8/13/2019 1 STAGES of Combustion in CI Engine
1/22
8/13/2019 1 STAGES of Combustion in CI Engine
2/22
The Combustion in C.I. engine is divided
into four stages
Delay Period(Pre-flame combustion) (A-B)
Uncontrolled combustion (B-C)
Controlled combustion (C-D)
After burning (D-E)
8/13/2019 1 STAGES of Combustion in CI Engine
3/22
TDC
8/13/2019 1 STAGES of Combustion in CI Engine
4/22
Delay period (A-B)
No noticeable deviation of pressure diagram from pure air
compression curve
This Period is counted from start of injection and start of
combustion
Physical DelaySpray disintegration and small droplets formation (friction)
Heating of liquid fuel and evaporation
Diffusion of vaporized fuel into air to form an ignitable
mixture within the air fuel range
Chemical Delay
Decomposition of heavy hydrocarbons into lightercomponents
Pre-ignition chemical reaction between the decomposed
components and oxygen
8/13/2019 1 STAGES of Combustion in CI Engine
5/22
Rapid or Uncontrolled combustion (B-C)
A considerable amount of fuel is accumulated in the
combustion chamber during the delay period.
So, enormous amount of energy produced
Ignition in one place is followed by else where .
The rate and quantity of combustion depends on duration of
delay period .
peak pressure is attained during this stage .
8/13/2019 1 STAGES of Combustion in CI Engine
6/22
Controlled combustion (C-D)
The temperature in the cylinder is so high that any fuel
injected after this time will burn as soon as it finds the
oxygen .
Further rise in pressure is controlled by injection rate .
This period is assumed from peak pressure to peak
temperature
8/13/2019 1 STAGES of Combustion in CI Engine
7/22
Late Burning phase or Afterburning (D-E)
Occurs in the expansion stroke of Engine.
Combustion does not stop after the completion of injection
process
The Thermal decomposition of the part of fuel takes place
during uncontrolled and controlled combustion.
The decomposed fuel molecules contain enough number of
hydrocarbons and carbon particles left in the combustion
chamber start burning as soon as they contact with oxygen,which have lower reaction rate. This process continues for
certain duration is called late burning
Produces smoky exhaust .
8/13/2019 1 STAGES of Combustion in CI Engine
8/22
Combustion in CI Engine
In a CI engine the fuel is sprayed directly into the cylinder and the vaporised
part of the fuel mixes with air and ignites spontaneously.
These photos are taken in a RCM under CI engine conditions with swirl
0.4 ms after ignition 3.2 ms after ignition
3.8ms after ignition Late in combustion process
1cm
Air flow
8/13/2019 1 STAGES of Combustion in CI Engine
9/22
In-Cylinder Measurements
This graph shows the fuel injection flow rate, net heat release rate and
cylinder pressure for a direct injection CI engine.
Start of injection
Start of combustion
End of injection
8/13/2019 1 STAGES of Combustion in CI Engine
10/22
Combustion in CI Engine
The combustion process proceeds by the following stages:
Ignition delay (ab)- fuel is injected directly into the cylinder towards the end ofthe compression stroke. The liquid fuel atomizes into small drops and
penetrates into the combustion chamber. The fuel vaporizes and mixes with
the high-temperature high-pressure air.
Premixed combustion phase (bc)combustion of the fuel which has mixed
with the air to within the flammability limits (air at high-temperature and high-
pressure) during the ignition delay period occurs rapidly in a few crank angles.
Mixing controlled combustion phase (cd)after premixed gas consumed, the
burning rate is controlled by the rate at which mixture becomes available for
burning. The burning rate is controlled primarily by the fuel-air mixing process.
Late combustion phase (de)heat release may proceed at a lower rate well
into the expansion stroke (no additional fuel injected during this phase).
Combustion of any unburned liquid fuel and soot is responsible for this.
8/13/2019 1 STAGES of Combustion in CI Engine
11/22
Four Stages of Combustion in CI Engines
Start of
injection
End of
injecction
-10 TC-20 10 20 30
8/13/2019 1 STAGES of Combustion in CI Engine
12/22
CI Engine Types
Two basic categories of CI engines:
i) Direct-injectionhave a single open combustion chamber into which fuel
is injected directly
ii) Indirect-injectionchamber is divided into two regions and the fuel is
injected into the prechamber which is connected to the main chamber via a
nozzle, or one or more orifices.
8/13/2019 1 STAGES of Combustion in CI Engine
13/22
Direct Injection
quiescent chamber
Direct Injection
multi-hole nozzle
swirl in chamber
Direct Injection
single-hole nozzle
swirl in chamber
Indirect injection
swirl pre-chamber
8/13/2019 1 STAGES of Combustion in CI Engine
14/22
Combustion Characteristics
Combustion occurs throughout the chamber over a range of equivalence
ratios dictated by the fuel-air mixing before and during the combustion phase.
