Ignition Systems MCQ Quiz in తెలుగు - Objective Question with Answer for Ignition Systems - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation:

Limitations:

i. The primary voltage decreases as the engine speed increases due to the limitations in the current switching capability of the breaker system.

ii. Time available for build-up of the current in the primary coil ad the stored energy decrease as the engine speed increases due to the dwell period becoming shorter.

iii. Because of the high source impedance (about 500 kΩ) the system is sensitive to side-tracking across the spark plug insulator.

iv. The breaker points are continuously subjected to electrical as well as mechanical wear which results in short maintenance intervals. Increased currents cause a rapid reduction in breaker point life and system reliability. Acceptable life for these systems is obtained with a primary current limited to about 4 amperes.

Contact Breaker:

This is mechanical device for making and breaking the primary circuit of the ignition coil. It consists essentially of a fixed metal point against which, another metal point bears which is being on a spring loaded pivoted arm. The metal used is invariably one of the hardest metals, usually tungsten and each point has a circular flat face of about 3 mm diameter. The fixed contact point is earthed by mounting it on the base of the contact breaker assembly whereas the arm to which the movable contact point is attached, is electrically insulated. When the points are closed the current flows and when they are open, the circuit is broken and the flow of current stops. The pivoted arm has, generally, a heel or a rounded part of some hard plastic material attached in the middle and this heel bears on the cam which is driven by the engine. Consequently, every time the cam passes under the heel, the points are forced apart and the circuit is broken.

Concept:

The pressure differential across the intake valves of a naturally aspirated engine is less than one atmosphere, while the pressure differential across the exhaust valves during blowdown can be as high as three or four atmospheres.

The higher pressures near exhaust valves assist the exhaust gases to escape from the combustion chamber. With a small space, they can easily go out of exhaust valve.

Due to these differences in pressure difference during intake and exhaust, it is tougher to induct the air than to exhaust the burnt gases and therefore the intake valve hole has to be bigger than the exhaust valve hole.

Concept:

• There is a portion of the operating cycle of a reciprocating engine in which, when the piston is passing TDC on the exhaust stoke, both the intake and exhaust valves are open.
• Valve overlap is the number of degrees of crankshaft rotation during which both intake and exhaust valves are open.
• Valve overlap increases the efficiency of the engine by allowing the low pressure caused by exhaust gases (leaving the cylinder) to help the fresh charge of the fuel-air mixture (in the induction system) to start moving into the engine.
• The opening and closing of the valves in an IC engine in relation to the movement of the piston and flywheel are called valve timing.
• The valve timing is represented by a diagram drawn on the face of the flywheel in degrees of the crankshaft rotation.

Valve timing diagram

• Inlet valve opens 9 degrees before T.D.C.
• Inlet valve closes 50 degrees after B.D.C.
• Exhaust valve opens 47 degrees before B.D.C.
• Exhaust valve closes 12 degrees after T.D.C.
• Overlap period 21 degrees

Concept:

Carburetor:

• The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to the engine cylinder is called carburetion and the device which does this job is called a carburetor. 
• Thus carburetor supplies a mixture of air and fuel with a maintained air petrol ratio, generally used in the petrol engine.
• As in the CI engine, only air is compressed and then fuel is injected into the cylinder by an injector. So the CI engine does not require the carburetor.

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Principle of Carburetion

Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the piston.

The Simple Carburetor:

• The simple carburetor mainly consists of a float chamber, fuel discharge nozzle, a metering orifice, a venturi, a throttle valve, and a choke
• The float and a needle valve system maintain a constant level of gasoline in the float chamber
• To prevent blockage of the nozzle by dust particles, the gasoline is filtered by installing a Fuel strainer at the inlet to the float chamber.
• Strainer is a device that is used to filter the fuel before entering the float chamber. It consists of a fine wire mesh that filters the fuel and removes dust and other suspended particles from it. If these particles are not removed, it can cause blockage of the nozzle so option 3 is correct.
• If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve 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
• The float chamber is vented either to the atmosphere or to the” upstream side of the venturi; During suction stroke air is drawn through the venturi

Explanation:

Valve Timing Of Four-Stroke Petrol /Spark Ignition Engine. ( SI Engine):
As shown in the below figure, we can conclude that the exhaust/exit valve opens at 35° to 60° before the bottom dead centre.

• In the valve timing diagram, as shown we see that the inlet valve opens before the piston reaches TDC or in other words, while the piston is still moving up before the beginning of the suction stroke.
• Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC This is done as the incoming charge continues to flow into the cylinder although the piston is moving upwards from BDC Now the charge is compressed (with both valves closed) and then and temperature) push the piston downwards with full force and the expansion or working stroke takes place.
• Now the exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. Now the piston reaches BDC and then starts moving up, thus performing the exhaust stroke.
• The inlet valve opens before the piston reaches TDC to start a suction stroke. This is done as the fresh incoming charge helps in pushing out the burnt gases.
• Now the piston again reaches TDC, and the suction stroke starts. The exit valve closes after the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards. It may be noted that both the inlet and outlet valves are open for a small fraction of a crank revolution. This is known as valve overlap.

Explanation:

Ignition Switch

• A multi-position switch turns power to the ignition circuit on and off.
• In the start position, it triggers the starter motor and provides a parallel path for electrical flow to the ignition system.
• After starting, in the run position, it continues to allow current to flow to the ignition system and the entire electrical system.
• The ignition switch operates the steering wheel lock and a buzzer or light.
• Some fuel pumps are connected to electrical power through the ignition switch.

