Internal Combustion Engine MCQ Quiz - Objective Question with Answer for Internal Combustion Engine - Download Free PDF

Explanation:

Thermodynamic Cycles in Internal Combustion Engines

• Thermodynamic cycles are the theoretical models used to describe the energy conversion process in internal combustion engines. Among the most common thermodynamic cycles are the Otto cycle, Diesel cycle, and Dual cycle. These cycles differ based on the way heat is added to the system and the type of fuel-air mixture combustion process used.

The given problem discusses the comparison of these cycles for the same compression ratio and heat rejection. We aim to identify the correct representation of the Otto cycle based on the provided information.

Otto Cycle:

• The Otto cycle is defined by constant volume heat addition and heat rejection.
• From the figure, the process 1-2 represents the isentropic (adiabatic) compression phase.
• Process 2-6 corresponds to the isochoric (constant volume) heat addition phase, which is the hallmark of the Otto cycle.
• Process 6-5 represents the isentropic (adiabatic) expansion phase, where the thermal energy is converted into mechanical work.
• The final process, 5-1, corresponds to the isochoric (constant volume) heat rejection phase, completing the cycle.

Thus, the sequence 1-2-6-5 accurately represents the Otto cycle in the context of the given problem

Concept:

Brayton Cycle:

The Brayton cycle (or Joule cycle) represents the operation of a gas turbine engine.

The cycle consists of four processes

• Process a – b: Reversible adiabatic compression in the inlet and compressor
• Process b – c: Isobaric heat addition (fuel combustion)
• Process c – d: Reversible adiabatic expansion in the turbine and exhaust nozzle
• Process d – a: Cool the air at constant pressure back to its initial condition.

The efficiency of the Brayton cycle:

Where, rp = pressure ratio = 

Otto cycle:

Otto cycle consists of four processes. They are as follows:

Process 1 – 2: Reversible adiabatic or Isentropic compression

Process 2 – 3: Constant Volume Heat Addition

Process 3 – 4: Isentropic (reversible adiabatic) expansion

Process 4 – 1: Constant Volume Heat Rejection

The efficiency of the Otto cycle:

\({\eta _{otto}} = 1 - \frac{1}{{{r^{γ - 1}}}}\)

Where r = compression ratio 

We know that compression in both the Otto cycle and Brayton cycle is an isentropic process, so we may write

Thus for the same pressure ratio or compression ratio in Otto and Brayton, their efficiency is equal.

Explanation:

Ideal Four-Stroke Petrol Engine

• An ideal four-stroke petrol engine operates on the Otto cycle, which consists of four distinct strokes: intake, compression, power (expansion), and exhaust. In such an engine, the power stroke is where the combustion of the air-fuel mixture occurs, releasing energy to perform work. The assumption made about the burning process during the power stroke in an ideal engine is crucial for understanding its efficiency and operation.

Explanation of the Assumption:

• The assumption of instantaneous combustion at TDC is made to ensure that the combustion process occurs at a constant volume. This is because, at TDC, the piston momentarily stops moving before reversing direction, and during this brief moment, the volume of the combustion chamber remains constant.
• In reality, combustion takes a finite amount of time, and the piston is already moving down during the actual combustion process. However, for ideal cycle analysis, this finite time is neglected, and the process is modeled as if it occurs instantaneously at constant volume.
• This assumption allows for the simplification of the thermodynamic analysis of the Otto cycle, making it easier to calculate parameters such as thermal efficiency, work output, and heat input.

Concept:

In an ideal Otto cycle, if the temperatures at the beginning and end of isentropic compression are known, the air-standard efficiency is:

Given:

• Initial temperature, 
• Final temperature, 

Calculation:

Explanation:

Frictional Power:

• Frictional power in an engine refers to the power loss due to friction within the engine components. This includes friction between the piston and cylinder walls, bearings, and other moving parts. It is the difference between the indicated power (the power generated within the engine cylinder) and the brake power (the usable power delivered by the engine).
• When an engine operates, not all the power generated by the combustion process is converted into usable work. A portion of the power is lost due to friction between the moving components of the engine. The indicated power (I.P.) is the total power generated inside the engine cylinder without considering losses, while the brake power (B.P.) is the actual power delivered by the engine to perform useful work.

Concept:

Diesel cycle:

If cut-off happens at k % of the stroke, then

cut-off ratio (ρ) = 1 + k(r - 1)

Calculation:

Given:

r = 16, k = 8 % , ρ = ?

