution of crankshaft = 2 strokes of piston

  • Compression Ratio is defined as the ratio of the volume of the cylinder at the beginning of the compression stroke (when the piston is at BDC) to the volume of the cylinder at the end of the compression stroke (when the piston is at TDC)

    • Common sparkignition compression ratio:6:1 to 12:1

    • Common compression-ignition ratio: 16:1 to 23:1


Four stroke cycle

Four stroke cycle

  • Intake stroke: intake valve opens while the piston moves down from its highest position in the cylinder to its lowest position, drawing air into the cylinder in the process.

  • Compression stroke: intake valve closes and the piston moves back up the cylinder. This compresses the air & therefore heats it to a high temperature, typically in excess of 1000°F (540°C).

    Near the end of the compression stroke, fuel is injected into the cylinder. After a short delay, the fuel ignites spontaneously, a process called auto ignition.

  • Combustion stroke: The hot gases produced by the combustion of the fuel further increase the pressure in the cylinder, forcing the piston down

  • Exhaust stroke: exhaust valve opens when the piston is again near its lowest position, so that as the piston once more moves to its highest position, most of the burned gases are forced out of the cylinder.


Four stroke cycle1

Four stroke cycle


I c engine parts

I.C.ENGINE PARTS


I c engine parts1

I.C ENGINE PARTS


Operation of cam

OPERATION OF CAM


Multi cylinder i c engine

MULTI CYLINDER I.C. ENGINE


Two stroke i c engine

TWO STROKE I.C. ENGINE


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