Compression Ignition is generally known as Diesel Engine—named after its inventor Dr. Rudolf Diesel who invented it in 1899. The CI engines have high thermal efficiency and use relatively cheaper diesel fuel compared to gasoline. CI engines are extensively used for power generation, in commercial transportation, buses, marine engines, locomotives, tractors, pumping sets, stationary industrial application and machine applications. However due to its higher weight, smoke and odour its application in passenger cars is limited. The CI engines have been produced in wide power range.
Combustion in CI engines is entirely different than in SI engines. In CI engines are compressed to a much higher pressure than that in SI engine due to higher compression ratio.
Compression ratio is of the order of 12-22 due to which temperature and pressure of the air are quite high. The fuel is injected just before TDC in the form of high-pressure jet. The fuel enters the combustion chamber containing hot gases that vaporizes and after some time because of heat its combustion occurs. It is to be noted that time available for combustion is relatively quite small. Table 26.1 shows the time available for combustion in a typical CI engines.
Unlike SI engines where we have homogeneous air fuel mixture of required quality delivered to the combustion chamber by carburetor or Multi-Point Fuel Injector (MPFI) system, in CI engines air and fuel are injected separately at different times. The mixture is at best heterogeneous and mixing takes place in the combustion chamber. After the fuel in injected at high pressure in the combustion chamber containing air at high pressure and high temperature the jet disintegrates.
The fuel droplets vaporise while absorbing heat of vaporisation from the hot air resulting in decrease in the temperature of air in the vicinity of the fuel droplets. Fuel takes heat from the surrounding air in the combustion chamber. This air has to take heat from the nearby air. Until the temperature of air increases the combustion cannot occur till the full mixture reaches the ignition temperature.
The delay period for combustion cannot occur till the full mixture reaches the ignition temperature. The delay period for combustion process on account of this is physical delay. After the physical delay there would be chemical delaying which reaction starts slowly and then accelerates until ignition takes place. Ignition delay would comprise of the physical and chemical delay periods. To understand the combustion process in CI engines it would be desirable to compare salient features of combustion in SI and CI engines.
The combustion in CI engines takes place in four stages as given in Fig. 26.8:
1. Ignition delay
2. Rapid or uncontrolled trolled combustion
3. Controlled combustion
It is preparatory phase beginning from the time fuel injection commences and ends when the pressure-time curve separates from the motoring curve Figure (26.9). It is the stage in which fuel has been injected in the combustion chamber but has not ignited. The ignition delay stage should be as small as possible. However ignition delay cannot be reduced beyond a certain limit. The ignition delay can be divided into two distinct parts.
(a) Physical Delay:
It begins from the time of injection till the fuel in atomised, vaporised and get mixed with air till its temperature reaches the self-ignition temperature. Physical delay will depend on the factors which affect the atomisation and vaporisation of the fuel—such as the type of fuel, injection pressure, type of nozzle, temperature and extent of turbulence etc.
(b) Chemical Delay:
This is a period of pre-flame chemical reaction. During this part of the ignition delay the chemical reactions commence first slowly and then the reaction rates increase. Normally the chemical delay portion of the ID is longer than the physical delay. The total ignition delay is sum of the physical and chemical delays. The ignition delay time is shorter than injection period.
During this stage that starts from end of ignition delay and terminates when about maximum pressure is reached. During this phase the fuel that has been admitted during the ID period would have evaporated and is ready for combustion. There would be very rapid combustion and quiet marked rise in pressure. The rate of heat release is highest during this phase. If the pressures are quite high a violent pounding noise known as ‘Diesel knock’, is produced during this stage. During this stage 30-35% of total heat is generated.
It starts at the end of second stage the combustion fairly steady state. Whatever fuel is injected virtually same gets burnt. During this phase the pressures are more or less steady. At the end of this phase, which the end of injection, the temperature reaches maximum value. Up to the end of the third stage about 70% to 80% of total heat is released. This is known as controlled stage of controlled combustion as the fuel gets burned as shown as it enters the chamber in this stage.
