Thermodynamics in SI Engines

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Questions and Answers

What primarily causes the pressure drop between the cylinder and the environment during the exhaust process?

External environmental conditions

The size of the engine

The design of the cylinder

The exhaust manifold, tailpipe, catalytic converter, and muffler (correct)

How is the pumping work per cycle mathematically expressed?

Wp = Vd (pi + pe)

Wp = Vd (pi - pe) (correct)

Wp = Vd (pi * pe)

Wp = Vd (pe - pi)

What happens to the throttling work as the load is reduced in SI engines?

Throttling work increases while valve flow work increases

Throttling work increases while valve flow work decreases (correct)

Throttling work decreases while valve flow work increases

Throttling work decreases while valve flow work decreases

What could characterize the pressure in the cylinder during the intake process at Wide Open Throttle (WOT)?

It can be 10 to 20 percent lower than atmospheric pressure Signup and view all the answers

In the context of the first law of thermodynamics applied to engine intake, what is the value of Ek assumed to be?

Negligible, equivalent to zero Signup and view all the answers

What effect does increasing speed have on friction work in an engine?

Friction work increases. Signup and view all the answers

Which component contributes the most to friction work in a naturally aspirated gasoline SI engine?

Piston and conrod Signup and view all the answers

What is the primary cause of thermodynamic losses in a real engine cycle?

Heat transfer from the charge to the cylinder walls Signup and view all the answers

Which of the following is a critical factor in minimizing heat transfer losses in engines?

Maintaining adequate cooling temperatures Signup and view all the answers

What is the formula for mean effective pressure (mep) in relation to work done?

mep = W / Vd Signup and view all the answers

How much does thermodynamic efficiency of a finely tuned engine typically reduce the work output compared to an equivalent air-fuel cycle?

85% Signup and view all the answers

Which method allows for the most precise measurement of friction inside an engine?

Indicator diagram method Signup and view all the answers

What effect does heat transfer have on engine performance during combustion?

Lowers the average gas temperature at combustion Signup and view all the answers

In small turbocharged Diesel engines, what happens to the component due to pistons and conrods compared to naturally aspirated engines?

It is significantly higher. Signup and view all the answers

What is the role of the crank angle sensor in the indicator diagram method?

To obtain a pV diagram. Signup and view all the answers

What happens to the compression ratio to avoid the onset of knocking in an engine?

It must be lowered Signup and view all the answers

Which material types have specific cooling temperature limits of T < 400 C and T < 300 C respectively?

Cast iron and aluminum alloys Signup and view all the answers

When examining mean effective pressures, which pressure can typically be neglected in turbocharged engines?

pmep Signup and view all the answers

What is an essential tool needed for conducting the indicator diagram method?

Pressure transducers. Signup and view all the answers

In what manner does heat transfer affect maximum pressures in engine cycles?

It reduces maximum pressures Signup and view all the answers

Which parameter is NOT directly involved in calculating the heat flux between the gas and the cylinder wall?

Engine speed Signup and view all the answers

What factor primarily influences the change in direction of the thrust force in a piston?

The position of the piston at TDC and BDC Signup and view all the answers

Which statement accurately describes the relationship between gas pressure during expansion and compression?

Gas pressure during expansion is greater than during compression. Signup and view all the answers

What does the formula for inertial forces work (WfII) depend on?

Mass acceleration and piston speed squared Signup and view all the answers

What type of friction is included in generic friction work?

Ring tension acting against the liner Signup and view all the answers

What impact do accessories like pumps and compressors have on engine power delivery?

They decrease engine power due to additional workload. Signup and view all the answers

Which of the following describes pumping work in relation to intake pressure?

Intake pressure is lower than ambient pressure due to fluid resistance. Signup and view all the answers

What is represented by the formula WfIII in the context of friction work?

The work required to overcome generic friction and accessory work Signup and view all the answers

What primarily contributes to the largest pressure drops in an intake system?

The inlet manifold, port, and valve Signup and view all the answers

What is the primary goal of gas exchange processes in engine cylinders?

To induce the correct amount of air inside the cylinder Signup and view all the answers

Which factor is essential for achieving high volumetric efficiency?

A well-designed inlet manifold and valves Signup and view all the answers

In SI engines, what must be controlled closely to maintain optimal combustion?

The mass of air in relation to the mass of fuel Signup and view all the answers

How does a throttle valve function differently in CI engines compared to SI engines?

It helps improve EGR at low load conditions Signup and view all the answers

What does the volumetric efficiency formula primarily indicate?

Effectiveness of the gas exchange process Signup and view all the answers

What occurs when the throttle valve is closed in a CI engine?

EGR processes are enhanced and the engine stops promptly Signup and view all the answers

What does a high volumetric efficiency suggest about an engine's performance?

It efficiently mixes air and fuel within the cylinder Signup and view all the answers

What leads to the need for a throttle valve in CI engines?

To enhance the pressure difference for better EGR Signup and view all the answers

What factor primarily influences the mass of air inducted into the cylinder at high engine speeds?

The pressure level in the intake port Signup and view all the answers

Why is the intake valve typically closed 40 to 60 CA after BDC?

To maximize the ram effect Signup and view all the answers

At low engine speeds, what is the primary consequence of reverse flow into the intake manifold?

Inefficient cylinder charging Signup and view all the answers

What is the ideal instant valve closing (IVC) timing based on?

Velocity of the inducted mass being zero Signup and view all the answers

What is the impact of increasing the IVC crank angle on the ram effect?

It shifts the ram effect to higher speeds Signup and view all the answers

What occurs when airflow becomes choked at the intake valve at high engine speeds?

