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.
tt101 said:
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.