Theoretically combustion should end after the third stage when fuel injection has ceased. However this does not happen because there would be still unburnt or partially burnt fuel particle which would continue to burn as soon as they come in contact with O2 in the combustion chamber. Thus phase starting from the end of the third phase continues during part of the expansion stroke. The duration may last 70-80° from the TDC. During this phase 5% to 10% of total energy is released.
Block diagram given in Fig. 26.10 describes the entire sequence of events of combustion process in CI engine.
Variables Affecting Delay Period:
It is quite difficult to make division of physical and chemical portions of the ignition delay. At best only an idea can be made about them that tell that generally chemical delay is larger than physical delay. Ignition delay plays a very important role in both design and performance of the engine. It affects the pressure rise in the engine and thus is responsible for knocking. Figure 26.10 shows the exact points where ignition delay starts and ends.
Following are the factors affect the ignition delay:
Type of fuel plays a very important role in ignition delay because vaporisation and chemical delay would depend also on the characteristics of the fuel. The self-ignition temperature is a property of the fuel. A lower self-ignition temperature means and wider margin between the compressed air temperature and fuel temperature. A short ignition delay results in smooth operation of engine in the presence of good fuel. Cetane number is a measure of the ignition delay.
(b) Injection Advance:
The delay period increases with increase in injection advance angle in high-speed diesel engine.
(c) Injection Pressure:
Ignition delay increases with the increase in inlet pressure.
(d) Intake Temperature:
Ignition delay increases with the increase in inlet pressure.
(e) Compression Ratio:
Increase in CR would increase both pressure and temperature. The delay period is reduce with the increase in compression ratio.
(f) Cooling Water Temperature:
Ignition delay reduces with increases in jacket cooling water temperature.
With the increase in engine speed both temperature and pressure of air would increase thus reducing the delay period.
(h) Air Fuel Mixture:
Fuel air to ratio increases with load and vice versa. Any reduction in the fuel air ratio will reduce the cylinder wall temperature and hence delay period increase.
(j) Engine Side:
Large engines operate on slow speed because of size limitation. At lower speeds the ignition delay period increases.
(k) Combustion Chamber:
The combustion chambers can have import bearing on ignition delay. The combustion chambers with pre-combustions chambers have shorter ignition delays as compared to the open type.
Summary of variables which reduce ignition delay.
Diesel engines especially in passenger vehicles like buses and three wheelers have been condemned for being pollutant primarily due to smoke emission. It would be desirable if phenomenon of smoke were understood. The diesel engines normally induct fixed amount of air and only fuel quantity is adjusted according to the load. The air fuel ratio in diesel engines may vary widely from 1: 20 or even less at full load to 1 : 100 at no loads. At higher loads there is appreciable smoke.
Fuel which beings at the beginning of the ignition delay period gets accumulated till combustion appears at the end of the ignition lag phase. This fuel would auto-ignite after end of delay period giving very high pressure rise. If the ignition delay has been long resulting in large amount of fuel accumulation rate of rise of pressure and maximum pressure during the second phase of burning could be quite high. Such pressure rates may cause diesel knock.
Diesel knock primarily arises from long ignition delay for the following reasons:
(a) Accumulation of large amount of fuel and
(b) Homogenisation and preparation of fuel for combustion.
It is very difficult to distinguish between knocking in CI engines since normal working of these engines is also quite noisy. There are methods to reduce or minimise it. In the CI engines knocking occur in the initial phase only, whereas in SI engine it occurs towards the last phase but though the phenomenon sounds same there is difference as follows.
The following factors affecting knock in CI engines are given below:
(a) Tendency to knock reduces if the air fuel mixture has-
(i) High temperature
(ii) High density
(iii) Short delay
(v) Reactive mixture
(b) Also tendency of knock would increase if-
(i) Compression ratio is lowered
(ii) Inlet air temperature is reduced
(iii) Coolant temperature is reduced
(iv) Load is reduced
(v) Decreasing the injection pressure
(vi) Decreasing the speed
(vii) Decreasing the turbulence
(a) Cetane rating is quite similar Octane rating of gasoline which is measure of resistance to knock. The Cetane measure the ignition lag on which diesel knock depends. Cetane (C16H34) is straight can paraffin and has a good ignition quality. It is assigned arbitrarily a rating of 100 Cetane number (CN = 100). Alpha Methyl Napthalene (C11H10) which has poor ignition quality is assigned a Cetane value of size (CN = 0).
