Easy way to Calculate Engine RPM **AND now Speed**
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Easy way to Calculate Engine RPM **AND now Speed**
I've always wanted to know the engine RPM of many of my model engines without purchasing expensive equipment/sensors. After a bit of research I was able to figure out an easy way of determining engine RPM.
What I did is extracted the audio on one of my boat videos I had taken. I loaded this audio track on a program called Cool Edit 2.1, It is just an audio editing program. When the audio is loaded I can visually see a waveform across the screen. If I zoom in on the waveform I can see peaks where amplitude is at a maximum. Each peak represents combustion of the fuel, or you could look at it as 1 revolution of the crank
I attached a screenshot so you can see the waveform. I selected or highlighted an area to be studied. In the bottom right hand corner it says the total time selected is 0.138 seconds. I counted the number of peaks/combustions in this selected area and arrived at 34. If I take 34 and divide that by .138 I arrive at a number that represents RPM / second. It came out to 246.3768. Simply multiplying that by 60 gives RPM/min
I achieved 14782 RPM
Since this was my boat I know the speed I achieved on that run and the prop used.
62km/h
prather 235 - 81mm pitch
prop slippage on a tunnel would be around 20%
I get ~15903RPM. Both numbers are not 100% on but they did come out to be very close.
KEY POINTS ON THE AUDIO METHOD
- The Doppler Effect - I selected that waveform as the boat just passes by in front of me.
- Waveforms does not always come out clear
- Any other noise other than the engine will make it hard to pick out the wave pattern.
- If you run a 4 stroker don't forget to multiply by two! [8D]
I hope to challenge some of these manufactures stating you get "X" number of RPM's out of an engine.
Ryan
What I did is extracted the audio on one of my boat videos I had taken. I loaded this audio track on a program called Cool Edit 2.1, It is just an audio editing program. When the audio is loaded I can visually see a waveform across the screen. If I zoom in on the waveform I can see peaks where amplitude is at a maximum. Each peak represents combustion of the fuel, or you could look at it as 1 revolution of the crank
I attached a screenshot so you can see the waveform. I selected or highlighted an area to be studied. In the bottom right hand corner it says the total time selected is 0.138 seconds. I counted the number of peaks/combustions in this selected area and arrived at 34. If I take 34 and divide that by .138 I arrive at a number that represents RPM / second. It came out to 246.3768. Simply multiplying that by 60 gives RPM/min
I achieved 14782 RPM
Since this was my boat I know the speed I achieved on that run and the prop used.
62km/h
prather 235 - 81mm pitch
prop slippage on a tunnel would be around 20%
I get ~15903RPM. Both numbers are not 100% on but they did come out to be very close.
KEY POINTS ON THE AUDIO METHOD
- The Doppler Effect - I selected that waveform as the boat just passes by in front of me.
- Waveforms does not always come out clear
- Any other noise other than the engine will make it hard to pick out the wave pattern.
- If you run a 4 stroker don't forget to multiply by two! [8D]
I hope to challenge some of these manufactures stating you get "X" number of RPM's out of an engine.
Ryan
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RE: Easy way to Calculate Engine RPM
Ryan, kinda interesting of what u've doin[sm=thumbup.gif], in your example, is the boat running wide open? or that was the time when the boat was close to u? coz 15,000+ rpm is kind low if it's running wide open, btw what's the test engine u used here?
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RE: Easy way to Calculate Engine RPM
haha . . i just saw in the bottom . . WWJD for a Klondike bar . . haha
the motor is not running full out . the max rpm is about 25,000 Ryan said
the motor is not running full out . the max rpm is about 25,000 Ryan said
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RE: Easy way to Calculate Engine RPM
Hey Ryan
i just checked out my boat running!
i got about 19,990!
on a full throttle pass close by
howcan you say how you worked out the speed ??
i know my prop -a graupner 45mm surface drive
just wondering if my boat is running anywhere near optimum
Dead smart on the audio bit though-easy to see the peaks on a clean run.
i just checked out my boat running!
i got about 19,990!
on a full throttle pass close by
howcan you say how you worked out the speed ??
i know my prop -a graupner 45mm surface drive
just wondering if my boat is running anywhere near optimum
Dead smart on the audio bit though-easy to see the peaks on a clean run.
