Nitro RC Motor Dynometer

[dupanic]

I want to build a nitro rc engine dyno to tune engines dynamically, at present you can only a chassis dyno that after a run, will tell you engine rpm, peak torque and power curves etc for the whole run. I was thinking of using an inductive pulse generator on the flywheel but this will only tell me a calculated rpm from peak to peak voltage and frequency.
I was wondering if anyone knew of a way i can maybe use a generator with a known physical resistance(drag) that will give me a required power output to generate an output (1kw/1.34hp will generate so many watts etc..)
or
to activate a low power electric motor opposing crankshaft rotation, to give a known load, then measuring the back emf that is created by the force of crankshaft rotation.
thanks

 

[dknguyen]

Speed and temperature are no problem. Torque tends to be much trickier to measure as you need something like an electric motor of comparable size to the engine that is well characterized so a set amount of drag can be applied.

An outrunner with a similar power output whose 3 terminals are wired up to a 3-phase rectifier that lead to a very LARGE (ie. 1kW or more adjustable power resistor which in itself is very hard to find) is the among the simplest method. Gearing might also be required since outrunners are capable of spinning about 1.5-2x faster than nitros so they may need to be geared down by 2 or so to output torque comparable to the engine.

BTW, if you go this route, the output voltage of the outrunner will be representative of the engine speed while the current represntative of the engine torque. So no need for a separate RPM sensor.

 

[Sceadwian]

If you want simple and easy, a basic dyno can just be a bathroom scale applied to a mechanism which lets the motor apply force to it. The trick is putting the motor under a load that it will experiance under operating conditions and measuring that force, and pushing against a bathroom scale isn't usually something most nitro engines do =) What you need is an adjustable mechanical clutch. If you spend any time actually toying with your nitro engine in your car/plane/heli/boat you should know what it sounds like when it's really doing the work you want. Tuning a car is probably the hardest because it will under go several orders of magnitude more load change from second to second than any plane or heli would though boats come close when their props start skipping.
If you want reproducibility record the sound it makes so you can match it up later. Use the mechanical clutch to load the engine at the power band (RPM vs acoustical signature) you're interested in and use the bathroom scale to adjust for optimal pounds of force. You could calculate this into the ground and still not really get anywhere as far as real world tuning goes.
If this is a car it really has to be done by feel because the change in torque used by a car that's spinning it's tires on dirt, freewheeling in the air and in full contact with the ground has such a high dynamic range there's not much to compare it to, and without fuel injection needle trimming such a device is truly an art.

 

[yngndrw]

Okay I might be wrong here so if someone could check over this and correct me, here's my thinking for it:

Power is the torque at a given speed, therefore to calculate the power output you need to measure the speed and torque.

If you was to connect a DC brushed motor to the output and were to put a MOSFET and power resistor across the terminals you would get a variable load. (Using PWM.)

You can measure the voltage drop across the resistor and mosfet to get the current flowing through in and hence work out the power dissipated from the resistance of the resistor and the resistance of the MOSFET when on.

If you know the efficiency of the motor (Does this change over the motor's life ?) at the current RPM, you can work out the mechanical power used. (I'll assume a motor has the same efficiency generating electricity as it does using electricity.)

From the mechanical power and the speed, you can work out the torque at the given speed.

Are these assumptions correct ? I'd also like to make a Dyno for stepper motors ...

I think that what you would do is open the throttle fully and use the adjustable load to get readings across the full speed range. I don't know how you would do this with a stepper motor as there is no "full throttle", but instead you must tell the motor what speed to go at ...

 

[Sceadwian]

I think we're getting away from the original post here a bit. He wants to measure engine load real time during a run not statically. Unfortunately the only way that can be done is to actually measure the strain on the drive shaft from the engine, or at the drive train. Personally if possible I'd measure at all driven wheels and the engine itself. There are plenty of sensors on the market that do this, they're usually instrument quality devices and you probably don't want to know the price. That's after a brief google, I don't know about inexpensive shaft strain gauges. Calibration really isn't an issue as what you need is basic real time data, you can calibrate it at a real dyno if you want.

