Title: Automotive Powertrain
Date: 03/01/19
Objective: To understand the power train components of automobiles both
visually and conceptually
Observation/Theory:
The engine of the vehicle creates a rotational power. To move the car, we
need to transfer that rotational power to the wheels. That’s what the car’s drivetrain
does and the clutch, transaxle and transmission are part of it.
Manual Transmission
There are a few problems with power produced by an internal combustion
engine. First, it only delivers usable power, or torque, within a certain range of
engine speed (this range is called an engine’s power band). Go too slow or too fast,
and you don’t get the optimal amount of torque to get the car moving. Second, cars
often need more or less torque than what the engine can optimally provide within
its power band.
What is the difference between engine speed and engine torque?
Engine speed is the rate at which the engine’s crankshaft spins. This is measured in
revolutions per minute (RPMs). Engine torque is how much twisting force the
engine generates at its shaft for a particular speed of rotation.
The transmission ensures that the engine spins at an optimal rate (neither too slow
or too fast) while simultaneously providing the wheels with the right amount of
power they need to move and stop the car, no matter the situation you find yourself
in. It’s able to do this effective transmitting of power through a series of different
sized gears that leverage the power of gear ratio.
Inside the transmission are a series of variously sized, toothed gears that produce
torque. Because the gears that interact with each other are different sizes, torque
can be increased or decreased without changing the speed of the engine’s rotational
power all that much. This is due to gear ratios.
Gear ratios represent the gears’ relation to each other in size. When different sized
gears mesh together, they can spin at different speeds and deliver different amounts
of power.
Fig. gear ratio in a manual transmission
First Gear- It’s the largest gear in the transmission and enmeshed with a small
gear. A typical gear ratio when a car is in first gear is 3.166:1. When first gear is
engaged, low speed, but high power is delivered. This gear ratio is great for
starting your car from a standstill.
Second Gear- The second gear is slightly smaller than first gear, but still is
enmeshed with a smaller gear. A typical gear ratio is 1.882:1. Speed is increased
and power decreased slightly.
Third Gear- Third gear is slightly smaller than the second, but still enmeshed with
a smaller gear. A typical gear ratio is 1.296:1.
Fourth Gear- Fourth gear is slightly smaller than the third. In many vehicles, by
the time a car is in fourth gear, the output shaft is moving at the same speed as the
input shaft. This arrangement is called “direct drive.” A typical gear ratio is
0.972:1
Fifth Gear- In vehicles with a fifth gear (also called “overdrive”), it is connected
to a gear that’s significantly larger. This allows the fifth gear to spin much faster
than the gear that’s delivering power. A typical gear ratio is 0.78:1.
Reverse/Idler gear- The idler gear (sometimes called “reverse idler gear”) sits
between the reverse gear on the output shaft and a gear on the countershaft. The
idler gear is what allows your car to go in reverse. The reverse gear is the only gear
in a synchronized transmission that isn’t always enmeshed or spinning with a
countershaft gear. It only moves whenever you actually shift the vehicle into
reverse.
Fig. parts of a manual transmission
Input shaft- The input shaft comes from the engine. This spins at the same speed
and power of the engine.
Countershaft- The countershaft (layshaft) sits just below the output shafts. The
countershaft connects directly to the input shaft via a fixed speed gear. Whenever
the input shaft spins, so does the countershaft, and at the same speed as the input
shaft. In addition to the gear that takes power from the input shaft, the countershaft
also has several gears on it, one for each of the car’s “gears” (1st-5th), including
reverse.
Output shaft- The output shaft runs parallel above the countershaft. This is the
shaft that delivers power to the rest of the drivetrain. The amount of power the
output shaft delivers all depends on which gears are engaged on it. The output shaft
has freely rotating gears that are mounted on it by ball bearings. The speed of the
output shaft is determined by which of the five gears are in “gear,” or engaged.
Synchronizer collars/sleeves- Most modern vehicles have a synchronized
transmission, meaning the gears that deliver power on the output shaft are
constantly enmeshed with gears on the countershaft and are constantly spinning.
Whenever you shift a car into a gear, the synchronizer collar shifts over to the
moving gear you’re looking to engage. On the outside of the gear are a series of
cone-shaped teeth. The synchronizer collar has grooves to accept those teeth.
Thanks to some excellent mechanical engineering, the synchronizer collar can
connect to a gear with very little noise or friction even while the gear is moving,
and sync the gear’s speed with the input shaft. Once the synchronizer collar is
enmeshed with the driving gear, that driving gear is delivering power to the output
shaft. Whenever a car is “neutral” none of the synchronizer collars are enmeshed
with a driving gear.
Gearshift- is what you move to put a car into gear.
Shift rods- are what move the synchronizer collars towards the gear you want to
engage. On most five-speed vehicles, there are three shift rods. One end of a shift
rod is connected to the gearshift. At the other end of the shift rod is a shift fork that
holds the synchronizer collar.
Shift fork- holds the synchronizer collar.
Clutch- Sits between the engine and gearbox of the transmission. When the clutch
is disengaged, it disconnects power flow between the engine and transmission
gearbox.
The Clutch
Fig. the clutch assembly
The first stage in the transmission of a car with a manual gearbox is the clutch.
Operation
It transmits engine power to the gear box, and allows transmission to be interrupted
while a gear is selected to move off from a stationary position, or when gears are
changed while the car is moving.
Most cars use a friction clutch operated either by fluid (hydraulic) or, more
commonly, by a cable. When a car is moving under power, the clutch is engaged.
A pressure plate bolted to the flywheel exerts constant force, by means of a
diaphragm spring, on the driven plate. Earlier cars have a series of coil springs at
the back of the pressure plate, instead of a diaphragm spring.
The driven (or friction) plate runs on a splined input shaft, through which the
power is transmitted to the gearbox. The plate has friction linings, similar to brake
linings, on both its faces. This allows the drive to be taken up smoothly when the
clutch is engaged.
When the clutch is disengaged (pedal depressed), an arm pushes a release bearing
against the centre of the diaphragm spring which releases the clamping pressure.
The outer part of the pressure plate, which has a large friction surface, then no
longer clamps the driven plate to the flywheel, so the transmission of power is
interrupted and gears can be changed.
When the clutch pedal is released, the thrust bearing is withdrawn and the
diaphragm-spring load once again clamps the driven plate to the flywheel to
resume the transmission of power.
Some cars have a hydraulically operated clutch. Pressure on the clutch pedal inside
the car activates a piston in a master cylinder, which transmits the pressure through
a fluid-filled pipe to a slave cylinder mounted on the clutch housing. The slave-
cylinder piston is connected to the clutch release arm.
Parts of the clutch
The modern clutch has four main components: the cover plate (which incorporates
a diaphragm spring), the pressure plate, the driven plate, and the release bearing.
The cover plate is bolted to the flywheel, and the pressure plate exerts pressure on
the driven plate through the diaphragm spring or through coil springs on earlier
cars. The driven plate runs on a splined shaft between the pressure plate and
flywheel. It is faced on each side with a friction material which grips the pressure
plate and flywheel when fully engaged, and can slip by a controlled amount when
the clutch pedal is partially depressed, allowing the drive to be taken up smoothly.
