DIY Simple analog engine dynamometer -- How to do it?

[Luth]

Hi, I have to measure the HP of a small engine at the crankshaft. The only usable things are.

A dinamometer
A brake disk
An rpm meter

The brake caliper is floating so when we apply de load it pull the dinamometer.
The rpm meter show the revolution during the load apply. It can works?
How I should do the measure to find the HP every 1000 rpm?
Here the picture of the project.

Please if is possible show me the procedure step by step.

Thanks!
Luth

 

[jack action]

$$HP = \frac{2\mu rN \times RPM}{5252}$$
where:

• $2$: because I assume there are 2 brake pads
• $\mu$: the friction coefficient of the brake pads
• $r$: the disc radius, ft (to the middle of the brake pads)
• $N$: the normal force applied to the brake pads, lb (which I assume is directly measured by your "dynmometer" (fishing scale?))

The problem with this set-up will probably be the friction coefficient that will change as the disc heats up. You will need a really efficient cooling system to maintain the disc temperature.

 

[jrmichler]

Please if is possible show me the procedure step by step.

1) Set the engine throttle wide open.
2) Adjust the brake to get 1000 RPM. Read force on the scale.
3) Adjust the brake to get 2000 RPM. Read the scale.
4) Continue until done.
5) You now have a series of RPM and force measurements. You know the moment arm from a measurement. Force times moment arm is torque, and power is torque times RPM (watch your units).

 

[Luth]

Thanks for the answer!

In this video they have done a similar thing. But you shouldn't consider the exhaust and intake temp there are not my goal. My goal is just an hp measurement.

Go at 5.14


Here a screenshot to see this thing in function watch the video.

 

[Luth]

$$HP = \frac{2\mu rN \times RPM}{5252}$$
where:

• $2$: because I assume there are 2 brake pads
• $\mu$: the friction coefficient of the brake pads
• $r$: the disc radius, ft (to the middle of the brake pads)
• $N$: the normal force applied to the brake pads, lb (which I assume is directly measured by your "dynmometer" (fishing scale?))

The problem with this set-up will probably be the friction coefficient that will change as the disc heats up. You will need a really efficient cooling system to maintain the disc temperature.

Yes, correct! Just a fishing scale looool!
In the video I have posted they don't consider the disk temp for the HP measurement. But for a best result like you say I think it is an important factor.

 

[Luth]

1) Set the engine throttle wide open.
2) Adjust the brake to get 1000 RPM. Read force on the scale.
3) Adjust the brake to get 2000 RPM. Read the scale.
4) Continue until done.
5) You now have a series of RPM and force measurements. You know the moment arm from a measurement. Force times moment arm is torque, and power is torque times RPM (watch your units).

Hi, you think i can also measure:

Go to 1500 rpm than push the brake and reduce until 1000 (measure lbs) than stop to brake go to 2500 rpm than push the brake until 2000 (measure lbs) than stop to brake go to 3500 than push the brake until 3000 (measure lbs) and continue to the end.

In this modality can I find anyway the HP or its mandatory do like you said?

Because this is a small engine and reduce it from 10000 rpm until 1000 rpm could be destroy it immediately.

 

[jack action]

He didn't say to wait until you reach 10 000 rpm, just that the throttle needs to be wide open when taking the measurement (to get the maximum hp at that rpm). You could set the brake to a high force and let the rpm increase slowly and stop (by increasing slightly the braking force) when it reaches 1000 rpm; then release the brake and repeat the cycle again for the next rpm. 1000 rpm is 1000 rpm, no matter where it was before.

 

[Luth]

He didn't say to wait until you reach 10 000 rpm, just that the throttle needs to be wide open when taking the measurement (to get the maximum hp at that rpm). You could set the brake to a high force and let the rpm increase slowly and stop (by increasing slightly the braking force) when it reaches 1000 rpm; then release the brake and repeat the cycle again for the next rpm. 1000 rpm is 1000 rpm, no matter where it was before.

Hi Jack action! Thanks for your answer

OK, sorry but it's my first dyno project and I never do that before!

So tell me if you think I understand well:

1 start the engine warm up
2 full gas wide open throttle (but it is not necessary go until the rpm limit)
3 start to brake until 1000 rpm > lbs measurement
4 stop the engine and cooling the system
5 start engine warm up and do the same but until 2000 than 3000... 4000...5000

Its correct?

 

[jrmichler]

The best way is to increase the throttle with one hand while increasing the braking force with the other hand such that the RPM stays roughly constant until you get to wide open throttle. Then fine tune the brake to get the desired RPM and take your reading.

