Engine Management

Coming to the end of phase 1 of the project. I am attaching a zip file with the source to this stage. The next stuff starts getting into engine theory and why timing of certain events is more critical than others.... Oh dear, I have a nice zip file, and can't find how to attach it here. I will do some searching, then attach it as soon as I figure it out.  On to engine theory stuff...

When the ignition event occurs (piston ignition), there is a downward force on the piston which is converted into a rotational force by the piston rod and crankshaft. When this happens, there is a similar force in the opposite direction which produces a wobble of the motor. Because things are generally bolted down, the motor doesn't jump out of the compartment, however the force still is there. The goal is to reduce the effect caused by this rocking motion. The solution implemented is to have the pistons fire in a sequence to balance out the forces. This is why engines implement a firing sequence Chevy uses 18436572 for an 8 cylinder motor. It is possible to keep this sequence stored someplace in an array, and just read the array value based on the sequence number in the coil firing sequence, and for many, many operations, this is the method used.  But I looked at the problem, and said, how do I make this easier for the controller. The solution became simple, just save the angle in the array properly.  For the Chevy 8 cylinder example; cylinder 1 is zero degrees, cylinder 8 is at the first position past 0, or 90 degrees. cylinder 4 is in the second positon, or 180 degrees, cylinder 3 is next at 270, etc.  When the exercise is over, the array of positions is (using our radians and the tkEv array).

tkEv[0] = 0;

tkEv[1] = 57344;

tkEv[2] = 24576;

tkEv[3] = 16384;

tkEv[4] = 40960;

tkEv[5] = 32768;

tkEv[6] = 49152;

tkEv[7] = 8192;

Which is how it gets stored in the calibration section, along with the cylinder count (8). The calisthenics can be performed in the user interface for the calibration program. Returning to the program, the window is compared to each cell of the array and when the value is within the window, the event is scheduled. it can be seen that the counter for doing the comparison, now holds the proper cylinder number.

Next thing to look at is the issue of "advance". For those that don't know already, in a cylinder, fuel and air are mixed to produce a flammable mixture, then under pressure, the mixture is ignited by a spark. The ignition of the fuel mixture is not instantaneous. In super slow motion, it is interesting to see the burning of the mixture occurs in a traveling wave. The goal is to maximize the impact of the wave at the point of TDC, so the greatest amount of energy is transferred to the down stroke of the power cycle. Because the rate of burn changes based on pressure and does not happen instantaneously, the command to ignite fuel happens before the cylinder reaches TDC. This is called the advance, and is measured as an angle. As the speed of the motor increases, the angle of advance also increases to compensate for the burn rate remaining generally the same. However, the rate of burn is somewhat dependent on pressure as well, so there is also a pressure compensation that needs to be factored in. Earlier in the compression cycle means the pressure value is lower, also, if the intake volume, measured by manifold air pressure (MAP), is lower or higher, the compression value changes as well. The good news is, the change in compression based on cylinder position sort of balances out because it generally increases to a stable amount and the RMS value (this is sort of based on a sinusoidal curve), stays pretty static, the MAP is the governing factor and the lower the MAP the more time is needed, so the advance increases. We have identified two components that influence the value of advance, the RPM and the MAP. The ignition is actually a chemical reaction called oxidation. Because it is a chemical reaction, the ambient temperature of the container (cylinder) also factors into rate of reaction. This means that temperature will also need to be factored into the value of total advance.

Sounds a little like this is getting into some complex math during runtime. The rule in embedded systems is a table is always faster. So, how to use a table in this case. Quite simply, if the table is a two dimensional array, with one axis as the RPM and the other as MAP, the table can be constructed so the value of advance resides at the array cell based on the RPM and MAP coordinates. However, for a reasonable range of MAP values and RPM speeds, the array would tend to become very large. In order to keep the table size reasonable, some math is necessary. The technique is called planar interpolation. You may be familiar with linear interpolation where one value X₀ and another value X₁ and the f(x) values Y₀ and Y₁ that equate. Any point Y_n along the line segment, can be calculated based on the X_n point. The equation is:

                  X_n x (Y₁ - Y₀)

Y_n = X₀ + ---------------------

                      (X₁ - X₀)

Similarly this operation can be done in two dimensions. The difference is, instead of using two end points, the operation uses the four corner points of a plane. The corner points are Z₀, Z₁, Z₂, and Z₃ and relate to X₀, X₁, Y₀, and Y₁ where Z₀ is the value at (X₀,Y₀), Z₁ is at (X₁,Y₀), Z₂ at (X₀,Y₁), and Z₃ at (X₁,Y₁). The formula resolves to:

Z₀(X₁-X_n)(Y₁-Y_n) + Z₁(X_n-X₀)(Y₁-Y_n) + Z₂(X₁-X_n)(Y_n-Y₀) + Z₃(X_n-X₀)(Y_n-Y₀)

----------------------------------------------------------------------------------------------

                                       (X₁-X₀)(Y₁-Y₀)

Thats the formula, I'll add it to code in a little while. I want to save this so it isn't lost (lots of editing)

Jack

Coming to the end of phase 1 of the project. I am attaching a zip file with the source to this stage. The next stuff starts getting into engine theory and why timing of certain events is more critical than others.... Oh dear, I have a nice zip file, and can't find how to attach it here. I will do some searching, then attach it as soon as I figure it out.  On to engine theory stuff...

