IN MY OPINION
HYBRID COMBUSTION-HYDRAULIC SYSTEM
IS MORE EFFICIENT FOR THE CAR’S POWER TRANSMISSION THAN
HYBRID COMBUSTION-ELECTRIC SYSTEM
The state of works under the hybrid power transmission systems is nowadays advanced in regard both to combustion-electric and combustion-hydraulic systems. Intensive research has been undertaken by big enterprises like Toyota, Allison, Rexroth, Parker etc. The results have been implemented to production of new cars. These solutions are considered to be the most competitive of all which exist on the market.
When I started my project in 2003, the state of researches concerned the hybrid drive were significantly less advanced.
Business secret was the obvious reason for limitation of access to the knowledge in regard to realized researches and projects.
Hydraulic pumps with variable work efficiency and reversible functioning (pump-engines) type AVG produced by Rexroth are the key to the construction of the system I have developed. Because such pumps have been commercially available in the last few years (in 2004 Parker did not produce such devices), the pre-prototype was started to be constructed when such pumps had appeared in the catalogue. At the same time I have realized the project of hybrid combustion-hydraulic car’s power transmission system (named recuperative power transmission system) and it was tested in a pre-prototype. I would like to ensure you that such a system is more efficient for city cars than series (connected) combustion-electric systems most often concidered and used in new cars and buses (Lexus, Solaris).
Since You are competent in the discussed problems then I shall start the specification from definitions of elementary/fundamental problems used in further argumentation. Assuming that the definition of “hybrid system” is evident for You, without analysing the structure, car’s hybrid systems are the following types:
SERIES, where both energetic configurations of drive system are connected with the power wheels (possibly with couplings)
PARALLEL, where one of the energetic configuration power transmission systems is connected with the power wheels and the other configuration is not connected
The hybrid system has two energetic configurations:
BASIC – MECHANICAL
BUFFER – in considered cases this is an ELECTRICAL or HYDRAULIC system
In the cars’ power transmission systems hybrid system has the following tasks:
During recovery, the braking electric device or a pump-engine works as a electrical generator/pump charging the batteries
Stabilizing the work of the combustion engine by limiting its service load:
during maximum load the power transmission system is supported by the electric device/pump-engine working as the engine, which is fed with energy from the batteries
in case of combustion engine power surplus during driving with stabilized speed that requires less power drive than the maximum power of the combustion engine, the electric device/pump charges the batteries
The most important differences between parallel and series hybrid systems are as follows:
In SERIES hybrid system - during power feeding by the combustion engine (when it is not declutched in order for the system to work as an electric drive system), it works the same way as in the classic drive system, in full range of loadings forced by quick variable parameters of driving – discordant with the principle of efficiency maximization.
In PARALLEL hybrid system - combustion engine works always in accordance with the principle of efficiency maximization, because its work conditions do not depend on quick variable parameters of driving, but they depends on the state of a buffer system (electric or hydraulic) separated from the drive system by the batteries operation.
Taking into consideration the work of the combustion engine in the hybrid power
transmission system of the car, THE PARALLEL SYSTEM IS MORE
ADVANTAGEOUS (using in RECUPERATIVE drive) and ensures super
efficiency of combustion engine’s work in each situation.
The basic system considered in car transmission power systems is not the subject to be discussed as a principle (although it can have different technical solutions e.g. axles drive equipped with differential gear with the aid of one engine or individual drive of each wheel with the aid of separated engines). The usage of a buffer system is a subject to select. The characteristic of the buffer systems should be submitted for consideration to find the most advantageous for the car transmission power system:
THE CHARACTERISTIC OF BATTERIE EXPLOATATION
POSSIBLE TO OBTAIN POWER TO BE TRANSMITTED IN THE SYSTEM
II.1. in aspect of system’s characteristic
II.2. in aspect of batteries’s characteristic
The parameters of power transmission of the car equipped with the hybrid drive system should be used for the analysis. Hybrid system’s demand for energy accumulation has been established as a reference point:
energy accumulation during common braking
energy accumulation during emergency braking
energy accumulation necessary for dynamic drive in the city with minimization of power of the combustion engine and dynamics obtained from the batteries energy during acceleration
|
THE TYPE OF THE ACCUMULATOR |
NiMH |
Pb |
Hydraulic 40 Mpa |
|
|
UNIT VOLUME |
6 |
12 |
180 |
dm3/kWh |
|
UNIT MASS |
15 |
24 |
540 |
kg/kWh |
|
MAX POWER OF LOADING |
1*C |
3*C |
DEPENDS ON THE PUMP |
kW |
|
MAX POWER OF LOAD |
5*C |
3*C |
DEPENDS ON THE ENGINE |
kW |
|
15*C dla t<5s |
||||
|
NOTICE: C – is the capacity of the accumulator in kWh |
||||
Specified on the basis of professional literature data and calculations – exploiting characteristic of the batteries used in hybrid drive systems are presented in the chart:
