ABSTRACT
The powertrain system of a late model, front-wheel drive, small (C-segment) passenger car was selected for this project (Prado, 2012) The system involved a 14 L four-cylinder double-overhead cam (DOHC) turbo-charged gasoline engine coupled to a six-speed automatic transmission It delivered 140 HP and achieved 42 mpg fuel consumption in highway driving The powertrain system was decomposed into the following three major subsystems: (1) engine, (2) transmission, and (3) drivetrain Prado’s original description of the decomposition of the three subsystems was rewritten as shown in the following subsections
Engine Sub-Subsystem The sub-subsystems of the engine subsystems are as follows:
Engine block and cylinder head: this sub-subsystem holds all the components of the engine Basically, it works as a frame for the moving parts and the support devices
Power conversion: all the moving parts that generate mechanical power are part of this sub-subsystem, such as pistons, piston rings, piston pins, connecting rods, bearings, and crankshaft
Valvetrain: in combination with the intake system, this sub-subsystem allows the engine to breathe The main components are valves, springs, retainers, sleeves, camshafts, sprockets, and timing belts
Intake, exhaust, and turbo system: these three sub-subsystems provide an air and fuel mixture to the engine and remove exhaust gases The intake sub-subsystem has the throttle body, intake pipes, and location for sensors The exhaust is made up of an exhaust manifold, pipes, a catalytic converter, the location for sensors, and mufflers The turbocharger is merged between intake and exhaust, combining the kinetic energy of the exhaust gases and converting this sub-subsystem to boost the engine in the intake side
Fuel system: this sub-subsystem manages fuel delivery in the cylinders through a complex network of components such as a fuel tank, fuel lines, a fuel pump, fuel injectors, sensors, and a fuel pressure valve
Cooling system: this sub-subsystem’s function is to keep the engine running within the best operating temperature range regardless of the climate Its components are the radiator, the coolant pump, coolant lines, hoses, sensors, and actuators (such as a thermostat)
Accessories: they accomplish other functions in the engine to keep it running (eg, an oil pump and lubrication) or providing other needs onboard a vehicle such as supplying energy for the climate control system (heating, ventilating, and air-conditioning systems), steering system, electrical system, and so forth
Transmission Sub-Subsystems The sub-subsystems of the transmission subsystem are as follows:
Casing: it holds all the moving parts such as bearings, shafts, and gears It provides protection and isolation for all the components located inside It also provides for lubrication of the moving parts
Power conversion: it contains all the moving parts that transmit or change the mechanical power and are part of this group, such as transmission pistons, shafts, clutches, gears, and so forth
Differential: it is a special kind of gear arrangement that is located at the final drive and makes it possible to split mechanical power among the left and the right powered front wheels in the proportion needed for the vehicle during all vehicle maneuvers Its components are conical gears, carriers, special bearings, shims, and so forth
Valve-body system: it is a labyrinth structure with a set of control valves that regulates the pressure and the flow rate of the oil that circulates inside the transmission It is usually attached to or a part of the main casing
Lubrication system: it manages the flow of oil for lubrication of the moving parts For automatic transmission, it works in combination with the valve-body system
Drivetrain Sub-Subsystems The sub-subsystems of the drivetrain subsystem are as follows:
Driveshafts: they couple the differential outputs to the front wheels They are made of steel and feature constant-velocity joints at their pivot points
Wheels: they consist of wheels with mounted tires
Fasteners There are many different types of fasteners (bolts, nuts, pins, clips, etc) that provide mechanical joints between the subsystems and sub-subsystems described in the earlier subsections
Based on the description of subsystems and sub-subsytems given in preceding subsections, a decomposition tree for the powertrain system was created The decomposition tree is presented in Figure 171 It shows the major subsystems, sub-subsystems, and components of the powertrain system in a hierarchical arrangement
The vehicle powertrain system also interfaces with the following vehicle systems: body system, chassis system, fuel system, electrical system, and climate control system Figure 172 presents an interface diagram of the powertrain system The interface diagram shows interfaces between the powertrain system’s three subsystems and their sub-subsystems and other vehicle systems by means of arrows The type of each interface is indicated by a letter code placed next to each arrow The letter codes are P = physical, S = spatial-packaging space, F = functional, E = energy transfer, M = material flow, and I = information flow
