undefined | unit 3 si and ci engines

Unit – 3

SI and CI Engines

Q1) Explain the working of simple carburettor.

A1)

The Simple Carburettor

• The figure shows the details of a simple carburettor: -

Fig. Simple Carburetor

• The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and ametering orifice, a venturi, a throttle valve and a choke.

• The float and needle valve system maintain a constant fuel of gasoline in the float chamber.

• If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply wall and admitting fuel.

• When the designed level has been reached, the float closes the fuel supply valve, thus stopping additional fuel flow from the supply system.

• Float chamber is vented either to the atmosphere or to the upstream side of venturi.

• During suction, stroke air is drawn through the venturi.

• As the air passes through the venturi, the velocity increases reaching a maximum at the venturi throat.

• Correspondingly, the pressure decreases reaching a minimum.

• From the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the throat of the venturi.

• Because of the differential pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is discharged into the air stream.

• The fuel discharge is affected by the size of the discharge jet and it is chosen to give the required air fuel ratio. The pressure at the throat at the fully open throttle condition lies between 4 to 5 cm of Hg.

• A simple carburetor of the type described above suffers from a fundamental drawback in that it provides the required A/F ratio only at 1 throttle position.

• At the other throttle position, the mixture is either leaner or richer depending on whether the throttle is opened less or more.

Q2) What is Electronic Fuel Injection System: Write its Merits and Demerits.

A2)

Electronic Fuel Injection System

Modern gasoline injection systems use engine sensors, a computer and solenoid operated fuel injectors to meter and inject the right amount of fuel into the engine cylinders.

This system called electronic fuel injection (EFI) use electrical and electronic devices to monitor and control engine operation.

An electronic control unit (ECU) on the computer receives electrical signals in the form of current or voltage from various sensors.

It then uses the stored data to operate the injectors, ignition system and other engine related devices.

As a result, less unburnt fuel leaves the engine as emissions and the vehicles give better mileage.

The fuel injector in an EFIis nothing but a fuel valve.

When it is not energized, spring pressure makes the injector to remain closed and the fuel will enter the engine.

When the computer sends the signal through the injector coil, the magnetic field attracts the injector armature. Fuel then spurts into the intake manifold.

The injector pulse width is an indication of the period for which each injector is energized and kept open.

The computer decides and controls the injector pulse width based on the signals received from the various sensors.

Under full load, the computer will sense a wide-open throttle, high intake manifold pressure and high inlet air flow.

The ECU will then increase the injection pulse width to enrich the mixture which will enable the engine to produce high power.

Under low load and idling conditions, the ECU will shorter the pulse width by which the injectors are kept in the closed position over a longer period of time.

Because of this, the air fuel mixture will become leaner and will result in better fuel economy.

Electronic fuel injection system has a cold start injector too.

This is an extra injector that sprays fuel into the centre of the engine intake manifold, when the engine is cold.

It serves the same purpose as the carburet or choke.

The cold start injector ensures easy engine start-up in very cold weather.

Merits of EFI System

Improvement in the volumetric efficiency.

Atomization of fuel is independent of cranking speed and therefore starting will be easier.

Better atomization and vaporization will make the engine less knock prone.

Formation of ice on the throttle plate is eliminated.

Distribution of fuel being independent of vaporization, less volatile fuel can be used.

Variation of air fuel ratio is almost negligible even when the vehicle takes different positions like turning, moving or gradients, uneven roads etc.

Position of the injection unit is not so critical and thereby the height of the engine (and hood) can be less.

Demerits of EFI system

High maintenance cost.

Difficulty in servicing and

Possibility of malfunction of some sensors.

Q3) Explain the Stages of combustion in SI Engines.

A3)

Stages of combustion in SI Engines:

The combustion process in SI engine as consisting of three stages:

The pressure variation due to combustion in a practical engine is shown in fig.

In this figure, A is the point of passage of spark (say 20˚bTDC).

B is the point at which the beginning of pressure rise can be detected (say 8˚bTDC) and C the attainment of peak pressure.

Thus, AB represents the first stage and BC second stage and CD third stage.

