Difference Between Automatic and Semi-Automatic Car

The main difference between an automatic and semi-automatic car is the ability for the driver to manually change gear. In an automatic car, what gear is selected and when it gets changed is determined by the car’s computer and in a semi-automatic car, the driver decides what gear to select and when they want to change it.

Automatic Cars

On an automatic car, using the gear selector lever you’ll usually have the option of selecting Park (P), Reverse (R), Neutral (N) and Drive (D) – (PRND). Fully automatic cars are very easy to use, there’s no manual clutch pedal. All the driver needs to do is operate the accelerator and brake. All gear changes are done by the vehicle.

Various sensors on the vehicle link to a computer which then decides when it’s the right time to change up a gear or if the engine is struggling, it will change down gear. 

Throttle position sensors help to determine what action the driver requires. If the driver requires greater acceleration (if they need to overtake for example), all the driver needs to do is push firmly down onto the accelerator. The computer will detect this and depending on the current engine speed, the vehicle will change down and gear or two.

Fully automatic cars aren’t always great at determining the correct gear. For example, you could be driving down a slope and the automatic transmission when in ‘Drive’ will likely change up a gear. Whereas you would in fact want to do is either maintain your current gear or change down a gear to prevent you from picking up too much speed.

For this reason, automatic cars have limited options for allowing the driver to select a range of gears. For example, some transmissions have gear selection options alongside the PRND such as 1, 2 and 3. By selecting ‘2’, your vehicle would use gears 1 and 2 only and 3rd gear would be locked out. By limiting the gearbox from selecting too higher gear, it helps to maintain a safe speed for descending a hill.

Automatic car gear selector lever
This fully automatic car allows the driver to select Park, Reverse, Neutral or Drive (PRND)

There are different types of automatic transmissions (gearboxes). Certain automatic gearboxes are full of different size cogs, or ‘gears’ similar to that of a manual transmission and a device called a torque converter is used to change gear. Other types of auto gearboxes do not have gears at all and instead use a pulley and belt system that continuously varies it’s gear ratios. These types of automatic transmissions are called continuously variable transmission (CVT).

Clutches

Clutches are needed to break the connection between the engine and the gearbox. This connection break is used for changing gear and when the vehicle comes to a stop so that the engine doesn’t stall.

Though automatic cars do not have a manually operated clutch, many of them still use a clutch in various forms either to aid in changing gears, or to disconnect the engine from the wheels when the car is stationary.

Semi-Automatic Cars

Some say automatic cars are boring and that with a manual, you get to choose what gear you want and when, giving you more control and more enjoyment. This is where semi-automatic cars come in; they allow you to choose what gear you want, but take care of all the actual gear changing for you. No clutch pedal and no clutch control needed here!

Cars with a semi-automatic transmission vary in their technique for how the driver changes gear. Some have a ‘+’ and ‘-‘ (gear up and gear down) option alongside the traditional PRND on the gear selector. Becoming more commonplace however is the option to change up and down gears by use of paddle shifters located on the steering column. 

Semi-automatic paddle shifter
Semi-automatic paddle shifters located on the car's steering column

The more basic versions of the semi-auto gearbox use a single clutch just as a manual gearbox does. Some of these vehicles can be a little slow on changing gear and they’re not always particularly smooth at it. But as technology improves, so has the basic semi-automatic transmission and they’re not more driver friendly.

More expensive versions of the semi-automatic gearbox employ the use of a double clutch, or ‘dual-clutch transmission’ (DCT). Essentially, one clutch is used to engage or disengage the even gears and the second clutch engages or disengages the odd gears. Dual-clutch systems are commonly used in sports cars due to their ability at making extremely fast gear changes – in fact, much faster than a human can make in a manual transmission vehicle. When a driver is changing gear using a DCT, as one clutch is disengaging a gear, the second clutch is already engaging the next gear meaning that gear changes are almost instant.

Should I Get an Automatic or Semi-Automatic Car?

Cars with a semi-automatic transmission can be more expensive, particularly if it has a dual-clutch transmission. If you have zero interest in changing gears and simply use your car to get to where you need to go, then a standard auto transmission is all you’ll require.

If however you prefer greater driver input and more enjoyment from driving (particularly if you like sports cars), then a semi-automatic transmission is what you need. A dual-clutch transmission is better still. A great option with cars that have a semi-automatic transmission, is that there’s an option on the gear selector for full automatic mode, so if you don’t feel like changing gear, select automatic mode to let the car do it for you.

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