Then, leaving the throttle wide open, adjust the brake to get the next RPM. Repeat until you have all of your readings or the brake gets too hot. If it uses a ventilated brake disk, the best way to cool it is to release the brake completely while setting the throttle for a fast idle, say about 3000 or 4000 RPM (if the engine is rated for more than that).

In a properly designed and built brake dynamometer, it must be possible to simultaneously adjust throttle and brake. Also, you need a way to instantly shut the engine off by hitting something - a shutoff switch or at least knock the throttle closed.

 

[Luth]

The best way is to increase the throttle with one hand while increasing the braking force with the other hand such that the RPM stays roughly constant until you get to wide open throttle. Then fine tune the brake to get the desired RPM and take your reading.

Then, leaving the throttle wide open, adjust the brake to get the next RPM. Repeat until you have all of your readings or the brake gets too hot. If it uses a ventilated brake disk, the best way to cool it is to release the brake completely while setting the throttle for a fast idle, say about 3000 or 4000 RPM (if the engine is rated for more than that).

In a properly designed and built brake dynamometer, it must be possible to simultaneously adjust throttle and brake. Also, you need a way to instantly shut the engine off by hitting something - a shutoff switch or at least knock the throttle closed.

Understand...the engine is a 2 stroke and not have the rpm limiter how i can establish the max rpm it can do witouth destroy it?

 

[hutchphd]

$$HP = \frac{2\mu rN \times RPM}{5252}$$
where:

• $2$: because I assume there are 2 brake pads
• $\mu$: the friction coefficient of the brake pads
• $r$: the disc radius, ft (to the middle of the brake pads)
• $N$: the normal force applied to the brake pads, lb (which I assume is directly measured by your "dynmometer" (fishing scale?))

The problem with this set-up will probably be the friction coefficient that will change as the disc heats up. You will need a really efficient cooling system to maintain the disc temperature.

I believe the dynamometer is configured to stop the rotation of the "floating" brake caliper and so directly reads the tangential frictional force (not the normal force as you indicate). This will make the measurement a little cleaner and independent of the brake efficiency. It may require a calibration factor because of the moment of the forces/torques.

 

[Luth]

I believe the dynamometer is configured to stop the rotation of the "floating" brake caliper and so directly reads the tangential frictional force (not the normal force as you indicate). This will make the measurement a little cleaner and independent of the brake efficiency. It may require a calibration factor because of the moment of the forces/torques.

Correct the dinamometer is fixed to the floating caliper and on a stationary arm on the other side.

 

[jrmichler]

A standard precaution with dyno testing is to put the operator station at one end of the engine so that if it explodes, the fragments will not hit the operator. That would not be necessary for a stock engine with a governor, such as the diesel engine in the video you posted. It would be necessary for an engine without a governor.

When operating a manually adjusted brake dyno on an engine that does not have a governor or rev limiter, you expect that you will sooner or later blow up an engine. If you are lucky, a rod will just break a hole in the crankcase. If unlucky, the flywheel will explode. A scattershield is highly recommended. Two thicknesses of 3/4" plywood should do the job, but others may have a better suggestion. I saw a single cylinder research engine where the scattershield was 3/4" Lexan, but that engine had the operator station in line with potential exploding flywheel fragments.

 

[Luth]

A standard precaution with dyno testing is to put the operator station at one end of the engine so that if it explodes, the fragments will not hit the operator. That would not be necessary for a stock engine with a governor, such as the diesel engine in the video you posted. It would be necessary for an engine without a governor.

When operating a manually adjusted brake dyno on an engine that does not have a governor or rev limiter, you expect that you will sooner or later blow up an engine. If you are lucky, a rod will just break a hole in the crankcase. If unlucky, the flywheel will explode. A scattershield is highly recommended. Two thicknesses of 3/4" plywood should do the job, but others may have a better suggestion. I saw a single cylinder research engine where the scattershield was 3/4" Lexan, but that engine had the operator station in line with potential exploding flywheel fragments.

Poooor engine... I have to consider to buy 2 cases 2 piston 2 crankshaft 2 Cylinder!

My goal is test on the dyno a small 50cc turbocharger 2 stroke. I open a thread some days ago about the possibility of turbocharge a 2 stroke.

https://www.physicsforums.com/threads/turbocharging-carbureted-petrol-2-stroke-engines.993418/

Online there are not dyno tests so I want to try it. But I don't have a huge knowledge on dyno or mechanical engineering (and not a huge budget) I'm just really fascinated by this strange thing "the 2 stroke turbo"

Hope you, in this forum help me during this experience! It can be interesting for all!

 

[jack action]

Understand...the engine is a 2 stroke and not have the rpm limiter how i can establish the max rpm it can do witouth destroy it?

