Nozzle tip will get damaged because of sulphuric acid corrosion. That is the main reason most of the engines cooling water temperature of the nozzle cooling is kept at 85°C and in modern engines the cooling of the nozzle by separate cooling system has been abandoned.
Q.399 What occurs in the combustion space of a diesel engine cylinder shortly after ignition and before piston reaches TDC?
Ans. (c) Rapid increase in the pressure and temperature.
When the piston is nearing the TDC and the fuel is injected, the temperature inside the combustion space is sufficiently high near the self-ignition temperature of the fuel. Hence the fuel catches fire and because of this, sudden increase in the pressure and temperature takes place. The initial stage of ignition takes place under constant volume therefore the temperature and the pressure increases. In the later stage, the combustion takes place under constant pressure because the piston is moving downwards and the volume is increasing.
Q.400 The effective pump stroke of a port and helix fuel injection pump is determined by
Ans. (c) plunger control rack position.
The rack position will determine the position of the plunger helix with respect to the suction port of the barrel. This angular position will determine the quantity and time for which the fuel is injected. Hence more the quantity of fuel more the load and lesser the fuel lower the load. The time for which the fuel is actually injected into the cylinder is called effective delivery stroke of the plunger which is decided by the helix position and the position of plunger with respect to the barrel.
Q.401 The amount of fuel delivered by helical plunger type fuel injection pump is controlled by
Ans. (a) rotation of the pump plunger.
Rotation of pump plunger by means of rack and pinion arrangement will cause variation of angular position of helix with respect to the suction port on the fuel pump barrel. Due to this, the effective delivery stroke of the fuel pump will change which means that it will change the time for which the fuel is injected. Hence it will control the quantity of fuel delivered in the unit because the position of plunger helix whether the termination of the fuel injection is carried out earlier or later part of the injection stroke.
Q.402 Which of the following conditions will develop if the flow of raw cooling water to diesel engine is obstructed?
Ans. (c) Jacket water temperature will increase.
The jacket water-cooling is a closed circuit and it uses the fresh water. The jacket water is cooled by raw water by the use of heat exchanger. If the raw cooling water flow is obstructed, the quantity of heat removed from the jacket cooling water is reduced. At the same time, the jacket cooling water is receiving heat from the engine which is not being removed from the cooling water. Hence the jacket cooling water will go on rising continuously.
Q.403 The effect stroke of a constant stroke individual fuel injection pump is varied by the
Ans. (a) controlled rack.
The controlled rack is engaged is with the pinion which in turn is engaged with the controlled sleeve of the fuel pump. The controlled sleeve, when it rotates, will rotate the fuel pump plunger with respect to the barrel. This rotation will change the angular position of helix with that of the suction port on the barrel. Hence ultimately the effective delivery stroke of the pump will change and hence it will change the quantity of fuel injected.
Q.404 Which of the listed conditions occur if a diesel engine cylinder exhaust valve is leaking?
(a) loss of compression for the cylinder.
(b) misfiring or rough engine running.
(c) damaged valve.
(d) All of the above.
Ans. (d) All of the above.
When the exhaust valve is leaking automatically, the compression pressure drops. When the compression pressure drops the fuel will not ignite properly. Hence there is a possibility that misfiring takes place. At the same time, due to low compression pressure the fuel will not burn properly which may cause high exhaust temperature and loss of the peak pressure. When the valve starts to leak, the high-pressure gas at high temperature will start to pass through the leaky portion of the valve. This will result in wear or wire-drawing effect on the valve and valve seat. Thus causing excessive damage to the valve and valve seat and excessive high exhaust temperature which increases fast.
Q.405 High firing pressure and low exhaust temperature in a diesel engine may result from
Ans. (c) early fuel injection timing.
In case of v.i.t. controlled injection, the early injection cause the fuel to mix up better with the quantity of air available it will have more time to mix up and form combustible mixture. Due to this, fuel will burn efficiently and very near to the TDC which will cause high peak pressure and low exhaust temperature.
Q.406 Exhaust pyrometer reading provides an indication of
Ans. (b) distribution of the load between the engine cylinders.
Exhaust gas pyrometer reading which gives the exhaust temperature of individual unit is a rough estimation of power developed by the individual unit. As a thumb rule, higher the exhaust temperature more is the power developed and lesser the temperature less is the power developed but this may not be always true. If the exhaust temperatures are nearly equal, it indicates that all units are developing equal powers and hence the engine is properly balanced as far as power is concerned.
Q.407 A change in the degree of fuel atomization in a diesel engine cylinder has the greatest effect on the
Ans. (d) combustion of fuel in the cylinder.
The more degree of atomization, with proper penetration, will cause better fuel efficiency because the fuel will be divided into very minute particles which will have more surface area available and the evaporation of the fuel will be faster so as to form a combustible mixture. Hence the mixture will burn efficiently and promptly due to which the combustion inside the cylinder will be efficient.
Q.408 Diesel engine oil having low cetane number can result in
Ans. (c) combustion knock.
Lower cetane number of the fuel low is the grade of the fuel and because of that ignition delay will increase. Hence more fuel will get accumulated inside the cylinder before it catches fire. When it catches fire it will give knocking effect because of sudden formation of pressure wave. This knocking effect is called diesel knock which directly depends on the quality of the fuel.
Q.409 To successfully reduce an excessively high diesel engine exhaust gas temperature
Ans. (d) you should reduce the load on the engine.
As a general rule more the load on the engine more will be the exhaust temperature. Hence the first action to be taken if high exhaust temperature is noted is to reduce the quantity of fuel which is burnt. This is possible by reducing the load on the engine which reduces the quantity of fuel burnt and the exhaust temperature reduces immediately.
