Final answer:
Heat engines convert heat into work using different thermodynamic cycles: the Otto cycle used in gasoline engines, the Diesel cycle in diesel engines, and the Carnot cycle providing theoretical efficiency limits. Each cycle is characterized by varying processes that affect temperature, volume, and pressure within the system, influencing the engine's operational efficiency.
Step-by-step explanation:
Heat engines are primary machines which use heat to do work. They operate on different thermodynamic cycles, each having unique processes that describe their functions.
The Otto cycle is typical for a four-stroke gasoline engine and includes four steps: isochoric heat addition, adiabatic expansion, isochoric heat rejection, and adiabatic compression. This cycle requires high temperature during the combustion process for efficiency and involves both volume and pressure changes to convert heat into work.
The Diesel cycle is used in diesel engines and differs mainly in the way the fuel is combusted. It involves a combination of isobaric and isochoric processes during heat addition, leading to higher peak temperatures but lower efficiency compared to the Otto cycle.
The Carnot cycle is an idealized model that represents the maximum theoretical efficiency for a heat engine between two heat reservoirs. It consists of two isothermal processes and two adiabatic processes. While it provides insights into the limits of engine efficiency, it is not used in practical engines due to its requirements for reversible processes and an ideal working substance.
The common metrics for comparing these cycles in operational terms are the temperature of the heat source, the volume of the working substance, and the pressure within the system. The higher the temperature gradient the engine can achieve, the more work it can extract from the heat energy.