In general most of the combustion occurs under very rich conditions within the
head of the jet, this produces a considerable amount of solid carbon (soot).
1o
ASI5o
ASI
Diffusion flame
Fuel vapourHigh soot
Liquid fuel
8/13/2019 1 STAGES of Combustion in CI Engine
15/22
Ignition Delay
Ignition delay is defined as the time (or crank angle interval) from when
the fuel injection starts to the onset of combustion.
Both physical and chemical processes must take place before a significant
fraction of the fuel chemical energy is released.
Physical processesare fuel spray atomization, evaporation and mixing of
fuel vapour with cylinder air.
Good atomization requires high fuel pressure, small injector hole diameter,
optimum fuel viscosity, high cylinder pressure (large divergence angle).
Rate of vaporization of the fuel droplets depends on droplet diameter,
velocity, fuel volatility, pressure and temperature of the air.
Chemical processessimilar to that described for autoignition phenomenon
in premixed fuel-air, only more complex since heterogeneous reactions
(reactions occurring on the liquid fuel drop surface) also occur.
8/13/2019 1 STAGES of Combustion in CI Engine
16/22
Fuel Ignition Quality
The ignition characteristics of the fuel affect the ignition delay.
The ignition quality of a fuel is defined by its cetane numberCN.
For lowcetane fuels the ignition delay is long and most of the fuel is
injected before autoignition and rapid combustion, under extreme cases
this produces an audible knocking sound referred to as dieselknock.
For highcetane fuels the ignition delay is short and very little fuel is injected
before autoignition, the heat release rate is controlled by the rate of fuel
injection and fuel-air mixingsmoother engine operation.
8/13/2019 1 STAGES of Combustion in CI Engine
17/22
Cetane Number
The method used to determine the ignition quality in terms of CN is analogous
to that used for determining the antiknock quality via the ON.
The cetane number scale is defined by blends of two pure hydrocarbon
reference fuels.
By definition, isocetane (heptamethylnonane, HMN) has a cetane number of
15 and cetane (n-hexadecane, C16H34) has a value of 100.
The higher the CN the better the ignition quality, i.e., shorter ignition delay.
The cetane number is given by:
CN = (%hexadecane)+ 0.15 (% HMN)
8/13/2019 1 STAGES of Combustion in CI Engine
18/22
Factors Affecting Ignition Delay Time
Injection timingAt normal engine conditions the minimum delay occurs
with the start of injection at about 10-15 BTC.
Earlier or later injection timing results in a lower air temperature and
pressure during the delay periodincrease in the ignition delay time
Injection quantityFor a CI engine the air is not throttled so the load is
varied by changing the amount of fuel injected.
Increasing the load (bmep) increases the residual gas and wall temperature
which results in a higher charge temperature at injectiondecrease in the
ignition delay.
Intake air temperature and pressurean increase in ether will result in a
decrease in the ignition delay, an increase in the compression ratio has the
same effect.
ff f l l
8/13/2019 1 STAGES of Combustion in CI Engine
19/22
Effect of Multiple Injections on
combustion processInjection
System
Start of Injection
deg, bTDC
Fuel injection
Nomenclaturea(b)-c-d(e)
Injection
Profile
Single step 12 0(0)-0-100(20)
Split 1 12 10(2)-6-90(18)
Split 2 12 20(4)-6-80(16)
Split 3 12 40(8)-6-60(12)
Split 4 12 50(10)-6-50(10)
a - Percentage mass injected during pre-injection pulse
b - Duration of pre-injection pulse (in degrees)
c - Dwell period between pre and main injection pulses (in degrees)
d - Percentage mass injected during main injection pulse
e - Duration of main injection pulse (in degrees)
Eff t f M lti l I j ti H t
8/13/2019 1 STAGES of Combustion in CI Engine
20/22
-5
5
15
25
35
45
55
65
75
8595
105
348 358 368 378 388 398 408
Crankangle,deg
HRR,J
/de
Single
Split1
Split2
Split3
Split4
Injection rate
shape
Ignition delay,
deg
Peak Heat Release
Rate, J/deg
Combustion
Duration, deg
Single step 6.6 97.1 55.5
Split1 6.1 47.9 61.3
Split2 6.3 74.3 60.1
Split3 6.4 90.2 57.2
Split4 6.5 95.2 56.4
Effect of Multiple Injections on Heat
Release Rate(HRR)
8/13/2019 1 STAGES of Combustion in CI Engine
21/22
8/13/2019 1 STAGES of Combustion in CI Engine
22/22
Heat release rate