Bill V in Md
501 Club
My reasons for submitting this post are as follows:
1) Provide a follow-up to my recent post on this subject (see link below).
2) Provide pressure wave form data and analysis from tests I conducted on the Farmall Cub engine.
3) Confirm my conclusions.
First I apologize for the length of this post, but there is a lot of data to present and felt others would find it beneficial. Second let me say that my cub is entirely a hobby, except for the occasional snow plowing here in the mid-Atlantic. And being retired I don’t have to adhere to the wise adage of “leave well enough alone”. Further I have the utmost respect for those that have to keep their tractors running to complete all the necessary chores required.
So having said that, I have been exploring whether my Cub’s engine is running as perfectly as it could. To begin with I really do not have any major concerns about how the cub performs. This is a 1948 Cub with 6v magneto ignition. It has excellent compression, starts easily (usually by hand cranking on the 2nd or 3rd pull) and the condition of the spark plugs is always good. However, I have always suspected that something could be fine tuned to produce a smoother running sound, especially after start-up. Seems like there is a barely perceptible miss or “puffing” sound immediately following a cold start. Once it warms up, it sounds fine.
So, I analyzed the pressure waveform of the cylinders during cranking. This is a follow-up to my recent post in the following link:
http://farmallcub.com/phpBB2/viewtopic.php?f=1&t=114607&p=913150#p913150
As noted in that post I was only able to look at the positive pressure waveform on the oscilloscope. Since then, I was fortunate to locate an affordable compound transducer that has a pressure range from full vacuum to 150 psig. This allowed me to see the full waveform during all four strokes of each cylinder (albeit one at a time). I was primarily interested in the shape of the curve during the opening and closing of the exhaust and intake valves. I should also point out that I searched this forum as well as the internet for oscilloscope tracings of other Farmall Cubs but came up empty. Below is an oscilloscope tracing of Cylinder #1. I have the tracings of the other cylinders, but the curves are very similar. Below the tracing is a pdf file of my analysis of the pressure readings during ten (10) cranking strokes.
Similar to previous compression tests, I ran the engine for 30 minutes to warm up, followed by turning fuel off, switching magneto off, removing all spark plugs, leaving choke off (open) and opening throttle wide open. Using the starter, I allowed it to crank over about a dozen times. This allowed me to eliminate the first and last piston strokes in the analysis, since I don’t know what position they might be at the start or finish. It was nice to finally see the vacuum segments of the tracing, but the shape did not exactly match what I expected to see. In that regard, there were two conditions that made me question whether the valves were opening/closing properly. The 1st condition was that the highest vacuum readings occurred at the end of the power (or expansion) stroke instead of during the intake stroke. However, the more I think about it, this is likely based on the design of the camshaft timing. In particular, the exhaust valve opens 42° before bottom dead center (BDC) and therefore the piston is creating vacuum for 138° of crankshaft rotation. The intake valves opens 5° after top dead center TDC , but there is also an overlap of 5° when both valves are open. So although the piston is creating vacuum during the down stroke, the exhaust/intake valve is letting fuel and air enter the cylinder for 170° of crankshaft rotation. I believe this would develop less vacuum pressure compared to the 138° rotation on the power stroke with all valves closed.
The 2nd condition that was unexpected was the apparent short duration of the vacuum pressure while the intake valve is open. As you can see from the oscilloscope tracing below, the intake valve opens shortly after TDC on the exhaust stroke. There is an obvious spike in the negative pressure that lasts only 30 milliseconds (msec). However, closer examination of this curve segment reveals that the spike is a short duration, but the vacuum pressure extends for 70-75 msec. This correlates with my calculation for the duration of the intake vacuum. I made the assumption that vacuum is only created during the downward intake stroke, and only after the exhaust valve closes. Since the exhaust valve closes 10° after TDC, this leaves only 170° of crankshaft rotation to produce vacuum on the intake stroke. We know that the crankshaft makes two revolutions in 330 msec and 170° rotation equates to 0.472 revolution. Therefore, vacuum pressure would theoretically be created for 77.9 msec, which is very close to the 70-75 msec that is graphically shown on the curve.
I was also interested in the magnitude of the vacuum pressure during the intake stage. I seem to remember reading somewhere that the expected range is 15”-22” Hg (atm). I assume this range would be during normal operation and probably a little less vacuum during cranking. The vacuum pressures I measured during cranking ranged from 24.8” – 29.0” Hg (atm) with an average of 26.8” Hg. The other cylinders had similar vacuum readings. This seems a little on the low side, but I really do not have anything to compare it to.
The other condition that I am unsure about is the consistency of the cylinder compression readings. This is something that is missing from a normal compression test with a gauge. The data shows a range from 110 – 122 psig with an average of 116 psig, based on ten cranking strokes. This is a standard deviation of 5.2 with 60% of the readings being within 1 std. dev. 0f the average. This is a little less than what is considered a normal bell curve distribution. This is obviously not terrible, but I was hoping for a steeper bell curve with less dispersion. However, without any data to compare against I am not sure what to expect. Cylinder 2 had a little higher average compression (117 psig) with an almost identical dispersion. Cylinder 3 had an average compression of 123 psig with 70% of the readings within 1 std. dev. of the average. Cylinder 4 had an average compression of 114 psig with 80% of the readings within 1 std. dev. of the average.
Hopefully, you will find this information beneficial. I have more curves and data, but since the data was consistent, I decided to limit the amount of noise. When I first looked at the data, I thought the valves might need adjusting, but after further examination I believe they are ok. I checked the adjustment about five years age and insured they were all set to 0.013”. I would appreciate any opinions regarding different conclusions or recommendations for fine tuning.
