(SEM V) THEORY EXAMINATION 2018-19 L.C. ENGINES & COMPRESSORS

I.C. ENGINES & COMPRESSORS (RME-051)

B.Tech (SEM-V) – AKTU
                                                                                                     Time: 3 Hours  Total Marks: 70

SECTION A

(Attempt all questions in brief – 2 × 7 = 14 marks)

Q1 (a) Differentiate between two-stroke and four-stroke engines.

In a two-stroke engine, the cycle is completed in two strokes of the piston, whereas in a four-stroke engine the cycle is completed in four strokes. Two-stroke engines produce one power stroke per revolution, while four-stroke engines produce one power stroke per two revolutions. Two-stroke engines are lighter and simpler but less efficient compared to four-stroke engines.

Q1 (b) Draw P-V and T-S diagrams for Otto, Diesel and Dual cycles.

The Otto cycle consists of constant volume heat addition,
the Diesel cycle consists of constant pressure heat addition,
and the Dual cycle combines both constant volume and constant pressure heat addition.
P-V and T-S diagrams are used to represent pressure-volume and temperature-entropy relationships of these cycles.

Q1 (c) Explain stoichiometric air-fuel ratio.

Stoichiometric air-fuel ratio is the exact ratio of air to fuel required for complete combustion of fuel without excess air or fuel. For petrol engines, it is approximately 14.7:1 by mass.

Q1 (d) What are the alternate fuels for I.C. engines?

Alternate fuels include CNG, LPG, biogas, ethanol, methanol, biodiesel, hydrogen, and producer gas. These fuels help reduce emissions and dependence on conventional fuels.

Q1 (e) Explain delay period in C.I. engine.

Delay period in a compression ignition engine is the time interval between the start of fuel injection and the start of combustion. It depends on fuel quality, temperature, pressure, and engine speed.

Q1 (f) Explain octane number and cetane number in brief.

Octane number indicates the anti-knock quality of petrol used in spark ignition engines.
Cetane number indicates the ignition quality of diesel fuel used in compression ignition engines.

Q1 (g) Differentiate between reciprocating and rotary air compressor.

Reciprocating compressors use piston-cylinder arrangement and are suitable for high pressure and low flow rate. Rotary compressors use rotating elements and are suitable for low pressure and high flow rate with smoother operation.

SECTION B

(Attempt any one part – 7 × 1 = 7 marks)

Q6 (a) Describe advantages and disadvantages of air cooling and water cooling. Explain thermostat cooling system with diagram.

Air cooling system uses fins to dissipate heat directly to air. It is simple, light, and requires less maintenance but is less efficient for large engines.
Water cooling system uses coolant circulated through jackets around the cylinder. It provides uniform cooling but is heavier and costlier.

In a thermostat cooling system, a thermostat valve controls coolant flow based on engine temperature. When the engine is cold, the valve remains closed; when temperature rises, it opens and allows coolant circulation, maintaining optimal engine temperature.

Q6 (b) Explain the Willan’s line and Morse test in detail.

Willan’s line method is used to determine friction power of an engine by plotting fuel consumption against brake power and extrapolating to zero brake power.
Morse test is used for multi-cylinder engines by cutting out one cylinder at a time to determine indicated power and friction power.

SECTION C

(Attempt any one part – 7 × 1 = 7 marks)

Q7 (a) Explain the advantages of multistage compression with the help of P-V and T-S diagrams. Derive optimum pressure ratio for two-stage compressor and also find minimum work of compression.

Multistage compression reduces work input, improves efficiency, and lowers discharge temperature.
With intercooling between stages, the compression process approaches isothermal compression.

For two-stage compression, the optimum intermediate pressure is the geometric mean of inlet and delivery pressures:

Popt=P1P3P_{opt} = \sqrt{P_1 P_3}Popt​=P1​P3​​

At this pressure ratio, minimum work of compression is achieved. P-V and T-S diagrams show reduced area, indicating lower work input.

Q7 (b) A two-stage air compressor compresses air from 1 bar and 17°C to 65 bar. The air is cooled in the intercooler to 30°C and intermediate pressure is 7.7 bar.

Low-pressure cylinder diameter = 16 cm, stroke = 11.25 cm.
Assuming PV¹·³ = constant, find the indicated power.**

The problem is solved by calculating work of compression for each stage using polytropic relation.
Volumetric flow rate is obtained from cylinder dimensions.
Total indicated power is the sum of work done in both stages per second.