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

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.

Explanation:

Importance of Piston Fit in Cylinder

The piston is a crucial component in internal combustion engines, reciprocating pumps, and other machinery involving fluid dynamics. It moves within the cylinder to create pressure differences necessary for the engine's operation. For optimal performance, the piston must fit snugly into the cylinder, balancing two critical requirements: allowing easy movement and providing a gas-tight space.

Correct Option: The correct option is 3) both, allow for easy movement and provide a gas-tight space. This dual requirement is fundamental to the efficiency and functionality of machinery involving pistons and cylinders.

Detailed Solution:

Piston-Cylinder Dynamics: The interaction between the piston and the cylinder is central to the operation of engines and pumps. The cylinder houses the piston, allowing it to move back and forth, creating pressure changes that drive the engine. For this system to work effectively, the fit between the piston and cylinder must be precise.

Gas-Tight Space: One of the primary requirements for the piston-cylinder fit is to create a gas-tight space. This ensures that the combustion gases or fluids do not escape, maintaining the necessary pressure for the engine's operation. A gas-tight fit is critical in internal combustion engines where the compression of the air-fuel mixture is essential for the engine to produce power. If the piston does not fit snugly, gases can leak, leading to a loss of pressure, reduced efficiency, and potential engine failure.

Easy Movement: While maintaining a gas-tight space is crucial, the piston must also move easily within the cylinder. The movement should be smooth and without excessive friction to prevent wear and tear on the piston and cylinder walls. Excessive friction can lead to heat generation, material degradation, and ultimately, engine malfunction. Therefore, the piston must be able to slide smoothly within the cylinder, facilitated by lubrication and precise engineering.

Balancing Both Requirements: Balancing the need for a gas-tight space with the requirement for easy movement is achieved through meticulous engineering and manufacturing processes. The materials used for the piston and cylinder, their surface finishes, and the lubrication system all play a role in ensuring this balance. Engineers design pistons with rings and seals that maintain the gas-tight fit while allowing smooth movement. These components are often made from materials that can withstand high temperatures and pressures, ensuring durability and performance.

Consequences of Poor Fit: If the piston does not fit snugly, several issues can arise. A loose fit can lead to gas or fluid leaks, reducing the efficiency of the engine or pump. On the other hand, an overly tight fit can cause excessive friction, leading to wear and potential seizure of the piston. Both scenarios can result in reduced performance, increased maintenance costs, and potential failure of the machinery.

Conclusion: The correct fit of the piston in the cylinder is essential for the efficient and effective operation of engines and pumps. It must provide a gas-tight space to maintain pressure while allowing easy movement to reduce friction and wear. Option 3 correctly identifies the need for both these requirements, highlighting the importance of precise engineering in ensuring the optimal performance of piston-cylinder systems.

Analysis of Other Options:

Option 1: Neither allow for easy movement nor provide a gas-tight space is incorrect. Both these requirements are essential for the efficient operation of engines and pumps. Without a gas-tight space, pressure cannot be maintained, and without easy movement, the piston and cylinder will suffer from excessive wear and potential failure.

Option 2: Provide a gas-tight space alone is insufficient. While a gas-tight space is crucial for maintaining pressure, the piston must also move easily within the cylinder to prevent excessive friction and wear. This option overlooks the importance of smooth movement in the system's overall performance.

Option 4: Allow for easy movement alone is also insufficient. Easy movement is necessary to reduce friction and wear, but without a gas-tight space, the system cannot maintain the required pressure for efficient operation. This option fails to address the need for maintaining pressure within the cylinder.

Conclusion: In summary, the correct fit of the piston within the cylinder must balance the need for a gas-tight space with the requirement for easy movement. This dual requirement ensures the efficient and effective operation of engines and pumps, highlighting the importance of precise engineering in achieving optimal performance.

Explanation:

Four-Stroke Cycle Diesel Engine

Definition: A four-stroke cycle diesel engine is an internal combustion engine that operates on the diesel cycle, completing a power cycle in four strokes of the piston: intake, compression, power, and exhaust. Diesel engines use compression ignition to burn the fuel, which is injected into the combustion chamber.

