Engine Performance Parameter MCQ Quiz - Objective Question with Answer for Engine Performance Parameter - Download Free PDF

Explanation:

Two-Stroke Petrol Engine:

• A two-stroke petrol engine is a type of internal combustion engine that completes a power cycle in two strokes of the piston during only one crankshaft revolution. In this type of engine, the inlet port, transfer port, and exhaust port play crucial roles in ensuring the proper intake of the air-fuel mixture, transfer of the mixture to the combustion chamber, and expulsion of exhaust gases.
• The opening and closing of the inlet port in a two-stroke engine are controlled by the movement of the piston itself. Unlike in four-stroke engines, there are no valves in a traditional two-stroke engine; instead, ports are used. The timing of these ports is critical for the efficient operation of the engine.

30° to 40° before BDC:

• The inlet port begins to open when the piston is moving towards the bottom dead center (BDC), which allows the fresh air-fuel mixture to enter the crankcase. This occurs slightly before the piston reaches BDC, typically in the range of 30° to 40° before BDC, to ensure that the mixture has enough time to flow into the crankcase under the influence of the pressure difference created by the piston's motion.

Working Mechanism:

1. Induction Phase: As the piston moves downward during its power stroke, it simultaneously uncovers the inlet port. This movement creates a low-pressure area in the crankcase. The fresh air-fuel mixture is then drawn into the crankcase through the inlet port due to the pressure difference.

2. Timing of Inlet Port Opening: The inlet port opens slightly before the piston reaches BDC, as specified in the range of 30° to 40° before BDC. This ensures that the air-fuel mixture starts entering the crankcase at the optimal time, maximizing the engine's efficiency and power output.

3. Transfer and Exhaust Phases: Once the piston starts moving upward after reaching BDC, it compresses the air-fuel mixture in the crankcase. At the same time, the transfer port and exhaust port open to allow the fresh mixture to enter the combustion chamber and the exhaust gases to exit, respectively.

Explanation:

Mean Effective Pressure (MEP) in Engine Performance Analysis

• The Mean Effective Pressure (MEP) is a critical parameter in engine performance analysis. It represents the average pressure acting on the piston during the entire engine cycle, which would produce the measured work output if it were applied uniformly. MEP is not an actual physical pressure but a theoretical concept used to evaluate the performance and efficiency of internal combustion engines.
• In simpler terms, MEP provides a way to compare the work output of engines irrespective of their size, speed, or displacement. This parameter is especially useful for engineers and designers to assess and optimize engine designs.

For Otto cycle:

Explanation:

Indicated Power (IP):

• Indicated power refers to the total power generated within the engine’s cylinders as a result of the combustion process. It is calculated based on the pressures exerted by the combustion gases on the piston during the power stroke. This power is theoretical in nature and does not account for the losses incurred due to friction and other mechanical inefficiencies.

The formula for indicated power is as follows:

IP = (P_m × L × A × N × K) / 60

Where:

• P_m: Mean effective pressure in the cylinder (N/m²)
• L: Stroke length (m)
• A: Area of the piston (m²)
• N: Engine speed (RPM)
• K: Number of power strokes per revolution (for a four-stroke engine, K = 0.5)

Indicated power is derived from the pressure-volume (P-V) diagram of the engine and represents the gross power developed by the engine before any losses are considered.

Brake Power (BP):

• Brake power, on the other hand, is the net useful power available at the engine’s output shaft (crankshaft). It is the power that can be harnessed for performing external work, such as driving a vehicle or running machinery. Brake power accounts for mechanical losses due to friction, heat dissipation, and other factors within the engine.

Brake power is measured using a dynamometer, which applies a controlled load to the engine and measures the torque and rotational speed. The formula for brake power is:

BP = (2 × π × N × T) / 60

Where:

• N: Engine speed (RPM)
• T: Torque applied (Nm)

Brake power represents the actual, usable output of the engine after accounting for internal losses.

Correct Option Analysis:

The correct option is:

Option 3: Indicated power is derived from combustion chamber pressures, whereas brake power is the net output at the crankshaft after mechanical losses.

This option accurately describes the distinction between indicated power and brake power:

• Indicated power is determined based on the pressure generated in the combustion chambers during the power stroke. It is a theoretical measure of the engine’s gross power output.
• Brake power, in contrast, is the practical, net power output available at the crankshaft. It accounts for losses due to friction, heat, and other inefficiencies within the engine.

Thus, indicated power represents the engine’s potential power output, while brake power indicates the actual power that can be utilized for external applications. The difference between the two is attributed to the mechanical losses within the engine.

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.

Explanation:

Specific Fuel Consumption (SFC)

• Specific Fuel Consumption (SFC) is a critical parameter in evaluating the performance of internal combustion engines. It measures the fuel efficiency of an engine by calculating the amount of fuel consumed per unit of power produced over a specific time period. This parameter is particularly useful in comparing the performance of engines and determining their operational efficiency. SFC is typically expressed in units such as grams per kilowatt-hour (g/kWh) or pounds per horsepower-hour (lb/hp-hr), depending on the unit system being used.

Formula for SFC:

The specific fuel consumption is calculated using the formula:

SFC = Fuel consumption rate / Power output

Where:

• Fuel consumption rate: The amount of fuel consumed by the engine per unit of time (e.g., kg/hr or lb/hr).
• Power output: The power produced by the engine during the same time period (e.g., kW or hp).

