Thermodynamics in SI Engines

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.