By P Sanchez
It’s interesting that there is at least one situation where going “neutral” actually polarizes people. This is a certain case when driving cars with automatic transmission. The use of neutral gearing has stirred more debate among regular drivers and car enthusiasts than the blue-or-gold dress, or even the Laurel vs Yani sound clip. Even experts in the field like long-time mechanics and purported engineers have opposing opinions whether shifting to neutral is advisable at any point of typical driving.
In this installment of the Auto Repair Blog, we’ll weigh in on the merits of both camps and hopefully arrive at a conclusion regarding the use of neutral gearing.
What’s Neutral for?
All internal combustion engines need to maintain minimum rpm to stay running, usually between 500 to 1000 rpm. Lower than that, the engine stalls (dies) requiring unnecessary engine restart. Engine restarting is not only inconvenient but hazardous in traffic situations, and frequent restarting shortens the life of the engine (Most mechanics agree that engine wear and tear happens mostly during start-ups when engine oil is not circulating). Also, frequent restarts shorten the starter motor’s service life, which is rated at about 100,000 restart cycles.
To prevent engine stalls whenever you need to brake to a halt, there has to be a means for the engine to continue running without driving the wheels. In cars with manual transmission (MT), the engine can be either temporarily disengaged from the transmission by depressing the clutch (like during gear shifts) or for longer periods, the transmission can be shifted to neutral. Neutral on a stick tranny disengages the transmission gears themselves, allowing the engine to spin freely while the drive shaft and wheels keep still.
Things work differently with an automatic transmission (AT). In an AT, whether it’s belt-driven continually variable transmission (CVT) or the more conventional type that uses planetary gears, the transmission gears can remain “engaged” with the engine all the time and not stall the engine even when a vehicle is at a stand-still. This is possible because of a nifty device called a torque converter (TC for short or we’ll be here all day).
The TC connects the engine’s flywheel/face-plate assembly to the rest of the AT. Housed within the decidedly torroidal shaped TC (that’s donut-shaped in plain speak) are basically two propellers submerged in oil, one spun directly by the engine (the impeller), the other is connected to the gear assembly (the turbine).
The usual analogy for the TC is two electric fans facing each other. Switch-on the first fan and it’ll blow air into the second fan, causing the later spin too (technically, this is called “coupling” in an AT). The real trick is when you hold the fins of the second fan which stops it from spinning. Stopping the second fan has little effect on the first fan. The latter will continue to spin and blow air. This is pretty much how a TC keeps the engine at rev even if the gears are locked in place.
Cool Analogy But Why have a Neutral in an AT at All?
There are situations when it’s imperative to disengage the transmission gears in an AT. Examples would be when the drive assembly has to be serviced by a mechanic or when the vehicle has to be towed. There’s really no debate about that.
Another area of consensus on when NOT to use neutral is in coasting. People do this as a fuel saving effort. For purposes of safety and vehicle control, you’ll want your engine engaged with the entire drive system even while coasting. This is in case you’ll need to gain speed for some emergency maneuver.
Additionally, there are doubts about any significant fuel savings that could be made in disengaging the transmission while coasting. Most modern automobile ECUs are smart enough to adjust fuel consumption when the engine is not under load, engaged transmission or not.
And God forbid you accidentally shift in reverse while you’re busy paying all your attention to the road ahead. Doing so will turn your transmission into metal mush, at which point your only repair option will be a very expensive transmission rebuild.
Neutral on Stoplights, To Do or Not To Do.
This is really where the debate lies. Should a driver just hold down the brake or put it in Park and let the engine spin against the TC and locked gears, OR disengage the gears in Neutral and allow the engine to spin more freely?
Neutral naysayers say it’s unnecessary, just let the TC do what it’s designed for. Some will also add that re-engaging to Drive exposes the gear assembly (clutch bands, meshed gears and all) to greater stress compared to just leaving everything engaged all the time, ready to receive power from the engine at any moment.
Other never-neutral proponents reason that constant shifting will wear down the shift lever’s linkages prematurely. These concerns for the mechanicals of the system seem sound although there haven’t been concrete examples of damages directly linked to the use of neutral.
Probably the best rationale for remaining in Drive at a red stoplight is similar to the reasons why you shouldn’t coast in neutral: safety and control. Staying in drive affords you to roll out with the rest of the traffic as soon as possible and it will make you ready to do quick maneuvers if needed. Lastly, it helps you avoid accidentally engaging the transmission in reverse, which can endanger you and the vehicles behind you in traffic.
Y to the N
On the opposing view, there are reasons why it’s recommended to shift to neutral if your vehicle has to stop on the road for a length of time. First and foremost has something to do with heat. As mentioned, the TC allows the engine to remain at idle speed even when it’s engaged with the transmission gears. Within the TC enclosure, the impeller and the encasing fluid (automatic transmission fluid or ATF) spins at engine speed, while the TC’s turbine that's connecting to the rest of the transmission is locked in place. This arrangement is said to generate a lot of pressure and heat.
Excess heat is the bane of ATF. Prolonged exposure leads to premature oil breakdown. Degraded lube leads to premature wear and tear of the entire transmission system. As such, putting the AT in neutral disengages the transmission gears and allows the impeller, oil, and turbine to couple and the entire TC to spin freely.
Anti-neutrals will say that the excess heat concern is unfounded because of the fact that an AT has an oil-cooling system. The oil-pump is powered by the engine (via the TC) and it’s always circulating hot ATF to the oil radiator for heat dissipation and it does this whether the transmission is engaged or not.
Pro-neutrals would argue back that any preventive measure to keep ATF breakdown at bay—including only engaging the transmission to move the vehicle to assure cooling airflow around the cooling system and transmission case – and relieving the gear assembly from unnecessary stress, are all worth doing to prolong the service life of the transmission.
So is it neutral on the stoplight or not? This might sound like a cop-out but the short answer is: it depends! Fretting about overheating oil is probably less of a concern in colder climates. Newer ATs will also likely use a variable-displacement oil-pump which will provide the same rate of ATF circulation regardless of engine or transmission speed. More importantly, as long as you have the right kind of oil in the transmission, get regular maintenance on your car, and you drive conservatively, then your car should be fine either way.
Me? I put my 15-year-old econobox on neutral at the stop signs to take some load off the engine and tranny. Plus, it runs an older fixed-displacement transmission oil pump so putting the gears on neutral allows the engine to run the pump faster on idle, ensuring more protective oil is cooled and circulated about while also avoiding the excess generation of heat.