Difference between revisions of "Otto Cycle"
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| − | + | =Cycle Overview= | |
| + | [[File:1200px-P-V Otto cycle.svg.png|right|middle|thumb|Otto Cycle]] | ||
| + | A typical gasoline [[Engine|engine]] has four strokes:(1) [[Intake|Induction]], (2) Compression, (3) Combustion, and (4) [[Exhaust|Exhaust]]. | ||
| + | Strokes (4) and (1) are used to exchange the burned fuel air mixture from each stroke and replace it with clean fresh air from outside respectively. Stroke (2) is used to increase the density of the charge, greatly improving the amount of work that can be obtained from each unit of fuel. Stroke (3) is the result of the detonation and explosion of the charge forcing the combustion chamber to expand, converting the chemical energy of the gasoline to mechanical work. When each stroke is plotted on a chart comparing pressure and volume of the combustion chamber, the cycle analysis becomes easier. | ||
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Strokes (4) and (1) perform the gas exchange process, allowing the engine to “breathe”. In a simplified case, the engine inducts and exhausts to atmospheric pressure. This is called pump work, but is negligible for the model used here. The difference in the work done by the engine to compress the charge and the work done by the exploding charge on the piston, strokes (2) and (3), is the engine work. From this analysis, we have two simple ways to characterize an engine: how much work we get each cycle, and how much we have to pay for the work we get, or the cycle’s efficiency. | Strokes (4) and (1) perform the gas exchange process, allowing the engine to “breathe”. In a simplified case, the engine inducts and exhausts to atmospheric pressure. This is called pump work, but is negligible for the model used here. The difference in the work done by the engine to compress the charge and the work done by the exploding charge on the piston, strokes (2) and (3), is the engine work. From this analysis, we have two simple ways to characterize an engine: how much work we get each cycle, and how much we have to pay for the work we get, or the cycle’s efficiency. | ||
The sum of the work for the entire cycle can be visualized as the area circled by the cycle on the PV diagram. This is called the net cycle work. Because it is the net cycle work found in a simplified, ideal case, we call it the ideal net work or otto cycle net work.The efficiency of the cycle is more difficult to visualize as it is a ratio of the work produced by the engine to the chemical energy supplied. From the PV diagram, this can be the ratio of the area inside the curve to the line segment 23, or the difference in the lengths of line segments 23 and 41. However, these visualizations are non-intuitive and can be better expressed numerically. However, this simplified model won’t hold much water when we need to improve engine performance. | The sum of the work for the entire cycle can be visualized as the area circled by the cycle on the PV diagram. This is called the net cycle work. Because it is the net cycle work found in a simplified, ideal case, we call it the ideal net work or otto cycle net work.The efficiency of the cycle is more difficult to visualize as it is a ratio of the work produced by the engine to the chemical energy supplied. From the PV diagram, this can be the ratio of the area inside the curve to the line segment 23, or the difference in the lengths of line segments 23 and 41. However, these visualizations are non-intuitive and can be better expressed numerically. However, this simplified model won’t hold much water when we need to improve engine performance. | ||
| + | {{clear}} | ||
| − | = | + | =Concessions to real life= |
| − | There are a few key things glossed over for ease of theoretical calculations that play a significant role in the operation of the engine. The first, and largest factor in differentiating real engine performance from theoretical is that all of the processes in the cycle take time. The key example of this is the combustion of the charge in a | + | [[File:Realistic combustion cycle.png|right|middle|thumb|Realistic Combustion PV Diagram]] |
| + | There are a few key things glossed over for ease of theoretical calculations that play a significant role in the operation of the engine. The first, and largest factor in differentiating real engine performance from theoretical is that all of the processes in the cycle take time. The key example of this is the combustion of the charge in a spark ignited engine (HCCI is basically instantaneous) is not instantaneous and occurs over a drawn out period of time. This removes the sharp peak on the PV diagram. Our limited power over the physics of flame propagation speed will put this factor far beyond the scope of this section and we will not discuss it’s finer points here. - we should absolutely talk about this somewhere | ||
