The term “drivetrain” (or “driveline”) refers to all the components between a vehicle’s engine and the its driving wheels, typically including the transmission, driveshaft(s), and axle(s). The purpose of a vehicle’s drivetrain is to transmit power from the engine to the wheels. Let’s look at the various drivetrain layouts and components: Passenger cars and light-to-medium duty trucks generally have either a front-wheel drive (FWD) or rear-wheel drive (RWD) configuration. Even vehicles with 4-wheel drive (4WD) or all-wheel drive (AWD) arrangements are typically based on a FWD or RWD design. In a typical RWD design, the engine is mounted longitudinally (meaning the centerline of the engine’s crankshaft runs fore and aft in the vehicle, rather than side-to-side). The transmission bolts to the back of the engine, and a coupling device (a clutch or torque converter, located inside the transmission’s hollow bellhousing) transmits the engine power to the input shaft of the transmission. A driveshaft (also known as a propeller shaft, or propshaft for short) transmits power from the transmission’s output shaft to the rear axle. The rear axle splits the power from the driveshaft (through an internal differential) and sends it to the rear wheels (which are mounted at the outer ends of the axle assembly). Let’s consider each of these components – what they do and why they are needed: · A clutch is a coupling device (typically used with a manual transmission) that allows the engine’s output power to be disconnected from (or reconnected to) the transmission. In a typical manual transmission vehicle, the driver depresses a clutch pedal to disengage engine power from the transmission. This allows the transmission to be shifted between gears (manual transmissions are not designed to be shifted while under power), and also allows the vehicle to remain stopped (at a traffic light, for example) while the engine remains running. Without a coupling device of some kind (or, more accurately, a de-coupling device!), the engine would have to stop whenever the wheels were not turning. In addition to being a coupling / de-coupling device, the clutch is also a launch device. When the vehicle is launched from a stop, the engine’s crankshaft (which is rotating) must be connected to the wheels (which are not rotating). The clutch is a friction device that allows the driver to gradually engage the engine with the rest of the drivetrain. The clutch allows some torque to be transmitted to the wheels (to launch the vehicle), while at the same time allowing slippage between the engine and transmission (to avoid stalling the engine). In summary, then, the clutch has three modes of operation: — Decoupled (clutch released, clutch pedal depressed): No engine power is transmitted to the drivetrain. Vehicle can be stopped while the engine continues to run. — Slipping (clutch pedal in mid-travel): Some engine torque is transmitted, but transmission input speed is less than the engine speed (in revolutions per minute [RPM]). This occurs when launching the vehicle from a stop. — Coupled (clutch engaged, clutch pedal released): All the engine torque / power is transmitted to the drivetrain. This is the typical condition during normal driving. Note that there are also typically several smaller clutches inside the transmission, but in most cases when we speak of “the clutch” we are referring to the clutch between the engine and the transmission’s input shaft. · A torque converter is also a coupling device, but is typically used with an automatic transmission. A torque converter is a fluid coupling. That is, it is a fluid-filled housing containing two sets of opposing blades or fins. One set of blades (called the impeller) is connected directly to the housing, which is connected to the engine’s crankshaft. So the impeller blades rotate with the engine, whenever the engine is running. The second set of blades (called the turbine) is connected to the transmission’s input shaft. As the engine turns, the rotating impeller blades force fluid (automatic transmission fluid) against the turbine blades, which makes the turbine want to rotate. This can be visualized by thinking of two fans, pointed towards each other. If one fan is turned on, it will blow air against the other fan, making it spin. In a similar way, the impeller (powered by the engine) transmits torque to the turbine (and thus, to the transmission). Since the engine torque is transmitted by the fluid (rather than a direct mechanical connection), the torque converter also allows the engine to continue running while the vehicle is at a stop. Considering the two-fan example again, the second fan could be held to stop it from rotating, even if the first fan were still running. The force of the air against the second fan could be easily overcome (to hold it at a stop). In the same way, applying the brakes in an automatic transmission vehicle (with a torque converter) can easily hold the drivetrain (and ultimately, the turbine) from turning, even if the engine is still running. And in the same way that the second fan would begin to rotate when released (since the blowing air is always trying to turn it), an automatic transmission vehicle will typically begin to move as soon as the brakes are released (since the fluid from the impeller is always trying to turn