Hybrid-drive prospects

A neat description of the problems of hybrid-drive vehicles has come out of the results of the 3 year HYZEM research programme undertaken by European manufacturers, Fig. 6.2. According to Rover participants¹, controlled comparisons of different hybrid-drive configurations, using verified simulation tools, are able to highlight the profitable fields of development needed to arrive at a fully competitive hybrid-drive vehicle and demonstrate, in quantitative terms, the trade-off between emissions, electrical energy and fuel consumption. Only two standard test points are required to describe the almost linear relationship: fuel consumption at point of no overall change in battery state-of-charge (SOC) and point of electrical consumption over the same cycle in pure electric mode. A linear characteristic representing an ideal lossless battery can also be added to the graph, to show the potential for battery development, as at (a).

Confirmation was also given to such empirical assessments that parallel hybrids give particularly good fuel economy because of the inherent efficiency of transferring energy direct to the wheels as against the series hybrids’ relatively inefficient energy conversion from mechanical to electrical drive. The need for a battery which can cope with much more frequent charge/discharge cycles than one for a pure electric-drive vehicle was also confirmed. Although electric energy capability requirement is less stringent, a need to reduce weight is paramount in overcoming the problem of the redundant drive in hybrid designs.

A useful analysis of over 10 000 car journeys throughout Europe was undertaken for a better understanding of ‘mission profile’ for the driving cycles involved. Cars were found to be used typically between one and eight times per day, as at (b), and total daily distances travelled were mostly less than 55 km. Some 13% of trips, (c), were less than 500 metres, showing that we are in danger of becoming like the Americans who drive even to visit their next door neighbours! Even more useful velocity and acceleration profiles were obtained, by data recoding at 1 Hz frequency, so that valuable synthetic drive cycles were obtained such as the urban driving one shown at (d).

6.2.1 MAP-CONTROLLED DRIVE MANAGEMENT

BMW researchers² have shown the possibility of challenging the fuel consumption levels of conventional cars with parallel hybrid levels, by using map-controlled drive management, Fig.

6.3. The two-shaft system used by the company, seen at (a), uses a rod-shaped asynchronous motor, by Siemens, fitted parallel to the crankshaft beneath the intake manifold of the 4-cylinder engine, driving the tooth-belt drive system as seen at (b); overall specification compared with the 518i production car from which it is derived is shown at (c). The vehicle still has top speed of 180 kph (100 kph in electric mode) and a range of 500 km; relative performance of the battery options is shown at (d). Electric servo pumps for steering and braking systems are specified for the hybrid vehicle and a cooling system for the electric motor is incorporated. The motor is energized by the battery via a 13.8 V/50 A DC/DC converter. The key electronic control unit links with the main systems of the vehicle as seen at (e).

To implement the driving modes of either hybrid, electric or IC engine the operating strategy is broken down into tasks processed parallel to one another by the CPU, to control and monitor engine, motor, battery and electric clutch. The mode task determines which traction condition is appropriate, balancing the inputs from the power sources; the performance/output task controls power flow within the total system; the battery task controls battery charging. According to accelerator/braking pedal inputs, the monitoring unit transfers the power target required by the driver to the CPU where the optimal operating point for both drive units is calculated in a continuous, iterative process. The graphs at (f) give an example of three iterations for charge efficiency, also determined by the CPU, based on current charge level of the battery.

(d)

(e) (f) (b)

(c)

Fig. 6.3 Map-controlled drive management: (a) BMW parallel hybrid drive; (b) parallel hybrid drive mechanism; (c) vehicle specification; (d) ragone diagram for the two battery systems; (e) vehicle management; (f) optimized recharge strategy.

clutch system E-gas Digital motor

electronics

Engine torque 162 Nm (119 lb-ft). 162 Nm (119 lb-ft)

power 83 kW (113 bhp) 83 kW (113 bhp)

MUCEUBM Battery 210 V/35 Ah

ASM 3M

0 20 30 40 50 70 100 200

6.2.2 JUSTIFYING HYBRID DRIVE, FIG. 6.4

Studies carried out at the General Research Corporation in California, where legislation on zero emission vehicles is hotly contested, have shown that the 160 km range electric car could electrify some 80% of urban travel based on the average range requirements of city households, (a). It is unlikely, however, that a driver would take trips such that the full range of electric cars could be totally used before switching to the IC engine car for the remainder of the day’s travel. This does not arise with a hybrid car whose entire electric range could be utilized before switching and it has been estimated that with similar electric range such a vehicle would cover 96% of urban travel requirements. In two or more car households, the second (and more) car could meet 100% of urban demand, if of the hybrid drive type.

Because of the system complexities of hybrid-drive vehicles, computer techniques have been developed to optimize the operating strategies. Ford researchers³, as well as studying series and parallel systems, have also examined the combined series/parallel one shown at (b). The complexity of the analysis is shown by the fact that in one system, having four clutches, there are 16 possible configurations depending on state of engagement. They also differentiated between types with and without wall-plug re-energization of the batteries between trips.

6.2.3 MIXED HYBRID-DRIVE CONFIGURATIONS

Coauthor Ron Hodkinson argues that while initially parallel and series hybrid-drive configurations were seen as possible contenders (parallel for small vehicles and series for larger ones) it has been found in building ‘real world’ vehicles that a mixture of the two is needed. For cars a mainly parallel layout is required with a small series element. The latter is required in case the vehicle becomes stationary for a long time in a traffic jam to make sure the traction battery always remains charged to sustain the ‘hotel loads’ (air conditioning etc.) on the vehicle’s electrical system. Cars like the Toyota Prius have 3–4 kW series capability but detail configuration of the system as a

(b)

Fig. 6.4 Justifying the hybrid: (a) EV traffic potential; (b) combined series–parallel mode.

(a)

ENGAGEMENT DRIVE ROUTE

Electric plate clutch A Engine to motor - starting + generation Manual dog clutch B Motor to final drive

Clutch A + B Motor + engine to final drive Manual lock C Parking lock

Fig. 6.5 The hybrid power unit.

whole is just a matter of cost vs performance. Generally the most economical solution for passenger cars is with front wheel drive and a conventional differential/final-drive gearbox driven by a single electric motor. No change-speed gearbox is required, where the motor can give constant power over a 4:1 speed range, but reduction gearing is required to match 13 500 rpm typical motor speed with some 800 rpm road-wheel speed. This is usually in the form of a two-stage reduction by epicyclic gear trains, the first down to 4000 rpm, and the final drive gearing providing the second stage – typically two stages of 3–4:1 are involved. A change-speed gearbox only becomes necessary in simple lightweight vehicles using brushed DC motors and Curtis controllers. Weight can be saved by using a motor of one-quarter the normal torque capacity and multiplying up the torque via the gearbox.