Supporting subsystems

It should be understood that hybrid vehicles require electrically augmented steering, braking and climate control systems. The vehicle steering system must be full electric assist, or electric over hydraulic, as a minimum to ensure that steering boost is avail-able even with the engine off, regardless of the vehicle at rest or in motion, and simi-larly for the brakes since engine vacuum is not available during idle-off mode. In fact, some mild hybrid implementations use separate electrically driven vacuum pumps for the brakes during engine off periods. Cabin climate control is the most energy intensive engine off load. The following subsections elaborate on each of these topics.

4.7.1 Steering systems

As a general rule of thumb, when a vehicle steering mechanism rack load exceeds about 8 kN, a low voltage, dc brush motor, electric assist may be inadequate for acceptable steering boost performance. The range of rack loads from 8 kN to roughly 12 kN defines a transition during which 14 V electric assist must give way to 42 V PowerNet systems. The low voltage 14 V power supply is not adequate to source the instantaneous power demanded by steering systems having high rack loading. Above 12 kN of rack load, regardless of vehicle type, the electric assist steering is best served from a 42 V PowerNet vehicle power supply.

Battery EVs will generally operate their electric assist steering from the traction battery. However, this requires attention to high voltage cabling and proper circuit protection. For distribution voltages greater than 60 V, it is accepted practice to contain high voltage cabling within orange jacketed sleeves or to use orange colored cable insulation.

4.7.2 Braking systems

A hybridized vehicle does not inherently require electric assist (electro-hydraulic or electromechanical) brake gear. Vehicle operation can be maintained in hybrid mode with conventional foundation brakes, but energy recuperation will fall significantly

(a) Hydraulic electronic control unit (b) Actuator control unit

Figure 4.46 Electro-hydraulic brake system components

short of expectations. Even grade holding does not require any special brake sub-systems. Some mild hybrid vehicles rely on simple electric driven booster pumps to maintain brake line pressure to hold a grade.

When performance is required it is common to implement electro-hydraulic brakes, EHB, in order to offer optimum energy recuperation, grade holding and vehicle stability. An electro-hydraulic brake system consists of two main compo-nents: (1) a hydraulic electronic control unit (HECU), which replaces the production ABS unit (pump, accumulator and pressure modulators); and (2) an actuator control unit (ACU), which replaces the conventional master cylinder and booster assembly.

Figure 4.46 illustrates some typical HECU and ACU hardware that constitute an EHB system.

In Figure 4.46 the ACU consists of a conventional master cylinder, a reservoir, plus brake pedal pressure and speed sensors. The HECU houses the motor-pump, an accumulator, valve body to regulate line pressures, and electronics to control the valve operation. It should be appreciated that during the first pressurisation of the HECU accumulator, hydraulic lines between the motor driven high pressure pump and accu-mulator may become very hot until the accuaccu-mulator pressure builds up sufficiently so that fluid flow is reduced.

In addition to providing full regenerative brake capability, the EHB system also maintains proper front–rear brake balance, provides ABS functionality when commanded, and is fully compatible with all vehicle stability programs. Vehicle stability programs were discussed in Chapter 3, Section 3.

4.7.3 Cabin climate control

Actively controlled air conditioning is a necessity in hybrid vehicles. Cabin climate control ranges from cold storage boxes, such as the cold storage unit used in the pro-totype ES³environmental vehicle build by Toyota, to hybrid drive air conditioning

compressors. A hybrid drive air conditioning compressor unit consists of the conven-tional A/C belt driven compressor plus a clutch mechanism and linkage to a separate electric motor and controller that is used to drive the pump when the engine is off.

In such a system a brushless dc motor rated 1.5 to 2.0 kW at 42 V is used to maintain cabin cooling during idle-off intervals.

A/C compressors used in hybrid vehicle climate control systems are of the two stage, rotary vane, variable displacement type. When the A/C compressor is engine driven the displacement is highest to provide sufficient coolant flow to the passenger cabin evaporator assembly during cabin temperature pull-down. When the A/C com-pressor is brushless dc motor driven the displacement is lower, since only 1.0 to 1.5 kW of drive power is needed to maintain cabin temperature within the comfort zone.

4.7.4 Thermal management

Managing the thermal environment within the complexity of a hybrid powertrain requires close attention to package locations, air flow patterns and vibration modes.

