Lecture 9: Gas Power Cycles

Vocabulary practice flashcards covering Chapter 9: Gas Power Cycles, including reciprocating engines, Otto, Diesel, Stirling, Ericsson, Brayton, jet propulsion cycles, and combined power cycles.

Air-Standard Assumptions

A set of four simplifying assumptions for gas power cycle analysis: (1) the working fluid is air circulating continuously in a closed loop behaving as an ideal gas, (2) all cycle processes are internally reversible, (3) combustion is replaced by a heat-addition process from an external source, and (4) exhaust is replaced by a heat-rejection process restoring the fluid to its initial state.

Cold-Air-Standard Assumptions

The air-standard assumptions with the additional simplification that the air working fluid has constant specific heats evaluated at room temperature ($25\,^\circ\text{C}$).

Air-Standard Cycle

Any thermodynamic power cycle for which the air-standard assumptions are applicable.

Compression Ratio ($r$)

The ratio of the maximum volume to the minimum volume in a reciprocating engine cylinder: $r = \frac{V_{\max}}{V_{\min}} = \frac{V_{\text{BDC}}}{V_{\text{TDC}}}$.

Top Dead Center (TDC)

The position of the piston in a reciprocating engine where cylinder volume is minimized ($V_{\min}$), forming the clearance volume.

Bottom Dead Center (BDC)

The position of the piston in a reciprocating engine where cylinder volume is maximized ($V_{\max}$).

Displacement Volume

The volume displaced or swept by the piston as it moves between Top Dead Center (TDC) and Bottom Dead Center (BDC), equal to $V_{\max} - V_{\min}$.

Clearance Volume

The minimum volume remaining in the cylinder when the piston is at Top Dead Center (TDC), designated as $V_{\min}$ or $V_{\text{TDC}}$.

Mean Effective Pressure (MEP)

A fictitious constant pressure that, if acting on the piston throughout the power stroke, would produce the same net work output as the entire cycle: $\text{MEP} = \frac{W_{\text{net}}}{V_{\max} - V_{\min}}$.

Four-Stroke Cycle

An engine cycle in which the piston completes four strokes (intake, compression, power, and exhaust) corresponding to two crankshaft revolutions to execute one thermodynamic cycle ($1\,\text{cycle} = 4\,\text{strokes} = 2\,\text{revolutions}$).

Two-Stroke Cycle

An engine cycle in which the intake, compression, power, and exhaust functions are completed in two strokes, corresponding to one revolution of the crankshaft ($1\,\text{cycle} = 2\,\text{strokes} = 1\,\text{revolution}$).

Otto Cycle

The ideal thermodynamic cycle for spark-ignition reciprocating engines, consisting of isentropic compression (1-2), constant-volume heat addition (2-3), isentropic expansion (3-4), and constant-volume heat rejection (4-1).

Autoignition (Engine Knock)

The premature self-ignition of the air-fuel mixture during the compression stroke due to elevated temperature and pressure, which limits the allowable compression ratio in spark-ignition engines.

Diesel Cycle

The ideal cycle for compression-ignition reciprocating engines, where air alone is compressed past fuel ignition temperatures and combustion occurs via fuel injection at constant pressure.

Cutoff Ratio ($r_c$)

The ratio of cylinder volumes after and before the constant-pressure combustion process in a Diesel cycle: $r_c = \frac{V_3}{V_2}$.

Stirling Cycle

A regenerative closed-loop power cycle consisting of isothermal heat addition from an external source (1-2), constant-volume regeneration (2-3), isothermal heat rejection to a sink (3-4), and constant-volume regeneration (4-1), with efficiency equal to the Carnot efficiency.

Ericsson Cycle

A regenerative cycle similar to the Stirling cycle, except that the two constant-volume regenerative processes are replaced by two constant-pressure regenerative processes.

Regenerator

A thermal storage device that borrows thermal energy from the working fluid during one portion of the thermodynamic cycle and returns it back to the fluid during another portion.

Brayton Cycle

The ideal cycle for gas-turbine engines consisting of isentropic compression in a compressor (1-2), constant-pressure heat addition (2-3), isentropic expansion in a turbine (3-4), and constant-pressure heat rejection (4-1).

Pressure Ratio ($r_p$)

The ratio of the compressor discharge pressure to the compressor inlet pressure in a Brayton cycle: $r_p = \frac{P_2}{P_1}$.

Back Work Ratio (bwr)

The fraction of the mechanical work output generated by the turbine that is consumed to drive the compressor, defined as $\text{bwr} = \frac{w_{\text{comp}}}{w_{\text{turb}}}$, typically ranging from $40\%$ to $80\%$ in gas turbines.

Intercooling

The process of cooling the working fluid between compression stages in a multistage compressor to reduce total compressor work input.

Reheating

The process of reheating the working fluid between expansion stages in a multistage turbine to increase overall turbine work output.

Jet-Propulsion Cycle

An open gas-turbine cycle where high-pressure and high-temperature gases exiting the turbine are expanded and accelerated through an exhaust nozzle to produce thrust, resulting in zero net shaft work output.

Propulsive Power ($\dot{W}_P$)

The rate at which mechanical work is done by the aircraft engine's thrust acting across a distance per unit time, given by $\dot{W}_P = F \cdot V$, where $F$ is propulsive thrust force and $V$ is aircraft velocity.

Turbofan Engine

A modified jet engine where a large fan bypasses a significant portion of ambient air around the combustion core through an outer duct to produce quiet and fuel-efficient thrust.

Turboprop Engine

A gas-turbine engine that drives a propeller through a gear reduction system, imparting moderate acceleration to a large mass of air.

Ramjet Engine

An airbreathing jet engine with no mechanical compressor or turbine, which relies completely on high forward aircraft speed to ram and compress intake air prior to combustion.

Combined Gas-Vapor Power Cycle

A high-efficiency power generation arrangement where a high-temperature gas-turbine (Brayton) topping cycle rejects heat to generate steam for a steam-turbine (Rankine) bottoming cycle, achieving overall thermal efficiencies exceeding $50\%$.