Working Principle: The four strokes of a diesel engine are:

• Intake Stroke: The intake valve opens, and the piston moves down from the top dead center (TDC) to the bottom dead center (BDC), allowing air to enter the cylinder.
• Compression Stroke: The intake valve closes, and the piston moves up from BDC to TDC, compressing the air inside the cylinder. The temperature and pressure of the air increase significantly.
• Power Stroke: Near the end of the compression stroke, diesel fuel is injected into the cylinder, and the high temperature and pressure cause the fuel to ignite. The resulting combustion forces the piston down from TDC to BDC, generating power.
• Exhaust Stroke: The exhaust valve opens, and the piston moves up from BDC to TDC, expelling the combustion gases from the cylinder.

In a four-stroke diesel engine, the intake valve begins to open before the piston reaches Top Dead Center (TDC) on the exhaust stroke. This early opening, typically 10°−25° before TDC, ensures that the valve is fully open when the intake stroke begins, allowing optimal airflow into the cylinder. This timing is part of the engine's valve lead for efficient scavenging and charging.

Explanation:

Thermal Efficiency in Internal Combustion Engines

Definition: Thermal efficiency in the context of internal combustion engines is defined as the ratio of the useful work output to the total chemical energy input from the fuel. This metric is crucial as it measures how effectively the engine converts the energy stored in the fuel into mechanical work.

Importance: Thermal efficiency is a key indicator of an engine’s performance. Higher thermal efficiency means that more of the fuel’s energy is being converted into useful work, which can lead to better fuel economy and lower emissions. This is particularly important for reducing operating costs and environmental impact.

Calculation: Thermal efficiency (\( \eta_{th} \)) is calculated using the formula:

\( \eta_{th} = \frac{\text{Useful work output}}{\text{Total chemical energy input from fuel}} \)

Where:

• Useful work output: The mechanical work that the engine produces, which can be measured by the power delivered to the wheels or other machinery.
• Total chemical energy input from fuel: The total energy content of the fuel consumed, which can be determined by the fuel's calorific value and the amount of fuel used.

Factors Affecting Thermal Efficiency:

• Engine Design: The design of the engine components, such as the combustion chamber, piston, and valves, can significantly impact thermal efficiency. Optimizing these components can lead to better combustion and heat transfer, thus improving efficiency.
• Fuel Type: Different fuels have different energy contents and combustion characteristics. Using a fuel with a higher calorific value can improve thermal efficiency.
• Operating Conditions: The operating conditions, including engine load, speed, and temperature, can affect thermal efficiency. Engines typically have an optimal operating range where efficiency is maximized.
• Technological Advances: Technologies such as turbocharging, direct fuel injection, and variable valve timing can enhance thermal efficiency by improving the combustion process and reducing energy losses.

Importance in Automotive Industry: In the automotive industry, improving the thermal efficiency of internal combustion engines is a primary focus. Higher thermal efficiency translates to better fuel economy, which is a significant selling point for consumers. Additionally, it helps in meeting stringent emission regulations by reducing the amount of fuel burned and the associated pollutants.

Conclusion: Thermal efficiency is a critical measure of an internal combustion engine's performance. By maximizing thermal efficiency, manufacturers can produce engines that are more economical, environmentally friendly, and competitive in the market.

Important Information:

Analysis of Other Options:

Option 1: The ratio of indicated power to frictional power losses

This option describes the mechanical efficiency of an engine, not the thermal efficiency. Mechanical efficiency focuses on the losses due to friction and other mechanical factors within the engine but does not account for the chemical energy input from the fuel.

Option 2: The ratio of the engine displacement to the fuel mass used

This option is unrelated to thermal efficiency. The engine displacement refers to the volume of the cylinders, while the fuel mass used pertains to the amount of fuel consumed. This ratio does not provide information about how efficiently the fuel’s energy is converted into useful work.

Option 3: The ratio of exhaust gas temperature to the intake air temperature

This option is also incorrect. The ratio of exhaust gas temperature to intake air temperature may provide some insights into the engine's thermal processes, but it does not directly measure thermal efficiency. Thermal efficiency is specifically about the conversion of chemical energy in the fuel to mechanical work.