Q.410 High exhaust temperature and black smoke exhausting from an engine can be caused by
Ans. (a) engine overload.
When engine overload takes place, the amount of fuel burnt is always higher than the quantity of the air supplied. At the same time, high quantity of fuel is injected. This high quantity of fuel injected cannot burn in the available time. Under such circumstances, afterburning takes and along with lack of proper quantity of air it will have higher exhaust temperature and black smoke.
Q.411 Combustion gas venting through the expansion tank can be caused by a
Ans. (b) cracked cylinder head.
When the cylinder head is cracked, during compression stroke when high peak pressure and high exhaust temperature takes place, these high pressure combustion gas will enter through the cracked cylinder head valve into the water space. This gas being lighter it will try to rise upwards and get accumulated inside the expansion tank and it will come out through the expansion tank weld. The gas coming out through the expansion tank weld will have exhaust gas smell and water in the expansion tank will get discoloured. Also if the crack is big, more quantity of exhaust gas will find its way into the expansion tank, the expansion tank may get bubbled out and may get empty due to trapped exhaust gas. Also, the jacket cooling outlet temperature of that particular unit will also be high.
Q.412 Which of the following problems can cause fluctuating pressures in the closed cooling system of a main propulsion diesel engine?
Ans. (b) Cavitation in the cooling water pump.
If the inlet in the cooling water pump is restricted or the water is having high temperature, cavitation will take place in the form of bubble at the suction of pump. Due to this, there will be partial loss of suction. This will cause the variation in discharge pressure which will result in fluctuations in the pressures of closed cooling water system.
Q.413 A fuel leak occurs in a high pressure piping between the fuel pump and fuel nozzle. This requires immediate repairs because of the
Ans. (b) serious fire hazard.
The high pressure pipe leakage will cause the high pressure fuel to leak through the cracked portion of the fuel pipe. Due to this, the oil coming out through the cracked portion will be in the form of a fine mist. This mist when it comes in contact with the exhaust manifold or hot surface of the engine, as there is sufficient amount of air in the engine, it will result in fire in the engine room. Hence immediate repair is necessary to avoid this fire.
Q.414 Individual cylinder performance in the diesel engine is routinely determined by the exhaust gas
Ans. (c) pyrometer readings.
As a general rule, the exhaust temperature will give a fairly good idea of power developed by the individual units. The exhaust gas temperature from individual unit is taken into consideration as a measure to determine the power developed by the individual unit. Hence as long as all units’s temperatures are equal or they are within a prescribed limit, this indicates that all units are developing nearly same power and the engine is properly balanced.
Q.415 Excessive diesel engine cylinder exhaust backpressure will be caused by
Ans. (c) an obstruction in the exhaust silencer.
When the exhaust valve opens and exhausting takes place through the exhaust valve, if the exhaust gas silencer valve is blocked or choked, there will not be easy flow of exhaust gas and complete emptying of the exhaust gas from the cylinder will not possible. Under such circumstances, when the piston is on the upward stroke it will give a backpressure over the piston and there will be loss of power in overcoming this backpressure.
Q.416 The injection pressure of a hydraulic fuel injection nozzle can be increased by
Ans. (d) increasing compression on the pressure spring.
The fuel is injected when the pressure developed by the fuel injection pump will act on the annular space around the fuel injection nozzle will overcome the spring compression from top on the fuel injection nozzle. As the compression pressure increases, more pressure will be required by the fuel injector nozzle to lift up against the spring compression. Hence more the compression pressure more will be pressure required for the fuel injector to open. Hence the injection pressure will increase as the compression pressure of the spring increases.
Q.417 The combustion knock occurring in a diesel engine can be caused by
Ans. (d) prolonged ignition lag.
Due to prolonged ignition lag the time between the fuel injection and fuel ignition will be large. During this time, large amount of fuel will get accumulated into the combustion space. When this accumulated fuel suddenly gets ignited it will cause an explosive pressure wave which will create the knocking effect.
Q.418 Which of the following problems could develop due to the accumulation of oil vapours into the crankcase of diesel engine?
Ans. Crankcase explosion.
Accumulation of oil vapours in the crankcase and presence of hot spot will cause the vapour to convert into oil mist. When the oil mist comes in contact with the hot spot in presence of large quantity of air, which is available in the crankcase, it will form a lethal combustible mixture and it will cause a crankcase explosion.
Q.419 One problem resulting in a diesel engine fuel injector opening pressure being lower than specified by the engine manufacturer is
Ans. (d) quantity of fuel injected tends to increase.
As the fuel injection pressure is lower than recommended by the engine manufacturer, the fuel injector valve will open much earlier but the closing will not alter. Due to this, the quantity of fuel injected will be more than what is required.
Q.420 Which of the listed reasons is the most likely cause of a sudden drop in compression pressure in one diesel engine cylinder?
Ans. (b) Malfunctioning of the valves.
Malfunctioning of the valves may be because of improper adjustment of the tappet clearance or the valves may be sticky or leaky. Under all these conditions, during compression, the compressed air will leak through the leaky valve and automatically the compression pressure will drop.
Q.421 repeated in Q.420
Q.422 Failure to open the diesel engine test cock after a long period of shutdown prior to starting may result in
Ans. (b) damage to cylinder head and piston.
If the test cock or the indicator cock is not opened and the engine is started and due to any reason there is accumulation of fluid on top of the piston, this fluid will get compressed. As the fluid is incompressible, excessive pressure will be developed between the piston top and cylinder head bottom portion. This will cause the cracking of the cylinder head, damage to the piston, bending of the connecting rod or in worse case it may cause damage to the crankshaft in the way of slippage.