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RE: Easy way to Calculate Engine RPM
Thanks for the replies guys. I was hoping more of you would try it. I found it fairly interesting. This will work for pretty much anything that produces a constant frequency/noise. Resulting unit is cycles per second.
What I mean by "when the boat was close to me" is the sound is least likely to produce a large doppler effect skewing results.
I used a 7.5cc K&B engine. It is running WOT but many people (maybe you) said I am not getting peak RPM from it. This was from the video I posted on RCU a while ago. I have tried everything to get it to rev higher but I just can't. I also viewed other 7.5cc tunnel video's and they are moving at the same speed as mine and revving around the same speed as mine.
K&B claims practical RPM's of 3000-25 000RPM and peak hp at 2.5hp. I don't believe even 20 000RPM is possible on this stock engine. 2.5hp is just crazy, it's most likely around 1.5-2, the rossi is rated at 2.4hp. I'm planning to go the Rossi route by next summer.
Yes but no. My laptop stays inside where water does not exist in large quantites.
GPS!
I worked from GPS max speed to engine RPM to show that my sound analysis came with in fairly close to the calculated theoretical engine RPM using prop slippage.
If you wish to go from engine RPM to speed it is just as easy.
1) Convert engine RPM and prop pitch, not diameter, by multiplying them. (use metric system!!!!!!(mm)) Resulting unit is mm/minute.
2) Convert mm/minute to km/minute by dividing by 1 000 000 (one million).
3) Convert km/minute to km/h by multiplying by 60.
4) Multiply this figure by one minus your prop slippage. so 80km/h and 20% slippage would be calculated b 80*(1-.2) = 64km/h
Prop slippage typically comes from a chart
Mono - 25%
Hydro - 20%
Rigger - 15%
5) If you prefer to have your final speed in miles per hour simply divide by 1.609.
Any problems send me your prop pitch, engine RPM and hull type for prop slippage.
Ryan
ORIGINAL: wave waker
in your example, is the boat running wide open? or that was the time when the boat was close to u? coz 15,000+ rpm is kind low if it's running wide open, btw what's the test engine u used here?
in your example, is the boat running wide open? or that was the time when the boat was close to u? coz 15,000+ rpm is kind low if it's running wide open, btw what's the test engine u used here?
I used a 7.5cc K&B engine. It is running WOT but many people (maybe you) said I am not getting peak RPM from it. This was from the video I posted on RCU a while ago. I have tried everything to get it to rev higher but I just can't. I also viewed other 7.5cc tunnel video's and they are moving at the same speed as mine and revving around the same speed as mine.
K&B claims practical RPM's of 3000-25 000RPM and peak hp at 2.5hp. I don't believe even 20 000RPM is possible on this stock engine. 2.5hp is just crazy, it's most likely around 1.5-2, the rossi is rated at 2.4hp. I'm planning to go the Rossi route by next summer.
You have that on a laptop to carry with you to the lake?
i got about 19,990!
on a full throttle pass close by
howcan you say how you worked out the speed ??
on a full throttle pass close by
howcan you say how you worked out the speed ??
I worked from GPS max speed to engine RPM to show that my sound analysis came with in fairly close to the calculated theoretical engine RPM using prop slippage.
If you wish to go from engine RPM to speed it is just as easy.
1) Convert engine RPM and prop pitch, not diameter, by multiplying them. (use metric system!!!!!!(mm)) Resulting unit is mm/minute.
2) Convert mm/minute to km/minute by dividing by 1 000 000 (one million).
3) Convert km/minute to km/h by multiplying by 60.
4) Multiply this figure by one minus your prop slippage. so 80km/h and 20% slippage would be calculated b 80*(1-.2) = 64km/h
Prop slippage typically comes from a chart
Mono - 25%
Hydro - 20%
Rigger - 15%
5) If you prefer to have your final speed in miles per hour simply divide by 1.609.
Any problems send me your prop pitch, engine RPM and hull type for prop slippage.
Ryan
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RE: Easy way to Calculate Engine RPM
ORIGINAL: Ron Olson
Somebody was on T-shirt Hell!
Somebody was on T-shirt Hell!
[8D]
any how . Ryan your sure you can only do it on internal combustions motors ? not BL motors?
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RE: Easy way to Calculate Engine RPM
BL motors don't produce a pattern that I can identify. If there was I would not know how to relate it to speed. BL motors have continuous power.