(b) Cetane Number is defined as the percentage by volume of Cetane in a mixture of Cetane and a-methyl naphthalene that produces the same ignition delay as the fuel being tested in the same standard engine under the same standard operating condition. Originally α-methyl naphthalene was used as reference fuel along with Cetane. However these days instead of α -methyl naphthalene heptamethyl none is used with CN = 15.
(c) Thus Cetane rating of 60 indicates that fuel would give the same ignition delay in a standard engine under standard test conditions as a mixture of 60 parts by volume of Cetane and 40 parts of α-methyl-naphthalene.
Thus Cetane No. calculated by-
C N = (% of Cetane) + (% α-methyl-naphthalene)
(d) Test Conditions:
The test is carried out in a single cylinder standard engine at the following condition:
(i) Speed —900 RPM
(ii) Jacket water temperature —100° C
(iii) Injection advance — 23°C
(iv) Ignition delay — 13°C
The test fuel is first tested and the fuel pump delivery is adjusted to give particular air/fuel ratio. The injection timing is adjusted to give injection advance of 14°. Then compression ratio is adjusted so that combustion occurs at TDC. Thus the test fuel has ignition delay of 13° in this case. Now the charts showing relationship between compression ratio at 13° and the Cetane number are referred to find out the Cetane number. Thus the knock in the engine is directly related to the ignition delay of fuel.
Some additives serve to reduce the self-ignition temperature of fuel by acting as local ignition points. Such additives can be mixed in the diesel to reduce its tendency to knock. Thus they improve the ignition quality of fuel by reducing ignition delay. Some of these additives are given in Table 26.4.
Importance of Turbulence in CI Combustion Engines:
(a) It is essential to have well organised movement of air within the combustion chamber for – (i) speedy evaporation of fuel (ii) to enhance air fuel mixing (iii) to increase combustion speed and (iv) to increase efficiency. Due to high velocities involved the flow of air within the combustion chamber is turbulent. Due to turbulence the transfer rates like vaporisation of fuel, heat transfer rates, mixing and combustion rates are greatly increased.
(b) The main aim of the CI combustion chamber design is to ensure proper mixing of fuel with the air in a very short time. To achieve this well organised movement of air is essential. Turbulence in the CI engines is quite systematic and built into the design. It would be desirable to understand various types of movements of the air within the combustion chamber.
(i) Turbulence is quite high during suction and recedes towards BDC.
(ii) Again during compression the turbulence increases as swirl, squish and tumble.
The rotational motion of air within the cylinder is called swirl. The swirl is achieved by a variety of methods like shaping the intake manifold, valve, valve port and at times by suitably contouring the piston. Swirl enhances mixing and makes the flue air mixtures homogeneous. Swirl is the main mechanism to spread the flame within the combustion chamber.
The swirl ratio used to indicate the extent of rotational motion within the combustion chamber. If the ratio of average angular speed to the engine RPM.
Following are the main types of swirls:
(a) Induction Swirl:
In this air flow is directed in a particular path while entering the cylinder. This method is generally employed for open combustion chamber.
(b) Compression Swirl:
Here the swirl is produced during the compression stroke air is forced through a passage into swirl chamber made separately during the compression stroke, as shown in Fig. 26.11.
(c) Combustion Induced Swirl:
This swirl is produced by using the phenomenon of high initial pressure rise due to partial combustion.
Radial inward movement of air while the piston reaches the TDC is called Squish. “Squish” can be defined as radically inward flow of air towards the combustion recess by squeezing it out from between the piston and cylinder head during the end of compression stroke. This movement of air is due to the fact that most of the modern chambers have clearance volume in the central portional only. While the piston goes moves the TDC the air from the outer edges where there remains no space rushes towards the central portional giving rise to inwards radial motion called squish.
