Internal Combustion Engines MCQ || IC Engine MCQ

Ans: 3. 227°C

Explanation

Given,

η = 30%

T₂ = 77°C = 77 + 273 = 350K

The efficiency of heat engine

$\begin{array}{l} \eta = 1 – \frac{{{T_2}}}{{{T_1}}}\\ \\ \frac{{30}}{{100}} = 1 – \frac{{350}}{{{T_1}}} \end{array}$

T₁ = 500 K

T₁ = 500 – 273 = 227°C

Ans: 1. Low heat value of oil

Explanation

All heat engines utilize the low heat value of whatever fuel is used. The high heat value of fuels containing hydrogen includes some heat not available for conversion into work in any internal – combustion engine.

Ans: 4. m.e.p. and I.H.P.

Explanation

An engine indicator is used to determine the mean effective pressure. The engine indicator diagram is plotted between pressure and piston displacement; during an engine’s stroke. An engine indicator is an instrument for graphically recording the pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine.

Ans: 2. High heat value of oil

Explanation

• Higher heating value is defined as the amount of heat released when fuel is combusted, and the products have returned to a temperature of 25°C. The heat of condensation of the water is included in the total measured heat.
• The higher heating value (HHV) is measured using a bomb calorimeter.
• Fuel oil consumption guarantees for I .C. engines are usually based on the High heat value of oil.

Ans: 4. 14%

Explanation

The efficiency of an engine working in an otto cycle is given by

$\eta = 1 – \frac{1}{{{r^{\gamma – 1}}}}$

We know that the heat capacity ratio for the air is approximately is 1.4.  So we substitute γ = 1.4 in above expression

$\begin{array}{l} \eta = 1 – \frac{1}{{{r^{1.4 – 1}}}}\\ \\ \Rightarrow \eta = 1 – \frac{1}{{{r^{0.4}}}} \end{array}$ —— (1)

The engine has an initial compression ratio of the engine is equal to 5 . So substituting r = 5 in eqn 1

${\eta _1} = 1 – \frac{1}{{{5^{0.4}}}}$

η₁ = 0.47

Now, the engine has a final compression of the engine is equal to 7 . So substituting r = 7 in eqn 1

${\eta _2} = 1 – \frac{1}{{{7^{0.4}}}}$

η₂ = 0.54

Now, the increase in efficiency can be given by

Δη = η₂ − η₁

The fractional change in efficiency can be given by

$\frac{{{\rm{\Delta }}\eta }}{{{\eta _1}}} = \frac{{{\eta _2} – {\eta _1}}}{{{\eta _1}}}$

And the percentage change in efficiency can be written as

$\frac{{{\rm{\Delta }}\eta }}{{{\eta _1}}} \times 100 = \frac{{{\eta _2} – {\eta _1}}}{{{\eta _1}}} \times 100$

∴ $\frac{{{\rm{\Delta }}\eta }}{{{\eta _1}}} \times 100 = \frac{{0.54 – 0.47}}{{0.47}} \times 100$

$\begin{array}{l} \frac{{{\rm{\Delta }}\eta }}{{{\eta _1}}} \times 100 = \frac{{0.54 – 0.47}}{{0.47}} \times 100\\ \\ \Rightarrow \frac{{{\rm{\Delta }}\eta }}{{{\eta _1}}} \times 100 = 14.9{\rm{\% }} \approx 14{\rm{\% }} \end{array}$

Thus, the percentage increase in the efficiency is equal to 14%

Ans: 2. Low Heat Value

Explanation

Low heat value is used where there is no occurrence of condensation of combustion product. In other words, the fuel’s latent heat of vaporization is not recovered. In the case of gas turbines, there is no recovery of phase of the fuel as the temperature range is high.

Ans: 1. Opens at 20° before top dead center and closes at 35° after the bottom dead center

Explanation

The inlet valves or intake valves are opened to allow the air/fuel mixture to flow into the engine’s cylinders before compression and ignition. So, basically, these are the valves through which a fluid is drawn inside of an engine. The induction valve is another name given to these valves.

For a typical medium-speed 4stroke cycle diesel engine, the inlet valves open at 20°C before the top dead center and close at 35°C after the bottom dead center.

Ans: 2. 6 – 12 kg/cm2 and 250 – 350°C

Explanation

Four-stroke spark ignition (SI) engines or Petrol Engines:

• During the compression stroke, pressure varies from 6 to 10 kg/cm² and temperature of about 260°C.

Four Stroke compression ignition (CI) engine:

• During the compression stroke, the engine attains high pressure ranging from 30 to 45 kg/cm² and a high temperature of about 500°C.

Ans: 4. 35 kg/cm²

Explanation

Four Stroke compression ignition (CI) engine:

• During the compression stroke, the engine attains high pressure ranging from 30 to 45 kg/cm² and a high temperature of about 500°C.

Four-stroke spark ignition (SI) engines or Petrol Engines:

• During the compression stroke, pressure varies from 6 to 10 kg/cm² and temperature of about 260°C.

Ans: 3. 1500-2000°C

Explanation

• Maximum temperature in an IC engine is approx in between 1500 to 2000 °C
• The temperature of the burning gases in the engine cylinder reaches up to 1500 to 2000 °C, which is above the melting point of the material of the cylinder body and head of the engine.
• To reduce the maximum temperature various cooling system are used.