TECHNICAL SPECS
CHASSIS
- Steel tubular spaceframe with aluminium differential box and carbon-fibre cockpit
- Sequential manual transmission with flipper paddles on the steering wheel
- Number of gears 6
- 4 wheels Brembo steel brake discs, adjustable brakebalance
- Unequal double wishbone suspension with push-rod, adjustable dampers and ARBs
- Custom magnesium wheels, 13”x6”, 29 mm offset (Fr) / 13”x7”, 41,7 mm offset (Rr)
- Tyres 152x62 R13 R25B Hoosier (Fr) / 178x48 R13 R25B Hoosier (Rr)
- Overall length 2795 mm
- Overall width 1468 mm
- Height 1005 mm
- Whellbase 1650 mm
- Front track 1200 mm
- Rear track 1190 mm
- Weight (with water, lubrificant and batteries) 298 kg
INTERNAL COMBUSTION ENGINE
- Type Kawasaki Ninja 250 R 2009
- Number of cylinders 2
- Total displacement 248 cm3
- Max power 33 Hp @ 11000 rpm
- Max torque 22 Nm @ 8200 rpm
ELECTRIC MOTOR
- Perm pms 150
- DC brushless
- Max power 14 kW
- Max torque 80 Nm
- Batteries Kokam lithium-ion polymer
DESCRIPTION
Drivetrain
System outline. Hybrid powertrain requirements are energy efficiency and adequate power in order to be able to compete with Formula SAE cars. For this reason powertrain design driver is the balance between the performance and energy consumption. For this reason a parallel hybrid solution is developed; moreover parallel hybrid does not need heavy components, like the alternator/rectifier or generator. Other features of the parallel hybrid are the ability to operate with engine alone, electric motor only, or with both motor and engine supplying torque. The electric-only mode is very important to improve energy consumption during stop-and-go operations. The parallel hybrid can respond to the demand for large, near instantaneous changes in either torque or power. Integration of mechanical components, electrical components, and the control software is as important as hardware, if not more so. There are numerous minor functions of the control system as components protection by monitoring battery state of charge (SOC), battery temperature, electric motor overheating, and engine overheating. The control system provides the fail-safe failure modes that yield the limp home capability.
The hybrid system consists of an electrical motor connected directly to the final drive of an IC engine. The electrical energy used for motion is stored in 96V 40Ah Lithium cells battery (27s). The control of the petrol engine is conventional, via an throttle pedal. The control of the electric machine is more complex, and involves driver intervention. To have regenerative braking, a dual purpose motor/generator (M/G) is needed. The same electrical device provides, on command, either motor or generator. Regenerative braking returns a portion of the kinetic energy of HEV motion to the battery. Regenerative coasting, either downhill or slowing speed, also uses the M/G in G-mode to charge the battery. In order not so overcharge the battery this function is actually non exploited.
In a little more detail, the key components are:
IC engine and gearbox. For simplicity and economy this is a standard Kawasaki Ninja 250R motorcycle power unit, a two cylinders 250cc four stroke petrol engine with a six speed gearbox. This unit also has an alternator and rectifier which will be used. Little modifications of intake and exhaust manifolds have been done in order to fit the engine in the car. Fuel, Ignition and A/F target 3D maps were tuned in order to increase power and to adapt to the new intake and exhaust systems. This solution allows a light and powerful stock engine (25 kW @ 11000 rpm).
Electric Motor. This is an ordinary brushless permanent magnet DC motor. The motor is supplied by Perm GmbH and (PMG 150S model). This technological solution of electric motor is the best compromise of power to weight ratio, efficiency and manufacturing costs. The buy approach reduces costs, and allows reasonably simple control. The maximum power is about 12kW at our peak operating voltage of 96V. The electric motor shaft is directly connected to the final output shaft on the gearbox.
Electric Motor Driver. It is supplied by Digital Motor Control GmbH and it is a four quadrants driver that utilizes the Insulated Metal Substrate IMS technology which principle advantage is that cost effective SMD Mosfet power devices can be mounted and soldered directly onto the IMS PCB, which provides immediate and excellent 'integral' heat sinking. Consequently, reliability and efficiency are significantly enhanced due to the power switching devices running cooler and therefore inherently more efficiently. This approach also leads to significantly improved continuous power delivery (1 hour current rating), as a ratio to peak power, with the controllers continuous rating normally being one of the most important aspects in determining the vehicles performance.
