Mechanically speaking, which gear can be started,
From the mechanical structure, the automatic transmission car can start the engine normally regardless of whether it is hung P gear or N gear or R gear D gear.
For manual cars, hanging gears can also start the engine, because you can step on the clutch to cut off the power transmission between the engine and the gearbox. But if you don't step on the clutch, you can't do it, because the power of the engine is directly transmitted to the gearbox, and if you start directly, the car will leave immediately.
However, automatic transmissions are not prone to such a situation, and the general automatic transmission car engine and transmission are transmitted according to the multi-plate clutch, dry clutch or torque converter.
The multi-piece clutch and the dry clutch are in a separate situation in the case of flameout, so even if the gear is running, it is equivalent to the manual transmission car hanging the gear and stepping on the clutch, there is no harm.
The torque converter itself will have the effect of flexible connection, although the torque converter can transmit a little driving force to the transmission when running the car engine with a gear, it can only be a little bit and cannot let the car develop, so even if it is hung up, there is no problem.
In addition, the automatic transmission must rely on the transmission gasoline pump to give the pressure to change gears, in other words, even if you run in D gear, the transmission is still neutral until the car engine runs thoroughly, because there is no gasoline pump to give the drive to shift.
So in theory, the automatic transmission can operate in all gears. A very simple example is today's start-stop technology, you hang up the D stop parking, automatically start and stop after running, and then turn off, when you give oil, the car engine will run immediately in the D gear and develop the car.
But in fact, you yourself in the D gear or R gear to fire then the car engine can not be reflected, because this is also a full consideration of some of the elements of the operation error, if some people forget the transmission hanging gear and run the car engine, then after the operation in case of loosening the brake pedal vehicle will move, it is likely to cause risk.
Although such accidents are almost difficult for their own car owners, it is only necessary for manufacturers to produce one case, which is all the obligation of the manufacturer.
In addition, for the car with torque converter for a moment of cold start the car speed is likely to be very high, the torque converter transmission of a large number of driving force at this moment, if the gear running well will lead to significant impact, and even may cause the running moment of the car suddenly forward, is not safe.
Therefore, in order to better safety and customer experience considerations, the manufacturer has set up only P or N gears to operate.
There is almost no difference in running a car engine in P or N gear. We can think of P gear as an additional posture that locks the transmission output shaft on the basis of N gear, and for transmission gear, whether it is P gear or N gear, the transmission is in neutral condition. Therefore, it can run in P gear or N gear.