VEHICLE POWER PLANT AND TRANSMISSION CHARACTERISTICS
The two factors that limit conventional vehicle performance are the maximum tractive effort that the tire–ground vehicle power plant and transmission characteristics adhesion can support,
and the tractive effort that the overall powertrain can provide.
The performance limits of the vehicle are dictate by the lesser of the tractive efforts.
With the transmission in low gear and the engine throttle at maximum.
The tractive effort may be limit by the nature of tire-road adhesion and loss of traction.
As the transmission is shift into higher gears, the tractive effort is often determine by the engine and transmission characteristics,
which have to be taken into consideration when predicting the overall performance of a road vehicle.
[caption id="attachment_4983" align="alignnone" width="300"] vehicle power plant and transmission[/caption]
Power Plant Characteristics
For conventional vehicles, the ideal performance characteristic of a power plant is an unchanging power output across the entire operating range.
At low speeds, the motor torque is force to maintain a constant value so as not to exceed the adhesion limit between the tire–ground contact area.
After the constant torque low-speed range, the torque varies with speed steeply.
Constant power characteristics provide conventional vehicles with a high tractive effort at low speeds, which improves acceleration, grade climbing, and towing capacity.
The internal combustion engine has been the standard power plant in conventional
vehicles for over a century because of its relatively high power-to-weight ratio and low cost,
but it is not without its shortcomings.
Typically, an internal combustion engine has torque-speed characteristics
that do not directly correlate with the ideal performance characteristics require by traction.
When compared with an ideal torque-speed profile
When compare with an ideal torque-speed profile,
typical internal combustion engines have a relatively flat profile, which is why a multiple gear transmission is usually employ to modify it.
As an internal combustion engine moves through its typical torque and power curves,
representative characteristics are develop in each speed range that can be use to predict vehicle performance.
The engine begins operating smoothly at idle speed, and as the engine speed approaches an intermediate-range,
good combustion quality and maximum engine torque are reached.
As the speed increases further, the increasing losses in the air-induction manifold and the steady decline in engine torque cause the mean effective pressure to decrease.
Power output continues to increase to its maximum at a specific high engine speed.
Beyond this peak power point vehicle power plant and transmission characteristics the engine torque decreases more rapidly as the engine speed is increase,
which results in the decline of engine power output.
The term “transmission” refers to all of the systems or subsystems.
Used for transmitting the engine power to the driven wheels or sprockets.
The principal requirements for the transmission are to attain the desired maximum vehicle speed with an appropriate engine,
to be able to move the vehicle on a steep slope as well as maintain speed on a gentle slope in high gear,
and to properly match the engine characteristics to achieve the desired operating fuel economy and acceleration rate.
The two most common types of transmissions for conventional vehicles are the manual gear transmission and the automatic transmission with a torque converter.
Other types of transmissions, such as the continuous variable transmission, are also in use and are beginning to gain popularity due to their relatively high overall efficiency.
Manual transmissions were the first gearbox designs use in conventional vehicles and were use for decades before automatic transmissions were introduce.
These transmissions are still popular due to their simplicity, low cost, and high efficiency.
The term “manual” implies that the driver must perform the shifting from gear to gear manually.
A manual gear transmission consists of a clutch, gearbox, final drive, and driveshaft.
The gearbox provides a number of gear reduction ratios, between three and five for passenger cars,
and more for heavy vehicle power plant and transmission characteristics commercial vehicles.
In determining maximum and minimum gear ratios,
we can think back to the two factors that limit conventional vehicle performance.
The maximum speed requirement determines the ratio of
the highest gear.
On the other hand, the gear ratio of the lowest gear is determin by the requirement
of the maximum tractive effort or the gradeability, which is often assumed to be 33%.
Choosing the ratios between
Choosing the ratios between high and low gear should be spaced in such a way that they will provide the tractive
effort–speed characteristics as close to the ideal as possible, but unfortunately, this is not always
Traditionally, in conventional vehicles, the gear ratios are often chose to minimize the
time require to reach a specific speed or the maximum speed of the vehicle. With fuel economy
moving to the forefront of vehicle design,
a geometric progression approach, where consecutive gear
ratios are very vehicle power plant and transmission characteristics close to one another, could be used. The basis for this is to have the engine operating
within the same speed range in each gear. This would ensure that in each gear, the operating fuel
economy is similar.
With an automatic transmission, a driver no longer needs to actively change gears during driving,
thus making the vehicle easier to drive. Automatic transmissions use luid to transmit power in the
form of torque and speed and are widely use in conventional passenger vehicles.
In a conventional automatic transmission, the clutch is replace with a luid coupling or torque converter to eliminate engaging and disengaging action during gear changes. The torque converter’s three major components,
are the pump impeller, the turbine runner, and the stator.
When the impeller is drive by the engine, the luid in the impeller rotates with it, and as the speed increases,
centrifugal force causes the fluid vehicle power plant and transmission characteristics to low into the turbine. The hydraulic luid in the converter transfers
torque through the kinetic energy of the transmission luid as it is forced from the pump impeller
to the turbine. The higher the engine speed the greater the torque applied to the turbine. The
stator is located between the pump impeller and the turbine. The vanes of the stator catch the luid
as it leaves the turbine runner and redirects it so that it strikes the back of the vanes of the pump
impeller giving it added torque.
The major advantages of the automatic transmission are that it prohibits the engine from stalling,
and it provides torque-speed characteristics that approach the
ideal characteristics discussed previously.
The major disadvantages of the automatic transmission
are its low efficiency in a stop and go drive cycle and its very complicated construction.
Continuously Variable Transmission
As the interest in improving the fuel economy of automotive vehicles continues to grow, continuously
variable transmissions (CVT) have attracted a great deal of attention.
This type of transmission
provides a continuously variable reduction ratio within a certain range,
thus providing an ininite number of gear ratios.
This allows the engine
This allows the engine to operate under the most economical conditions over a
wide range of vehicle speeds. It is therefore vehicle power plant and transmission characteristics possible to achieve an ideal torque-speed profile because
any engine power output to the transmission can be applied to the wheels at any speed.
A belt CVT system is similar to a standard belt–pulley drive, with one pulley connected to the
engine shaft and the other connected to the output shaft. The exception, however, is that the half
pulleys are not fixed and are able to move apart. Both pulleys have fixed axes of rotation at a distance
from each other. The sides of each pulley are controlled so that they may move apart or together
laterally, varying the effective diameter on which the belt grips. The overall lateral displacement of
each pulley is the opposite of the other.