fundamentals and designs of various types of combustion ... 5_6_diesel... · fundamentals and...

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IIT Kanpur Kanpur, India (208016)

Fundamentals and Designs of Various Types of Combustion Chambers for Diesel Engines

Dr. Avinash Kumar Agarwal Engine Research Laboratory,

Department of Mechanical Engineering, Indian Institute of Technology, Kanpur

INDIA [email protected]

Engine Research Laboratory, IIT Kanpur

The shape of the combustion chamber is one of the decisive factors: Determines

the quantity of combustion: Performance & exhaust characteristics.

Diesel Engine combustion is greatly influenced by air turbulence: Created by the

shape of combustion chamber area.

Each Combustion chamber shape creates its own unique turbulence pattern that is

right for some application while wrong for others.

Combustion Chambers in Diesel Engines

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To optimize the filling and emptying of the cylinder with fresh (unburnt) charge

respectively over the engines operating range (All loads and speeds).

To create the conditions in the cylinder for the air and fuel to mix thoroughly: Get

Excited into a highly turbulent state: Burning of the charge to be completed in the

shortest possible time.

The Objective of Good Combustion Chamber Design


Heat loss to combustion chamber walls Injection pressure. Nozzle design: Number, size, & arrangement of holes in the nozzle Maintenance Ease of starting Fuel requirement: Ability to use less expensive fuels Utilization of air: Ability to use maximum amount of air in cylinder Weight relation of engine to power output Capacity for variable speed operation Smoothness with which forces created by expanding gases are transmitted to the

piston.

Important Factors Considered in Combustion Chamber Design

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Characterization of Common Diesel Combustion systems

DirectInjection IndirectInjection

System Quiescent MediumSwirl HighSwirlM HighswirlMediumSpray

Swirlchamber Prechamber

Size Largest Medium Mediumsmaller Mediumsmall Smallest smallest

Cycle 2/4stroke 4stroke 4stroke 4stroke 4stroke 4stroke

TC/SC/NA TC/SC TC/NA TC/NA NA/TC NA/TC NA/TC

Mediumspeed(rpm)

1202100 18003500 25005000 35004300 36004800 4500

Bore,mm 900150 150100 13080 10080 9570 9570

Stroke/bore 3.51.2 1.31.0 1.20.9 1.10.9 1.10.9 1.10.9

Compressionratio

1215 1516 1618 1622 2024 2224

chamber Openorshallowdish

Bowlinpiston Deepbowlinpiston

Deepbowlinpiston

Swirlprechamber

Single/multiorificeprechamber

Airflowpattern Quiescent MediumSwirl HighSwirl Highestswirl Veryhighswirlinprechamber

VeryturbulentinPrechamber

No.ofNozzleholes

Multi Multi Single Multi Single single

Inj.Press. Veryhigh High Medium High Lowest Lowest


Direct Injection Combustion Chamber

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The proportion of DI is increasing due to their more economical fuel consumption (up to 20% savings).

For DI engine, piston crown recess is most widely used. In this design, the fuel is injected directly into the cylinder chamber.

Lower combustion surface wall area compared to combustion volume in comparison with IDI.

More combustion taking place in and on the piston and less contact with coolant.

DI chamber has highest fuel efficiency rating compared to other chamber design.

Direct Injection Combustion Chamber


Direct Injection CI Engine Combustion Systems

Quiescent chamber with multi hole nozzle

typical of larger engines

Bowl-in-piston chamber with swirl

and multi hole nozzle

Bowl-in-piston chamber with swirl

and single hole nozzle

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Direct injection engines have two design philosophies: High-swirl design, which have a deep bowl in the

piston, a low number of holes in the injector and moderate injection pressures.

Low-swirl or quiescent engines are characterized by having a shallow bowl in the piston, a large number of holes in the injector and higher injection pressures.

Smaller engines tend to be of the high-swirl type, while bigger engines tend to be of the quiescent type.

Direct Injection Engines

Air Swirl in DI Engine


DI engines are designed so that the adequate mixing of air and fuel is enhanced by a swirling action within the combustion chamber.

