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Easy way to Calculate Engine RPM **AND now Speed**

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Easy way to Calculate Engine RPM **AND now Speed**

Old 11-28-2006, 08:50 PM
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ryan_t888
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Default 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
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Old 11-28-2006, 10:02 PM
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wave waker
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Default 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?
Old 11-28-2006, 10:07 PM
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Default 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
Old 11-28-2006, 11:08 PM
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Default RE: Easy way to Calculate Engine RPM

You have that on a laptop to carry with you to the lake? Will need it while trying different props/setups.
Old 11-29-2006, 12:14 AM
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Default RE: Easy way to Calculate Engine RPM

Somebody was on T-shirt Hell!
A lot of testing was and still is being done on .21 engines in Belfast, Northern Ireland on our 2-strokes. What you might have noticed is that you can lose around 500 RPM on evey upstroke.
Old 11-29-2006, 06:15 AM
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mrfixitwhite
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Default 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.
Old 11-29-2006, 04:16 PM
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Default 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.

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?
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.


You have that on a laptop to carry with you to the lake?
Yes but no. My laptop stays inside where water does not exist in large quantites.

i got about 19,990!
on a full throttle pass close by
howcan you say how you worked out the speed ??
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
Old 11-29-2006, 04:25 PM
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Default RE: Easy way to Calculate Engine RPM


ORIGINAL: Ron Olson

Somebody was on T-shirt Hell!
no i wasn't pfff
[8D]
any how . Ryan your sure you can only do it on internal combustions motors ? not BL motors?
Old 11-29-2006, 04:33 PM
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Default 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
Old 11-29-2006, 07:47 PM
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LES301M
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Default 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.
Old 11-29-2006, 08:59 PM
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Default 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
Old 11-29-2006, 09:49 PM
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Default 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
Old 11-29-2006, 10:12 PM
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Default 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.
I'm trying to keep it as easy as possible, if you have a question about anything just ask. Not one fancy tool as in a data logger is used. There is a few oscilloscope programs on the web downloadable for free. I got my music editiing program off of a P2P downloading program.

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

Old 11-30-2006, 11:31 AM
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Default RE: Easy way to Calculate Engine RPM **AND now Speed**

My brain just exploded...
Old 11-30-2006, 11:55 AM
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Default 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!
Old 11-30-2006, 04:05 PM
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Default RE: Easy way to Calculate Engine RPM

speed (MPH) = df / f * 2 * 760
Ryan,
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?
Old 11-30-2006, 04:21 PM
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Default 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
Old 11-30-2006, 06:40 PM
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Default 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
Old 11-30-2006, 07:07 PM
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Default 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
Old 11-30-2006, 08:24 PM
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Default 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.
Ryan, I like what Your working on here!

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
Old 11-30-2006, 09:22 PM
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Default 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


Old 11-30-2006, 10:06 PM
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Default 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


Old 12-01-2006, 02:43 AM
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Default 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!
Old 12-01-2006, 11:30 AM
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Default RE: Easy way to Calculate Engine RPM


ORIGINAL: LES301M

Ok Ryan, Nice math work.
I don't know if you are trying to be funny or just don't understand what I explained to you.

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?
Not at all. My inaccurate results from taking cycles per second (counting the pattern divided by time selected) twice then substituting them into the in two different areas caused the error. Example one of 15456RPM has no relation with Example two of 15376RPM, I don't know why you are trying to relate them.

Did you raise the prop 1/4" and get (only) an 80 RPM delta?
Why would I raise the prop 1/4"? I'll give you a hint because I know what you are going to tell me next if you still don't understand. The examples I gave you are from a real video of my RC boat. Then you had introduced the problem with slippage error saying, depending on different prop height, slippage can vary up to 8% or more.

my prop pitch is 63mm and rpm was 19900
any chance of some figures guys!!
Give me you hull type, eg. cat, vee, hydro, rigger, and I will come up with a rough figure for you!

Ryan
Old 12-01-2006, 12:55 PM
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Default 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

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