Posted by frustrated

SPREADSHEETS April 08, 2019 11:02AM |
AdminRegistered: 16 years ago Posts: 1,927 |

I'm starting this thread to post a few spreadsheets that I have on hand. The first spreadsheet is used to calculate 4-5-6-7 polynomial cam profiles. This profile allows for extremely smooth cam operation at high speeds and lift, resulting in much more capable and durable valve operation. These cams were first described in the 1950s but I think they came into wide use circa the 1970s and 80s and are now pretty much the default design. The file employs User Defined Functions (UDFs) which are Visual Basic functions that Excel can use just as though they were the typically supplied functions such as SUM, AVERAGE, POWER and so on.

Given the total number of degrees of valve lift or fall and the amount of lift in inches and the RPM you can calculate the correct valve lift at any particular degree of cam rotation and also find the valve velocity, acceleration and jerk (jerk measures the changes in rate of acceleration and is the variable that determines smoothness … or shock). If you understand spreadsheets, I think this should be easy to use.

I have to reiterate that these particular functions are for the 4-5-6-7 polynomial cam … there are a wide variety of options. The 3-4-5 polynomial allows lower acceleration for the same lift but has unconstrained jerk at the ends of the profile (though peak jerk through the profile is possibly smaller).

Edited 1 time(s). Last edit at 04/08/2019 11:38AM by frustrated.

Given the total number of degrees of valve lift or fall and the amount of lift in inches and the RPM you can calculate the correct valve lift at any particular degree of cam rotation and also find the valve velocity, acceleration and jerk (jerk measures the changes in rate of acceleration and is the variable that determines smoothness … or shock). If you understand spreadsheets, I think this should be easy to use.

I have to reiterate that these particular functions are for the 4-5-6-7 polynomial cam … there are a wide variety of options. The 3-4-5 polynomial allows lower acceleration for the same lift but has unconstrained jerk at the ends of the profile (though peak jerk through the profile is possibly smaller).

Edited 1 time(s). Last edit at 04/08/2019 11:38AM by frustrated.

Re: SPREADSHEETS April 09, 2019 05:09PM |
Registered: 3 years ago Posts: 62 |

Re: SPREADSHEETS April 10, 2019 12:05PM |
AdminRegistered: 16 years ago Posts: 1,927 |

Re: SPREADSHEETS April 10, 2019 12:20PM |
AdminRegistered: 16 years ago Posts: 1,927 |

STEAM CYCLE CALCULATOR

Unfortunately, this next one is a bit too big to upload as it is nearly 9 megabytes. I think it's the most useful sheet I've written. It allows you to input steam conditions and engine configuration and then calculate horsepower, efficiency and so on. Usually this is done by using polynomial coefficients of expansion and compression but the problem with this approach is that the coefficients are variable and you are stuck with "best guess" values. Unfortunately, I've noticed that a lot of calculations are kind of optimistic in their selection of coefficients.

I wasn't sure that the method I developed was any good until I found out that Art Gardiner had worked it out independently, before I did. My methods a lot easier because he manually determines values from a set of tables whereas I was able to use existing steam functions to allow EXCEL to do the calculations automatically.

The way the table works is that it assumes expansion and compression is essentially isentropic. If you think about it, the only value of which we are entirely certain at any point in a stroke is the steam density. If the density is "ONE" at cutoff, it will be "ONE-THIRD" when the piston has dropped to the point where cylinder volume is three times greater. Since the entropy is generally unchanged throughout the stroke, we now have a second value. All we need then is a function that gives either temperature or pressure for any combination of entropy and density.

Unfortunately, there are no such functions. I had to download a free automation program and with a lot of work managed to get it to work with EXCEL such that the computer was able to automatically assemble a very large lookup table giving pressure based on entropy and density. The spreadsheet uses a spline function to interpolate values between the table rows and columns, a spline being used because it gives more accurate answers for data occurring in curves rather than lines.

I should note that the spreadsheet takes the values of the incoming and compression steam and then calculates the resulting mixture of the two for computation of the expansion cycle.

Of course, you're on your own as regards the selection of "car factor" (which includes mechanical efficiency) plus the exhaust pressure. This is sort of obvious if exhausting to atmospheric, if your engine breathes well, but is a bit more problematic for condensing systems.