Decreased flow rate at the valve outlet Signup and view all the answers

What must be considered when selecting the optimal speed $n^*$ for IVC timing?

The inertia of flow and reverse flow Signup and view all the answers

Which phenomenon occurs due to the motion of the piston back towards TDC in the compression stroke?

Net reverse flow into the intake manifold Signup and view all the answers

Study Notes

Real Engine Cycles

Real engine cycles differ from ideal cycles due to thermal effects.
A spark ignition engine is considered, similar analysis applies to compression ignition engines.
Figure 5.1 shows a graphical comparison between a real cycle and an air-fuel cycle.
Thermodynamic efficiency accounts for losses.
Geometrical and chemical parameters (compression ratio, fuel composition) are similar in both cycles.
Five types of thermodynamic losses are analyzed (heat transfer being the most significant).

Heat Transfer

The primary loss mechanism is heat transfer from the charge to cylinder walls.
High temperatures (up to 2500K) during combustion necessitate efficient cooling (less than 400°C for cast iron).
Heat flux (Q) is calculated as h(Tg - Tw)A where h is the heat transfer coefficient, Tg is the gas temperature, Tw is the wall temperature, and A is the area.
Heat transfer reduces the work per cycle.
Knock is more likely due to heat transfer from walls, valves, and piston to the charge in the compression phase, requiring a lower compression ratio to avoid it.
Lower exhaust gas temperatures affect after-treatment of CO and hydrocarbons.

Finite Combustion Time

Real combustion isn't instantaneous, occurring while the piston moves.
Ideal cycles initiate combustion at top dead center (TDC) to maximize work, but this is not practical.
Combustion is initiated before TDC to reduce exhaust gas temperature, which is important for minimizing after-combustion and maximizing work.

Spark Timing and Spark Advance

The spark timing can optimize engine performance according to desired output.
Timing is critical, and often determined through experimentation.
Modern engines adjust spark timing based on operational conditions and goals.
Spark advance angle (measurement from TDC) is related to maximum pressures and torque.
Optimal is between 40° and 10° before TDC .

Exhaust Blowdown Losses

Exhaust valves open before bottom dead center (BDC) to minimize pumping work during the exhaust stroke.
This loss of expansion stroke work is offset by reduced pumping work in the next cycle.

Crevice Effect and Leakage

Combustion chamber crevices can increase leakage of gases.
Blow-by gases are gases that escape these crevices and enter the crankcase.
Crankcase gases are generally recycled and combusted during the next cycle.

Incomplete Combustion

Incomplete combustion introduces combustible species in exhaust gasses.
Rich mixtures often have significant incomplete combustion, resulting in lower than expected chemical energy released from the fuel.
Rich mixtures also mean lower exhaust gas temperatures, affecting catalytic converters.

Engine Energy Balance

This analysis examines the energy balance in the engine across all parts.
Chemical energy from the fuel is converted into other energy forms.
Energy loss through cooling systems (radiator, oil cooler) is substantial.
Inefficiencies are reflected in exhaust enthalpy.
Detailed energy balance calculations are complex and depend on engine and fuel parameters.

Effect of Combustion Chamber Shape on Heat Transfer

The shape of the combustion chamber impacts heat transfer.
A spherical shape minimizing surface area, but leads to high thermal stresses, inefficient and excessive thermal stresses.
Cylindrical shapes are used with slight modifications for better heat transfer performance and operational characteristics to minimize loss.
Square-cylinders are used for better heat-transfer efficiency.

Gas Exchange Processes

The purpose of gas exchange is to induct and retain a sufficient amount of air.
Maintaining the correct fuel-to-air ratio is critical.
Proper mixture preparation and establishing turbulence within the combustion chamber are other important goals.
Volumetric efficiency is a key indicator of gas-exchange process performance.

Intake and Exhaust Processes in the Four-Stroke Engine

Intake system pressure drops due to component resistances.
The mass of inducted air can be lower than expected due to burned gas pressure.
Maintaining the correct mixture amounts is important.
Intake valve timing and exhaust valve timing both influence gas exchange performance.
Heat transfer between the cylinder walls and gases affects the mixture temperature, density and volumetric efficiency.

Phenomena Affecting Volumetric Efficiency

Intake and exhaust flow characteristics are critical to volumetric efficiency.
Flow limitations, like choked flow, can occur at high speeds.
Quasi-static flow effects are present for idealized models but deviate from real behavior.
Heat transfer between the valves and mixture influences the temperature and volumetric efficiency.

Additional Considerations on Flow Through Valves

Intake and exhaust port designs affect flow characteristics.
Minimizing flow resistance is important, which can be accomplished through tuning.
The discharge coefficient (CD) can be used to model flow rate.

Variable Valve Timing and Actuation

Variable valve timing improves performance through flexibility in valve operation to accommodate load and speed conditions.
Variable valve actuation (VVA) and cam phasing (VVT) are employed for this purpose.
These enable the tuning of valve opening and closing times to optimize performance.
Variable valve opening and closing improves airflow through the valves effectively.

Cam Phasing

Cam phasing is an approach to adjust intake and exhaust cam timings to optimize airflow.
Cam phasing impacts the pumping work, particularly the mass flow rate and the volumetric efficiency of the intake and/or exhaust system.

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Description

This quiz explores key concepts in thermodynamics as they relate to Spark Ignition (SI) engines. Questions cover pressure dynamics, work calculations, and the effects of engine speed and load on performance. Test your knowledge on factors influencing thermodynamic efficiency and friction work in engine cycles.