Chapter 9: The Drive Train

Chapter 9: The Drive Train
    Chapter 9: The Drive Train 

    Chapter 9: The Drive Train

    The drive train (drivetrain or powertrain) is a series of parts that transfer energy from the combustion engine to the wheels. If needed, review Chapter 5 to recall how the internal combustion engine works to convert chemical energy to rotational energy. The drive train has three main parts: the transmission, drive line or drive shaft, and differential. In some four-wheel drive vehicles a transfer case is also necessary. 

    The rotation of the crankshaft in the internal combustion engine does not equal the rotation of the wheels. Just like a bicycle, motor vehicles have gears. The lower gears are used for lower speeds, while the higher gears are used for higher speeds. Yet, the RPM which is read on the dashboard tachometer may be relatively constant (or stay within a range) through all speeds. The transmission, which is where the actual gears are located, takes an input speed from the crankshaft and reduces it to the output speed until the vehicle is in direct drive. In other words, it changes the gear ratios. So, in first gear you might have an input to output ratio of 4 to 1. This means that as the input shaft goes around four times, the output shaft goes around one time. At 4,000 RPMs input you would have 1,000 RPMs output. Second gear may have a 3 to 1 ratio and third gear a 2 to 1 ratio. Fourth gear would be direct drive with a 1 to 1 ratio.

    Think of the sprockets on a bicycle. There is a large sprocket on the front that rotates as the bike is pedaled, and there are also large and small sprockets on the back wheel with a chain that connects the front sprocket to the rear sprockets. As the gears are changed, so is the mechanical advantage for the person that is pedaling.
    In direct drive, the input speed and the output speed are the same; there is no gear reduction. Modern vehicles have overdrive, which is a ratio of 0.75 to 1. This means that the input speed is less than the output speed, which results in better fuel economy on modern vehicles.

    These numbers and ratios are used to present the general idea. Every vehicle has its own input to output ratios.

    The drive line (or drive shaft) is a round tube that transfers power from the transmission to a differential. It is situated longitudinally underneath the car. A front-wheel drive vehicle won t have a drive shaft. Some four-wheel drive vehicles have more than one drive shaft.

    A differential (aka rear differential in rear-wheel drive vehicles, rear end, or part of the rear axle) has hypoid gears. These are specialized gears which transmit the rotation of the drive shaft 90 degrees in order to turn the axles and correlating wheels and tires. There are spider gears inside the center section of the differential that allow the wheels to travel different distances while turning. Consider a car that is turning right. The driver s side rear wheel travels a farther distance than the passenger side rear wheel. Think of using a protractor to draw a circle. One leg of the protractor simply rotates on one center spot, while the other leg draws the circle. While the rear wheels of a vehicle don t turn as sharply as a protractor, the principle is the same. 

    to be Continued ....

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