Another key characteristic of real engine performance is cylinder pressure during gas exchange processes. To induce induction into the combustion chamber, the pressure inside must be lower than the pressure of the atmosphere. Similarly, the exhaust pressure inside the cylinder must be higher than atmospheric or else the charge would not exit the cylinder. Thus, the work done by the air on the engine during induction is less than the work done on the engine to the exhaust charge. This work done to facilitate the gas exchange process is known as pump work. | Another key characteristic of real engine performance is cylinder pressure during gas exchange processes. To induce induction into the combustion chamber, the pressure inside must be lower than the pressure of the atmosphere. Similarly, the exhaust pressure inside the cylinder must be higher than atmospheric or else the charge would not exit the cylinder. Thus, the work done by the air on the engine during induction is less than the work done on the engine to the exhaust charge. This work done to facilitate the gas exchange process is known as pump work. | ||
| − | == | + | ==Volumetric Efficiency== |
| − | Because the intake charge is at a lower pressure and thus less dense than the theoretical maximum of atmospheric air, this allows us to analyze the performance of the engine based on how much air it actually breathes in versus the theoretical maximum. This is known as volumetric efficiency. The lower the manifold, or cylinder, pressure is, the lower its volumetric efficiency. Special tricks can be played with manifold tuning and resonances to increase engine performance or even temporarily achieve greater than 100% volumetric efficiency. | + | Because the intake charge is at a lower pressure and thus less dense than the theoretical maximum of atmospheric air, this allows us to analyze the performance of the engine based on how much air it actually breathes in versus the theoretical maximum. This is known as volumetric efficiency. The lower the manifold, or cylinder, pressure is, the lower its volumetric efficiency. Special tricks can be played with [[Intake#Intake_tuning|manifold tuning]] and resonances to increase engine performance or even temporarily achieve greater than 100% volumetric efficiency. |
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| + | ==Scavenging== | ||
One trick that is particularly important in high revving engines is scavenging. Scavenging is the harnessing of exhaust manifold kinetic energy to help draw additional clean air into the cylinder or using similarly using intake air energy to aid in the removal of low energy exhaust gasses. This is achieved by elegant cylinder head and port geometry, but also camshaft timing. If the intake and exhaust valves are open simultaneously, the scavenging effect can be further tuned. This is known as valve-overlap. For high engine speeds, the volumetric efficiency plummets as it is much more difficult to fill the cylinder adequately, so greater valve overlap is used to compensate. | One trick that is particularly important in high revving engines is scavenging. Scavenging is the harnessing of exhaust manifold kinetic energy to help draw additional clean air into the cylinder or using similarly using intake air energy to aid in the removal of low energy exhaust gasses. This is achieved by elegant cylinder head and port geometry, but also camshaft timing. If the intake and exhaust valves are open simultaneously, the scavenging effect can be further tuned. This is known as valve-overlap. For high engine speeds, the volumetric efficiency plummets as it is much more difficult to fill the cylinder adequately, so greater valve overlap is used to compensate. | ||
| − | ==== | + | ==Throttling== |
| − | Another factor that is seen in real engine performance is what happens when the engine is not allowed to breathe as much as it wants. This is known as throttling. This is done to modulate speed when driving, as one opens and closes the throttle. Throttling is an isenthalpic process that drops the pressure and temperature of a gas at the expense of increase entropy, turbulence and/or friction against the throttle body. This pressure drop has three large effects. One, the volumetric efficiency plummets. Two, the pump work of each cycle increases. Three, less power can be drawn from the engine. | + | [[File:ThrottlingPV.png|right|middle|thumb|Effect of Throttling on Combustion Cycle]] |
| + | Another factor that is seen in real engine performance is what happens when the engine is not allowed to breathe as much as it wants. This is known as throttling. This is purposefully done to limit power (link to restrictor calcs) in the FS competition or to modulate speed when driving, as one opens and closes the [[Throttle|throttle]]. Throttling is an approximately isenthalpic process that drops the pressure and temperature of a gas at the expense of increase entropy, turbulence and/or friction against the throttle body (throttling element?). This pressure drop has three large effects. One, the volumetric efficiency plummets. Two, the pump work of each cycle increases. Three, less power can be drawn from the engine. [citation needed for 1 and 2, 3 is handled by restrictor calcs] | ||
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| + | [[Category:Internal_Combustion]] | ||
Latest revision as of 14:46, 30 January 2023
Contents
Cycle Overview
A typical gasoline engine has four strokes:(1) Induction, (2) Compression, (3) Combustion, and (4) Exhaust.
