The **Air
Standard Otto cycle** is the ideal cycle
for **Spark-Ignition**
(SI) internal combustion engines, first proposed by
Nikolaus Otto over 130 years ago, and which is currently used most
motor vehicles. The following link by the **Kruse
Technology Partnership** presents a
description of the **four-stroke
Otto cycle** operation including a short
history of Nikolaus Otto. Once again we have excellent animations
produced by **Matt
Keveney** presenting both the **four-stroke**
and the **two-stroke**
spark-ignition internal combustion engine engine operation

The analysis of the Otto cycle is very similar to
that of the Diesel cycle which we analysed in the **previous
section**. We will use the ideal
"air-standard" assumption in our analysis. Thus the working
fluid is a fixed mass of air undergoing the complete cycle which is
treated throughout as an ideal gas. All processes are ideal,
combustion is replaced by heat addition to the air, and exhaust is
replaced by a heat rejection process which restores the air to the
initial state.

The most significant difference between the ideal Otto cycle and the ideal Diesel cycle is the method of igniting the fuel-air mixture. Recall that in the ideal Diesel cycle the extremely high compression ratio (around 18:1) allows the air to reach the ignition temperature of the fuel. The fuel is then injected such that the ignition process occurs at a constant pressure. In the ideal Otto cycle the fuel-air mixture is introduced during the induction stroke and compressed to a much lower compression ratio (around 8:1) and is then ignited by a spark. The combustion results in a sudden jump in pressure while the volume remains essentially constant. The continuation of the cycle including the expansion and exhaust processes are essentially identical to that of the ideal Diesel cycle. We find it convenient to develop the analysis approach of the ideal Otto cycle through the following solved problem:

**Solved Problem 3.7**** ****-
**An ideal air-standard Otto cycle engine has a
compression ratio of 8. At the beginning of the compression process,
the working fluid is at 100 kPa, 27°C (300 K), and 800 kJ/kg heat is
supplied during the constant volume heat addition process. Neatly
sketch the pressure-volume [*P-v*]
diagram for this cycle, and using the specific heat values for air at
a typical average cycle temperaure of 900K determine:

a) the temperature and pressure of the air at the end of each process

b) the net work output/cycle [kj/kg], and

c) the thermal efficiency [η

_{th}] of this engine cycle.

**Solution Approach:**

The first step is to draw the *P-v* diagram of
the complete cycle, including all the relevant information. We notice
that neither volume nor mass have been provided, hence the diagram
and solution will be in terms of specific quantities.

We assume that the fuel-air mixture is represented by pure air. The relevant equations of state, internal energy and adiabatic process for air follow:

We recall from the previous section that the nominal
specific heat capacity values used for air at 300K are C_{v}
= 0.717 kJ/kg.K,, and k = 1.4. However they are all functions of
temperature, and with the extremely high temperature range
experienced in internal combustion engines one can obtain significant
errors. In this problem we use a typical average cycle temperature of
900K taken from the table of **Specific
Heat Capacities of Air**.

We now go through all four processes in order to determine the temperature and pressure at the end of each process, as well as the work done and heat transferred during each process.

Note that the pressure P_{4} (as well as P_{2}
above) could also be evaluated from the adiabatic process equation.
We do so below as a vailidity check, however we find it more
convenient to use the ideal gas equation of state wherever possible.
Either method is satisfactory.

We continue with the final process to determine the heat rejected:

Notice that we have applied the energy equation to all four processes allowing us two alternative means of evaluating the "net work output per cycle" and the thermal efficiency, as follows:

Note that using constant specific heat values over the cycle we can determine the thermal efficiency directly from the ratio of specific heat capacities k with the following formula:

where r is the compression ratio

**Quick Quiz:** Using the heat
and work energy equations derived above, derive this relation

**Problem 3.8**** ****-
**This is an extension of Solved Problem 3.7,
in which we wish to use throughout all four processes the nominal
standard specific heat capacity values for air at 300K. Using the
values C_{v} = 0.717
kJ/kg.K, and k = 1.4, determine:

a) the temperature and pressure of the air at the end of each process [P

_{2}= 1838 kPa, T_{2}= 689K, T_{3}= 1805K, P_{3}= 4815 kPa, P_{4}= 262 kPa, T_{4}= 786K]b) the net work output/cycle [451.5 kJ/kg], and

c) the thermal efficiency of this engine cycle. [η

_{th}= 56%]

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Engineering Thermodynamics by Israel
Urieli is licensed under a Creative
Commons Attribution-Noncommercial-Share Alike 3.0 United States
License