Primer on Automobile Fuel Efficiency and Emissions - Pollution Probe

The Drivetrain
In a conventional car, the drivetrain (sometimes referred to as the driveline) consists of several components (i.e.,
driveshaft, transmission, differential and axle) that transmit energy from the engine to the wheels. The transmission
connects the crankshaft to the driveshaft. The transmission also converts the engine speed (the rate at which the
crankshaft rotates) into the speed required for the driveshaft to turn the wheels at the desired rate. Said another
way, power is the product of force and speed, and so the purpose of the transmission is to convert the force and
speed combinations that the engine can produce into force and speed combinations that the driver wants at the
wheels. The transmission contains low gears that provide extra force for accelerating the car from a rest position
or for climbing hills, and high gears primarily used at cruising speeds. The idea is to turn the driveshaft and wheels
at revolutions needed to propel the automobile forward at any speed (and in reverse) while permitting the engine
to run in a narrower, optimal speed range (say, between 1,500-3,500 rpm). The axle upon which the wheels turn
is connected to the driveshaft through the differential. The differential is simply a set of bevelled gears that directs
power from the driveshaft to the wheels. A vehicle’s wheels can rotate at different speeds, especially when turning
corners. The differential is designed to drive a pair of wheels with equal force, while allowing them to rotate at
different speeds.
The drivetrain accounts for about 5.6 per cent of energy losses in both urban and highway driving. Some of this
energy is lost in the transmission. There are two types of transmissions: manual transmissions and automatic
transmissions. Manual transmissions are controlled by the driver by way of a stick shift and are in direct contact
with the engine crankshaft through a clutch. To shift gears in a manual transmission, a driver will momentarily
disengage the clutch, which disconnects engine power to the driveshaft, make the shift and then re-engage the
clutch to bring power back to the driveshaft. The benefit of the direct connection is that little power is lost across
the clutch. Automatic transmissions, on the other hand, shift gears without driver intervention and are indirectly
connected to the engine through a fluid-coupling device known as a torque converter. Some power is lost in the
fluid between the engine and the driveshaft, meaning that automatic transmissions can lead to lower levels of
fuel efficiency. A trained driver, shifting at efficient engine speeds, can use less energy with a manual transmission
than can be achieved with an automatic transmission. However, most drivers lack such training and awareness;
this, coupled with technical advancements in some automatic transmissions, means that the average driver may
lose less energy with an automatic than a manual transmission.
<strong>Primer</strong> on Automobile Fuel Efficiency & Emissions
36