As the piston reaches close to the TDC the squish generates secondary motion about circumferential axis near the outer edges. This motion is called ‘tumble’.
To achieve this either the fuel is directed towards air or air is directed towards fuel.
Basically there are three methods to create air movement or swirl during:
(ii) Compression or
(iii) Partial combustion.
(a) Open Combustion Chambers:
The combustion chambers in which swirl is produced during induction are called ‘open combustion chambers’ or Direct injection chambers.
(b) Turbulent Combustion Chambers:
The combustion chamber in which compression stroke is used to produce swirl are called ‘Turbulent or swirl combustion chambers’.
(c) Pre-Combustion Chambers:
The combustion chambers in which partial combustion is used to create swirl are ‘Turbulent or air-cell combustion chambers’.
Combustion Chambers for CI Engines:
In CI engine only air is admitted in the cylinder. The field is injected in the hot compressed air at about 15° before TDC in the compression stroke. The mixing of air and fuel as well as combustion of the mixture is to be done in the cylinder with a short time of 15-25° of crank rotation after TDC in the working stroke for best efficiency. Good mixing requires entire mass of air in the combustion chamber must sweep past in the fuel jet in an ordinary fashion. Turbulence is not enough. Orderly movement is called ‘Swirl’. The main objective of CI combustion chamber is to create the swirl.
Basic functions of CI Engine combustion chamber are:
(i) To assist and support the fuel injection system in preparation of cyl-charge.
(ii) To obtain efficient combustion for high thermal efficiency.
(iii) To assist in achieving smooth and noiseless combustion.
In general, combustion chambers are classified as follows:
Open combustion chambers of various shapes are shown in Fig. 26.12.
These are open to the cylinder. It is also known as direct injection or quiescent chamber. This chamber is often an extension of main cylinder only. The compressed air is confined in an undivided space. Fuel is injected directly into the combustion space. The mixing solely depends upon the spray characteristics and air motion present. Fuel injector is placed on the cylinder head. The chamber is often symmetrical in space.
There are three basic methods to create swirl in CI engine combustion chambers.
Each method is used in one of the three types of CI combustion chambers:
1. Induction Swirl:
This is generated during induction stroke by bringing the intake flow of air into cylinder with an initial angular momentum. Induction swirl is used in open chambers.
There are two general methods of creating induction swirl:
(i) The air flow is tangential to the cylinder wall. There is sideways and downward motion due to swirl- (a) Deflected port, (b) Shrouding the inlet valve (also masking),
(ii) The swirl is achieved within the port about the valve axis before entry to cylinder since whole periphery of the valve is used.
(iii) Squish – Induction swirl is usually augmented by a secondary movement called squish.
2. Compression Swirl:
This is generated in a separate chamber adjacent to main CC. It is called a divided chamber system and chamber is called swirl chamber. Compression creates a very strong swirl.
3. Combustion Induced Swirl:
This also takes place in a separate chamber adjacent to the CC and connected by a small throat. It is called pre-combustion chamber. Fuel is injected in this chamber. Here part of combustion takes place. Pressure created forces the hot mixture along with burning fuel with high velocity. This creates a very- strong swirl. This type of swirl is used in pre-combustion chamber and air-cell type turbulent CC.
Open combustion chamber using induction swirl have following advantages and disadvantages:
(a) High thermal efficiency—low turbulence and low heat transfer.
(b) Cold starting easier.
(c) No work for producing swirl. Therefore, High Thermal efficiency and low BSFC.
(d) Suitable for large, slow speed engines (d > 2000 mm) cheap low cetane rating field can be used. Ignition delay unimp.
(a) Weak swirl necessitate multi-hole nozzles and high injection pressure. More maintenance.
(b) Small nozzle openings make regulation of fuel supply.
(c) Shrouded valves—Low volumetric efficiency.
(d) Weak swirl requires excess air. Bulk engine.
(e) Induction swirl independent of speed. Difficult to maintain efficiency.
A chamber that depends upon primary turbulence of air to break up the fuel spray and form a homogeneous mixture is called a turbulent chamber.