View attachment Cylinder Pressure Wave Form Readings_Rev1.pdf
1) Provide a follow-up to my recent post on this subject (see link below).
2) Provide pressure wave form data and analysis from tests I conducted on the Farmall Cub engine.
3) Confirm my conclusions.
First I apologize for the length of this post, but there is a lot of data to present and felt others would find it beneficial. Second let me say that my cub is entirely a hobby, except for the occasional snow plowing here in the mid-Atlantic. And being retired I don’t have to adhere to the wise adage of “leave well enough alone”. Further I have the utmost respect for those that have to keep their tractors running to complete all the necessary chores required.
So having said that, I have been exploring whether my Cub’s engine is running as perfectly as it could. To begin with I really do not have any major concerns about how the cub performs. This is a 1948 Cub with 6v magneto ignition. It has excellent compression, starts easily (usually by hand cranking on the 2nd or 3rd pull) and the condition of the spark plugs is always good. However, I have always suspected that something could be fine tuned to produce a smoother running sound, especially after start-up. Seems like there is a barely perceptible miss or “puffing” sound immediately following a cold start. Once it warms up, it sounds fine.
So, I analyzed the pressure waveform of the cylinders during cranking. This is a follow-up to my recent post in the following link:
http://farmallcub.com/phpBB2/viewtopic.php?f=1&t=114607&p=913150#p913150
As noted in that post I was only able to look at the positive pressure waveform on the oscilloscope. Since then, I was fortunate to locate an affordable compound transducer that has a pressure range from full vacuum to 150 psig. This allowed me to see the full waveform during all four strokes of each cylinder (albeit one at a time). I was primarily interested in the shape of the curve during the opening and closing of the exhaust and intake valves. I should also point out that I searched this forum as well as the internet for oscilloscope tracings of other Farmall Cubs but came up empty. Below is an oscilloscope tracing of Cylinder #1. I have the tracings of the other cylinders, but the curves are very similar. Below the tracing is a pdf file of my analysis of the pressure readings during ten (10) cranking strokes.
Similar to previous compression tests, I ran the engine for 30 minutes to warm up, followed by turning fuel off, switching magneto off, removing all spark plugs, leaving choke off (open) and opening throttle wide open. Using the starter, I allowed it to crank over about a dozen times. This allowed me to eliminate the first and last piston strokes in the analysis, since I don’t know what position they might be at the start or finish. It was nice to finally see the vacuum segments of the tracing, but the shape did not exactly match what I expected to see. In that regard, there were two conditions that made me question whether the valves were opening/closing properly. The 1st condition was that the highest vacuum readings occurred at the end of the power (or expansion) stroke instead of during the intake stroke. However, the more I think about it, this is likely based on the design of the camshaft timing. In particular, the exhaust valve opens 42° before bottom dead center (BDC) and therefore the piston is creating vacuum for 138° of crankshaft rotation. The intake valves opens 5° after top dead center TDC , but there is also an overlap of 5° when both valves are open. So although the piston is creating vacuum during the down stroke, the exhaust/intake valve is letting fuel and air enter the cylinder for 170° of crankshaft rotation. I believe this would develop less vacuum pressure compared to the 138° rotation on the power stroke with all valves closed.
The 2nd condition that was unexpected was the apparent short duration of the vacuum pressure while the intake valve is open. As you can see from the oscilloscope tracing below, the intake valve opens shortly after TDC on the exhaust stroke. There is an obvious spike in the negative pressure that lasts only 30 milliseconds (msec). However, closer examination of this curve segment reveals that the spike is a short duration, but the vacuum pressure extends for 70-75 msec. This correlates with my calculation for the duration of the intake vacuum. I made the assumption that vacuum is only created during the downward intake stroke, and only after the exhaust valve closes. Since the exhaust valve closes 10° after TDC, this leaves only 170° of crankshaft rotation to produce vacuum on the intake stroke. We know that the crankshaft makes two revolutions in 330 msec and 170° rotation equates to 0.472 revolution. Therefore, vacuum pressure would theoretically be created for 77.9 msec, which is very close to the 70-75 msec that is graphically shown on the curve.
I was also interested in the magnitude of the vacuum pressure during the intake stage. I seem to remember reading somewhere that the expected range is 15”-22” Hg (atm). I assume this range would be during normal operation and probably a little less vacuum during cranking. The vacuum pressures I measured during cranking ranged from 24.8” – 29.0” Hg (atm) with an average of 26.8” Hg. The other cylinders had similar vacuum readings. This seems a little on the low side, but I really do not have anything to compare it to.
The other condition that I am unsure about is the consistency of the cylinder compression readings. This is something that is missing from a normal compression test with a gauge. The data shows a range from 110 – 122 psig with an average of 116 psig, based on ten cranking strokes. This is a standard deviation of 5.2 with 60% of the readings being within 1 std. dev. 0f the average. This is a little less than what is considered a normal bell curve distribution. This is obviously not terrible, but I was hoping for a steeper bell curve with less dispersion. However, without any data to compare against I am not sure what to expect. Cylinder 2 had a little higher average compression (117 psig) with an almost identical dispersion. Cylinder 3 had an average compression of 123 psig with 70% of the readings within 1 std. dev. of the average. Cylinder 4 had an average compression of 114 psig with 80% of the readings within 1 std. dev. of the average.
Hopefully, you will find this information beneficial. I have more curves and data, but since the data was consistent, I decided to limit the amount of noise. When I first looked at the data, I thought the valves might need adjusting, but after further examination I believe they are ok. I checked the adjustment about five years age and insured they were all set to 0.013”. I would appreciate any opinions regarding different conclusions or recommendations for fine tuning.
View attachment Cylinder Pressure Wave Form Readings_Rev1.pdf