Engines are designed with a specific swirl ratio typically 2.5 (swirling rotation within cylinder versus engine speed).

Swirl ratio is defined as the ratio of the air rotation speed about cylinder axis to crankshaft rotational speed.

Swirl in Diesel Engine

Air intake being directed and swirled as in enters in combustion chamber

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Types of Swirl

Induction swirl

Compression swirl

Combustion swirl


During suction stroke forcing air for rotational movement (a)

By masking one side of inlet valve (b)

By lip over one side of inlet valve (c)

Induction Swirl

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Air from periphery is forced to the centre cavity of the piston during compression

stroke.

The squishing of air is created and forced to enter tangentially in the piston cavity

when the piston reach to TDC.

Compression Swirl


Created due to partial combustion so called as a COMBUSTION INDUCED

SWIRL.

Only for pre-combustion chamber.

Combustion during delay period in pre-combustion chamber so A/F mixture

becomes rich and forces the gases with high velocity into the main combustion

chamber.

Creates high temp and provides better combustion.

Combustion Swirl (For IDI Engines only)

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Suitable for large, slow and

medium speed engines running up

to 1500 rev/min

There is sufficient time for the fuel

to be injected into the cylinder and

for it to be distributed and

thoroughly mixed with the air

charge so that combustion takes

place over the most effective crank

angle movement just before and

after TDC, without having to resort

to induction swirl and large

amounts of compression squish.

Direct Injection Open Quiescent Quadruple Valve CC


Without air swirl in the combustion-chamber there is no high hot gas velocity,

which would increase the thermal impingement on the surfaces surrounding the

chamber space. Accordingly, there will be more heat available to do useful work

so that higher brake mean effective pressures can be obtained where mixing of

the fuel and air is achieved purely by the intensity of the spray and its ability to

distribute and atomize with the surrounding air.

This is made possible by locating the injector in the center of a four valve cylinder

head and using an injector nozzle with something like 8 to 12 holes all equally

spaced and pointing radially outwards so that they are directed towards the

shallow wall of the combustion chamber

The air movement is almost quiescent (the air is inactive) and mixing depends

entirely on the discharged spray distribution and atomizing fuel particles are

therefore known as quiescent open chambers.


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The piston crown has a flat narrow annular zone inside of which is the chamber

recess,, the base of the chamber from the center to the wall has a downward dish

shape which curves up and merges with the vertical wall of the chamber. The

contour of the chamber is such that it conforms to the expanding spray formation

so that it conforms to the expanding spray formation so that fuel particles do not

normally touch the chamber surfaces.

The heat loss with this open chamber is the least compared with all other semi-

open or divide combustion chambers, which is due to its very low ratio of surface

area to volume, and thus its relative efficiency is the highest.

Generally, open quiescent combustion chambers provide good cold starting and the

lowest specific fuel consumption values relative to semi open and divided

combustion chambers.



It consists of semi-swirl induction port with an inclined centrally located injector.

It has a slightly offset bowl in the piston combustion chamber surrounded by a large annular squish zone formed between the piston crown and flat cylinder head

The incoming air enters in a tangentially and downward direction due to the valve port and seat being positioned to one side of the cylinder axis.

Air is thus forced to spiral its way down and around the cylinder as it fills the space previously occupied by the outward moving piston.

Direct Injection Semi-Open Volumetric CC Phases

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At the end of the combustion stroke the piston

reverses its direction and then commences its

compression stroke. Towards the end of the

compression stroke the bump-clearance between the

flat annular piston crown and the cylinder head

quickly decreases causing it to squeeze the swirling

air charge inwards towards the inner chamber bowl.

The air stream from all sides of the annual squish

zone flows radially inwards meeting at the center

where it is then deflected downward into the bowl. At

the bottom, the air disperses radially outward and

then upwards to the lip of the chamber wall.

Direct Injection Volumetric Combustion Chamber Phases (Compression and Injection)


The upward moving air will be met by more inwardly moving compression squish

which again pushes the air towards the center and down.