These chambers generate swirl due to compression. Figure 26.13(a) shows a Ricardo swirl chamber Comet Mark-II. It consists of spherical swirl chamber giving strong rotary motion. It is intense at 15° BTDC. Higher the speed, greater the swirl. The operation of turbulent chamber combines open chamber and pre-combustion chamber operation.
Compression swirl has the advantages:
(a) Strong swirl-single hole and low pressure
(b) High pm due to greater utilisation of air
(c) Injector located on one side of cylinder Therefore, large diametre valves can be used
(d) Swirl and speed
Therefore, Variable speed operation is possible
(e) Delay period reduces due to high temperature Therefore, Low cetane rating
(f) Smooth operation
(a) Compression work increases. Therefore, low ηm, ηth and high BSFC
(b) Complex construction of cylinder makes it costly
(c) Strong swirl increases heat losses starting difficult
Divided (Turbulent) Combustion Chamber using Combustion Swirl:
These chambers are not widely used nowadays.
1. Pre-combustion chamber
2. Air cell combustion
3. MAN combustion chamber.
1. Pre-Combustion Chamber:
The most popular design in the small, high speed engine range is the pre-combustion chamber. In this type of engine the clearance volume is divided into two parts by a restricting passageway as illustrated in Figs 26.14 and 26.15. The small ante chamber is called the pre-combustion chamber (25 to 40% of clearance volume).
(1) Single orifice nozzle
(2) Takes initial stock of peak pressure
(3) Low cetane rating fuel can be used
(4) It has multifuel capability
(5) Tendency to knock is minimum—short or no delay period
(6) High injection pressure not required
(7) Suits for high speed engine
(1) Cold starting difficult
(2) Heat loss is high
2. Air Cell Combustion Chamber:
The air-cell diesel is essentially an open combustion chamber engine with a small ante-chamber (air cell) that is remote from the nozzle. The chamber requires high primary turbulence, if a single orifice nozzle is used or else requires a multiorifice nozzle.
An engine of the former type is illustrated in Fig. 26.14 and an American version of the latter type in Fig. 26.15. In operation of the engine, the sequence of events is similar to the open chamber until ‘after combustion’ has been initiated. In Fig. 26.15 the air cell is called the cup wiper and contains about 5% of compressed volume.
A combination of the pre-combustion chamber and air-cell principles in one design is illustrated in Fig. 26.16. It is called the energy cell or lanova combustion chamber.
A throttling pintle type nozzle with an opening pressure of 150 bar and small spray angle is recommended, and a fuel of good quality 50 cetane or better is used.
The energy cell discharges either into a double lobe main chamber or into a single-lobe chamber. Either construction assists the formation of high velocity swirl in the main chamber.
Combustion probably begins in the main chamber, where the spray envelope resides, but spreads immediately to the fuel in the energy cell, and the initial high pressure is confined in that chamber. The blast from the minor cell is directed against the final portion of the fuel spraying from the nozzle, and therefore a mixture of fuel and burning gases is swept around the main chamber.
As the piston descends, the pressure difference between cell and chamber will assist discharge from the energy cell. The discharge from the main major cell will pick up fuel in the minor cell and pass into the main chamber, to renew the turbulence and complete combustion.
3. M-Combustion Chamber (MAN C.C.):
This combustion chamber was developed by Dr. Meurer of MAN Germany. This is in the form of a spherical cavity in the piston. Contrary to the common belief that the fuel should not impinge on the combustion chamber walls, the fuel in this chamber is directed tangentially on to the combustion chamber walls.
It is believed that a small but sufficient quality of fuel (particles) will burn before the fuel strikes the chamber walls so that the temperature and delay period is normal. The fuel that impinges must evaporate before it burns. This process being relatively slow there will be much of the fuel to take part in the second stage of combustion. This means the tendency to knock is greatly reduced.
This leads us to doubt that combustion during third stage may occupy considerable times which lower the efficiency due to advancement of piston in the expansion stroke. But the temperatures are sufficiently high to cause rapid evaporation and combustion of fuel and the time losses will not increase significantly. This combustion chamber is not sensitive to quality of fuel and better performance can be expected from smaller engines.