the turbine). So with a torque converter, the engine is never truly de-coupled from the transmission’s input, since there is always some torque being transmitted when the engine is running. But the turbine (transmission input) can be held at zero speed without stalling the engine. So the torque converter (with an automatic transmission) provides the same functions as the clutch (with a manual transmission). It allows the engine to continue running while the vehicle is at a stop, and it serves as a launch device (since it allows slippage between the impeller and turbine). But a torque converter also includes a third member (the stator) that provides yet another advantage: torque multiplication. At low turbine (transmission input) speeds, the impeller causes the fluid inside the torque converter to travel in a kind of "spiral donut" path (as if you wrapped a string around and around a donut, going through the center hole and around the outside, then back through the center hole, over and over, with each winding spaced slightly away from the last one). The fluid passes through the impeller blades (in the rear portion of the converter), which force it against the turbine blades (in the front portion). But the turbine blades are curved, so as the fluid travels through the turbine blades, its flow direction gets turned. When the fluid exits the turbine blades, it is moving in a direction opposite the rotation of the impeller (which would tend to impede the impeller). The stator is yet another set of curved blades, which is mounted in between the inner portion of the turbine and impeller. Fluid coming from the turbine strikes the stator blades, which redirect the fluid (turn it back around) so it is moving in the same direction as the impeller. Now a fundamental equation for transmissions (and torque converters) is: Torque In = Torque Out (neglecting frictional losses). The "Torque In" to a torque converter is the engine torque. The "Torque Out" is actually the sum of two torques: the turbine (transmission input shaft) torque, and the stator torque. The blades in the stator act like the buckets in a water wheel; as the fluid flows against the stator blades, it tries to turn the stator (in the direction opposite the impeller and turbine). But the stator is mounted on a fixed shaft (called the stator shaft or the reaction shaft, a hollow shaft which surrounds the transmission input shaft) which is bolted or welded to the transmission pump housing or case, so it can't turn in that direction. Even though the stator can't turn, it still has a torque (rotational force) applied to it, which is then transmitted to the stator shaft. So at low speeds, there is torque in the normal driving direction (on the turbine and the transmission input shaft), and there is also torque (in the opposite direction) on the stator shaft. The "Torque Out" of the converter is the sum of both of these torques, but since the stator torque is in the opposite direction, it is noted as a negative torque value. Since: Torque In = Torque Out and Torque Out = Turbine (Input) Torque + Stator Torque we find that: Torque In = Turbine (Input) Torque + Stator Torque and therefore: Turbine (Input) Torque = Torque In - Stator Torque Suppose, for example, that our engine torque (Torque In) is 200 ft-lbs, and that when the vehicle is stopped (turbine speed of zero, which is typically the point of maximum stator torque), the stator torque is -180 ft-lbs (again, a negative value since it is in the opposite direction of the engine and input torque). Then: Turbine (Input) Torque = Torque In - Stator Torque Turbine (Input) Torque = 200 - (-180) Turbine (Input) Torque = 200 + 180 Turbine (Input) Torque = 380 ft-lbs So the transmission input torque is actually being multiplied within the torque converter, because of the stator. As noted, this torque multiplication is highest at a stop, so its greatest benefit is that it assists in launching the vehicle from a stop. As the vehicle accelerates (turbine speed increases), the torque on the stator decreases, as does therefore the amount of torque multiplication. Once the turbine reaches about 85% of the engine's speed, the stator torque drops to zero and there is no more torque multiplication. The stator is mounted (on the stator shaft) on a mechanical one-way clutch. This clutch locks when the stator tries to turn opposite the normal driving direction (imparting torque to the stator shaft and producing the torque multiplication), but as the turbine speeds up, eventually the fluid flow starts to push the stator in the same direction as the turbine and impeller. At that point, the one-way clutch unlocks and all three members rotate in the same direction. It should be noted that modern torque converters also include another mechanical clutch (known as the Torque Converter Clutch [TCC] or “Lockup” clutch) that can be used to physically connect the impeller directly to the turbine. This improves efficiency (fuel economy), since even at highway speeds the fluid coupling by itself would still allow some slippage between the engine and the transmission. The TCC, then, provides for fully coupled operation. · The transmission (whether manual or automatic) is a mechanical device that provides a range of various ratios (typically called gears, or gear ratios) between the transmission