That won't be a sine-wave generator.
I disagree.
All mechanically-coupled, direct engine-driven alternator heads produce a true sine wave AC output waveform. In fact, it would be tough to design such a direct driven alternator head to produce anything else. It appears the OP's generator is a mechanically-coupled direct engine driven alternator.
In contrast, all indirectly-coupled inverter-type generator sets must synthesize the output AC waveform from an input DC voltage (typically 12Vdc or 24Vdc). There are many approaches to doing so, with the results varying from simple step function voltage output to staircase output to true sine wave output, in order of increasing complexity and cost.
n.b.
The primary advantage of an indirectly-coupled inverter-type generator set is that the engine RPM is disassociated from the output frequency.
With a mechanically-coupled, direct engine-driven alternator, the attached engine must rotate at either 1800RPM (4 pole wound alternator) or 3600RPM (2 pole wound alternator) to maintain a 60Hz output. If the engine slows down, the output voltage and frequency both decrease.
With an indirectly-coupled inverter-type generator set, the attached engine powers an intermediate alternator; the AC output of that alternator is rectified (converted to DC) and is then used as the input to the inverter section -- which synthesizes a nominal 120Vac 60Hz waveform. The engine RPM can now be varied as needed against the power being drawn by the load; it is not necessary for the engine to run at a specific RPM. The result of this ability to vary speed as a function of load power leads to reductions in acoustic noise and fuel consumption.
The mechanically-coupled, direct engine-driven generator does have one significant performance benefit over it's inverter counterpart: it is superior in terms of handling motor starting and other high inrush current loads. This is due to the fact that there is far more rotating inertia in a mechanically-coupled, direct engine-driven generator.
Wrooster