The first stage(A-B) is referred to as the ignition lag or preparation phase in which growth and development of a self-propagating nucleus of flame takes place.

This is a chemical process depending upon both temperature and pressure, the nature of the fuel and the proportion of the exhaust residual gas.

Further, it also depends upon the relationship between the temperature and rate of reaction.

The second stage (B-C) is a physical one and it is concerned with the spread of flame throughout the combustion.

The starting point of the second stage is where the first measurable rise of pressure is seen on the indicator diagram.

During the second stage, the flame propagates practically at a constant velocity.

Heat transfer to the cylinder wall is low because only a small part of the burning mixture comes in contact with the cylinder wall during this period.

The starting point of the third stage is usually taken as the instant at which the maximum pressure is reached on the indicator diagrams (point C).

The flame’s velocity decreases during this stage. The rate of combustion becomes low due to lower flame velocity and reduced flame front surface.

Since the expansion stroke starts before this stage of combustion with the piston moving away from the top dead centre, there can be no pressure rise during this stage.

Q4) Explain the effect of engine variables on detonation in SI engines.

A4)

Intake Temperature:

Increased intake temperature reduces the delay period, therefore increases the detonation tendency. However, it should be noted that the increased temperature also increases the flame speed, thereby reducing the detonation tendency.

But the effect of increased temperature has more pronounced effect on delay period compared to flame speeds due to which the destination tendency is increased with increase in intake temperature.

Intake pressure:

Increased intake pressure increases the density of charge and reduces the delay period but increases the flame speed. The overall effect is to increase the detonation tendency.

Compression ratio:

Increased compression ratio increases both the pressure and temperature and reduces the delay period, hence the tendency to detonation increases.

Ignition advance:

Advancing the Spark timing increases the peak pressures of the cycle and thus reduces the delay period of end part of the gas in the combustion chamber, hence tendency to detonate increases.

Coolant temperature:

Raising the coolant temperature will increase the cylinder wall temperature and reduce the heat transfer rate between gas and cylinder walls.

Increased temperature of the gases would reduce the delay period and increase the detonation tendency.

Engine load:

Higher loads on the engine increases the heating of the engine and reduces the delay period.

Therefore, the increased loads increase the detonation tendency of the engine.

Engine speed:

Increase in engine speed increases the turbulence in the combustion chamber, thereby increasing the flame speed while the effect on the delay period is negligible.

Due to this, the increased speed of the engine reduces the detonation tendency.

Air-fuel ratio:

It has been mentioned earlier that about 10% rich mixtures have the minimum delay period and the flame speeds are high.

But it is observed that the effect of slightly rich mixtures on delay period is more dominant compared to flame speeds due to which the detonation tendency increases.

Engine size:

Similar engines of various sizes have the delay period nearly the same. However, in case of larger sized engines, the flame has to travel longer distance of combustion space compared to smaller sized engines.

Therefore, the larger engines have more tendency to detonate compared to smaller engines.

Combustion chamber design:

In general, more the compact combustion chamber, shorter will be flame travel and combustion time, hence it will give better antiknock characteristics.

Also, if the combustion chamber design is such that it promotes turbulence, then the flame speed will increase which would reduce the tendency to detonate.

Location of spark plug

In case The Spark plug is located centrally in the combustion chamber, it reduces the length of flame travel and reduces the tendency to detonate.

The flame travel can be reduced by using two spark plugs.

Q5) Explain the rating of fuel in SI Engine.

A5)

Resistance of knocking is an extremely important characteristic of fuel for Spark ignition engines.

These fuels differ widely in their ability to resist knock depending on their chemical composition.

A satisfactory rating method for comparing the antiknock qualities of the various fuels has been established.

In addition to the chemical characteristics of hydrocarbons in the fuel, other operating parameters such as fuel air ratio, ignition timing, dilution, engine speed, shape of the combustion chamber ambient conditions, compression ratio, etc affect the tendency to knock in the engine cylinder.

Therefore, in order to determine the knock resistance characteristic of the fuel, the engine and its operation variables must be fixed at Standard values.