Table 171 presents an interface matrix of the powertrain system It shows all the sub-subsystems of the powertrain system and other major vehicle systems in its columns and rows Each interface is represented by an individual cell of the matrix The letters inside each cell indicate the type of interfaces associated with the systems defined by the row and column of the cell The letter code used here is the same as that used in the interface diagram in Figure 172 The blank cells show absence of interfaces between the rows and columns corresponding to the cells As expected, the engine block and cylinder head subsystem, the transmission casing, and the powertrain conversion subsystems (for the engine and the transmission) have the most interfaces Thus, powertrain engineers involved in the design of the aforementioned subsystems have to spend a lot of time in understanding all the interfaces and their requirements to ensure that all the sub-subsystems operate to provide the necessary subsystem performance and they function well with other systems in the vehicle Since the entire powertrain system is installed in the vehicle using the vehicle body and vehicle chassis system, the body and chassis engineers have to work very closely with the powertrain engineers to ensure that all subsystems of the powertrain system can be packaged and installed to provide their respective functions
The powertrain system must be designed to meet requirements of the following three major attributes: (1) performance, (2) fuel economy, and (3) costs Brief descriptions of the three attributes are provided in the following subsection
Attributes of the Powertrain System Fuel Economy The vehicle must meet the US Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration fuel economy and emission standards To verify that the vehicle meets the requirements, it must undergo a series of EPA fuel economy and emissions verification tests including city and highway duty cycles and idling The powertrain can also be tested on a dynamometer and the required EPA tests can be simulated by varying combinations of wheel speed and torque in a test cell
Performance Several methods can be used to test powertrain performance For example, the vehicle can be tested using a dynamometer, road tests, long-term durability
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tests, cold and hot environmental tests under different maneuvers, and so forth A professional driver can be used to conduct the acceleration-time test from 0 to 60 mph on a test track The powertrain can be tested in a dynamometer at combinations of wheel speed and torque over different simulated vehicle maneuvers
Costs The vehicle cost can be based on the manufacturer suggested retail price (MSRP) and benchmarking prices of similar automotive products The powertrain costs can be estimated based on an accepted percentage of the MSRP minus the sum of dealer and marketing costs and the desired company profit The powertrain cost can be also estimated from the manufacturing and/or supplier costs of the powertrain system Generally, developing a cost target of the powertrain from a competitive benchmarking exercise followed by reductions in manufacturing costs to meet target costs is considered to be a reasonable approach
Table 172 presents a tabular chart created to understand the role of vehicle attributes and their subattributes in the development of powertrain system design requirements The table presents all the vehicle attributes in the first column The subattributes of each of the vehicle attributes are presented in the second column of the table And the third column presents the powertrain design requirements developed to support each of the vehicle subattributes shown in the second column The last three columns briefly describe key requirements on the three powertrain subsystems These subsystem requirements are cascaded (or resulted) from the vehicle attributes and their subattributes listed in the first two columns of this table The tabular format is very useful in realizing that the powertrain system needs to meet certain requirements of each of the vehicle subattributes that have a relationship with it The tabular format thus shows traceability of requirements from the product level to the subsystems level
There are several trade-offs between the three major attributes of the powertrain system Higher vehicle performance requires a more powerful engine and higher capacity transmission and drivetrain, which increase the powertrain costs Achieving a higher performance requires a more powerful powertrain system, which results in challenges in packaging its larger and heavier hardware This also negatively impacts achieving a better fuel economy Similarly, greater levels of fuel economy (ie, higher miles per gallon) are also linked to higher costs due to the extra hardware (eg, turbo charger), costly light-weight and/or high-strength materials (eg, aluminum, magnesium, and titanium), more sophisticated engine and transmission (eg, turbo-boost, low-friction bearings, and eight-speed transmissions), low drag aerodynamic body shape, engine start-stop feature, and well-integrated vehicle systems (eg, to reduce weight and increase crashworthiness capabilities) neededTA
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