The BSG system has to be designed to meet a number of vehicle performance, cost, and timing requirements. The most signiicant performance requirements are listed in Table 10.4. The most primary function of the BSG system is to provide fast auto-start feature during both warm and cold cranking. The selected electric motor/generator ixed on the FEAD to replace the alternator and potentially a starter requires a power rating between 8 and 15 kW to meet the power requirements for auto-start, regenerative braking, electric assist, as well as a small portion of electric-only drive.
The key beneit of the stop/start system is the improvement in fuel economy by minimizing the fuel consumption during vehicle idle periods. This feature will provide automakers with fuel economy credits by achieving a stop/start at each vehicle idle period of the EPA FTP drive cycle with the exception of idle periods soon after a key start. This section discusses the component and system requirements, control strategy, and implementation overview to maximize this beneit.
Other opportunities to further improve the fuel economy of the vehicle, reduce emissions, and ease vehicle integration complexities will also be discussed. There are multiple enabling conditions in a vehicle that deine stop/start function. These conditions are unique to the vehicle and the automotive supplier. Some high-level enabling conditions include:
For auto-stop
• Brake and gear position is valid
• Ambient and coolant (liquid or air) temperatures are within the range for all components
• Battery SOC is above the threshold
• Engine and vehicle speed is below the threshold
• No inhibit
• Onboard diagnostic (OBD) conditions are met For auto-start
• Brake and gear position is valid
• Completed key start
• OBD conditions are met
The approach to stop/start operation will vary based on multiple vehicle features, initially being the transmission system.
TABLE 10.4
Performance Requirements from a 48-V BSG System
Requirement Value
Operating voltage 48 V
Peak power 8–15 kW
Number of auto-starts 350 k–450 K depending on location and drive cycle
Engine start time 300–500 ms
Number of key starts 30,000
Engine start time—cold start at −25°C 1 s—since key start only, longer duration is accepted
Service life 15 years/100,000 miles
346 Advanced Electric Drive Vehicles
10.4.1.1 Automatic Transmission
The system will generally provide automatic engine stops and zero vehicle speeds when the brake pedal is applied and all vehicle enablers have been met. These enablers will be unique to the auto- maker and the vehicle. Releasing the brake pedal will restart the engine. The engine may also restart during the brake condition if the 48-V battery SOC is too low, if it is required by the auxiliary loads, or if the component requirements for a start are at their threshold.
10.4.1.2 Manual Transmission
Unlike in an automatic transmission, the system will implement an engine stop at low vehicle speeds, provided that the transmission is in “Neutral” and the clutch is not engaged. In case the vehicle is geared and the clutch is engaged, the brake pedal must be applied prior to an engine stop so as to prevent vehicle roll. Engine start will be based on both clutch and brake position require- ments unique to both neutral and gear conditions. Similar to the above, other vehicle conditions will also initiate engine start, provided the vehicle safety is met.
Vehicle-level objectives of a start–stop system will be generated by the respective vehicle groups.
They will in general include requirements for
• Vehicle speed at shutdown
• Engine restart time—pedal driven
• Engine restart time—change of mind
• Start/stop vibration
• Noise—inside the vehicle
System-level objectives of a start–stop system will be generated by the power-train team for each unique application. They will in general include requirements for
• Engine restart time
• Cranking time
• Time to engine starting speed (RPM)
• Motor ramp rate
• Power-train jerk
• Power-train vibration
• Power-train-radiated noise Component requirements
• Max torque capability
• Continuous torque capability
• Generating capability
• Slew rate
10.4.2 deSign ChangeSFora bSg SyStem
A BSG system will add components to the base vehicle, such as the inverter, motor, and a 48-V bat- tery pack. However, it must be noted that implementing a 48-V BSG system will also modify, and respecify requirements of some fundamental components. In some cases, these components can be optimized as a result of the functional capabilities of the BSG to further improve the fuel economy of the vehicle or removed reducing the delta cost increase of the system.
347 48-V Electriication
Some of these key component areas that will be impacted by the addition of the BSG system are
• Alternator: The conventional alternator will potentially be removed.
• Accessory drive: Pulleys (alternator-decoupled pulley), tensioners, idlers, belt, and other propulsion system components.
• Controller hardware: Processing capability of controllers and input/output capability.
• Underhood environment: Modiications to the underhood packaging to create space for the motor and inverter.
• Revised wiring: Additional of both 48-V and 12-V wiring and rerouting existing wire harnesses.
• Redesigned underhood coolant system: On the basis of the coolant strategy for the added motor and inverter, either the existing engine coolant loop must be modiied or a new cool- ant system must be added.
• Potential addition of 48-V electric air compressor (EAC).
• Potential addition of 48-V EPS: The current 12-V power- steering systems are one of the largest loads on the auxiliary system. Designing to meet edge-to-edge steering (~50+ A) has driven the DC/DC to be much larger than required for normal operation. By switching to 48 V, the current required can be reduced and a DC/DC converter will not be required to support it.
• Engine mounts: Since the motor and most likely the inverter is directly mounted to the engine, the mounting point location and mounts themselves must be reevaluated for struc- tural and noise, vibration, and harmonics (NVH) requirements.