You can derive it from an estimation of the typical maximum piston speed. You can find typical values here.

 

[Luth]

You can derive it from an estimation of the typical maximum piston speed. You can find typical values here.

Really interesting link! Thanks for post it!

 

[jrmichler]

There's another way to dyno test an engine. Put it in a vehicle, a motorcycle in your case, and do an acceleration / coastdown test. The coastdown test gives you total drag vs speed. The acceleration test gives you acceleration vs speed.

The rate of acceleration and total weight gives you the net acceleration force at the rear wheel from $F = ma$. The total driving force at any speed is the sum of the total drag plus the net acceleration force. The engine power to the rear wheel is $Hp = F*V/550$, where:
$HP$ = horsepower
$F$ = total driving force
$V$ = speed in ft/sec
550 is the conversion factor from ft-lbs/sec to horsepower

The acceleration test is done in one gear, usually the highest gear. Start at the lowest speed at which the engine runs smoothly without slipping the clutch, and accelerate until maximum speed or the engine reaches red line. Then pull in the clutch and let it coast down. Here is a link to a coastdown test in a truck: https://ecomodder.com/forum/showthread.php/coastdown-test-06-gmc-canyon-20405.html. Note the importance of finding a level road on a no wind day, and doing the test in both directions.

Now you have power vs speed over the full engine speed range. Since you also know RPM vs speed, you have power vs RPM.

 

[DaveE]

$$HP = \frac{2\mu rN \times RPM}{5252}$$
where:

• $2$: because I assume there are 2 brake pads
• $\mu$: the friction coefficient of the brake pads
• $r$: the disc radius, ft (to the middle of the brake pads)
• $N$: the normal force applied to the brake pads, lb (which I assume is directly measured by your "dynmometer" (fishing scale?))

The problem with this set-up will probably be the friction coefficient that will change as the disc heats up. You will need a really efficient cooling system to maintain the disc temperature.

Yes, but I think he has a simpler set up. Since he can measure all of the tangential force applied, 2μN can be replaced by whatever his force measurement is. The only problem with the coefficient of friction changing with temperature (or anything else) is that it will change the operating point (RPM & Torque). presumably this will happen slowly enough that it can be adjusted in real time.

 

[Luth]

I don't have a frame with a tire or the possibility to go on the road. The engine is just on a bench with these instruments fixed on it. Ok, so I'll go in this modality:1 start the engine warm up

2 full gas wide open throttle
QUESTION:but it is not necessary go until the rpm limit?

3 start to brake until 1000 rpm than while I measure lbs

4 stop the engine and cooling the system
5 start engine warm up and do the same but until 2000 than 3000... 4000...5000

Please can you tell me if this modality can work?

Luth

 

[Luth]

C

I don't have a frame with a tire or the possibility to go on the road. The engine is just on a bench with these instruments fixed on it. Ok, so I'll go in this modality:1 start the engine warm up

2 full gas wide open throttle
QUESTION:but it is not necessary go until the rpm limit?

3 start to brake until 1000 rpm than while I measure lbs

4 stop the engine and cooling the system
5 start engine warm up and do the same but until 2000 than 3000... 4000...5000

Please can you tell me if this modality can work?

Luth

Could be the right modality? Or it's wrong?

 

[jack action]

It could work.

You don't have to blow up the engine by going over the rpm limit. You probably will see the horsepower go down before that limit; so there is no need to measure hp pass that point.

 

[Luth]

It could work.

You don't have to blow up the engine by going over the rpm limit. You probably will see the horsepower go down before that limit; so there is no need to measure hp pass that point.

Thanks Jack action!

 

[Luth]

It could work.

You don't have to blow up the engine by going over the rpm limit. You probably will see the horsepower go down before that limit; so there is no need to measure hp pass that point.

But after open completely the throttle at how many rpm I can start to braking?

When I see the engine Rev enough to accept the braking force without switchoff could be correct?

 

[jack action]

You can apply the brakes at the same time you open the throttle, such that the rpm doesn't go out of control.

 

[Tinker]

Hi Jack action! Thanks for your answer

OK, sorry but it's my first dyno project and I never do that before!

So tell me if you think I understand well:

1 start the engine warm up
2 full gas wide open throttle (but it is not necessary go until the rpm limit)
3 start to brake until 1000 rpm > lbs measurement
4 stop the engine and cooling the system
5 start engine warm up and do the same but until 2000 than 3000... 4000...5000

Its correct?

Note all measurements should be taken at full throttle unless you want to know the power output at various throttle settings, which might be useful for tuning air/fuel mixture.