When opening the spreadsheet, note that there are tabs at the bottom of the page allowing you to select the control panel where you enter data and view results. There are pages which show the lookup table and calculations. A couple of pages also show feature steam calculations done by publicly available steam EXCEL functions. These can act as a simple lookup table, as presented, or you can copy the functions into your own work sheet.

Hope this works well for someone.

You can download at the link, below, by right clicking on the little "X" to the left of the file name which is table lookup 2019 Ab

STEAM CYCLE CALCULATOR

Edited 2 time(s). Last edit at 04/10/2019 12:41PM by frustrated.

Unfortunately, this next one is a bit too big to upload as it is nearly 9 megabytes. I think it's the most useful sheet I've written. It allows you to input steam conditions and engine configuration and then calculate horsepower, efficiency and so on. Usually this is done by using polynomial coefficients of expansion and compression but the problem with this approach is that the coefficients are variable and you are stuck with "best guess" values. Unfortunately, I've noticed that a lot of calculations are kind of optimistic in their selection of coefficients.

I wasn't sure that the method I developed was any good until I found out that Art Gardiner had worked it out independently, before I did. My methods a lot easier because he manually determines values from a set of tables whereas I was able to use existing steam functions to allow EXCEL to do the calculations automatically.

The way the table works is that it assumes expansion and compression is essentially isentropic. If you think about it, the only value of which we are entirely certain at any point in a stroke is the steam density. If the density is "ONE" at cutoff, it will be "ONE-THIRD" when the piston has dropped to the point where cylinder volume is three times greater. Since the entropy is generally unchanged throughout the stroke, we now have a second value. All we need then is a function that gives either temperature or pressure for any combination of entropy and density.

Unfortunately, there are no such functions. I had to download a free automation program and with a lot of work managed to get it to work with EXCEL such that the computer was able to automatically assemble a very large lookup table giving pressure based on entropy and density. The spreadsheet uses a spline function to interpolate values between the table rows and columns, a spline being used because it gives more accurate answers for data occurring in curves rather than lines.

I should note that the spreadsheet takes the values of the incoming and compression steam and then calculates the resulting mixture of the two for computation of the expansion cycle.

Of course, you're on your own as regards the selection of "car factor" (which includes mechanical efficiency) plus the exhaust pressure. This is sort of obvious if exhausting to atmospheric, if your engine breathes well, but is a bit more problematic for condensing systems.

When opening the spreadsheet, note that there are tabs at the bottom of the page allowing you to select the control panel where you enter data and view results. There are pages which show the lookup table and calculations. A couple of pages also show feature steam calculations done by publicly available steam EXCEL functions. These can act as a simple lookup table, as presented, or you can copy the functions into your own work sheet.

Hope this works well for someone.

You can download at the link, below, by right clicking on the little "X" to the left of the file name which is table lookup 2019 Ab

STEAM CYCLE CALCULATOR

Edited 2 time(s). Last edit at 04/10/2019 12:41PM by frustrated.

Re: SPREADSHEETS April 16, 2019 07:46AM |
AdminRegistered: 16 years ago Posts: 1,927 |

Here's a pretty elaborate spreadsheet covering boiler design functions. It was created by the Wolverine tube people.

[drive.google.com]

[drive.google.com]

Re: SPREADSHEETS May 04, 2019 08:13PM |
Registered: 14 years ago Posts: 135 |

Re: SPREADSHEETS May 06, 2019 09:58AM |
AdminRegistered: 16 years ago Posts: 1,927 |

Re: SPREADSHEETS May 09, 2019 01:31PM |
Registered: 12 years ago Posts: 663 |

The steam property formulations I have used are the IAPWS scientific steam properties. There is also a simpler indesteral set of formulations. The scientific are huge formulas but cover a larger range and are single formulas. The industrial are much shorter (number of terms) but seperate the properties into ranges.

[www.iapws.org]

[www.iapws.org]

Re: SPREADSHEETS May 09, 2019 08:30PM |
AdminRegistered: 16 years ago Posts: 1,927 |

Sorry, only registered users may post in this forum.

File Name | File Size | Posted by | Date | ||
---|---|---|---|---|---|

POLYNOMIAL CAM.xlsm | 25.1 KB | open | download | frustrated | 04/08/2019 | Read message |