The pressure behind the discharged fuel projects it radially outwards until it strikes

the chamber wall. Some of this fuel bounces off the wall while the remainder clings

and spreads over the wall.

The completion of the fuel injection period simply increases the amount of fuel

deposited or rebounded from the chamber wall until the metered quantity of fuel

per injection has been ejected.

Direct Injection Volumetric Combustion Chamber Phases (Compression and Injection)

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As the center core of the spray moves

radially outwards from the injector nozzle,

its outer layers first become finely atomized

and then transform into clouds of vapour.

The Compressed air occupying the spaces

between the spray will have reached the

fuels ignition threshold temperature, and so

the oxygen contained in the air in the

vicinity of the fuel spray therefore reacts

with the vapour, causing ignition to occur.

Direct Injection Volumetric Combustion Chamber Phases (Ignition)


The nuclei of flames, established randomly

around the vapor clouds, then propagate

rapidly towards the bulk of the mixture

concentration near the chamber walls, the

flames are then distributed and spread

throughout the bowl due to the general air

movement within the chamber.

During expansion on the power stroke the

outward movement of the piston enables

mixing of air and fuel to continue by the

combined effect of air swirl and reversed

squish.

DI Volumetric CC (Burning and Expansion)

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Direct Injection Volumetric Combustion Chamber Illustrating Phases of Combustion


The extra air movement is achieved by

utilizing a helical or partial vortex form of

induction port passage.

The incoming air flow is given a helical twist

or semi vortex motion about the valve stem

before it passes out between the opened

valve head and its seat in a tangential

direction to the cylinder axis.

As a result, a high degree of air swirl is

generated within the curved port passage

before it is expelled into the cylinder.

Small Direct Injection Semi-Open Combustion Chamber with Helical Induction Part

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It uses a two valve cylinder head with a high swirl or vortex type induction port

with an inclined injector, which is located to one side of the cylinder axis.

The combustion chamber is a spherical cavity in the piston crown with a small

secondary recess on one side which aligns with the injector in the cylinder head to

provide access for the fuel spray discharge.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Induction)


Air from the high swirl generating induction port enters the cylinder where it is

forced to rotate about the cylinder axis in a progressive spiral fashion as the piston

moves away from the cylinder head on its induction stroke.

After the cylinder has been filled with air having a high intensity of swirl, the inlet

valve closes and the air is compressed between the cylinder head and the inwardly

moving piston crown.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Induction)

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As the piston rapidly approaches TDC, air from

the annular squish area surrounding the

chamber recess is squeezed towards the center

of the chamber, it is then forced downwards

and, at the bottom, outwards, where it then

follows the contour of the spherical chamber

wall until it again emerges at the mouth of the

chamber, where further compression squish as

the bump clearance reduces, causes the

transverse rolling movement to repeat itself.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Compression and Injection)


The transference of air from the annular squish area to the inner chamber causes

the rotational movement of the air around the cylinder to be considerably

increased as it moves into the much smaller spherical chamber.

Just before the end of the compression stroke, fuel is injected into the cylinder

from two nozzle hole set at acute angles to the chamber walls so that after the

spray penetrates the swirling air and reaches the cylinder wall, it is not reflected

but spreads over the surface in the form of a thin film.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Compression and Injection)

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The discharge of liquid spray through the hot air charge causes the surrounding air to resist partially the jet penetration so that initial outer layers of fuel partially the jet penetration so that initial outer layers of fuel particles slow down very quickly to transform into vapour.

Immediately, this vapour commences to oxidize and to ignite

5 to 10% of the total quantity of fuel discharged per cycle burns in the spray stream near the injection nozzle with the minimum of delay.

The vaporized fuel is carried away by the air stream and burns in the flame front spreading from the initial ignition zone slightly beyond the injector nozzle and very nearly in the center of the chamber.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Ignition)


The energy released by the propagating combustion in the chamber bowl causes a rapid pressure rise and simultaneously an expansion of the burning charge.