In conclusion, thermal efficiency is a vital parameter in evaluating the performance of internal combustion engines. It is defined as the ratio of useful work output to the total chemical energy input from fuel, which is correctly described in Option 4. Understanding and improving thermal efficiency is essential for developing more efficient, cost-effective, and environmentally friendly engines.

Concept:

Brake specific fuel consumption (BSFC) =m_f/BP

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

\(Brake\;thermal\;efficiency\left( {{\eta _b}} \right) = \frac{{BP}}{{{m_f} \times CV}} = \frac{1}{{BSFC \times CV}}\)

CV = Calorific Value

Calculation:

Given:

CV = 40 MJ/kg = 40 × 10⁶ J/kg.

\(BSFC = 200\;gm/kWh = \frac{{200 \times {{10}^{ - 3}}}}{{\left( {3600 \times {{10}^3}} \right)}}\;kg/J = \frac{1}{{18}} \times {10^{ - 6}}\;kg/J\)

\(\eta = \frac{1}{{\left( {\frac{1}{{18}}} \right) \times {{10}^{ - 6}} \times 40 \times {{10}^6}}} = \frac{{18}}{{40}} = 0.45 = 45\% \)

Concept:

Mechanical efficiency at half load \( = \frac{{BP}}{{BP+ FP}}\)

Calculation:

Given: 

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

Mechanical efficiency at half load \( = \frac{{BP}}{{BP+ FP}}\)

Mechanical efficiency at half load \( = \frac{{100}}{{125 }}\)

Mechanical efficiency at half load = 0.8 ⇒ 80 %

Concept:

Brake specific fuel consumption \( = \frac{{Mass\;of\;fuel/Hour}}{{Brake\;power\;\left( {kW} \right)}}\)

Calculation:

Given:

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

\(BSFC = \frac{{Mass\;of\;fuel/Hour}}{{Brake\;power\;\left( {kW} \right)}}\)

\(BFSC = \frac{{0.44 × 60}}{{105}}\)

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

\(Compression\ ratio\left( {{r_c}} \right) = \frac{{Effective\ Swept\ Volume}}{{Clearance\ Volume}}\)

\({r_c} = \frac{{{V_S} + {V_C}}}{{{V_C}}}\)

\({r_c} = \frac{{{V_S} + {V_C}}}{{{V_C}}}=1+\frac{{{V_S} }}{{{V_C}}}\)

\(8 =1+\frac{{{450} }}{{{V_C}}}\Rightarrow \frac{{{450} }}{{{V_C}}}=7 \Rightarrow V_c=64.3\; cc\)

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:

\(\eta_m = \frac{Brake\ Power}{Indicated\ Power} = \frac{B.P.}{I.P.}\)

Indicated thermal efficiency:

\(\eta_{ith} = \frac{Indicated\ Power}{Heat\ added\ per\ second}\)

Heat added per second HA/s =  \(\dot m_f\ \times \) (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

\(η_m = \frac{B.P}{I.P.}\)  ⇒  \(I.P = \frac{B.P}{η_m} = \frac{4.5}{0.85} \) = 5.294 kW

\(η_{ith} = \frac{I.P}{HA/s}\)

\(HA/s = \frac{I.P}{η_{ith}} = \frac{5.294}{0.3} = 17.647\ kW\) 

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

\(HA/s = \dot m_f\times {C.V}_f\) = 17.647 kJ/s

\(\dot m_f = \frac{17.647\ \times\ 3600 }{40000}\) = 1.6 kg/hr.

Concept:

Brake thermal efficiency\(\left( {{\eta _{bth}}} \right) = \frac{{BP}}{{{m_f}\; × \;CV}}\)

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,  

∴ \(0.3 = \frac{{15}}{{40 ~\times~ 10^3\; × \;{m_f}}}\)

\({m_f} = \frac{{15}}{{0.3}} × \frac{1}{{40000}} × 3600\frac{{kg}}{{hr}}\)

∴ mf = 4.5 kg/hr

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.

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.