Mechanic Diesel MCQ Quiz - Objective Question with Answer for Mechanic Diesel - Download Free PDF

Explanation:

Spark plug:

• A spark plug is basically two electrodes positioned to form a gap.
• The gap is between the insulated center electrode and the ground electrode.
• This is the gap that the spark jumps to start the ignition of the compressed air-fuel mixture in the engine cylinder.

Working of spark plug:

• The number one job of the ignition system is to produce the high-voltage surges that cause the sparks at the spark plug gaps.
• The ignition coil has two windings: a primary winding of a few hundred turns of relatively heavy wire, and a secondary winding of thousands of turns of very fine wire.
• When the ignition switch is turned on, and the trigger has closed the circuit between the ignition coil primary winding and ground (the other battery terminal).
• Battery current will flow through the primary winding.
• This causes a magnetic field to form around the winding.
• Now, when the trigger opens the circuit between the winding and the ground, the current stops flowing.
• The magnetic field collapses.
• Spark plug with end cap designed to improve combustion swirl.
• The Spark plug has an end cap with a gap to the end cap.
• When the air-fuel mixture is compressed, some of it enters the cap.
• Then, when the spark occurs, ignition starts in the cap.
• The burning mixture streams out through the orifices to ignite the rest of the compressed mixture.
• Note that the tangential orifices are at an angle.
• As the burning mixture streams out through these orifices, it sets up a swirling motion that speeds up the burning of the mixture. This is said to improve engine performance.

Explanation:

Anti-lock braking system (ABS):

• An anti-lock braking system (ABS) is a safety anti-skid braking system used on vehicles, such as cars, motorcycles, trucks, and buses.
• ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface and allowing the driver to maintain more control over the vehicle.
• Typically ABS includes a central electronic control unit (ECU), four-wheel speed sensors, and at least two hydraulic valves within the brake hydraulics.
• The ECU constantly monitors the rotational speed of each wheel.
• If it detects the wheel rotating significantly slower than the speed of the vehicle, a condition indicative of impending wheel lock, it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel; the wheel then turns faster.
• Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel.
• This process is repeated continuously and can be detected by the driver via brake pedal pulsation.
• Some anti-lock systems can apply or release braking pressure 15 times per second.
• Because of this, the wheels of cars equipped with ABS are practically impossible to lock even during panic braking in extreme conditions.
• The ECU is programmed to disregard differences in wheel rotative speed below a critical threshold because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two.
• For this same reason, a differential is used in virtually all roadgoing vehicles.

Explanation:

Engine:

• It is a power pack for any automobile vehicle where in which the power is developed by means of the combustion of fossil fuel.
• In heavy vehicles the major fuel is high-speed diesel oil. Simply we can say diesel.
• Almost all engines are now a day’s internal combustion engines.
• A few decades ago external combustion engines also existed in practice.
• The engines are capable of propelling larger loads with considerable speed.
• So that the constructions are heavier when compare with the passenger cars and other utility vehicles.
• Engines are classified according to the many factors. One such is on the basis of stroke:
  • ​Two-stroke
  • Four stroke
• ​The difference between a two-stroke and a four-stroke engine is their thermodynamic cycle.

Comparison between four-stroke engine and two-stroke engine:

Four-stroke engine	Two-stroke engine
Four operations (suction, compression, power and exhaust) take place in the four strokes of the piston.	The four operations take place in two strokes of the piston.
It gives one power stroke in two revolutions of the crankshaft. As such three strokes are idle strokes.	The power stroke takes place in every two strokes i.e. one power stroke for one revolution of the crankshaft.
Engine design is complicated and heavier flywheel is used.	Engine design is simple.
Engine require more space	The engine has more uniform load as every time the piston comes down it is the power stroke. As such a lighter flywheel is used.
The engine has more parts such as valves and its operating mechanism.	Engine requires less space.
More thermal efficiency.	The engine has no valves and valve - operating mechanism
The engine efficiency is more.	The engine is less expensive
Engine has heavy weight.	The engine efficiency is less.
Complicated lubricating system.	Simple lubricating system.

Explanation:

Wheel alignment:

• Wheel alignment is the positioning of the front wheel and steering mechanism to:
  • minimize tyre wear
  • reduce the driver’s effort to turn the vehicle
  • achieve self-centring after turning
  • achieve directional stability of the vehicle while running straight ahead.

