Introduction
We were asked to analyze a current heat engine to obtain a better understanding of the thermodynamic properties of heat engine processes and to report such findings. A heat engine is a system which operates continuously, while only work and heat may pass across its boundaries. Also, we want to show how thermodynamic cycles, such as the Otto cycle, are used in internal combustion gasoline powered engines. The Otto cycle is applicable to all four-stroke engines, which include all automobile, petrol, and lawn mower engines. The car engine undergoes a number of thermodynamic operations, and thus can be studied through analysis of the Otto cycle. The understanding of the Otto cycle is pertinent to the automobile industry. The purpose of this report is to provide our results and conclusions in context of efficient heat engines.
Summary
The research we found led us to conclude that the Otto cycle is a reliable method for converting heat energy into mechanical energy. The PV diagram for an Otto cycle shows that there are four main processes in operating an automobile engine. The PV diagram provides the background for the analysis of a heat engine which uses a gas as working substance. The cycle is arranged as follows:
1-2 Constant volume heat addition.
Volume remains constant at V1 = V2
Pressure increases from P1 to P2
Temperature increases from T1 to T2
Entropy remains constant as s1 = s2
2-3 Adiabatic expansion of the gas
Pressure decreases from P2 to P3
Volume increases from V2 to V3
Temperature decreases from T2 to T3
Entropy remains constant at s2 = s3
3-4 Constant volume heat rejection
Volume remains constant at V3 = V4
Pressure decreases from P3 to P4
Temperature decreases from T3 to T4
Entropy decreases from s3 to s4
4-1 Adiabatic compression of the gas
Pressure increase from P4 = P1
Volume decreases from V1 to V2
Temperature increases from T1 to T2
Entropy remains constant at s1 = s2
(isentropic compression)
The cyclic manner in which heat engines are operated, that is, they add energy in the form of heat to one part of the cycle and use the energy to do useful work in another part of the cycle, makes them one of the most effective means by which work can be done. Thermal efficiency is the best indicator of how effective a heat engine cycle is, since is it a ratio of net work output to heat supplied. The thermal efficiency of an Otto cycle can be expressed in terms of the volumetric compression ratio:
h = Qin - Qout
—————
Qin
= 1 - 1/(rk-1)
where k is the specific heat ratio ( = Cp / Cv).
Thus we can improve the cycle’s efficiency by increasing the compression ratio. However, mechanical constraints, such as pressure and temperature limit the ability to increase the compression ratio. The pressure must be maintained at a level indicative of what the cylinder, piston, and engine constraints may be.
Procedure
The methodology which was used to analyze the importance of the Otto cycle in four-stroke engines in terms of its efficiency is to mathematically calculate the work accomplished.
Applying the cyclic process of heat engines to a four-stroke engine can be illustrated in the figures below:
PV diagram for a four-stroke engine
1-2 A crank shaft turns the piston and pulls in a gas mixture from the carburetor. This takes place at constant pressure (not a key step in the Otto cycle).
2-3 As the crank shaft turns the piston compresses the gas fuel mixture adiabatically. This happens as volume decreases and pressure increases (work done on gas).
3-4 Once the piston has compressed all available fuel, the spark plug causes the fuel to ignite and gives a rapid rise to temperature and pressure within the piston assembly (heat added).
4-5 Once ignition has occurred the gases expand adiabatically and do work
on the piston thrusting it downwards and lowering the pressure while increasing the volume (work done by gas).
5-6 Once the piston has reached the bottom with maximum volume within the assembly, the exhaust valve opens lowering pressure to equal external pressure. This external pressure can be atmospheric depending on the assembly (heat extracted).
6-1 The piston drives out the burned fuel and once this is complete the exhaust valve closes and the intake valve opens bringing new fuel into the engine and starts the cycle over again (not a key step in the Otto cycle).