input and output speeds, including at least one reverse ratio. For example, a transmission might have the following gear ratios: o First gear: 3.23 (sometimes written as 3.23 : 1) o Second gear: 1.84 o Third gear: 1.41 o Fourth gear: 1.00 o Fifth gear: 0.82 o Sixth gear: 0.67 In this example, the first gear ratio of 3.23 means that the transmission input shaft (which is generally connected to the engine) will rotate 3.23 times for each one revolution of the transmission output shaft (which is connected to the driveshaft, and ultimately to the wheels). So the engine would be spinning over 3 times as fast as the driveshaft. Similarly, in second gear, the transmission input RPM will be 1.84 times its output RPM. Gears where the transmission output speed is less than input speed are known as underdrive gears (in our example, first, second, and third gears are underdrive gears). A gear ratio of 1.00 (sometimes written as 1:1) is known as direct gear, where the transmission input and output shaft speeds are equal. In our example, fourth gear is direct drive. Gears where the transmission output turns faster than the input are known as overdrive gears (fifth and sixth gears in our example). In a reverse gear (rather obviously), the transmission output shaft turns in the opposite direction from the input (and typically at a much slower speed), allowing the vehicle to be driven backwards. So, why do we need different gears? Well, the need for a reverse gear is pretty obvious. For forward gears, there are two main reasons we need multiple gears: o Engine speed is limited to a certain range (generally about 700 – 6000 RPM for gasoline engines, and 700 – 4000 RPM for diesel engines). The clutch or torque converter will allow slip at launch, but suppose we want the coupling device to be fully engaged (no slip) when the vehicle speed reaches 10 mph. And suppose that at that point we would allow the engine speed to be running at just 800 RPM (which is too low to provide adequate acceleration, but we’ll ignore that). That would mean (if we only had one forward gear) that at 70 mph, the engine speed would be 5600 RPM! We need multiple gear ratios so we can keep the engine speed at a reasonable value, across a wide range of vehicle operating speeds (both slow and fast). o In addition, we need to allow different engine speeds (under varying conditions) at the same vehicle speed. For example, cruising at 55 mph, we would want to run at a low engine speed, for good fuel economy. But if we stomp on the accelerator to pass another vehicle, we want maximum engine power, which means we’d want the engine at a much higher speed. This is why an automatic transmission will typically “kick down” to a lower gear during a passing maneuver (to raise the engine speed into the region of maximum horsepower). Being able to choose between several different gear ratios allows us to vary the engine speed for either high power or optimal fuel economy. So by changing the gear ratio within the transmission, we can select the engine operating speed that is best for the current driving conditions. · The driveshaft simply connects the transmission output shaft to the rear axle. The driveshaft includes at least two universal joints, allowing for movement of the rear axle (due to rear suspension travel or windup) relative to the transmission. · The rear axle has two main drive components: a ring and pinion gear that provide yet another gear ratio (in addition to the transmission gear ratio), and a differential, which splits the power between the two rear driving wheels, and also allows for differentiation (that is, a difference in speed) between the two rear wheels. The ring and pinion gear ratio (commonly known as the axle ratio) is always an underdrive ratio (so the wheels spin slower than the driveshaft), and is necessary because the wheel speed is significantly lower than the engine or driveshaft speed. For example, a common light truck tire size (LT265/70R17) rolls about 638 revolutions per mile. Therefore, at 60 mph (one mile per minute) the wheel speed with this tire size will be only 638 RPM. At 20 mph, the wheel speed will be about 212.7 RPM. Using our example transmission gear ratios, even first gear (3.23 ratio) would only allow 687 RPM engine speed (at 20 mph) if we didn’t have the additional gear ratio in the axle. But because of the rear axle ratio (for example, 3.55, which is a common axle ratio), the 212.7 RPM wheel speed (at 20 mph) would give about 755 driveshaft RPM. Therefore, in first gear the engine speed would be about 2440 RPM. The differential is necessary because, when the vehicle turns, the inner wheels (which are travelling along a smaller radius) turn slower than the outer wheels. The axle’s ring gear is bolted to the differential carrier, which transmits power to gears mounted on the inner ends of the right and left axle shafts (which are connected to the right and left rear wheels). But the gears on the axle shafts are not connected to each other, so they are able to turn at different speeds. Normally (when driving in a straight line), both axle shafts turn at the same speed as the differential carrier. But when going around a corner, one wheel will slow down while the other speeds up. Their average speed will remain equal to the differential carrier speed. For example, suppose