Direct Injection Semi Open Film (M-type) Combustion Chamber (Burning and Expansion)

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Direct Injection Film (M type) Combustion Chamber Illustrated Phases of Combustion


Indirect Injection Combustion Chamber

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IDI combustion chamber types Pre-combustion chamber type Swirl chamber type

Good

Excellent mixing, turbulence characteristics Can burn lower quality fuel Lower injection pressure Less pronounced knock Low noise & exhaust emissions

Bad Very high temperature/pressure in injection chamber Higher emissions, especially NOx Harder to start - glow plugs Less efficient

Indirect Injection Engines


Small Indirect-Injection Diesel Engine Combustion System

Swirl Prechamber Turbulent Prechamber

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With swirl chambers, combustion is also initiated in a separate chamber that has approx. 60% 0f the compression volume.

As soon as combustion starts, the air/fuel mixture is forced under pressure through the connecting channel into the cylinder chamber where it is turbulently mixed with the remaining air.

Swirl Chamber System


The combustion process actually takes place in a two stage divided chamber

system. Initially, combustion takes place in a spherical swirl chamber housed in

the cylinder head whereas the second half of the process is completed in the twin

disc shaped recesses in the piston crown.

The swirl chamber in the form of sphere is located to one side and above the

cylinder wall in the cylinder head.

The upper half of the sphere is cast directly in the cylinder head whereas the lower

half is a separate heat resisting nimonic alloy member flanged and cylindrical in

shape with an upward facing semi-hemispherical chamber, it fits in a machined

recess so that its underside is flush with the flat face of the cylinder head.

It is located and secured by a ball and flange while the outer cylindrical vertical

wall stands away from the machined cylinder head recess to create an isolating air

gap.

Indirect Injection Divided Chamber Swirl-Combustion Chamber

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This chamber separator is commonly known as a heat regenerative member since

it absorbs heat from combustion and dissipates it during the compression stroke.

An inclined passage through the base of the lower regenerative member forms a

throat or neck between the spherical swirl chamber and twin adjacent circular

cavities cast in the piston crown.

A pintle soft conical spray injector is positioned over the chamber at an acute

angle to the swirl chamber whereas a cold start heater plug projects horizontally

into the side of the chamber wall.

The lower regenerative member forms the lower half of the combustion chamber.

Indirect Injection Divided Chamber Swirl-Combustion Chamber


The delivery and expulsion of air and exhaust gases are provided by the inlet and exhaust valve ports.

With the indirect-injected swirl chamber method of combustion control, a high level of induction swirl is not so critical so that the intake port can be designed to cater more for improved breathing rather than the generation of high intensity induction swirl.

Air is drawn tangentially into the cylinder via the twin induction port where it then moves in a circular downward path around the cylinder wall as the piston moves away from the cylinder head.

Indirect Injection Divided Chamber Swirl Combustion Chamber (Induction)

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On the return stroke, the air change is compressed causing something like 40% of the air mass per induction stroke to be transferred through the throat of the regenerative member into the spherical swirl chamber.

The angle of the inter-linking throat passage guides the air stream tangentially into the swirl chamber so that it is forced to follow the contour of the chamber wall in a vertical circular swirl many times during the compression stroke.

As air flows through the throat passage, it absorbs heat from the hot alloy mass and from the chamber walls as it circulates around the chamber so that once combustion has been established, any fresh air entering the swirl chamber quickly attains a temperature well above the threshold ignition temperature of the liquid fuel.

Indirect Injection Divided Chamber Swirl Combustion Chamber (Compression and Injection)


When the crankshaft is of the order of 20 to 25 before TDC, fuel is injected at an acute angle in a downstream direction to the air swirl to one side of the chamber, the spray penetrates the dense air change and impinges onto the spherical surface of the regenerative member. Instantly the liquid fuel spreads out to form a thin film, which then vaporizes and is immediately swept around with the air stream.

The fuel vapour, oxygen and heat then combine to cause the oxidation reaction which is essential for ignition at random nuclei sites surrounding the vapour clouds within the swirl chamber.

Rapid flame spread follows as unburnt vapour seeks out the oxygen in the dense but rapidly rotating air charge.