Bolting modules directly to the engine or transmission has historically been a very challenging if not a daunting task [22]. The vibration levels alone on the powertrain can reach magnitudes of 20g peak over a broad frequency spectrum. Temperature extremes on the high end can reach 115^◦C on the transmission to 150^◦C on the engine (exclusive of exhaust bridge and manifold areas) with a potential to reach 175^◦C for underhood packaging that restricts air flow or creates air dams. It is this simultaneous temperature plus vibration regime that dictates the durability of electronic modules in the automobile. Given a service life requirement of 6000 hours it is no wonder that few modules are packaged directly on the powertrain. Figure 4.47 illustrates schematically the various regions of temperature and vibration extremes.

The temperature and vibration extremes illustrated in Figure 4.47 are sufficient to shake conventional electronic assemblies to pieces. Today’s electronic modules are fabricated with very low mass, surface mounted devices (SMD) plus chip and wire on ceramic substrates, to tolerate such conditions. Vibration transmitted along

Temperature,°C Simultaneous temperature and vibration

Test level

In cabin

On transmisson On engine

Vibration level,

g 30 20

5

–30 0 25 65 85 115 125 150

Cabin/engine compartment

Figure 4.47 Powertrain package environmental zones

Vehicle body/chassis:

–30°C to + 65°C with vibration levels of up to 20g peak over 200 Hz to 1 kHz

Engine:

–30°C to + 150°C with vibration levels of up to 20g peak over 200 Hz to 1 kHz

Powertrain:

–30°C to + 115°C with vibration levels of up to 20g peak over 200 Hz to 1 kHz

Cabin/trunk:

–30°C to + 65°C with vibration levels of up to 5g peak over 200 Hz to 1 kHz

Figure 4.48 Vehicle thermal environment by zone

Table 4.19 Thermal environment conditions

Condition Unit Value

Vehicle speed kph 48

Cooling fan speed rpm 2100

Ambient pressure kPa 101

Ambient temperature C 43

the vehicle powertrain originates from the engine itself due to misfire (now very infrequent) to pre-detonation due to improper timing and/or improper fuel blends, to engine hop due to its moving components. Resonance can also play a role, but these tend to be at low frequencies in the range of powertrain bending and engine hop.

Higher frequencies are generated by crankshaft whirl due to imbalance and journal bearing wear-out. Figure 4.48 summarises the automotive temperature and vibration environment by zone.

Modeling and simulation of the powertrain thermal environment along the cen-treline of the vehicle is shown in Figure 4.49 for a vehicle under the conditions listed in Table 4.19.

In Figure 4.49 it is clear that hot locations include those in close proximity to the engine or radiator (vertical hot zone) plus all zones where air damming is prominent – for example, on surfaces where air flow is blocked and flow restricted such as in front of the engine, on the outside surface of air induction components, between the lower portion of the radiator and front of the engine block, and along the engine compartment bulkhead. Also evident are good package locations such as up front in the vicinity

80°C

67°C

67°C 43°C

92°C

Figure 4.49 Underhood CFD thermal mapping (along plane through vehicle centreline)

of the headlamps and also around the cowl top. Packaging of high replacement cost components in crush zones such as areas immediately behind the front bumper or headlamps is not recommended. The cowl top area (where the windshield wiper linkages reside) and in locations above the powertrain in the air induction component areas also appear benign.

Thermal mapping is performed using computational fluid dynamics (CFD) using colour gradients to identify hot zones. In Figure 4.49 ambient air enters at the vehicle grill and exits beneath the chassis. Hot zones occur at radiator coolant inlet (bottom) and near coolant outlet (top) as shown. The high temperature zone from the lower radiator to front of the engine represents the thermal load of both engine coolant and air conditioner condenser. The engine compartment air wash beneath the powertrain is also evident. Air wash beneath the vehicle flows generally from the driver side to the passenger side due to ram air plus engine cooling fan patterns. The remaining area to note is the zone in front of the bulkhead and cowl top. Along the vehicle centreline the temperatures here are higher than along the sides such as by the front shock towers near the cowl top. Typically this zone is used to package the vehicle battery and/or electrical power distribution boxes.

Trends in product integration continue to drive actuator power processing to the actuator itself with control and intelligence located remotely to eventually becoming distributed in the vehicle’s communications and control architecture. At the present time thermal design and thermal management remain the most significant barriers to power electronics reliability. Nearly all vehicle installations of power electronics for traction and electrification of ancillaries rely on liquid cooling systems such as shown in Figure 4.50.

Notable exceptions are novel two phase, or boiling pool, cooling systems that rely on complete immersion of the power chips in an evaporative bath. The process