It's not just for internal combustion engines, there's many other uses.
If you own a machine gun, record a five second clip and take the audio from it. Resulting unit is shots(cycles) per second
Not very practical but a paint ball gun/marker with an unknown amount of paint balls per second would be.
Ryan
It's not just for internal combustion engines, there's many other uses.
If you own a machine gun, record a five second clip and take the audio from it. Resulting unit is shots(cycles) per second
Not very practical but a paint ball gun/marker with an unknown amount of paint balls per second would be.
Ryan
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RE: Easy way to Calculate Engine RPM
Hey Ryan,
You can also measure the speed of the boat using the doppler effect. assuming constant RPM. The closer to the boat you are, the more accurate the reading.
f1 = frequency coming towards you
f2 = frequency going away from you
df = f1 - f2
f = (f1 + f2) /2 ( = rpm)
speed (MPH) = df / f * 2 * 760
ie, coming toward you = 16,000 , away from you = 14,000
df = 2000
f = 15000 RPM
speed = 2000 / 30000 * 760 = 50.6 MPH
note: 760 is the speed of sound in MPH at 70 degrees. You have to adjust for temperature to be accurate.
You can also measure the speed of the boat using the doppler effect. assuming constant RPM. The closer to the boat you are, the more accurate the reading.
f1 = frequency coming towards you
f2 = frequency going away from you
df = f1 - f2
f = (f1 + f2) /2 ( = rpm)
speed (MPH) = df / f * 2 * 760
ie, coming toward you = 16,000 , away from you = 14,000
df = 2000
f = 15000 RPM
speed = 2000 / 30000 * 760 = 50.6 MPH
note: 760 is the speed of sound in MPH at 70 degrees. You have to adjust for temperature to be accurate.
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RE: Easy way to Calculate Engine RPM
LES, great point but I believe you have something wrong. I'm trying to remember what I learned in physics.
I believe the formula for the doppler effect is -
(change in f) / f = v / vw
Where:
change in f is - frequency coming towards observer minus frequency going away from observer
f = average freqency
v - is the speed of the object
vw - speed of waveform, in our case sound @ 1225km/h
I reviewed my video and found an area where I am achieving slightly higher RPM
F1 is 15 652RPM
F2 is 15 260RPM
Average F is 15 456RPM
Fdif is 392
From the calculation my object speed is 31.06km/h. NOW if I multiply this by two I get 62.14km/h. I am very unsure where I am missing this multiplication of two but it works.
From my original post I GPS'd my boat at 62km/h, my calculation using the doppler effect came out to 62.14km/h
THIS IS UNREAL!!!
Ryan
I believe the formula for the doppler effect is -
(change in f) / f = v / vw
Where:
change in f is - frequency coming towards observer minus frequency going away from observer
f = average freqency
v - is the speed of the object
vw - speed of waveform, in our case sound @ 1225km/h
I reviewed my video and found an area where I am achieving slightly higher RPM
F1 is 15 652RPM
F2 is 15 260RPM
Average F is 15 456RPM
Fdif is 392
From the calculation my object speed is 31.06km/h. NOW if I multiply this by two I get 62.14km/h. I am very unsure where I am missing this multiplication of two but it works.
From my original post I GPS'd my boat at 62km/h, my calculation using the doppler effect came out to 62.14km/h
THIS IS UNREAL!!!
Ryan
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RE: Easy way to Calculate Engine RPM **AND now Speed**
ughhhh and this is easy, geezzz ummm this like hard core math it looks like to me. But i guess I will have to try it out in the future, gonna be too cold real soon for boats for me but soon enough I will thanks.
Paul
Paul
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RE: Easy way to Calculate Engine RPM **AND now Speed**
ughhhh and this is easy, geezzz ummm this like hard core math it looks like to me.
Doing this only requires you to count and make very simple calculations.
The hardest part is the 4 variable equation. I'll even make it simpler for you.
Vboat = 2450 * ( |change in F| / Favg ]
(change in F) is the absoute( symbol - | | ) difference in the approaching/leaving RPM's. Absolute means no negative values, so you can subtract the frequencies in any which way.
Favg is your average engine RPM. Take the high and low RPM's and divide by two.
