1. The working cycle in case of four-stroke engine is completed in following number of revolutions of crankshaft
1/2
1
2
4
Ans: 3. 2
Explanation
In a four-stroke engine, the engine requires four strokes of the piston (intake, compression, power, exhaust) to complete one cycle. Hence, it requires two rotations of the crankshaft to complete one cycle.
2. In a diesel engine, the fuel is ignited by
Spark
Injected fuel
Heat resulting from compressing air that is supplied for combustion
Ignition
Ans: 3. Heat resulting from compressing air that is supplied for combustion
Explanation
In diesel, engine air is compressed to a sufficiently high temperature to ignite diesel fuel injected to a cylinder. The piston is actuated by combustion and expansion.
They convert chemical energy into mechanical energy. It operates on a two strokes cycle or four strokes cycle.
They gain their energy by burning fuel injected or sprayed into the compressed, hot air charge within the cylinder.
The air molecule present should be heated at a temperature greater than the temperature at which the injected fuel can ignite.
Now combustion starts and the temperature reaches the fuel’s ignition temperature. Hence it generally starts with heat resulting from compressing air that supplies for combustion.
3. Scavenging air in diesel engines means
Air used for combustion sent under pressure
Forced air for cooling cylinder
Burnt air containing products of combustion
Air used for forcing burnt gases out of engine’s cylinder during the exhaust period
Ans: 4. Air used for forcing burnt gases out of engine’s cylinder during the exhaust period
Explanation
Scavenging is a process in a diesel engine, in which exhaust (burnt) gases are removed from the cylinder after combustion, done by fresh air coming inside the cylinder.
4. Supercharging is the process of
Supplying the intake of an engine with air at a density greater than the density of the surrounding atmosphere
Providing forced cooling air
Injecting excess fuel for raising more load
Supplying compressed air to remove combustion products fully
Ans: 1. Supplying the intake of an engine with air at a density greater than the density of the surrounding atmosphere
Explanation
Supercharging is the process of supplying the intake of an engine with air at a density greater than the density of the surrounding atmosphere such that every intake cycle of the engine receives more oxygen, which burns more fuel and thus generates more power.
5. Does the supply of scavenging air at a density greater than that of atmosphere mean engine is supercharged?
Yes
No
To some extent
Unpredictable
Ans: 2. NO
Explanation
Supercharging is the process of supplying the intake of an engine with air at a density greater than the density of the surrounding atmosphere such that every intake cycle of the engine receives more oxygen, which burns more fuel and thus generates more power.
Scavenging air is defined as the air used for forcing burnt gases out of the cylinder during the exhaust period.
6. The ratio of indicated thermal efficiency to the corresponding air standard cycle efficiency is called
Net efficiency
Efficiency ratio
Relative efficiency
Overall efficiency
Ans: 3. Relative efficiency
Explanation
The relative efficiency of an I. C. engine is the ratio of the indicated thermal efficiency to the air standard efficiency.
7. Compression ratio of LC. engines is
The ratio of volumes of air in cylinder before compression stroke and after compression stroke
Volume displaced by piston per stroke and clearance volume in cylinder
Ratio of pressure after compression and before compression
Cylinder volume/swept volume.
Ans: 1. The ratio of volumes of air in cylinder before compression stroke and after compression stroke
Explanation
The ratio of larger volume to lesser volume during any process is known as the volume ratio of that process.
The volume ratio during the compression process is known as the compression ratio and the volume ratio during the expansion process is known as the expansion ratio.
Thus compression ratio is given by the ratio of volume before compression to volume after compression.
8. The air standard efficiency of an Otto cycle compared to diesel cycle for the given compression ratio is
Same
Less
More
More or less depending on power rating
Ans: 3. More
Explanation
For the given compression, the area of the P-V diagram of the Otto cycle is greater than the diesel cycle. This means the Otto cycle gives more work than the diesel cycle for the same compression i.e. the air standard efficiency of an Otto cycle is more than a diesel cycle for given compression.
9. The calorific value of gaseous fuels is expressed in terms of
Kcal
Kcal/kg
Kcal/m2
Kcal/n
Ans: 2. Kcal/kg
Explanation
The Calorific value or the Heating value refers to the amount of energy released when a known volume of gas is completely combusted under specified conditions and is dependent upon the composition of the gas.
It may be expressed with the quantities:
Energy/mole of fuel (kJ/mol)
Energy/mass of fuel (kcal/kg or kJ/kg)
Energy/volume of the fuel is usually expressed in kcal/m3
10. If the intake air temperature of I.C engine increases, its efficiency will
Decreases
Increases
Remains same
Depending on other factors
Ans: 1. Decreases
Explanation
If the intake of an I.C. engine increases, its efficiency decreases.
The efficiency of the engine decreases as the temperature of inlet air increases. As, the air becomes thinner at a higher temperature so it does not expand much so doing less work and thus, reducing engine efficiency. The efficiency of the engine increases as the temperature of inlet air decreases, increasing the density of air. Thus, air expands more producing more work. The turbocharger and supercharger are employed for taking more air to increase engine efficiency.