According to a standard practice, the antiknock valve of SI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels. Iso-octane

and normal heptane

Iso- Octane chemically being a very good antiknock fuel arbitrarily assigned a rating of 100 octane number.

Normal heptane

on the other hand has very poor antiknock qualities and is given a rating of zero Octane number.

The Octane number fuel is defined as the percentage, volume of the isooctane in a mixture of isooctane and normal heptane which exactly matches the knocking on the fuel in a standard engine under a set of standard operating conditions.

The addition of certain compound (e.g., Tetraethyl lead) to iso-octane produces fuels of greater antiknock quality.

The antiknock effectiveness of the tetraethyl lead for same quantity of lead added decreases as the total content of the lead in fuel increases.

Furthermore, each Octane number at the higher range of the Octane scale will produce greater antiknock effect compared to the same unit at the lower end of the scale.

Because of this nonlinear variation, a new scale was derived which expresses the approximate relative engine performance and the units of this scale are known as the performance numbers, PN.

Octane numbers, ON above 100 can be computed by

ON (>100) =100 +

Where A is TEL in ml/gal of fuel or from the performance number, PN.

Octane Number = 100 +

Q6) Describe the construction and working of fuel pump.

A6)

• The plunger is driven by a cam and tappet mechanism at the bottom. The plunger reciprocates in the barrel. There are as many plungers as the number of cylinders in the engine. The plunger has a rectangular vertical groove.

• The delivery valve is lifted off its seat under the pressure of the fuel against the spring. The fuel from the delivery valve goes to the injector. When the plunger is at the bottom of its stroke, the supply port and spill are uncovered, the fuel from a low-pressure pump after filtration is forced into the barrel.

• Now the plunger is pushed up by the cam movement and both the parts are closed. On further movements of the plunger, the fuel above it is compressed which lifts the delivery valve and the fuel through it goes to the injector.

• The plunger rises up still further, and at a particular moment, the helical groove connects the spill port, through the rectangular groove to the fuel in the upper part of the plunger. Consequently, there is a sudden pressure drop due to which the delivery valve falls back on its seat under the spring force. The pressure in the delivery pipe also drops. Thus, the discharged from the nozzle of the injector is cut off suddenly. The cycle is repeated again and again.

• During each stroke of the plunger, the duration of the delivery is more or less according to as the spill port is made to communicate earlier or later, the high-pressure fuel in the upper part of the barrel. This depends upon the position of the helical groove which can be changed by rotating the plunger by the rack.

• The rack is connected to the accelerator. It meshes with a geared quadrant. The motion of the rack rotates the gear quadrant which ultimately rotates the plunger. The driver simply operates the accelerator which controls the fuel supply to the engine cylinder.

Q7) Explain the Combustion stages in CI Engines in detail.

A7)

Combustion stages in CI Engines

Pressure variation along with crank angle in CI engine Stages of combustion in CI engine:

1. Ignition delay period

2. Pre-mixed burning

3. Mixing controlled combustion

4. Tail region / After burning

Ignition delay period:

After the injection of the fuel in combustion chamber, fuel does not ignite immediately it takes a definite period from the time of injection to actual burning this delay is known as the ignition delay period.

Ignition delay further can be divided into two part:

a) Physical Delay: Since the time of the injection to the beginning of chemical reactions is called physical delay. In this period fuel is atomized, vaporized and mixed with air and raised to self-ignition temperature.

b) Chemical Delay: In the chemical delay, reactions start slowly and then accelerate until inflammation takes place. It depends various factors like temperature, property of the fuel.

Pre-mixed burning:

In this phase the combustion of pre-ignited fuel occurs which was raised to self-ignition temperature after vaporization. The rate of heat release is maximum in this period.

The period of pre-mixed combustion depends on the duration of the delay period (the longer the delay higher the pressure rises due to higher heat release).

Mixing controlled combustion:

This phase is called rapid phase because the temperature and the pressure are so high that fuel injected droplets burn as soon as they are injected and any further pressure rise can be controlled by injection rate.

It is assumed to be end when maximum temperature is reached inside the combustion chamber.