• Engine housing: Modiications to the engine housing as additional mounting points maybe required.
• Exhaust system: On the basis of the packaging feasibility of the component and coolant strategy, the exhaust may have to be redesigned.
The belt drive of the BSG system should be capable of transmitting the high torque based on the vehicle demands. As opposed to conventional vehicles, BSG requires a bidirectional drive-tension- ing system to cater for the negative torque during regenerative braking. The belt must also be wider and made of material that supports high load and tension.
Components in a typical electriied vehicle are not integrated and implemented in aggressive environments. However, with BSG systems, electriication components are prone to the more extreme environments. The BSG design therefore needs to sustain these harsh environmental fac- tors and based on where the components are mounted, these environmental requirements tend to vary. The two locations for component packaging in the vehicle are underhood or trunk/cabin.
Irrespective of their location, all components of the vehicle must meet the NVH requirements, par- ticularly those that directly impact propulsion. The motor and the power electronics in the case of integrated components are directly mounted to the engine. The vibration of these components over lift and the shock at each start will be extreme. It is essential that any device under speciication does not emit any unwanted, undesirable, whining, disturbing, or annoying noise that a customer can hear during normal vehicle operations over the entire vehicle design life. The importance of meeting NVH requirements for BSG components, speciically the motor, is extremely important.
The motor/generator is directly coupled with the engine during starts and any NVH effect will be experienced by the driver. It needs to be ensured that vibrations caused by an electric motor should not be more disturbing under any operating conditions than a pure combustion engine operation.
These operating conditions cover the entire vehicle speed range and the entire ambient temperature range, including but not limited to, the motoring mode, the regeneration mode, acceleration/decel- eration, slow start-up or wide open throttle, tip-in or tip-out, and so on. Note that tip-in and tip-out refers to engaging (or disengaging) the engine by stepping in (or out) of the pedal.
348 Advanced Electric Drive Vehicles Another environmental factor that needs to be taken into consideration is the high temperature under the hood. The proximity of the components to the engine and the exhaust results in high ambient temperatures above the common 105°C and will be required to qualify testing at 125°C or 150°C. This high-temperature environment and the considerably high-power ratings in BSG com- ponents warrant a suitable cooling system. The components can be either forced air or liquid cooled depending on the design speciications. Typically, air cooling is considered suitable for components rated under 10 kW and liquid cooling for higher power ratings.
10.4.3 deSign ChallengeSand implementation
There are several challenges in designing the BSG system in a cost-effective manner without adding complexity to the process. To ensure an acceptable power density, the packaging of the components needs to be compact. The motor/generator and the power inverter module must it underhood and the power pack unit should it in the cabin/trunk and has to be designed accordingly. A BSG system will add weight to the base car at times, resulting in the vehicle moving up on weight classiica- tions. Component and interface weight must be factored into the assessment of such a system. Key weight factors include the battery pack, motor, inverter, mounting brackets, and wiring. The break- down of extra weight due to additional components or component redesign is given in Table 10.5.
Minimizing this added weight is one of the crucial concerns in BSG design. Moreover, it is highly desirable to develop a global system that can be reused and integrated across different platforms and into various classes of vehicles to make this technology available to a wider consumer base.
As with any new technology, it is important to validate the design prior to implementation. This is conducted by simulation, functional, performance and reliability tests, and studies among others. The usage cycle is one of the most important requirements to validate the design. The usage cycle must comprise of the load of the system (by component) over the expected life of the vehicle. It should also account for environmental proiles and performance degradation of speciic functions where appli- cable. On average, automotive life is deined as the total number of years and miles. However, for a start–stop system, the number of starts over this period, the duration of the start, and the time between starts and the environmental conditions at each event is most important. Assuming a 10-year, 200-km life cycle in Europe, the total number of starts can be calculated as shown in Table 10.6.
OBD requirement applies to all vehicles sold in North America and is a regulatory requirement that must be considered when implementing a 48-V BSG system. The stop/start is a power-train feature that will impact the overall emissions of the vehicle and must thus be compliant.
Tailpipe emission certiication will be conducted with both start–stop active and disabled to cap- ture worst-case values. If the emission delta between the cases is within a provided margin, OBD compliance requirement may be waived. However, a properly designed BSG will not (should not) fall within this range. The BSG will therefore require an indicator on the dashboard to indicate that
TABLE 10.5
BSG System-Driven Weight Analysis
Component /Integration Weight (kg)
Motor, brackets, and cooling ~+8
Inverter, brackets and cooling ~+2
ESS, brackets, and cooling ~+10
DC/DC converter, brackets and cooling ~+5
Engine adaptation ~+5
Interfaces (wiring, coolant lines, etc) ~+8
Removal of alternator ~−8
Total ~30
349 48-V Electriication
the stop/start feature has been disabled for any reason. A typical system will utilize the “Malfunction Indicator Lamp” (MIL) for this purpose. If the BSG system is unable to perform as designed with the functions becoming inappropriately, unintentionally, or accidently nonfunctional, the MIL will serve as the indicator to the consumer. The reasons for this behavior include BSG subcomponents failure, BSG subcomponent fault codes, BSG enablers, inhibit codes, and so on.