Significance of SFC:

• Fuel Efficiency: SFC serves as a direct indicator of an engine’s fuel efficiency. Lower values of SFC suggest that the engine consumes less fuel to generate the same power output, which is desirable in most applications.
• Cost Savings: Improved fuel efficiency, as indicated by a lower SFC, leads to reduced operating costs for vehicles, machinery, and equipment.
• Environmental Impact: Engines with lower SFC values produce fewer emissions for the same power output, contributing to reduced environmental pollution.
• Design Optimization: By analyzing SFC, engineers can identify areas for improvement in engine design and optimize performance for specific applications.

Factors Affecting SFC:

• Engine Design: The design and configuration of the engine, such as the number of cylinders, compression ratio, and turbocharging, significantly influence SFC.
• Operating Conditions: SFC varies with engine load, speed, and temperature. Engines tend to be most efficient within a specific range of operating conditions.
• Fuel Quality: The type and quality of fuel used, including its calorific value and combustion characteristics, impact SFC.
• Maintenance: Regular maintenance and proper tuning of the engine can enhance fuel efficiency and lower SFC.

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:

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 %

Concept:

Brake specific fuel consumption 

Calculation:

Given:

Brake power = 105 kW, ṁ = 4.4 kg per 10 min = 0.44 kg/min ⇒ 0.44 × 60 = 26.4 kg/hr.

BSFC = 0.251 kg/kW-hr.

The compression ratio is the ratio of volume before compression and after compression.

Let VS = Stroke volume

VC = Clearance volume

Concept:

A dynamometer is a device used for measuring the torque and brake power required to operate a driven machine. It has a device to measure the frictional resistance.

The following are the two types of dynamometers, used for measuring the brake power of an engine.

• Absorption dynamometers: The entire energy or power produced by the dynamometer is absorbed by the friction resistance of the brake and is transformed into heat, during the process of measurement.
  • Example: Prony brake dynamometer, Rope brake dynamometer, Hydraulic dynamometer, Eddy current dynamometer.
• Transmission dynamometers: The energy is not wasted in friction but is used for doing work. The energy or power produced by the engine is transmitted through the dynamometer to some other machines where the power developed is suitably measured.
  • Examples: Epicyclic-train dynamometer, Belt transmission dynamometer, Torsion dynamometer.

Concept:

Four-stroke engines: In this type of engine, one power stroke is obtained in two revolutions of the crankshaft.

Two-stroke engines: In this engine, one power stroke is obtained in each revolution of the crankshaft.

Concept:

Mechanical efficiency:

Indicated thermal efficiency:

Heat added per second HA/s =   (C.V.)_f, where C.V. = Calorific value of fuel 

Calculation:

Given:

Brake power, B.P = 4.5 kW, Indicated thermal efficiency η_ith = 30% = 0.3, Mechanical efficiency η_m = 85%, Calorific value (C.V)_f = 40000 kJ/kg

  ⇒   = 5.294 kW

where I.P = indicated power, HA/s = Heat added per second

 = 17.647 kJ/s

 = 1.6 kg/hr.

Concept:

Brake thermal efficiency

where, BP = Brake power of IC engine, m_f = fuel consumption, CV = Calorific value of the fuel

Calculation:

Given:

ηbth = 0.3, CV = 40 × 10³ kJ/kg, BP = 15 kJ/s

Now,  

∴ 

∴ mf = 4.5 kg/hr

Concept:

The difference between the indicated and the brake power of an engine is known as friction power. The frictional power of an engine can be determined by the following methods:

1. Willan’s line method
2. Morse test
3. Motoring test
4. From the measurement of indicated and brake power
5. Retardation test

Willan’s line method:

• This method is also known as the fuel rate extrapolation method.
• A graph connecting fuel consumption on Y-axis and brake power on X-axis at constant speed is drawn and it is extrapolated on the negative axis of brake power.
• The intercept of the negative axis is taken as the friction power of the engine at that speed. 

Morse test:

• The Morse test is used for measuring the indicated power of the multi-cylinder engines.
• This test measures the indicated power by cutting out the spark plug of the cylinder by keeping the speed of the engine constant.
• To understand this test better, suppose the brake power for a cylinder with the spark plug on is B.P₁ and with the spark plug cut is B.P₁' then the indicated power will be (B.P₁ - B.P₁').
• It is assumed that pumping and friction losses are the same when the spark plug is cut-off or in operation.
• Friction power = Indicated power of cylinder – Break the power of the cylinder.

Motoring test:

• In this test, a swinging field-type electric dynamometer is used to absorb the power developed during the steady-state operation.
• The ignition is cut off and by suitable electric switching devices, the dynamometer is converted to run as a motor so as to crank the engine at the same speed at which it was previously operated.
• The power supply is then measured which gives the friction power of the engine at that speed.

Concept:

• Mean effective pressure is the average pressure inside the cylinder of an internal combustion engine based on the calculated or measured power.

• Mean effective pressure increases with compression ratio because of an increase in efficiency.
• At a fixed compression ratio, maximum MEP occurs at a slightly rich fuel-air ratio (Higher than stoichiometric)similar to the case of maximum combustion temperature.

Requirement of richer mixture for maximum MEP:

While complete combustion happens at the stoichiometric AFR, a slightly richer mixture (more fuel than stoichiometric) leads to:

• Faster and hotter combustion: The additional fuel provides more readily available fuel molecules for rapid burning, leading to a quicker pressure rise during the power stroke.
• Increased cylinder temperature: The extra fuel burning generates more heat, further contributing to a higher average pressure throughout the power stroke.

While a richer mixture maximizes MEP, it comes at the cost of slightly reduced thermal efficiency. This means the engine converts less of the fuel's energy into usable work.