Indirect Injection Divided Chamber Swirl Combustion Chamber (Ignition)

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The high burning rate produces a corresponding rapid pressure rise in the swirl chamber. As a result, the burning charge will be blown down the throat of the regenerative member, after which it divides into two separate flame fronts as they enter the twin, shallow, disc shaped recess formed in the piston crown.

The tangential entry compels the flame fronts to swirl clockwise and anticlockwise around their respective cavity walls, which gives the burning and unburnt vapour the maximum opportunity to search out the oxygen and, simultaneously, to displace the burnt products of combustion.

Indirect Injection Divided Chamber Swirl Combustion Chamber (Burning and Expansion)


Indirect Injection Swirl Combustion Chamber Illustrating Phases of Combustion

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In Pre-combustion chamber, the fuel is injected through a pintle nozzle at a low pressure (upto 450 bar)

A specially shaped baffle in the

centre of the chamber diffuses the

jet of fuel that strikes it & mixes it

thoroughly with the

Pre-combustion Chamber


This divided-chamber two-stage combustion system incorporates a heat resisting

alloy pre-combustion chamber mounted in the cylinder head slightly to one side of

the single inlet and exhaust valve seats.

The pre-combustion chamber is a two-piece cylindrical unit consisting of a large

diameter flanged section which, houses the combustion chamber and a small

diameter extended nozzle section. At the end of the enclosed nozzle are five radial

holes, which communicate with the main chamber, while the upper flanged end is

opened up to accommodate the pintle injector.

Within the cylindrical casing is a spherical chamber with a narrow parallel passage

or throat leading to the radial nozzle holes. A transverse bar with a spherical bulge

in the middle is positioned in the lower half of the spherical chamber, whereas a

cold start heater plug intersects from the side of the upper half of the chamber

wall.

Indirect Injection Divided-Chamber Pre-Combustion Chamber

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When the inlet valve opens and the piston moves away from the cylinder head, air enters the cylinder tangentially so that it rotates in a downward direction about the cylinder axis.

Once the piston has reached BDC it reverses its direction and commences to move inwards towards the cylinder head until the inlet valve closes, this then completes the induction period.

Indirect Injection Divided-Chamber Pre-Combustion Chamber (Induction)


As the piston approaches TDC, the air charge

is compressed between the cylinder head and

the piston crown so that something like 35%

to 45% of the air is forced through the five

nozzle holes which protrude below the flat

cylinder head.

Air will the n be transferred from the cylinder

in to the pre-combustion chamber via the

nozzle holes and parallel throat passage

where it is exited into a vigorous and highly

turbulent mass.

Indirect Injection Divided-Chamber Pre-Combustion Chamber (Compression and Injection)

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Towards the end of the compression stroke, fuel is discharged from the injector

towards the center of the chamber where it strikes the transverse located semi-

spherical bar.

The spray is very slightly angled so that a proportion of the spray misses the

spherical bar and reaches the base of the chamber to one side of the nozzle throat.

The liquid fuel now spreads out over the spherical bar and the mouth or throat of

the nozzle passage. Immediately, the liquid film vaporizes and is torn away from

the bar and chamber wall by the turbulent air movement.

Indirect Injection Divided-Chamber Pre-Combustion Chamber (Compression and Injection)


Oxidation then commences causing random

nuclei flame sites to form, these quickly

propagate and spread throughout the hot

dense air mass.

The resulting pressure rise created by the

burning charge reverses the direction of air

flow. The burnt and unburnt rich vapour

charge is now blown down the throat of the

nozzle where it then expands radially

outwards through the five nozzle holes into

corresponding shallow guide channels

formed in the piston crown.

Indirect Injection Divided-Chamber Pre-Combustion Chamber (Ignition)

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The thrust of combustion projects these

directional jet-like flame-fronts towards the

cylinder walls and, in doing so, sweeps the

burnt gases and soot to one side while

exposing the remaining fuel vapour to fresh

oxygen.

