Controling the Voltage of alternator
Firstly, your circuit maybe doesn't do what you might think you want. With an input voltage of (say) 10.1 volts, the output voltage (under load) will be about 9.4 volts. However, if this is acceptable then you could use a buck regulator with under-voltage lock-out (UVLO). At voltages slightly about 10V (as per your design) the output voltage will be slightly below 10V and as the input voltage rises to about 11 volts, the output will rise but lag below the input by maybe 0.5 volts.

Alternatively you could use a buck-boost regulator that would attempt to produce 14V from inputs as low as a few volts to well over 20V. Again if the 10V lock-out is important, most of the chips available come with UVLO pins or you can use a disable pin driven from a comparator that senses the input supply voltage.

Here's one that Linear tech's parametric search engine (8A, 14V o/p with input between 8V and 22V) found: -



It will be capable of 8A output but with different output transistors. Power efficiency will be around the 95% mark. The RUN pin can force the device into shut-down and serves as an UVLO pin with a potential divider from the incoming power rail. Read the data sheet for details.