 

[dknguyen]

I think we're getting away from the original post here a bit. He wants to measure engine load real time during a run not statically. Unfortunately the only way that can be done is to actually measure the strain on the drive shaft from the engine, or at the drive train. Personally if possible I'd measure at all driven wheels and the engine itself. There are plenty of sensors on the market that do this, they're usually instrument quality devices and you probably don't want to know the price. That's after a brief google, I don't know about inexpensive shaft strain gauges. Calibration really isn't an issue as what you need is basic real time data, you can calibrate it at a real dyno if you want.

I thought about that, but he said "to give a known torque" so I assumed he meant a bench dyno. No such thing as a cheap torque gauge they were all a few grand and up from what I saw.

 

[dknguyen]

Okay I might be wrong here so if someone could check over this and correct me, here's my thinking for it:

Power is the torque at a given speed, therefore to calculate the power output you need to measure the speed and torque.

If you was to connect a DC brushed motor to the output and were to put a MOSFET and power resistor across the terminals you would get a variable load. (Using PWM.)

You can measure the voltage drop across the resistor and mosfet to get the current flowing through in and hence work out the power dissipated from the resistance of the resistor and the resistance of the MOSFET when on.

If you know the efficiency of the motor (Does this change over the motor's life ?) at the current RPM, you can work out the mechanical power used. (I'll assume a motor has the same efficiency generating electricity as it does using electricity.)

From the mechanical power and the speed, you can work out the torque at the given speed.

Are these assumptions correct ? I'd also like to make a Dyno for stepper motors ...

I think that what you would do is open the throttle fully and use the adjustable load to get readings across the full speed range. I don't know how you would do this with a stepper motor as there is no "full throttle", but instead you must tell the motor what speed to go at ...

Ah, yeah. Doing PWMing a giant fixed load resistor to change the load on the dyno is a much better way than trying to find a giant variable load resistor.

 

[dupanic]

I looked in the Jaycar catalogue that i have and found some stepper motors and other basic brush motors that will give a decent load to the engine - about 101g/cm i think was the biggest torque output. if i was to hook up an ammeter to the power side, when the engine turns the motor backward, will giive an output that could calculate the torque output of the engine by the back emf minus the input current. only problem is i think this will badly damage the motor or even cause a fire. btw, torque is twisting or turning force, whereas power is the rate of force, doing work, which is, as said, a calculated number, max torque is reached at rpm which the intake stroke of an engine can completely fill the cylinder in the duration of that stroke and max power is when the intake stroke is so short that the cylinder is unable to accept any air'fuel to continue creating combustion and therefore power. incase you were wondering

 

[Mikebits]

I tried building an RC dyno tuner for one of my Nitro cars. I had it set up so slick and it was cool I thought. Precision bearings for the rollers, expensive load motors to measure torque. Spent weeks on the damn thing. In principle it was perfect, but what pitfall I never expected was this. My Nitro car was too light, and when I tried to run it on the machine it would fly off, so I made restraints as that should solve the problem. After I restrained the car it would just spin across my rollers. So then I pulled the car tighter on the rollers. Now I have defeated the whole purpose of the testing as I had to put excess down pressure on the Nitro so it would spin my rollers. This meant all my numbers were meaningless. I eventually gave up on the whole idea. Not to say it can't be done, just thought I would mention the pitfalls I encountered.

 

[yngndrw]

You need larger diameter rollers, that's all.

 

[Mikebits]

Yeah, that probably would have done it, but I built the thing based on the rollers I had at the time. They were vey precise shafts that fit my bearing mounts. At the time it would have entailed a whole mechanical redesign, so I just scrapped the whole thing.
I just thought I mention that so the OP is aware of some issues and not make the same mistake I did.