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| Know Your Car : Manual Transmission System |
If you drive a stick-shift car, then you may have several questions floating in your head :
- How does the funny "H" pattern that I am moving this shift knob through have any relation to the gears inside the transmission? What is moving inside the transmission when I move the shifter?
- When I mess up and hear that horrible grinding sound, what is actually grinding?
- What would happen if I were to accidentally shift into reverse while I am speeding down the highway? Would the entire transmission explode?
If you have ever had questions like these, then you will love this section as we explore the interior of a manual transmission.
The Goal of a Transmission
Cars need transmissions because of the physics of the petrol engine. First, any engine has a redline - a maximum rpm value above which the engine cannot go without exploding. Second, if you have read the section on horsepower, then you know that engines have narrow rpm ranges where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm.
The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so that the engine can stay below the redline and can stay near the rpm band of its best performance.
Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, best-performance rpm value. That is the idea behind the Continuously Variable Transmission (CVT). A CVT has a nearly infinite range of gear ratios. However, CVTs have not been able to compete with 4-speed and 5-speed transmissions in terms of cost, size and reliability, so you usually do not see them in production automobiles.
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The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore turns at the same rpm as the engine. A 5-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm value at the output shaft. Here are some typical gear ratios :
|
| Gear |
Ratio |
RPM |
| First |
2.315 |
1,295 |
| Second |
1.568 |
1,913 |
| Third |
1.195 |
2,510 |
| Fourth |
1.000 |
3,000 |
| Fifth |
0.915 |
3,278 |
|
| ( In table, we have shown RPM at transmission output shaft if engine is at 3,000 rpm ) |
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