Why is exhaust stroke isochoric in otto cycle? Please help me clarify this simple concept. In otto cycle, after the power stroke, the gas expands adiabatically and during the exhaust stroke why is the volume of the gas constant? Doesn't the volume of the gas change after the exhaust valve is open? Is the volume considered constant because the temperature of the gas drops quickly at the end of the power stroke? If so, how does the temperature of the gas drop so quickly in such small time interval? I am confused, please help me. Thank you!
 A: 
Doesn't the volume of the gas change after the exhaust valve is open?

Yes it does, for the real Otto cycle which is an irreversible open system. But it does not for the reversible closed system model of the cycle.
The ideal reversible model of the Otto cycle does not include the intake or exhaust strokes because it is a closed thermodynamic system model of the cycle. Instead of thermal energy being lost along with mass in the exhaust stroke for an open system, it is shown as heat rejected at constant volume for the reversible closed system model. See the PV diagram of the ideal reversible cycle.
For reference I have included arrows to represent the intake and exhaust strokes of a real open system cycle which are not normally shown for the reversible closed system model. This has no impact on the net work done, since there is no net work done for the combination of the exhaust and intake strokes both of which occur at the same constant external pressure and the same increase and decrease in volume.  All the thermal energy lost during the real exhaust stroke is accounted for as heat rejected at constant volume for the reversible cycle model.
Hope this helps.