Front-wheel alignment depends upon the following:

• Camber
• Castor
• Kingpin inclination
• Toe-in and Toe-out

Combined angle:

• The combined angle or included angle (1) is the angle between the vertical plane of the wheel centre line (2) and the kingpin centre line (3) called the steering axis.
• The combined angle is the sum of camber and kingpin inclination called steering axis inclination.

Kingpin inclination:

• Kingpins are mounted in such a way that they tilt inward.
• The angle between the centre line of the kingpin and the vertical line is called kingpin inclination.

Purpose of kingpin inclination:

• It provides directional stability along with the castor angle.
• It helps in the self-centring of wheels after negotiating a turn.

Camber:

• The inward or outward tilt of a wheel from the vertical is termed camber.
• The angle between the vertical line (1) from the central point of the tyre and the central line (2) of the tyre is known as the camber angle (3).
• When the wheels are tilted outward at the top it is called positive camber, and if tilted inward at the top, it is called negative camber.
• An equal camber angle is provided on both front wheels.
• With the positive camber wheels becoming vertical under load the tyre will have full contact with the road.
• If the positive camber is excessive then the tyre’s outer edge will wear out faster.
• If the negative camber is excessive the tyre’s inner edge will wear out faster.
• Unequal camber on both front wheels will result in wheel shimming at low speed (abnormal vibration).

Explanation:

Propeller shaft:

• The propeller shaft connects the gearbox and final drive.
• The pinion shaft of the differential is connected to the propeller shaft.
• One universal joint is used between the propeller shaft and the pinion shaft of the differential.
• Another universal joint with one slip joint is also used between the propeller shaft and the gearbox.
• The propeller shaft rotates at high speed and bears a heavy torque.
• So it is made of a strong steel tube.
• In some vehicles, a solid propeller shaft is also used.
• Vehicles having a larger wheelbase use two propeller shafts.
• Whenever the distance between the gearbox and rear axle is very large (example-passenger buses) more than one propeller shaft is used for torque transmission.
• The Centre support bearing is used to connect the two propeller shafts.
• A two-piece propeller shaft requires a center support bearing.
• If there is a large wheelbase, a long propeller shaft is needed.
• So a two-piece propeller shaft is used in such cases with a center support bearing.
• By incorporating this there is no sag or whirl. 

Concept:

• Whenever the engine is started from cold, the coolant temperature has to be brought to the desired level in order to minimize the warm-up time.
• This purpose is achieved by a thermostat fitted in a system which initially prevents the circulation of water below a certain temperature through the radiator so that the water gets heated up quickly.
• When the preset temperature is reached, the thermostat allows the water to flow through the radiator.

In normal atmospheric conditions water boils at 100°C. At higher altitude, atmospheric pressure is low and water boils at a temperature below 100°C. To increase the boiling temperature of water the pressure of the cooling system is increased. This is achieved by providing pressure caps to seal the system.

Pressure caps also permits the engine to operate at higher temperatures so that better efficiency of engine is achieved.

Brushes are part of the commutator; the brushes must conduct electricity as to keep contact with the rotating armature. Now a days, carbon graphite is used as brush material in dynamo.

Most likely causes of engine over heating are

• Less air drafts
• Less circulation of water
• No water circulation of water in cylinder block
• Incomplete combustion of fuel
• Cylinder block water passage blocked/scale formation

For a mechanic, initial and most likely check for engine overheating will be loose fan belt as this is common for less air draft or less circulation of water.

Explanation:

Sludge Formation:

• In the Crankcase, water collects in two ways:
  • Water formed as a product of combustion and water entering because of crankcase ventilation.
  • Water mixed with oil when churned up, leads to the formation of sludge.
• This prevents normal circulation of oil to the engine parts.
• The problem of sludge formation is more acute When the engine runs for a too short period of time.
• In such cases, the oil may have to be changed more frequently.
• In the engine running for a considerable period of time, the crankcase ventilation takes care of the sludge formation.