EDIT: Final unit is Km/h
Ryan
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RE: Easy way to Calculate Engine RPM **AND now Speed**
You haven't seen exploded yet! On another forum site they get over the top technical talking in a language that I don't understand. The University in Belfast that I was typing about earlier uses sensors all over the engine but will take a less than one second sample and analyze that showing RPM, intake and exhaust temps, surrounding weather conditions plus figuring in every mathematical factor from engine timing to the tuned pipe used. What he showed is nothing compared to those guys!
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RE: Easy way to Calculate Engine RPM
speed (MPH) = df / f * 2 * 760
using your numbers, And my original equation above
F1 is 15 652RPM
F2 is 15 260RPM
Average F is 15 456RPM
Fdif is 392
392 / 15,456 * 2 * 760 = 38.55 MPH
38.55 * 1.609 = 62.02 KPH
I think we got the same answer?
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RE: Easy way to Calculate Engine RPM
Very cool LES!! I was confused with the order of operation using your formula because it's missing brackets, and ended up with a very small number. After looking at it the way you calculated it , it is the very same formula.
I know the formula for the doppler effect is just a ratio - (change in F) / F = V / Vw but why is the multiplication of two needed? I can't figure that one out.
Hey mrfixitwhite
What speed did you come up with using the prop pitch and engine RPM method?
Ryan
I know the formula for the doppler effect is just a ratio - (change in F) / F = V / Vw but why is the multiplication of two needed? I can't figure that one out.
Hey mrfixitwhite
What speed did you come up with using the prop pitch and engine RPM method?
Ryan
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RE: Easy way to Calculate Engine RPM
Ryan,
I worked this out with my Pops long ago, but I don't remember the answer. I do remember it was not simple!
We were looking at it because we bought a box that had a speaker, a couple of dials (pots) and buttons. You would set the tone by turning the first pot to match the pitch of the engine coming towards you. Then set the second pot with the boat going away from you. Then read the two dials, do some math, and Wow, The speed was calculated. With digital stuff today, this might be worth doing again with a little microprocessor and microphone. In theory, with temperature input, Could be more accurate than GPS.
L
I worked this out with my Pops long ago, but I don't remember the answer. I do remember it was not simple!
We were looking at it because we bought a box that had a speaker, a couple of dials (pots) and buttons. You would set the tone by turning the first pot to match the pitch of the engine coming towards you. Then set the second pot with the boat going away from you. Then read the two dials, do some math, and Wow, The speed was calculated. With digital stuff today, this might be worth doing again with a little microprocessor and microphone. In theory, with temperature input, Could be more accurate than GPS.
L
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RE: Easy way to Calculate Engine RPM
LES, that sounds pretty cool!
The way I'm doing it, it can never be more accurate than GPS. My RPM readings will never be right on all the time as I'm visually taking the reading. When ever you use a figure that already contains an error in it you are just creating a larger possibility for error.
I believe taking the RPM from the extracted sound then going by prop pitch, slippage and RPM will result in a higher degree of accuracy for speed.
Ryan
The way I'm doing it, it can never be more accurate than GPS. My RPM readings will never be right on all the time as I'm visually taking the reading. When ever you use a figure that already contains an error in it you are just creating a larger possibility for error.
I believe taking the RPM from the extracted sound then going by prop pitch, slippage and RPM will result in a higher degree of accuracy for speed.
Ryan
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RE: Easy way to Calculate Engine RPM
I believe taking the RPM from the extracted sound then going by prop pitch, slippage and RPM will result in a higher degree of accuracy for speed.
If you are right, You should be able to get the same accuracy if you raise or lower your prop by 1/4" depth.
This assumes that slippage remains 20%, regardless of prop height. I know for a fact that on my full scale hydro, I can change slippage by 8% with a 1/4" change in prop depth.
L
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RE: Easy way to Calculate Engine RPM
LES, I know exactly what you are saying. You are totally right. BUT, you completely missed my point. I will explain:
What I am saying is if I'm out by 200RPM either away, my results will be totally off. Totally as in TOTALLY. This can be as simple as missing 1/2 of a wave during the initial calculation. Also, my program only allows for a precision of 3 decimals. This is not enough, resulting in error. As I said above "When ever you use a figure that already contains an error in it you are just creating a larger possibility for error. " This is of course taking the figure with an error and using it to calculate another variable
Example:
original
F1 is 15 652RPM
F2 is 15 260RPM
Average F is 15 456RPM
Fdif is 392
392 / 15,456 * 2 * 760 = 38.55 MPH
38.55 * 1.609 = 62.02 KPH
new with slight error
F1l - 15452RPM
F2 - 15300RPM
Average F - 15376RPM
F diff - 152
(152/15376)*2(760)
Calculated out = 15mph moving speed. Total difference is 38-15=23mph
Notice how this was only 240RPM error in total.