that at 20 mph, the wheel speed (driving in a straight line) is 212.7 RPM. The differential carrier and ring gear would also be turning at 212.7 RPM (the pinion gear and driveshaft, in our earlier example, would be turning at 755 RPM). If the vehicle then drove around a curve, the inner wheel might slow down to 192.7 RPM (20 RPM slower than usual). In that case, the outer wheel would speed up by 20 RPM (to 232.7 RPM), so that the average wheel / axle shaft speed remains at 212.7 RPM. So the axle provides an additional gear ratio, and allows for differentiation in wheel speeds when driving through a curve. Now that we understand the basic components in a RWD vehicle’s drivetrain, let’s consider a front-wheel drive (FWD) configuration. Most FWD vehicles use a transverse-mounted engine, where the crankshaft centerline runs side-to-side rather than fore and aft. The transmission (and coupling device, either clutch or torque converter) are still mounted to the “back” of the engine (although the “back” of the engine is typically facing the left side, not the rear, of the vehicle). But since the engine and transmission must fit into the space between the front wheels, the transmission is much shorter than a typical RWD transmission. In addition, the ring and pinion gear, and the differential, that would be in the rear axle of a RWD vehicle, are typically part of the FWD transmission. Basically, it’s as if the center section of the rear axle has been stuck onto the side of the FWD transmission, and in place of the axle shafts there are two shafts (known as halfshafts, which include universal joints) that drive the front wheels. Since the FWD transmission includes both the transmission, and the “axle” (ring and pinion gears, and differential), it is often referred to as a “transaxle”. This basically means transmission and axle in combination. So a FWD vehicle has the same basic drivetrain components as a RWD vehicle, except the driveshaft is eliminated, and the axle components are typically incorporated within the transmission assembly. Now, what about four-wheel drive (4WD) and all-wheel drive (AWD) vehicles? First off, what’s the difference? They both drive all four wheels, so why two different names? A 4WD (or AWD) vehicle is a FWD or RWD vehicle with extra driveline components, to allow the normally non-driven wheels to also be driven. Typically, the 4WD designation indicates that the 4WD mode is engaged or disengaged by the driver, using a lever or switch. When the 4WD system is disengaged, the vehicle operates in normal two-wheel (FWD or RWD) mode. Many 4WD systems also include a “Low” position which adds yet another underdrive gear ratio (on top of the axle ratio and the transmission ratio) for extreme off-road situations. The AWD designation typically indicates that power distribution between the front and rear wheels is automatically and continuously controlled by the AWD system itself; no driver input is required. Some AWD vehicles always operate with all four wheels being driven, although power may be directed primarily to one axle depending on conditions. Other AWD vehicles will sometimes operate in true two-wheel drive mode, but they still engage four-wheel drive automatically when needed. So typically, a 4WD vehicle only operates in four-wheel drive mode when the driver selects that mode, whereas an AWD vehicle typically senses when four-wheel drive is needed, and engages it automatically. This 4WD/AWD nomenclature is not always consistently applied, however. For example, some completely automatic systems are still referred to as “4WD” in the manufacturer’s literature. In 4WD / AWD vehicles, a transfer case (sometimes called a power transfer unit) is added to the drivetrain, to split the transmission’s output power between the front and rear wheels. Another drive axle is added (to the normally non-driven wheels), and an added driveshaft connects this axle to the transfer case. The simplest type of transfer case merely provides the ability to engage (or disengage) driving power to the added axle. When 4WD mode is engaged, the front and rear driveshafts are locked together within the transfer case. This is known as “part-time” 4WD, and is intended only for off-road use. In normal driving (on pavement), the front and rear wheels turn at different speeds when going through a curve, but since “part-time” 4WD mode locks the front and rear driveshafts together (forcing them to turn at the same speed), using “part-time” 4WD on pavement results in tire scrubbing and rapid tire wear, and causes blatant “crow hop” (vehicle and steering wheel oscillation) in tight turns. For this reason, many 4WD systems include a differential (similar to the one in the axle) within the transfer case, to allow different front and rear driveshaft speeds. This allows the use of 4WD mode even on dry pavement, and is therefore called “full-time” 4WD since it can be used at any time. All AWD vehicles include a front-to-rear differential. As noted above, some 4WD systems also include a “Low” range (an additional available underdrive gear ratio) to enhance tractive ability for extreme situations. In the universe of vehicles, there are obviously some exceptions to the general descriptions given here, but hopefully this overview has provided a basic understanding of vehicle drivetrain components.