Indirect Injection Divided-Chamber Pre-Combustion Chamber (Burning and Expansion)


Characterization of Common Diesel Combustion systems

DirectInjection IndirectInjection

System Quiescent MediumSwirl HighSwirlM HighswirlMediumSpray

Swirlchamber Prechamber

Size Largest Medium Mediumsmaller Mediumsmall Smallest smallest

Cycle 2/4stroke 4stroke 4stroke 4stroke 4stroke 4stroke

TC/SC/NA TC/SC TC/NA TC/NA NA/TC NA/TC NA/TC

Mediumspeed(rpm)

1202100 18003500 25005000 35004300 36004800 4500

Bore,mm 900150 150100 13080 10080 9570 9570

Stroke/bore 3.51.2 1.31.0 1.20.9 1.10.9 1.10.9 1.10.9

Compressionratio

1215 1516 1618 1622 2024 2224

chamber Openorshallowdish

Bowlinpiston Deepbowlinpiston

Deepbowlinpiston

Swirlprechamber

Single/multiorificeprechamber

Airflowpattern Quiescent MediumSwirl HighSwirl Highestswirl Veryhighswirlinprechamber

VeryturbulentinPrechamber

No.ofNozzleholes

Multi Multi Single Multi Single single

Inj.Press. Veryhigh High Medium High Lowest Lowest

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Quiescent DI chamber

Multuihole nozzle DI chamber with swirl

Combustion of four sprays in DI engine with counter-clockwise swirl

Combustion of a single spray burning under large DI engine conditions

Photographs of CI Combustion Process in Different CCs


Photographs of CI Combustion Process in Different CCs

M.A.N. M DI chamber

Ricardo Comet IDI swirl chamber

Combustion of single spray in M.A.N M DI diesel

Combustion in pre-chamber(on left) and main chamber(on right) in Ricardo Comet IDI swirl chamber

diesel

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Cylinder pressure, injector needle lift and injection system fuel line pressure as functions of crank angle


Typical DI Engine Heat-release-rate Diagram Identifying Different Diesel Combustion Phases

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Cylinder pressure , rate of fuel injection and net heat release rate as functions of crank angle

During combustion process, the burning proceeds in three stages.

In the first stage, the rate of burning is very high and lasts for only a few crank angle degrees. It corresponds to the period of rapid cylinder pressure rise.

The second stage correspond to a period of gradually decreasing heat-release rate. This is the main heat-release period and lasts about 40.

The third stage corresponds to the tail of the heat release diagram in which a small but distinguishable rate of heat release persists throughout much of the expansion stroke.


A. Fuel injection across the chamber with substantial momentum. Mixing

Proceeds immediately as fuel enters the chamber and is little affected by

combustion

B. Fuel deposition on the combustion chamber walls. Negligible mixing during the

delay period due to limited evaporation. After ignition, evaporation becomes rapid

and its rate is controlled by access of the hot gases to the surface, radial mixing

being induced by radial differential centrifugal forces. Burning is therefore,

delayed by the ignition lag.

C. Fuel distributed near the wall: Mixing Proceeds during the delay, but a rate

smaller than in mechanism A. After ignition, the mixing is accelerated by the same

mechanism as in B.

Three Basic Injection, Burning, Mixing Pattern in Diesel Engines

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Schematic injection-rate and burning-rate diagrams in different types of naturally aspirated diesel combustion system

DI engine with central multihole nozzle

DI M-type engine with fuel injected on wall

IDI swirl chamber engine


Summary DI vs IDI:

IDI Higher RPM Rapid

Combustion Only works 400-800cc/cyl

(1.4 4 cyl to 6.4 ltr V8) Reduced ignition delay More swirl 5-15% fuel efficiency

penalty More complicated

combustion chamber design

May require ceramic liner in pre chambers to limit heat transfer

DI Lower RPM limited by

piston speed (flame front must keep up with piston)

Longer ignition Delay More efficient Unlimited size Injectors exposed directly

to cylinder pressures More exotic injectors

required

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fundamentals and designs of various types of combustion ... 5_6_diesel... · fundamentals and...

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