Strokes (4) and (1) are used to exchange the burned fuel air mixture from each stroke and replace it with clean fresh air from outside respectively. Stroke (2) is used to increase the density of the charge, greatly improving the amount of work that can be obtained from each unit of fuel. Stroke (3) is the result of the detonation and explosion of the charge forcing the combustion chamber to expand, converting the chemical energy of the gasoline to mechanical work. When each stroke is plotted on a chart comparing pressure and volume of the combustion chamber, the cycle analysis becomes easier.
Strokes (4) and (1) perform the gas exchange process, allowing the engine to “breathe”. In a simplified case, the engine inducts and exhausts to atmospheric pressure. This is called pump work, but is negligible for the model used here. The difference in the work done by the engine to compress the charge and the work done by the exploding charge on the piston, strokes (2) and (3), is the engine work. From this analysis, we have two simple ways to characterize an engine: how much work we get each cycle, and how much we have to pay for the work we get, or the cycle’s efficiency.
The sum of the work for the entire cycle can be visualized as the area circled by the cycle on the PV diagram. This is called the net cycle work. Because it is the net cycle work found in a simplified, ideal case, we call it the ideal net work or otto cycle net work.The efficiency of the cycle is more difficult to visualize as it is a ratio of the work produced by the engine to the chemical energy supplied. From the PV diagram, this can be the ratio of the area inside the curve to the line segment 23, or the difference in the lengths of line segments 23 and 41. However, these visualizations are non-intuitive and can be better expressed numerically. However, this simplified model won’t hold much water when we need to improve engine performance.
Concessions to real life
There are a few key things glossed over for ease of theoretical calculations that play a significant role in the operation of the engine. The first, and largest factor in differentiating real engine performance from theoretical is that all of the processes in the cycle take time. The key example of this is the combustion of the charge in a spark ignited engine (HCCI is basically instantaneous) is not instantaneous and occurs over a drawn out period of time. This removes the sharp peak on the PV diagram. Our limited power over the physics of flame propagation speed will put this factor far beyond the scope of this section and we will not discuss it’s finer points here. - we should absolutely talk about this somewhere
Another key characteristic of real engine performance is cylinder pressure during gas exchange processes. To induce induction into the combustion chamber, the pressure inside must be lower than the pressure of the atmosphere. Similarly, the exhaust pressure inside the cylinder must be higher than atmospheric or else the charge would not exit the cylinder. Thus, the work done by the air on the engine during induction is less than the work done on the engine to the exhaust charge. This work done to facilitate the gas exchange process is known as pump work.
Volumetric Efficiency
Because the intake charge is at a lower pressure and thus less dense than the theoretical maximum of atmospheric air, this allows us to analyze the performance of the engine based on how much air it actually breathes in versus the theoretical maximum. This is known as volumetric efficiency. The lower the manifold, or cylinder, pressure is, the lower its volumetric efficiency. Special tricks can be played with manifold tuning and resonances to increase engine performance or even temporarily achieve greater than 100% volumetric efficiency.
Scavenging
One trick that is particularly important in high revving engines is scavenging. Scavenging is the harnessing of exhaust manifold kinetic energy to help draw additional clean air into the cylinder or using similarly using intake air energy to aid in the removal of low energy exhaust gasses. This is achieved by elegant cylinder head and port geometry, but also camshaft timing. If the intake and exhaust valves are open simultaneously, the scavenging effect can be further tuned. This is known as valve-overlap. For high engine speeds, the volumetric efficiency plummets as it is much more difficult to fill the cylinder adequately, so greater valve overlap is used to compensate.
Throttling
Another factor that is seen in real engine performance is what happens when the engine is not allowed to breathe as much as it wants. This is known as throttling. This is purposefully done to limit power (link to restrictor calcs) in the FS competition or to modulate speed when driving, as one opens and closes the throttle. Throttling is an approximately isenthalpic process that drops the pressure and temperature of a gas at the expense of increase entropy, turbulence and/or friction against the throttle body (throttling element?). This pressure drop has three large effects. One, the volumetric efficiency plummets. Two, the pump work of each cycle increases. Three, less power can be drawn from the engine. [citation needed for 1 and 2, 3 is handled by restrictor calcs]