Batteries. Considering the high demand of electrical energy a use of well designed batteries is necessary. Main aspects are the safety, a rather lightweight construction and a wide range of possible charge / discharge current. For safety reasons we decided to choose high power lithium polymer cells with a nominal voltage of 3,7 V and capacity of 40 Ah. The whole batterypack contains 27 serial cells.
Battery management system (BMS). The BMS measures the cell and battery voltages, the cell temperatures and the battery current. It uses those values to do its job of managing the battery, and it reports them through the CAN Bus. The BMS measures the voltage of each set of cells in parallel. It determines the voltage of the most charged and of the most discharged sets, and calculates the average cell voltage. It adds all the voltages in a series string to get the pack voltage. The BMS measures the temperature of the cell boards, at each point that is located close to each set of cells in parallel (not the temperature of the cells themselves). The BMS controller calculates the total battery current as the sum of the charger current and the load current. The BMS uses the above measurements to calculate certain parameters: DOD (and SOC), Resistance, Capacity and current limits. It uses those values to do its job of managing the battery, and it reports them through the CAN Bus. Depth Of Discharge (DOD) is a measure of the electricity that is extracted from a battery. It starts at 0 when the battery is full, and increases as the battery is depleted. There is no direct way of measuring the DOD of a battery. No indirect method of measuring is accurate. The BMS controller uses a combination of the two different methods: it uses "Coulomb Counting" to calculate DOD in the middle range of DOD, when the voltage is very flat and doesn't give many clues on the DOD; it uses the voltage method, compensated for temperature and current, at the low DOD, to set a reference point (by calibrating the DOD). State Of Charge (SOC) is the reverse of DOD: it starts at 0 when the battery is empty, and increases as the battery is charged. The BMS manages the battery by balancing its cells, requiring cooling if too hot, and, as a last measure, entering a fault state, during which no battery current is allowed. A balanced battery will hold the maximum possible amount of energy. A balanced battery is one in which all the cells are at the same DOD at some point. Because all the cells have different capacity, it is not possible for all the cells to be equally charged no matter what the DOD. Therefore, that point can be chosen to be either at fully charged, or fully discharged, or somewhere in between. BMS uses the 0 DOD point for balancing, as it is acceptable by all applications. In order to Balance a battery, some charge must be removed from the most charged cells. This BMS uses passive balancing to dissipate some of the extra energy in the most charged cells as heat. Having done so, then the charger is able to put a bit more energy in all the cells. This process is repeated until all the cells are at the same DOD. At that point no more energy is dissipated for balancing. Moreover an on/off line that is driven active whenever the temperature exceeds a threshold. A variable PWM line that is at 0 % duty cycle until the battery reaches that threshold temperature, and goes up to 100% as the temperature reaches the maximum. This line drives a power circuit to drive a variable speed fan. BMS detects any abnormal conditions and sets a fault state. This may be because the rest of the system did not heed the requests from the BMS controller (the battery current is beyond what is specified by the current limits, the battery is excessively charged or discharged, the temperature is excessively hot or cold, a vehicle is on while still plugged into the wall).
Transmission. We adopt a limited slip Quaife helical gears differential that is a frameless differential so we had to design and manufacture the differential frame (two aluminum alloy end bells) and the sprocket flange. These components are made of 7075 aluminum alloy which allows low weight and high resistance. The torque is transmitted by a rimmed hub. The flange and the end bells have a structural anodized coating (20 μm thickness) .The frameless differential advantages are a consistent weight reduction and the freedom in fitting it in the car (center transversal position). A deep FEM structural optimization was done in order to lighten all the components, coping with stiffness and resistance issues. In order to test different gear ratio a set of 4 aluminum alloy split sprockets is set. The sprockets are split in two halves to allow a quick assembling and disassembling. The two halves are retained by two small harmonic steel plates. The final drive ratio is adjustable between 3 and 4. Taylor Race Engineering CVJ (tripods) and axles complete the transmission system; these components are specifically designed for a FSAE application so they are very light and small.
