Design of an Otto Cycle
We look at the design of an Otto cycle and at how its performance can be improved by changing its volumetric compression ratio. The Otto cycle a closed cycle (where the system is a control mass), commonly used to model the cylinders of spark-ignition, internal combustion, automobile engines, i.e. gasoline engines.
The General IdeaThe Otto cycle is very similar to the Diesel cycle in that both are closed cycles commonly used to model internal combustion engines. The difference between them is that the Otto cycle is a spark-ignition cycle instead of a compression-ignition cycle like the Diesel cycle. Spark-ignition cycles are designed to use fuels that require a spark to begin combustion.
Stages of Otto Cycles
Otto Cycles have four stages: expansion, cooling, compression, and combustion.
The P-v diagram for an Otto cycle is shown below.
Example Otto Cycle Design
For purposes of illustration, we will assume that we want to design an Otto cycle that takes 1kg of air at ambient conditions of 15°C and 100kPa, compresses it to one-eighth its original volume and adds 1800kJ of heat to it in its combustion process. With what we know about Otto cycles, that is all we need to completely describe the problem.
Below is a possible CyclePad design of an Otto cycle.
the working fluid
We the most common working fluid for an Otto cycle is air, since it is the cheapest thing in which to burn gasoline. We can choose air as our working fluid as air by selecting it as the substance in the meter window of any stuff.
Description of Cycle Stages
We will briefly examine each statepoint and process of the Otto cycle where design assumptions must be made, detailing each assumption. As we can see from the example design constraints, very few numbers need be specified to describe an ideal Otto cycle. The rest of the assumptions are determined by applying background knowledge about the cycle. The principle numerical design decision is the compression ratio.
Cycle PropertiesUnder the Cycle menu item, we can call up the Cycle Properties meter window. The only needed assumption here is that the cycle is a heat engine (a device to convert heat to work) so that CyclePad knows how to evaluate its efficiency.
Pre-Expansion (S1)No necessary specifications here, although it is as good a place as any to specify the working fluid to be air.
Expansion Process (EXP1)Since we are analyzing an ideal Otto cycle, we assume that the expansion is isentropic. If we knew how much heat loss occurred in the expansion and the work it produced, we might be able to specify those here instead to model a non-ideal expansion process.
Exhaust (Post-Expansion) (S2)No necessary specifications here. This is where we release the used air to the environment.
Cooling Process (CLG1)Since the replacement of spent air with fresh air occurs when the piston is at its top dead center position, we assume the cooling process to be isochoric.
Pre-Compression (S3)At this point, we have air entering the cylinder at ambient conditions, so we assume the temperature t be 15%deg;C and the pressure to be 100 kPa, as specified in the problem statement.
Compression Process (CMP1)Here we assume both that the compression for out ideal Otto cycle is isentropic and that our compression ratio is 8, as given in the problem statement.
Post-Compression (S4)No necessary specifications here.
Combustion Process (HTG1)Here we assume that the heating (which takes place while the piston is at bottom dead center and not moving, similar to the cooling) is isochoric and we also assume the heat added (Q) to be 1800 kJ.
Otto Cycle EfficiencyWe can look again in the Cycle Properties meter window to see that the thermal efficiency of the Otto cycle we have built is about 57.5%.
So we can improve the cycle's efficiency by increasing the compresion ratio. The following figure shows the relation graphically.
This begs the obvious question: Why not set the compression ratio to something very large to get the highest efficiency? The answer is twofold. First, our compression ratio is limited by mechanical constraints in the system. If the pressure in the cylinder is too high, the chance of breaking the piston, the cylinder, or some other part of the engine. For example, bearings are prone to failure in automobile engines run with overly high compression ratios. The plot below shows the relationship between maximum cycle pressure and the compression ratio.
In addition, as we increase the compression ratio, the increased pressure and temperature after the compression process increases the likelihood of dieseling, which describes a situation in which the fuel ignites on its own, before the ignition spark is applied. This conflicts with our assumption that ignition (and thus combustion) takes place when the piston is at the isochoric bottom dead center position. In addition, it can actually result in engine damage where the combustion takes place even before the piston has finished getting through the compression process and forces the piston backup before the crank shaft has rotated to the proper position (before it has gone from the orientation shown in figure 3 to the one in figure 4).
CyclePad Design Files
Download the CyclePad design of the Otto cycle.
Whalley, P.B. 1992. Basic Engineering Thermodynamics. Oxford University Press. ISBN: 0-19-856255-1
Van Wylen, Sonntag, Borgnakke. 1994. Fundamentals of Classical Thermodynamics, 4th edition. John Wiley and Sons. ISBN: 0-471-59395-8
Contributed by: M. E. Brokowski