(ρ) = 1 + k(r - 1) 

∴ 1 + 0.08 (16 - 1) = 2.20

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.

Concept:

Brake specific fuel consumption (BSFC) =m_f/BP

Where m_f = mass flow rate of fuel, BP = Brake Power

CV = Calorific Value

Calculation:

Given:

CV = 40 MJ/kg = 40 × 10⁶ J/kg.

Concept:

The capacity of engine is given by:

Capacity of engine = Swept volume × Numbers of cylinders(n)

Swept volume is given by:

Calculation:

Given:

d = 4 cm, L = 7 cm, n = 4

Clearance volume, V_{c ­}= 2 cm³

Capacity of engine is:

capacity of engine = Swept volume × Numbers of cylinders 

Concept:

Diesel cycle:

Processes in compression engine (diesel cycle) are:

Process 1-2: Reversible adiabatic compression

Process 2-3: Constant pressure heat addition

Process 3-4: Reversible adiabatic expansion

Process 4-1: Constant volume of heat rejection

cut-off ratio:

The cut-off ratio is the ratio of the volume after combustion to the volume before combustion.

Cut-off ratio: 

Compression ratio: 

The efficiency of the diesel cycle is given by

The mean effective pressure (p_m) which is an indication of the internal work output increases with a pressure ratio at a fixed value of compression ratio and the ratio of specific heats.

The expression for mean effective pressure for diesel cycle,

From the expression,

The mean effective pressure of the diesel cycle having a fixed compression ratio will increase if the cut-off ratio increases.

Concept:

Diesel cycle:

P-V and T-S diagram of Diesel cycle are:

Compression ratio (r) is given by:

Cut-off ratio (r_c) is given by:

Calculation:

Given:

Compression ratio (r) = 16 = 

v₃ - v₂ = 0.06(v₁ - v₂)

r_c = 1.9

Explanation:

There are three standard cycles that are used to perform analysis of IC engine:
1) Constant volume combustion (Otto) cycle

2) Constant pressure combustion (Diesel) cycle

3) Combination of constant volume and constant pressure combustion (Dual) cycle

Assumptions during analysis:

• The working fluid throughout the cycle is air and it is treated as an ideal gas 
• The compression and expansion processes are taken as frictionless and adiabatic (no heat loss) i.e. they are reversible 
• The chemical equilibrium of the working fluid is taken as constant   
• The combustion process is replaced by well-defined heat addition processes
• The exhaust process is replaced by a heat rejection process that returns the air of the cycle to intake conditions
• Since the gas is assumed as ideal the specific heats at constant volume and pressure are taken as constant 

​∴ There are no intake and exhaust processes because they are replaced by heat addition and heat rejection processes

Concept:

Thermal efficiency of Otto Cycle:

Compression ratio: r = v₁/v₂

It is given that 

Comparing it to the derived equation, T_a resembles T₁ and T_b resembles T_2. 

T₂ is the temperature where compression stops and the constant volume heat addition starts.

∴ T_b is the temperature where constant volume heat addition starts.

Concept:

Mechanical efficiency at half load 

Calculation:

Given: 

Brake power (BP) = 200 kW, Half load = 100 kW Friction Power (FP) = 25 kW

Mechanical efficiency at half load 

Mechanical efficiency at half load 

Mechanical efficiency at half load = 0.8 ⇒ 80 %

Explanation:

Petroil (Petro-oil lubrication system): In this method, the lubricating oil is mixed with petrol and fed into the engine cylinder during the suction stroke. The droplets of the partials cause the lubricating effect in the engine cylinder.

This method of lubrication is used in small engines like motorcycles and scooters. The system of lubrication is used in scooters and motorcycles, particularly for two-stroke engines about 3 to 6% of lubrication oil is added with petrol is the petrol tank.

The petrol evaporates when the engine is working. The lubricating oil is left behind in the form of mist. The parts of the engine such as piston, cylinder walls and connecting rod are lubricated by being waited with the oil mist left behind.

Splash lubrication system: The splashing action of oil maintains a fog or mist of oil that drenches the inner parts of the engine such as bearings, cylinder walls, pistons, piston pins, timing gears etc. The splash oil then drips back into the sump.

This system is commonly used in a single-cylinder engine with the closed crankcase.

This system is commonly used on the high-speed multi-cylinder engine in tractors, trucks and automobiles.