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Types of transmissions



The point of a transmission is to improve fuel economy, improve acceleration and reduce engine wear. To do this, your transmission uses leverage to create different outputs from the motor input. Basically, there is an ideal range of RPM’s for your motor to operate in, which we’ll refer to as the powerband. Too low, and you don’t have enough power. Too high, and your engine will be loud, run too hot, and consume copious amounts of fuel.


Different transmissions have different ways of creating the ideal torque output for any given situation. Over the last century, mechanical geniuses have pushed the envelope, creating the various transmissions that are available today.


Manual gearbox

The first type of vehicle transmission to ever be produced commercially was, predictably, the simplest to understand. Manual gearboxes need the driver to select the appropriate gear range for their driving conditions. This is done with the use of a clutch pedal and shift lever. The clutch pedal disengages the motor input from the transmission. That way, the gears are not being forcibly rotated by the motor as they mesh together.


Manual transmissions offer great fuel economy. They’re also cheap and easy to maintain. They require clutch fluid, which is easily checked and topped off under the hood, and a drain-and-refill service at regular intervals.


The main reason they are less popular today is that they’re tiresome to drive in stop-and-go traffic, and people find manual shifting to be an inconvenience. However, they are better than automatics for fast acceleration. Automatics run to a higher range before shifting, since they rely on the build-up of pressure to shift. With a manual, the driver can shift earlier, thus keeping the vehicle operating in a tighter and more efficient powerband.


Dual clutch transmission


The biggest drawback to traditional manuals are that they take time to shift. A DCT solves this by putting two different transmissions in one housing. One clutch operates the even numbered gears, and one operates the odd numbered gears. The two clutches are synchronized, so that when one releases, the other engages. The appropriate gear is already selected, and no interruption of power to the wheels needs to occur.


DCT’s are designed for use in performance cars. Their major drawback is that they’re jerky and uncomfortable to drive in in traffic or at low speeds in general. Some DCT’s require no fluid at all, so they’re largely maintenance-free over their (admittedly short) lifetime. Others do use fluid, and the required transmission maintenance is the same as traditional manuals.


Hydraulic automatic transmission


This is the modern automatic transmission that is in widespread use today. It lags behind the rest in fuel economy and performance, but has won out for its convenience. Safety is supposed to be another benefit, as the driver has to pay less attention operating and can pay more attention to the road.


However, this is not necessarily the reality, since automatic transmissions just make it easier for irresponsible drivers to turn their attention toward things like using their cell phone as they drive. The modern automatic is a hydraulic machine. Everything within the transmission functions on hydraulic pressure. For a breakdown of how automatic transmissions know when to shift, you can see this article.


The maintenance required is slightly more than most other transmissions. There’s typically more fluid needed. Additionally, there’s a filter that needs to be changed regularly in most models. The transmission is particularly sensitive to dirty or low fluid; the fluid is, after all, a hydraulic medium in a finely tuned hydraulic machine. Too much fluid, too little fluid, or dirty fluid can cause malfunctions and increase wear. For more on proper transmission maintenance, see this post.


Automated Manual Transmission


These transmissions are your conventional manual gearbox with a few added components to handle the shifting procedure. The driver has the advantage of being able to select the appropriate gear range, but doesn’t have to operate the clutch pedal. If the driver chooses not to select gears manually, the car will still shift gears on its own.


Tiptronic AKA manumatic


On the surface, these transmissions function exactly the same as the aforementioned AMT. The driver is allowed to select gears manually, but the car will shift by itself once the pressure builds. The Tiptronic transmission is found in many modern sport and economy-sport cars.


The key difference between the Tiptronic and the AMT is that a Tiptronic is truly an automatichydraulic transmission, and it uses sensors and computers to signal a manual gear selection from the driver. You can expect your maintenance to be the same as with an ordinary automatic. The upside, however, is that you have manual gear selection for swift acceleration when you want it.


CVT (Continuously variable transmission)


The CVT is as automatic as it gets; it uses a pulley and belt system that can create a gear of any size by expanding the pulley. That means that the car never needs to shift, and there are no gear ranges. With all other types of transmission, you have gears that are meant to keep the vehicle operating within the power band. But the real sweet spot is somewhere in the middle of the powerband, so in a transmission with gears, the car is typically operating above or below the ideal RPM. Not so with a CVT; its always right in the sweet spot.


The advantages of the CVT are fuel economy and ideal output for performance at all times. However, it has torque limitations, because belts tend to slip under extreme torque. This can be mitigated by using a chain instead of a pulley, but it doesn’t eliminate the problem. The stress is too much even for a chain in high-torque applications, like diesel trucks.


The maintenance for CVT’s is basically the same as for your conventional hydraulic automatic transmission. You mainly need to check the fluid regularly, keep it topped off, and change the fluid at regular service intervals or when it gets dirty.


For transmission repair and maintenance in the Des Moines area, you can bring your vehicle to ABC Auto. Reach us by phone at 206 395 5300 or fill out a contact form here.


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