Motor vehicles – Power – Vehicle has plural power plants
Reexamination Certificate
1999-10-15
2001-01-09
Johnson, Brian L. (Department: 3618)
Motor vehicles
Power
Vehicle has plural power plants
C180S065230, C180S165000, C477S002000
Reexamination Certificate
active
06170587
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to motive power systems for use in propelling vehicles. The invention particularly relates to motive power systems which include an arrangement of two or more power units coupled to a transmission, commonly termed “hybrid” propulsion systems, for road vehicles such as omnibuses.
BACKGROUND TO THE INVENTION
Road vehicles, especially omnibuses, are used for a variety of different types of operations, which may be conveniently categorised as follows:
(a) central business district (CBD) or local school operations, typically travelling up to 100 km/day;
(b) commercial non-transit operations, typically travelling around 120 km/day;
(c) suburban transit operations, typically travelling from 100 to 200 km/day; and
(d) long distance transit operations, typically travelling 400 km or more each day.
Table 1 (overpage) sets out some typical operating parameters for each of these categories. The parameters include the average number of stops likely to be made by the omnibus per kilometre, the hours of operation per day, the opportunities available to replenish the battery energy source, if used, the relative requirement for smooth vehicle operations, the relative importance of energy regeneration and transmission efficiency, and the number of passenger seats. It will be appreciated from the following parameters, that a wide variety of road load environments are encountered during omnibus operations. Such road load environments call for significant flexibility in specifying propulsion systems for these vehicles.
For the purposes of the specification, categories (a) and (b) will be hereinafter collectively referred to as “non-transit” operations, whilst categories (c) and (d) will be hereinafter collectively referred to as “transit” operations. Conventional omnibuses are generally propelled by a relatively high powered compression ignition engine fuelled by diesel. In some cases, typically for non-transit operations, omnibuses may be propelled by electric motors supplied from storage batteries.
TABLE 1
Typical parameters for vehicle operating categories
Commer-
cial
Transit
Transit
CBD
non-transit
School
Short haul
Long haul
Average
12-15
15-20
20
12-20
25-35
speed
(km/h)
Distance
100
120
2 × 35
120-200
400
(km/day)
Stops/km
3-4
3
2-3
2
1-2
Hours/day
8
14
2 × 2
10
16
Daytime
battery
charging -
* Opportu-
No
Several
One
No
No
nity
* Fast
Yes
Yes
No
Yes
No
Smooth
Essential
Important
Not
Important
Not
operation
Essential
Essential
Regenerative
Very Imp.
Important
Very
Important
Not
Important
Important
Transmission
Very
Very
Important
Important
Important
efficiency
Important
Important
Seats
30-45
20-30
45-50
30-45
45
The problems and drawbacks with these propulsion units include, in the case of compression ignition engines, high noise levels, environmental pollution and high fuel consumption resulting from operating at part load or idle for long periods. Omnibuses for transit operations are traditionally powered by diesel engines with power outputs in the range of 140 to 185 kW and typically the engine and transmission have a combined mass of 500 to 800 kg. The engine is usually coupled to an automatic transmission with 4 or 5 gears, with more recent variants including a lock-up torque converter in the top two gears. A half loaded 12 metre omnibus travelling at 60 km/h has approximately 1.8 MJ of energy, this is equivalent to 100 kW of continuous power available for absorption during a relatively slow stop of 18 seconds and 150 kW for a more usual stopping time of 12 seconds. This energy, which might otherwise be recovered, is merely dissipated through friction braking and/or engine retardation braking in prior art propulsion systems. The opportunity for energy recovery also exists during downhill running situations with this potential energy, also normally dissipated in conventional propulsion systems.
The average level road power requirement for CBD operations in dense traffic is about 1.8 kW/t and about 3 kW/t when moving with a velocity in excess of 5 km/h. This power consumption for a full sized bus results in power demands of 25 kW and 40 kW, respectively. This is well below the maximum power demand which, for a 15 tonne omnibus, is of the order of 150 kW. Braking losses are particularly significant during CBD operations, where up to four stops per kilometre are common. There are also noise and air pollution problems attendant with the use of diesel fuel, such as the production of soot which some health authorities state is carcinogenic.
In transit operations, there is generally a high peak:average power ratio which has customarily led to the specification of diesel engine for such applications, because of this engine's combination of constant compression ratio and low pumping losses at all torque levels. These characteristics of compression ignition engines are in sharp contrast to spark ignition engines wherein power output is controlled by throttling the engine intake fluid, thereby reducing the compression pressure (and hence the maximum combustion pressure and the efficiency of the combustion process) as well as incurring increased pumping losses.
Turning to conventional electric vehicles, a significant problem is the low energy density of standard batteries, such as the lead acid type, along with the relatively high capital cost of suitable power electronic systems for implementing regenerative operations. Furthermore, reduced vehicle performance is experienced as the batteries approach a low state of charge. Traction batteries typically possess an energy density of about 100 kJ/kg at a 3 hour rate of discharge, but only about 50 kJ/kg at a 30 minute rate of discharge. Conventionally battery packs in electric vehicles constitute up to 30% of the vehicle mass.
It is extremely difficult to transfer more than 70% of vehicle kinetic energy back into a battery pack of the above mentioned mass. For example, if a vehicle was braked from 60 km/h for 10 to 12 seconds, the electric machine and electronic system would need to deliver 50 W/kg to the battery at an efficiency of perhaps 80% for the electric machine and 85% for the battery, resulting in an overall efficiency of 68%. When braking from higher speeds the efficiencies are worse. Thus an electric drive is not really suited to stop-start operations. Furthermore, the electric machine has to have a sufficiently high power rating in order to be compatible with normal traffic, which requires peak powers of around 15 kW/t of vehicle mass. For example an AC machine rated at 180 kW for powering an omnibus has large losses when delivering the average level road power of 40 kW.
The prior art is replete with examples of hybrid propulsion systems for vehicles wherein a combustion engine and an electrical machine, operating as a motor, are used as propulsion units. U.S. Pat. No. 5,343,970 (Severinsky) describes a typical hybrid arrangement wherein an AC induction motor drives the vehicle at low speeds or in traffic, whilst an internal combustion engine drives the vehicle in highway cruising. The electric machine is supplied by a bidirectional AC/DC power converter and is operable as a generator to charge storage batteries, during braking or from the engine. Both propulsion units may together drive the vehicle during acceleration or hill climbing situations. The Severinsky arrangement is an example of a “parallel hybrid” system wherein the propulsion units can each provide power via a torque sharing device coupled directly to a vehicle's final drive. The specification also includes a useful review of prior art propulsion systems. U.S. Pat. No. 5,562,566 (Yang) is another example of a hybrid propulsion system of this type.
U.S. Pat. No. 5,318,142 (Bates et al.) is an example of a “series hybrid” wherein only one propulsion unit supplies torque directly to the final drive. A further example is disclosed in U.S. Pat. No. 5,515,937 (Adler et al.), which happens to employ individual motors at each wheel in the final drive. As set out in Severinsky the cost, weight and inefficiency limit
Fischmann Bryan
Johnson Brian L.
Transport Energy Systems PTY LTD
Workman & Nydegger & Seeley
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