Qualitative Introduction

Internal Combustion Terminology

Displacement Volume, the difference in volume between the bottom dead center and top dead center.

Compression Ratio, the ratio between the maximum and minimum volumes

Mean Effective Pressure, the pressure which would have to act on the piston to generate the same work as developed in a single cycle.

$mep=work/volume$

Air Standard Analysis

• Ideal air is the working fluid
• Combustion is modeled as heat transfer from an external source
• Intake and exhaust processes are ignored, rather the cycle is reset by a heat transfer process at the bottom-dead center
• Internally reversible processes

A cold air standard analysis includes the further idealization of constant specific heats

Area Interpretations of Work and Heat Transfer

Closed Systems

For closed systems, and internally reversible processes, the following area interpretations apply.

$$ \frac Wm=\int pdv $$

$$ \frac Qm=\int Tds $$

Control Volumes

For control volumes with one inlet and one exit, and internally reversible processes, the following area interpretations apply.

$$ \frac{\dot{W}}{\dot{m}}=-\int{v}{dp} $$

$$ \frac{\dot{Q}}{\dot{m}}=\int Tds $$

Otto Cycle

Process 1-2, isentropic compression

Process 2-3, constant volume heat addition

Process 3-4, isentropic expansion

Process 4-1, constant volume heat loss

$$ r=\frac{V_1}{V_2}=\frac{V_4}{V_3} $$

$$ \eta=\frac{W}{Q_H}=\frac{(u_3-u_2)-(u_4-u_2)}{u_3-u_2} $$