NOW I will take your 8% slippage error, actually make that 10%.
I will use the average RPM from above - 15376RPM The true speed at 20% slippage using the prop I used is 37mph
Same prop and RPM, I will increase prop slippage to 30%
Speed is 32mph
Difference is 5mph
Now I will re-state what I said above:
I believe taking the RPM from the extracted sound then going by prop pitch, slippage and RPM will result in a higher degree of accuracy for speed.
Ryan
What I am saying is if I'm out by 200RPM either away, my results will be totally off. Totally as in TOTALLY. This can be as simple as missing 1/2 of a wave during the initial calculation. Also, my program only allows for a precision of 3 decimals. This is not enough, resulting in error. As I said above "When ever you use a figure that already contains an error in it you are just creating a larger possibility for error. " This is of course taking the figure with an error and using it to calculate another variable
Example:
original
F1 is 15 652RPM
F2 is 15 260RPM
Average F is 15 456RPM
Fdif is 392
392 / 15,456 * 2 * 760 = 38.55 MPH
38.55 * 1.609 = 62.02 KPH
new with slight error
F1l - 15452RPM
F2 - 15300RPM
Average F - 15376RPM
F diff - 152
(152/15376)*2(760)
Calculated out = 15mph moving speed. Total difference is 38-15=23mph
Notice how this was only 240RPM error in total.
NOW I will take your 8% slippage error, actually make that 10%.
I will use the average RPM from above - 15376RPM The true speed at 20% slippage using the prop I used is 37mph
Same prop and RPM, I will increase prop slippage to 30%
Speed is 32mph
Difference is 5mph
Now I will re-state what I said above:
I believe taking the RPM from the extracted sound then going by prop pitch, slippage and RPM will result in a higher degree of accuracy for speed.
Ryan
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RE: Easy way to Calculate Engine RPM
Ok Ryan, Nice math work.
So, an 80 RPM loss (15456 - 15376 = 80 RPM) resulted in a 23 MPH loss?
Would a 80 RPM increase result in a 23 MPH gain?
Did you raise the prop 1/4" and get (only) an 80 RPM delta?
The difference between theory and practice, Is that, In theory, there is no difference. MJK, 1999
So, an 80 RPM loss (15456 - 15376 = 80 RPM) resulted in a 23 MPH loss?
Would a 80 RPM increase result in a 23 MPH gain?
Did you raise the prop 1/4" and get (only) an 80 RPM delta?
The difference between theory and practice, Is that, In theory, there is no difference. MJK, 1999
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RE: Easy way to Calculate Engine RPM **AND now Speed**
maths! new i would come in handy one day! lol now if only i'd listened in class[&:]
my prop pitch is 63mm and rpm was 19900
any chance of some figures guys!! cheers!
my prop pitch is 63mm and rpm was 19900
any chance of some figures guys!! cheers!
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RE: Easy way to Calculate Engine RPM
ORIGINAL: LES301M
Ok Ryan, Nice math work.
Ok Ryan, Nice math work.
So, an 80 RPM loss (15456 - 15376 = 80 RPM) resulted in a 23 MPH loss? Would a 80 RPM increase result in a 23 MPH gain?
Did you raise the prop 1/4" and get (only) an 80 RPM delta?
my prop pitch is 63mm and rpm was 19900
any chance of some figures guys!!
any chance of some figures guys!!
Ryan
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RE: Easy way to Calculate Engine RPM
DOH! sorry you'd already said put your boat type!
its a deep vee
so would the speed you come up with be the actual speed (near enough) or just the speed it SHOULD be travelling
i.e drag and very heavy boat
or would those things effect the speed of the motor anyway?
Cheers
Graham
its a deep vee
so would the speed you come up with be the actual speed (near enough) or just the speed it SHOULD be travelling
i.e drag and very heavy boat
or would those things effect the speed of the motor anyway?
Cheers
Graham