Crankcase ventilation:

• In the crankcase oil gets diluted due to the mixture of blow-by gases, carbon particles, metallic particles, sand, dust, dirt, and the acids formed out of the exhaust gas condensation such as sulphuric acid and phosphoric acid.
• This affects lubrication and forms sludge (accumulation of dirty oil).
• Frequent cleaning and change of oil are needed.
• To overcome this problem, crankcase ventilation is provided.
• Fresh air is allowed in the crankcase which passes out after circulation through a breather pipe (1) in the rear.
• This arrangement is known as open-type crankcase ventilation.
• In earlier vehicles, crankcase vapors were vented directly to the atmosphere through a breather tube, or road draught tube.
• It was shaped to help draw the vapors from the crankcase, as the vehicle was being driven.
• Modern vehicles are required to direct crankcase breather gases and vapors back into the inlet system to be burned.
• A general method of doing this is called positive crankcase ventilation, or PCV.

 

• The lead - acid battery is an electrochemical device for converting electrical energy into chemical energy and vice versa
• The main purpose of the battery is to store electrical energy in the form of chemical energy
• It provides a supply of current for operating various electrical accessories, when the engine is not running
• When the engine is running it gets electric supply from the dynamo/alternator
• It is also known as accumulator and storage battery
• A solution of sulphuric acid and distilled water is used in the Lead - acid battery as electrolyte

In compression ignition (CI) engines, spark plug is not required because the combustion takes places by high compression of air resulting in very high temperature. In CI engines, the injector is used for injecting fuel in highly compressed air. On the other hand, spark plug is used in spark ignition (SI) engines to ignite the air fuel mixture.

Explanation: 

Bleeding of the fuel system:

• Bleeding is the process by which air, which is present in the fuel system, is removed by using a priming pump.
• Air locking in the fuel system will result in the erratic running of the engine and may result in the stopping of the engine.
• Bleeding is carried out by priming the filter.
• A slight loosening of the bleeding screw allows locked air to escape as bubbles along with the fuel.
• When locked air escapes and the system is free of air, the screw is tightened finally.

Priming pump:

• A small, hand or electrically operated pump, is used to spray gasoline into the induction system of a reciprocating engine for starting the engine.
• It is used to remove an airlock in a fuel feed system.
• The fuel does not pass through the carburetor.
• The pump must be locked during flight to avoid excessive fuel being drawn into the cylinders, especially at low power settings, which may result in an overly rich mixture and the realization of much less range than anticipated.

Explanation:
Thermocouple:

• A thermocouple is a sensor used to measure temperature.
• Generally, temperature up to 1400° is measured through this device.
• The thermocouple is generally made up of Nickel and Chromium for the use of temperature measurement inside the engine.
• It consists of two wire legs made from different metals.
• The wire's legs are welded together at one end, creating a junction.
• This junction is where the temperature is measured.
• When the junction experiences a change in temperature, a voltage is created.
• The voltage can then be interpreted using thermocouple reference tables to calculate the temperature.
• It works on the principle of the Seebeck effect which states that the temperature difference between two dissimilar electric conductors produces a voltage difference between them.
• This potential difference is used to measure temperatures.

Types of thermocouple:

• Type K (Nickel – alumel or Nickel-Chromium)
• Type E (Chromel – constantan)
• Type J (Iron – constantan)
• Type B (Platinum – rhodium)

Explanation:

Engine Coolant:

• An engine coolant (also called an antifreeze) is a special fluid that runs through your engine to keep it within its correct operating temperature range.
• It is made from either ethylene glycol or propylene, water, some protection additives and is usually green, blue or even pink in colour.

Engine Coolant properties:

An efficient cooling system removes 30 to 35% of the heat generated in the combustion chamber.

• Coolant should remove heat at a fast rate when the engine is hot.
• Coolant should remove heat at a slow rate when the engine is started until the engine reaches its normal operating temperature.
• Coolant should not remove too much heat from the engine. Too much removal of the heat decreases the thermal efficiency of the engine.
• It should have a Low freezing point
• It should have high thermal conductivity.
• It should have high thermal stability.
• It should have a high thermal specific heat
• It should circulate freely in the coding system.
• It should prevent frequency and rust formations.
• It should be reasonably cheap.
• It should not waste by vaporization.
• It should not deposit any foreign matter in the water jackets/radiator.