Many drivers wonder why the "neutral" mode is needed at all on cars with an automatic transmission. Most often it is engaged because of the common belief that it can save fuel. Also, the lever is moved to this position when approaching a traffic light or driving downhill.
Let's immediately debunk the myth about savings: when the driver takes their foot off the gas pedal, a minimal portion of the fuel mixture enters the cylinders, just enough to keep the engine running. Therefore, shifting the gearbox into "neutral" for the sake of economy is pointless. This applies not only to "automatics" — on modern cars with a manual transmission, coasting in "neutral" also offers no fuel consumption benefit.
Fuel savings when driving in "neutral" are only possible on older cars with carbureted engines. With the advent of electronic engine management systems, this claim is completely outdated.
Even during a brief stop, such as at a traffic light, shifting into neutral is not advisable. This only leads to unnecessary shifts inside the gearbox and reduces its service life.
Now let's look at how an automatic transmission is designed.
Briefly about the design of an automatic transmission
Earlier we already talked about the operating principles of automatic gearboxes in articles about engaging reverse gear while moving and about simultaneously pressing the gas and brake pedals, where we examined the processes occurring during motion. Now we will focus in more detail on what happens in an "automatic" gearbox in the "neutral" position.
Let us recall that in such a transmission, torque from the power unit is transmitted through the torque converter — which acts as a clutch — to the input shaft of the gearbox.
To obtain different gear ratios, planetary mechanisms are used — a construction of several satellite gears that rotate around a central sun gear. The gear ratio changes by locking some elements relative to others.
Gear shifting (i.e., fixing the necessary parts) is performed by a hydraulic or electronic control system. The automatic system analyzes information from the engine and the output shaft of the automatic transmission and selects the optimal gear.
A valve is connected to the mode selector lever, which, depending on the selected position, prohibits the engagement of certain gears.
The oil pressure in the system is created by an oil pump. Transmission fluid, like engine oil, performs two tasks at once: it lubricates rubbing parts and removes excess heat from them.
Automatic transmission in cross-section (Mercedes):
1 - Turbine wheel of the torque converter,
2 - Shaft of the torque converter reactor,
3 - Drive shaft,
4 - One-way clutch of the torque converter,
5 - Drive flange of the main pump,
6 - Reactor,
7 - Main pump,
8 - Automatic transmission housing,
9 - Front cover,
10 - Clutch K1,
11 - Brake B3,
12 - Satellites,
13 - Sun gear of the middle planetary set,
14 - Ring gear of the rear planetary set,
15 - Freewheel clutch,
16 - Clutch K2,
17 - Breather,
18 - Speed governor drive gear,
19 - Output shaft locking gear,
20 - Output shaft speed sensor,
21 - Output shaft flange,
22 - Lower part of the housing,
23 - Pawl of the output shaft locking mechanism,
24 - Rod of the output shaft locking mechanism,
25 - Speed governor,
26 - Oil pan,
27 - Rear cover,
28 - Oil filter,
29 - Band brake B2,
30 - Satellites of the rear planetary set,
31 - Sun gear of the rear planetary set,
32 - Valve body,
33 - Valve body cover,
34 - Band brake B1,
35 - Drain plug,
36 - Pump wheel of the torque converter.
What happens when "neutral" is engaged
Key point: when the automatic transmission selector is moved to the "neutral" position, the input shaft of the gearbox is disconnected from the output shaft.
The engine then runs at idle speed (approximately 700–900 rpm), and the pressure in the gearbox lubrication system drops by almost half compared to the "drive" mode.
The control unit, of course, will select the gear corresponding to the current driving speed (although engaging first gear is possible — it all depends on the algorithm programmed into the software). But it is worth remembering: when engaging "drive" even on a stationary car, a slight jolt is felt — this is the gears meshing.
If you switch from "neutral" to "drive" while moving, then, in addition to a more noticeable impact when the gears engage, the gearbox experiences increased load — it has to synchronize the rotational speed of the shafts.
That is why it is recommended to engage "drive" only when the car is completely stopped.
Of course, a one-time shift of the lever while moving is unlikely to lead to serious consequences. But if the driver is accustomed to constantly "playing" with the selector, this could well cause an automatic transmission failure. Such enthusiasts can be advised to switch to a car with a manual gearbox.
Summary
- "Neutral" does not provide fuel savings.
- In "neutral," it is permissible to move only at speeds up to 40 km/h. The higher the speed, the faster oil starvation and overheating of the gearbox will occur.
- The "drive" mode should only be engaged when the vehicle is stationary.
- "Neutral" is intended exclusively for towing (for a distance of no more than 40 km at a speed of up to 30–40 km/h). And using it for other purposes makes no sense for the driver.
