I’ve read it helps with towing and steep hills, but what is actually happening and does it hurt the car by having it in L at any other time?
In: Engineering
If you ride a bike, this should be something you’ve experienced before.
L, or Low gear, is good for exerting less force yourself, the trade off being that you need to pedal more times. So you’ll be pedalling many many times with little effort, but each rotation will be moving your bike by a miniscule amount.
At a high gear, pedalling requires considerable effort, but each rotation of your feet moves the bike along much further. So when the terrain is flat and you can easily expend that effort, you switch to the high gear and zoom past. When the incline is steep, you switch to a low gear so you can pedal up without dying.
If you use L under normal conditions, you’re spinning your engine (pedal) much more than is needed, and will be inefficient with your fuel. This also increases the wear and tear on your engine, and caps your max speed, since even when spinning (pedalling) furiously, your car won’t move very fast.
Automatic transmissions, as the name would imply, automatically shift to a higher gear when the engine’s RPMs are too high and shift to a lower gear when the engine’s RPMs are too low.
When the car is in L, the transmission is locked into its lowest gears. In some cars, this just locks the transmission into first gear, regardless of RPMs. In others, it will allow the car to shift into second or third gear at very high RPMs.
The point of high gears in a transmission is that they transfer some of the strain of moving the car off of the engine and onto the transmission – which allows the car to go faster than the engine could otherwise accomplish by itself. By locking the car into L, you’re placing more of the strain on the engine and less on the transmission.
Strain on a transmission generally translates into mechanical strain in the gearbox, which can quite literally destroy it if you’re towing too much weight and give the car too much gas. Strain on the engine, on the other hand, just translates into higher gas usage and a higher rate of wear and tear on the engine. By tying the speed of the car more closely to the speed of the engine, you also increase engine braking and cause the car to feel more responsive to how much gas you’re giving it.
The downside is the aforementioned higher wear on the engine, reduced gas mileage, and the fact that the car is locked to an artificially low top speed due to being unable to shift into higher gears (often ~30 mph). But its better than destroying your transmission because you tried to gun it while towing a giant boat with your old Volvo sedan.
It forces it to stay in the lowest gear(1st gear) and this is usually not needed unless off-roading or going up or down a VERY steep hill. It gives your car the maximum amount of torque for going up steep inclines and the maxmimum amount of engine braking for going down steep inclines.
L is for low.
It’s a lower gear ratio, so your engine can exert more force at a lower speed.
W=F•d
Work is a measure of energy, and energy must be conserved
We can scale the force by a factor of mechanical advantage (by any simple machine, in this case gears) but we scale down the distance traveled by the same amount.
W=MA*F•d/MA
If you’re just driving the car normally, it’s more work for the engine to travel a given distance, but it’s not harmful to the engine in any way. A waste of gas more than anything. If you drive it in L all the time, it may wear out quicker over the lifetime of the car, and at high speeds, you will be redlining the engine, but otherwise it’s fine.
TL;DR you have no reason to use L unless you have a very specific reason to use L
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