Frame
To increase chassis performances we decided to analyze and to improve the following aspects:
Torsional Stiffness. Frame is a chrome moly steel tube structure, TIG welded. The structure design targets have been weight saving, maximum torsional stiffness without compromise the driver ergonomics. Our introductive FEA analysis show that the most critical regions of FSAE frames are the portions between front hoop and main hoop and the rear portion in back of the engine (suspension and transmission braces). The purpose of reinforcing the structure with a low weight increase has been reached including different functions in two new components: - a carbon fibre structural cockpit: designed to integrate the structural enforcement with safety driver environment and to be the carrier for all the cockpit issues(ECU, battery, seat etc.). - an aluminum alloy rear box designed to integrate the structural enforcement with driveline and rear suspension mounts. The components have been designed using a complete chassis finite element model in order to evaluate all the changes as impact on final vehicle performances. First of all our study moves from an intensive FEA analysis schedule in order to determine which are the most meaningful surfaces and the best way to connect the structural shell of the cockpit to the tube frame. From this preliminary study derives the archetype of our structural cockpit which consists of an internal shell that define the inner driver environment and two external panels that are glued to the internal shell and to frame tubes by epoxy structural adhesive. This kind of solution allows us to create a sort of large resistant section which exploits in the best way the resistance of carbon fibres. Then with an iterative simulation process and a synergic effort with frame design we froze first the geometry of shell and panels and then the lamination layout: antisymmetric lamination of 8 plies (90°,45°,135°,45°) T200 carbon fibre with 5mm PVC core panel, external structural panel made of 4 plies of T200 carbon fibre (90°,45°,135°,45°). The Aluminum Alloy rear box rises from an introductive study aimed to find a solution that best conciliate production cost and technology with suspension and transmission mounts positioning precision. The final result is an aluminum alloy welded plate structure. In order to avoid a manual iterative simulation process the box has been studied and refined by Altair Optistruct Structural Optimization tool. A big effort has been made to maintain all the suspension components linked to the box in order to make easier the drive-train marriage with frame and to limit the suspension mounts deformation during dynamical loads. All the frame, tubes and Aluminum Alloy rear box, are studied in order to support the engine and to reduce stresses on the engine structure: main hoop braces are connected to the rear box and, in the lowest part of the frame, behind the engine, there are two tubes that also connect the main hoop to the rear box.
Weight distribution. Weight distribution surely comes from a series of compromises in the overall design, the required weight distribution (45% on front axle and 55% on rear one) was identified by dynamic simulations (performed with MSC Adams sport car model we refined step by step in parallel with design progress) mix with centre of gravity height as low as possible. Weigh distribution is highly influenced by driver, engine motor and battery position so we worked according to two different strand of design: first of all we move back and lay down the driver, according to ergonomics layout of the driver. Secondly we placed engine in the correct position choosing also to adopt a dry sump solution achieving a further engine CoG lowering of 45mm respect of stock solution.
Kinematic and dynamic suspension behaviour. Suspension design can be divided in two different steps:
• kinematics were studied through a simulation software in order to:
- increase camber gain and roll center height
- reduce roll center lateral migration and bump steer
• dynamic: a second rewiew of suspension geometry made by MSC Adams sport car model in order to increase simulated vehicle performance. A bench analysis suggests us to adopt new shock absorber: Cane creak Db1 200mm free length. Adams model have been very useful to set first value of spring anti roll bars rate, braking balance and static camber. In order to validate and confirm our dynamic model a complete data aquisition system has been installed. Suspension layout has been revised with front and rear push rod in order to reduce loads on bell crank mounts. In order to minimize friction all the bell cranks are assembled by needle roller bearing.
Unsprung masses redesign. The unsprung masses design focuses on each components in order to improve vehicle performances:
• adoption of Magnesium Alloy casted wheels allow a weight saving of 4,8 kg respect normal Aluminum Alloy (flange+ rims) solution. The student designed geometry is the result of intensive FEA schedule aimed to test different configuration in order to obtain the best stiffness-weight ratio
• centre lock nut
• rear hubs now integrate tripod housing with a 0,4 Kg weight saving
• 90mm OD angular contact bearings in order to improve corner stiffness
• uprights (Al 7075 T6) and Hubs (TPR1 steal) configuration have been studied by FEA to determinate correct thicknesses and shapes
Driver accommodation, safety and car-interface
The analysis of our previous race performances, made during introductive design sessions, showed us that in the final endurance laps our drivers were much slower than in the initial ones: they were fatigued. The Sc08 solution to reduce drivers tiredness are: an adjustable carbon fibre seat studied to support driver during longitudinal and lateral accelerations fully integrated with structural cockpit; adjustable pedal box in order to maintain correct leg position; a carbon fibre steering wheel Lcd dash and Rpm indicator led integrated; steering column with double joints and roller bearing support; student designed shift actuators behind steering wheel with integrate clutch actuator. New shifter control rises from an intensive ergonomics and mechanic study. Motion and loads have been calculated to be compatible with an iterative actuation. The cable linkage to gear box has been chosen for its lightness and reliability. The hydraulic clutch actuation is designed to reduce the driver effort and to assure a precise driving feeling