Tail region / After burning:

The combustion does not stop with the completion of injection process because unburned and partially burnt fuel particles start burning as soon as they come in contact with oxygen. This burning may continue in expansion stroke up to 70 to 80% of crank travel from TDC.

Q8) Explain the phenomena of knocking in CI Engine.

A8)

Fuel Injection takes definite interval.

Injection & Combustion take place simultaneously.

Initial droplets while undergoing ignition delay, additional droplets are being injected.

If ignition delay of fuel is short, initial few droplets will commence actual burning.

As a result, mass rate of mixture burned produce rate of press.

As ignition delay is longer, the actual burning of first few droplets is delayed and accumulation of greater quantity of fuel take place. When actual burning take place under such conditions the rate of pressure rise increases.

If ignition delay is quite large and the actual burning is substantially delayed and the accumulation of fuel is high when actual burning take place the rate of pressure rise is almost instantaneous the knocking begins.

Knocking is characterized by extreme pressure differentials and violent gas vibrations evidenced by audible knock.

In CI engines knocking occurs in the beginning of combustion.

In order to decrease knocking the actual burning should start as early as possible after the fuel injection begins.

It has been found that provided the rate of press. rise does not exceed 3 bar per ºCA combustion is smooth.

Between 3 to 4 bar there is tendency to knock, above this rate of pressure rise the diesel knock will be prominent.

Normally audible knock is always present in CI engines, when it becomes sever and cause heavy vibrations in the engine, it is said to be knocking.

Q9) Explain the Direct Injection types of combustion chamber used in CI engine.

A9)

Combustion Chamber used in CI Engines

There are different types of combustion chamber used in CI Engine are as follows:

Direct Injection (DI) Type – These also called open combustion chambers. The entire combustion space lies in the main cylinder and the fuel is injected into this volume.

Indirect Injection (IDI) Type – In this type of combustion chambers, combustion space is divided into two parts, one in the cylinder head and other in the main cylinder. Fuel is generally injected in the part which lies in cylinder head.

DIRECT INJECTION COMBUSTION CHAMBERS: -

Shallow Depth Chamber:

In shallow depth chamber, depth of the cavity provided in the piston in small. Due to this, the squish is negligible.

It is usually adopted for large engines running at low speeds.

Hemispherical Chamber:

This design also gives small squish.

However, its depth to diameter radius can be varied to obtain desired type of squish for better performance.

Cylindrical Chamber:

This design is a modification of cylindrical chamber in the form of truncated cone. Swirl is produced by masking the valve up to 180 deg. of circumference.

Squish produced is better than the previous two designs.

Toroidal Chamber:

This design produces a powerful squish along with air movement.

The masking needed on the valves is small, and thus, it makes better utilization of oxygen.

Q10) Explain the In-Direct Injection types of combustion chamber used in CI engine.

A10)

INDIRECT INJECTION COMBUSTION CHAMBERS: -

Swirl Chamber:

Swirl Chamber is provided in the cylinder head. A throat which is tangent to this chamber connects it with the main cylinder.

During compression, air flows into the swirl chamber through the throat. Tangential direction of air motion generates rotary motion inside the swirl chamber. Fuel in injected into the chamber, and the combustion products travel back to the main cylinder at much higher velocity.

This type of chamber is used where fuel quality cannot be controlled, where reliability is more important than fuel economy.

Pre-ignition Chamber:

It consists of a pre-ignition chamber (40% of total combustion space) connected with the main cylinder through small no. of holes.

During compression, piston forces the air into pre-ignition chamber. Fuel is then injected into the chamber due to which combustion takes place.

The resulting pressure rise forces the combustion products at higher velocities through small holes into the main cylinder, where bulk of combustion takes place.

Air-Cell Chamber:

The clearance volume is divided into two parts – one in the main cylinder and rest in energy cell.

Energy cell is divided into two parts – major and minor, which are separated from each other and from the cylinder by narrow orifices. Pressure in the energy cell is low, therefore, air is forced at high velocity into the energy cell on compression.

After the injection of fuel, combustion takes place inside energy cell and hot burning gases blow out through small holes into the main chamber. These hot gases provide swirling movement in main chamber, which results in complete combustion.

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