Single cylinder engine dynamometer

[tt101]

i am working with a Honda Gx-35 engine (1.2kw and 1.9 N-m) for a project and wish to make a dynamometer to measure its torque and hp.Was wondering whether an electric motor or an alternator should suffice my needs?
wont the voltage regulator in the alternator keep my o/p constant? please help

 

[Baluncore]

The alternator would need to be rated to handle 1.2kW. It should not have an internal regulator.
You would connect the alternator output to a load, then adjust the field current to set the load on the motor under test.

 

[tt101]

rewiring an alternator to remove the regulator will be a tedious task ...will it be better if i use a motor instead ? couple the motor and the engine connect two wires for output will be sufficient. in case of an alternator i will also require a battery.

 

[Baluncore]

If you use an alternator with a regulator then you can use a variable number of headlamp and smaller globes as the main dump load. The voltage would be fixed at say about 14V. You can measure the load current and voltage to calculate power. It will only need a small battery to provide a voltage stabilising influence. Once an alternator is running it generates it's own field current from three small diodes on the stator output. The main three phase rectifier on the stator output provides the main output current.

rewiring an alternator to remove the regulator will be a tedious task

No, not tedious. It is only the field connection that needs to be cut and spliced to remove the regulator.

Continuous control of the alternator's small field current is easier than variation of the main load resistance. The advantage of the regulator is that only about one amp of field current is needed to regulate up to 100A of output current. One problem is that the load must withstand the maximum voltage it generates, a hand wound wire resistor load would be more robust than light globes.

I suspect that your biggest problem would be finding a cheap second hand alternator with a high current rating from a (truck?) wrecker. There would be an advantage in going to a 24V system with half the current. The 24V light globe loads would be harder to find, but you could use pairs of 12V globes connected in series.

Using a motor rather than an alternator would require control of the motor field current, with a permanent magnet field motor, it would require variation of the load. Because the speed of the IC engine under test is variable, the output voltage of a PM motor used as a generator would be speed dependent and so require a variable dump load. A vehicle starter motor might only be rated for short bursts of generation, maybe 15 to 30 seconds. A 1.2kW motor will cost more than an alternator.

 

[tt101]

will the engine be able to suffice the input needs of such high rating alternators? according to some research i have done the input requirements of the alternator varies around 1kw and my engine being 1.2 kw will be a problem ?Also i attain this max torque and hp at a speed range of 5500-7500 rpm which the alternator won't withstand.

The speed issues though can be solved by providing some kind of reduction like gears or pulleys etc but the initial loading of the alternator is a problem now..?

 

[Baluncore]

The load in kW presented by the alternator to the IC engine is determined by the alternator field current.
The alternator power rating must be sufficient for the maximum power you will test the IC engine.

Speed is not a problem if the maximum alternator RPM is not exceeded. Car alternators often run faster than engine RPM by using belt pulleys of different sizes. Alternator field current is adjusted to compensate for RPM.

Do you have to run the IC engine at maximum power? The limit will be the maximum power rating of the alternator and the power rating of the electrical load presented to the alternator.