Ignition Coil

• The coil Is the heart of the ignition system. Battery voltage is not enough electrical pressure to push a spark across the spark plug gap. 
• The coil is a transformer that transforms battery voltage into many thousand volts.
• A coil has a low voltage primary winding and a high voltage secondary winding.
• When electrical current flows through a coil it creates a magnetic field and delivers it to the distributor through High Tension lead.
• Coiling the conductor around an iron core creates a stronger field. 
• The high voltage spark is created by the ignition coil when the electrical current supply is discontinued and the magnetic field breaks down.
• The more windings there are in a coil, the stronger the magnetic field. 
• When there are fewer coils in the primary part of a transformer, It Is a step-up transformer. 
• Fewer coils in the secondary would make a step-down transformer. 
• An ignition coil is a step-up transformer. 

A ballast resistor is a device in your vehicle that limits the amount of current in an ignition circuit.

A circuit breaker is like a fuse to break a circuit path at a predetermined amount of current is passed.

Concept:

In a piston engine, the main bearings are the bearings on which the crankshaft rotates.

Minimum number of bearings = 2, one at each end of the shaft.

Maximum number of bearings = n + 1, one at each end and one between every two cylinders.

Greater the number of main journals, better will be the support for the crankshaft and smoother will be the operation of the engine at high speeds.

But it is a compromise between the extra size, cost and stability of a large number of bearings and the compactness and light weight of a small number.

In general,

• All single and twin cylinder engines have two main bearings.
• Four-cylinder engines in a car have 3 bearings.
• High luxury engines of six cylinders have 4 bearings.

Concept:

Equivalence ratio (ϕ) is defined as ratio of Fuel Air Ratio (FAR) under actual condition to the Fuel Air Ratio under ideal condition.

ϕ = \(\frac{(FAR)_{actual}}{(FAR)_{ideal}}\)⇒ ϕ = \(\frac{{\frac{{{\rm{actual}}~{{\rm{m}}_{\rm{f}}}}}{{{{\rm{m}}_{{\rm{air}}}}}}}}{{\frac{{{\rm{Ideal}}~{{\rm{m}}_{\rm{f}}}}}{{{{\rm{m}}_{{\rm{air}}}}}}}} = \frac{{{\rm{Actual}}~{{\rm{m}}_{\rm{f}}}}}{{{\rm{Ideal}}~{{\rm{m}}_{\rm{f}}}}}\)

Therefore, from above relation, ϕ ∝ m_f

∴ when ϕ < 1,

• Fuel supply will be less and temperature of the charge will also be less.
• The power producing capacity of engine will be lower.
• Due to less fuel, the knocking tendency decreases.

When ϕ > 1, the fuel supply will be more than that of ideal condition and tendency of knocking also be increases.

Explanation:

Ignition System:

• The ignition system is one of the most important systems used in Internal Combustion Engines.
• The spark ignition system requires some device to ignite the compressed air-fuel mixture.
• The ignition takes place inside the cylinder at the end of the compression stroke.
• It is a part of the electrical system which carries the electrical current to a current plug. It gives the spark to ignite the air-fuel mixture at the correct.

Types of ignition systems:

• Battery ignition system or coil ignition system
• Magneto ignition system
• Electronic ignition system

Additional Information

Magneto ignition system:

• Magneto is a special type of electric generator. It is mounted on the engine and replaces all the components of the coil ignition system except the spark plug.
• A magneto when rotated by the engine is capable of producing very high voltage and does not need a battery as a source of external energy.
• The use of magneto is best at high speeds and therefore widely used for sports, racing cars, motorcycles, scooters.

The figure shows the schematic diagram of a high tension magneto ignition system.

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• Magneto ignition system incorporates windings to generate as well as to step up the voltage and does not require a separate coil to boost up the voltage required to operate the spark plug.
• The breaker points are opened and closed with the help of cam the primary circuit flux discharge and a high voltage is produced in the secondary circuit.
• The cranking speed for starting is low the current generated by the magneto is very low as engine speed increases the current increases. 
• Thus with magneto, there is almost always a starting difficulty and a separate battery is needed for starting. Hence it is best to use magneto for high-speed engines.

Explanation:

Pre-ignition:

• Ignition of a homogenous mixture in the cylinder before initiation of spark is known as pre-ignition.
• It is caused by local overheating of the mixture.
• There is no pre-ignition in CI engine because fuel is absent during compression.

Ignition

• An ignition process can be considered as the initiation of burning in a small spherical volume called the minimal volume.
• Energy is supplied from an external source (spark) to the fuel charge, which burns, changes its chemical state and releases chemical energy proportional to its heating value.
• If this release of energy, which depends on the nature of the spark, is sufficient to make the flame self-sustaining, ignition is promoted and combustion takes place.

Detonation:

• A flame front travelling across the combustion chamber from A to D.
• As the flame propagates, the unburnt mixture called as end gas is compressed which causes its temperature to increase.
• The temperature of end gases increases further when it receives heat by radiation from burning charge.
• Hence some of the end charges may undergo pre-flame reaction.
• Due to which burning of end charges starts spontaneously and auto-ignition will occur. This is known as knocking or detonation.

Effects of detonation:

• Noise and roughness
• Mechanical damage
• Loss of power
• Loss in efficiency
• Pre-ignition

Rumble:

• It is a name assigned to intermittent roughness caused by combustion chamber deposits which create the secondary flame front.
• It is a low pitched noise distinctly different from spark knock.
• Rumble causes vibration of crankshaft arising from a high rate of pressure rise with consequent deflection of mechanical parts.
• Rumble is avoided or minimized by eliminating deposits usually by fuel additives.