Thermofluids Lecture 16 - Otto Cycle and Engine Performance

Flashcards covering key concepts and terminology related to the Otto Cycle and engine performance in thermofluids.

Otto Cycle

A thermodynamic cycle that describes the functioning of spark ignition engines, characterized by adiabatic compression, isochoric heat addition, adiabatic expansion, and isochoric heat rejection.

Thermodynamic Efficiency

The ratio of work output to heat input in a thermodynamic cycle, often expressed as a percentage.

Compression Ratio (r)

The ratio of the volume of the combustion chamber when the piston is at the bottom of its stroke to the volume when it is at the top of its stroke.

Adiabatic Process

A thermodynamic process in which no heat is exchanged with the surroundings.

Isobaric Heat Addition

A process where heat is added to a system at constant pressure.

Isochoric Heat Rejection

A process where heat is removed from a system at constant volume.

Real Cycle vs Ideal Cycle

The real cycle accounts for irreversibilities and inefficiencies, while the ideal cycle assumes all processes are reversible.

Q-in (Heat Transfer)

The amount of heat added to the system, typically during the combustion process in an engine.

Q-out (Heat Rejection)

The amount of heat rejected from the system, typically after combustion and expansion.

Thermal Efficiency (ηth)

A measure of the performance of an engine, defined as ηth = 1 - (Qout/Qin) or ηth = 1 - (T4-T1)/(T3-T2).

Ideal Gas Law

A relation between pressure, volume, and temperature of an ideal gas, stated as PV=nRT.

Heat Capacity Ratio (γ)

The ratio of specific heats, defined as Cp/Cv, where Cp is the specific heat at constant pressure and Cv is the specific heat at constant volume.

Carnot Efficiency

The maximum theoretical efficiency of a heat engine operating between two thermal reservoirs, defined as η_c = (Th-Tc)/Th.

Irreversibility in Engine Cycles

The loss of efficiency due to non-ideal processes such as friction, turbulence, and heat losses.

Knocking

A phenomenon in internal combustion engines where fuel auto-ignites due to pressure, leading to loss of performance and potential engine damage.

Specific Heats (Cp and Cv)

Cp is the specific heat capacity at constant pressure, and Cv is the specific heat capacity at constant volume, crucial for calculating energy changes in gases.

Work Input

The energy supplied to the system during the compression stage of the Otto cycle, usually defined by W = ∫pdV.

Heat Rejection in Otto Cycle

The heat rejected from the system during the final isochoric process, which is critical for evaluating efficiency.

Max Temperature in Engine

The maximum temperature in an Otto Cycle engine is typically limited by material properties and combustion stability.

Process Equations

Mathematical relationships that describe the behavior of thermodynamic processes, including P-V relationships during different states of the Otto cycle.

T-S Diagram

A temperature-entropy diagram used to illustrate the changes in temperature and entropy throughout a thermodynamic cycle.