When the ignition event occurs (piston ignition), there is a downward force on the piston which is converted into a rotational force by the piston rod and crankshaft. When this happens, there is a similar force in the opposite direction which produces a wobble of the motor. Because things are generally bolted down, the motor doesn't jump out of the compartment, however the force still is there. The goal is to reduce the effect caused by this rocking motion. The solution implemented is to have the pistons fire in a sequence to balance out the forces. This is why engines implement a firing sequence Chevy uses 18436572 for an 8 cylinder motor. It is possible to keep this sequence stored someplace in an array, and just read the array value based on the sequence number in the coil firing sequence, and for many, many operations, this is the method used.  But I looked at the problem, and said, how do I make this easier for the controller. The solution became simple, just save the angle in the array properly.  For the Chevy 8 cylinder example; cylinder 1 is zero degrees, cylinder 8 is at the first position past 0, or 90 degrees. cylinder 4 is in the second positon, or 180 degrees, cylinder 3 is next at 270, etc.  When the exercise is over, the array of positions is (using our radians and the tkEv array).

tkEv[0] = 0;

tkEv[1] = 57344;

tkEv[2] = 24576;

tkEv[3] = 16384;

tkEv[4] = 40960;

tkEv[5] = 32768;

tkEv[6] = 49152;

tkEv[7] = 8192;

Which is how it gets stored in the calibration section, along with the cylinder count (8). The calisthenics can be performed in the user interface for the calibration program. Returning to the program, the window is compared to each cell of the array and when the value is within the window, the event is scheduled. it can be seen that the counter for doing the comparison, now holds the proper cylinder number.

Next thing to look at is the issue of "advance". For those that don't know already, in a cylinder, fuel and air are mixed to produce a flammable mixture, then under pressure, the mixture is ignited by a spark. The ignition of the fuel mixture is not instantaneous. In super slow motion, it is interesting to see the burning of the mixture occurs in a traveling wave. The goal is to maximize the impact of the wave at the point of TDC, so the greatest amount of energy is transferred to the down stroke of the power cycle. Because the rate of burn changes based on pressure and does not happen instantaneously, the command to ignite fuel happens before the cylinder reaches TDC. This is called the advance, and is measured as an angle. As the speed of the motor increases, the angle of advance also increases to compensate for the burn rate remaining generally the same. However, the rate of burn is somewhat dependent on pressure as well, so there is also a pressure compensation that needs to be factored in. Earlier in the compression cycle means the pressure value is lower, also, if the intake volume, measured by manifold air pressure (MAP), is lower or higher, the compression value changes as well. The good news is, the change in compression based on cylinder position sort of balances out because it generally increases to a stable amount and the RMS value (this is sort of based on a sinusoidal curve), stays pretty static, the MAP is the governing factor and the lower the MAP the more time is needed, so the advance increases. We have identified two components that influence the value of advance, the RPM and the MAP. The ignition is actually a chemical reaction called oxidation. Because it is a chemical reaction, the ambient temperature of the container (cylinder) also factors into rate of reaction. This means that temperature will also need to be factored into the value of total advance.

Sounds a little like this is getting into some complex math during runtime. The rule in embedded systems is a table is always faster. So, how to use a table in this case. Quite simply, if the table is a two dimensional array, with one axis as the RPM and the other as MAP, the table can be constructed so the value of advance resides at the array cell based on the RPM and MAP coordinates. However, for a reasonable range of MAP values and RPM speeds, the array would tend to become very large. In order to keep the table size reasonable, some math is necessary. The technique is called planar interpolation. You may be familiar with linear interpolation where one value X₀ and another value X₁ and the f(x) values Y₀ and Y₁ that equate. Any point Y_n along the line segment, can be calculated based on the X_n point. The equation is:

                  X_n x (Y₁ - Y₀)

Y_n = X₀ + ---------------------

                      (X₁ - X₀)

Similarly this operation can be done in two dimensions. The difference is, instead of using two end points, the operation uses the four corner points of a plane. The corner points are Z₀, Z₁, Z₂, and Z₃ and relate to X₀, X₁, Y₀, and Y₁ where Z₀ is the value at (X₀,Y₀), Z₁ is at (X₁,Y₀), Z₂ at (X₀,Y₁), and Z₃ at (X₁,Y₁). The formula resolves to:

Z₀(X₁-X_n)(Y₁-Y_n) + Z₁(X_n-X₀)(Y₁-Y_n) + Z₂(X₁-X_n)(Y_n-Y₀) + Z₃(X_n-X₀)(Y_n-Y₀)

----------------------------------------------------------------------------------------------

                                       (X₁-X₀)(Y₁-Y₀)

Thats the formula, I'll add it to code in a little while. I want to save this so it isn't lost (lots of editing)

Jack

...as expected, I checked my work after my panic of loosing all my work and noticed the linear interpolation was a little wrong. The X₀ + should be a Y₀ +

sorry again

Jack