The parameters of the vehicles equipped with hybrid power transmission are shown in the chart below. Minimum capacity of the energy batteries is defined as equal to the kinetic energy of the vehicle during the driving with maximum speed. Operating capacity of the batterie:
HYDRAULIC is defined on the basis of the RESEARCHES OF THE CITY CAR PRE-PROTOTYPE and computer calculations of the city bus
ELECTRIC based on PROFESSIONAL LITERATURE DATA (Mercedes Benz, Solaris, Lexus cars)
|
VEHICLE |
mass |
velocity |
power of max. velocity |
exceleration |
common braking |
emergency braking |
|||||
|
max. excelera-tion |
power |
common decelera- tion |
power |
max. decelera-tion |
power |
||||||
|
kg |
km/godz |
kW |
m/s2 |
kW |
m/s2 |
kW |
m/s2 |
kW |
|||
|
CITY |
1000 |
80 |
25 |
3,5 |
78 |
4 |
89 |
6 |
133 |
||
|
50 |
49 |
56 |
83 |
||||||||
|
30 |
29 |
33 |
50 |
||||||||
|
TAXI |
2000 |
80 |
45 |
2,5 |
111 |
3 |
133 |
6 |
267 |
||
|
50 |
69 |
83 |
167 |
||||||||
|
30 |
42 |
50 |
100 |
||||||||
|
BUS |
25000 |
50 |
120 |
1,5 |
521 |
2 |
694 |
6 |
2083 |
||
|
35 |
365 |
486 |
1458 |
||||||||
|
15 |
156 |
208 |
625 |
||||||||
|
VEHICLE |
mass |
velocity |
min. capacity of battery equal to energy recovery of 1 braking from max.velocity to stop |
power of recovery braking |
BATTERY |
||||||
|
kJ |
electric accumu- lator |
hydraulic accumu- lator |
common |
emerge-ncy |
electric |
hydraulic |
|||||
|
kg |
km/godz |
|
kWh |
dm3 |
kW |
kW |
kWh |
kW |
dm3 |
kW |
|
|
CITY |
1000 |
80 |
250 |
0,07 |
8 |
90 |
130 |
15 |
25 |
17 |
60 |
|
50 |
55 |
85 |
|||||||||
|
17 |
120 |
||||||||||
|
30 |
35 |
50 |
|||||||||
|
TAXI |
2000 |
80 |
500 |
0,14 |
17 |
135 |
270 |
20 |
30 |
35 |
250 |
|
50 |
85 |
170 |
|||||||||
|
30 |
50 |
100 |
|||||||||
|
BUS |
25000 |
50 |
2400 |
0,67 |
80 |
700 |
2 100 |
30 |
50 |
200 |
1 200 |
|
35 |
490 |
1 440 |
|||||||||
|
15 |
210 |
600 |
|||||||||
|
REXROTH PUMPS TYPE AVG WITH VARIABLE WORK FLOW |
|||||||||||
|
|
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|
* ELECTRICAL PARAMETERS OF COMBUSTION-ELECTRICAL SYSTEM ARE CALCULATED BASED ON PROFFESIONAL LITERATURE TECHNICAL DATA OF BATTERIES |
|||||||||||
|
** BLUE MARKED DATA OF THE SYSTEM REALIZED IN THE LMK/SZEWCZYK PRE-PROTOTYPE |
A Hydraulic battery has unitary mass, capacity and volume much worse than both types of the electric batteries (NiMS, Pb), but POWER OF ITS LOADING IS OPTIONALLY HIGH. The loading power of an electric battery depends on its capacity, whereas the loading power of the hydraulic one depends only on pump’s power, by which it is loaded.
BASED ON THE PRE-PROTOTYPE RESEARCHES it is sufficient that the capacity of the battery in the car’s hybrid power transmission system is approximatelly 2 times more than minimum capacity. It allows for the use hydraulic battery with low capacity: such system has been used in pre-prototype and enables FULL RECOVER OF BRAKING ENERGY. It has been named RECUPERATIVE POWER TRANSMISSION.
ACCORDING TO ANALYSIS OF REALIZED CONSTRUCTIONS using secondary cells NiMH their capacity is 50 to 200 times higher than minimum capacity (see the chart above). It is the consequence of the necessity of supporting enough power recovery of braking energy. Using the electric batteries with high capacity does not allow for full recovery of braking energy even during common braking with the speed of more than 20km/h for the buses and 30 km/h for the city cars and taxis.
Using the
HYBRID COMBUSTION-HYDRAULIC SYSTEM (also RECUPERATIVE) allows for the
full recovery of common and emergency braking energy, while using
HYBRID COMBUSTION-ELECTRIC SYSTEM allows for recovery of 20 to 35%
of common braking energy and 10% of emergency braking.
SERIES HYBRID COMBUSTION-ELECTRIC SYSTEMS have a configuration forced by the connection between the drive transfer system, engine and driving axle (engine – gear box and clutches unit/system – drive shaft – differential gear axle). This solution determines the possibility of recovery of the braking energy only from one driving axle, if there is no electric drive for other axles installed. It limits the space in the vehicle because the engine must be located in such a way that the drive shaft has access to the driving axle. In electric battery NiMH there is the so called “memory effect” which makes its exploitation difficult. The NiMH batteries need advanced systems of regulation and thermostatic control, which protect the battery against excessive load current and unloading, overload and overheating. In spite of using batteries of high capacity (because of the possibility of recovery braking), the possibility of independent drive as electric vehicle (after switching off the combustion engine) is so limited that this kind of driving is ineffective.
PARALLEL HYBRID COMBUSTION-HYDRAULIC SYTEMS allow to install individual pump-engines at each wheel, it makes a more comfortable/handy layout of the drive system (the connection between the pump-engines and batteries and with other elements of the system is made with elastic hydraulic hoses) and in consequence it helps to take full advantage of space inside the car. In hydraulic battery loading and unloading the batteries do not cause the effects which requires special protective systems (hydraulic batteries have been exploited for many years in industry and in working machines). Low capacity of the battery allows for driving without using combustion engine only at very short distances.
From the
point of view of recovery braking ability, operational reliability,
service costs and operating comfort and the possibility of load space
organization - PARALLEL HYBRID COMBUSTION-HYDRAULIC SYTEMS and also
RECUPERATIVE SYSTEM is better than SERIES HYBRID COMBUSTION-ELECTRIC SYSTEMS.
The series combustion-electric hybrid systems, which do not meet all constructive assumptions, require usage of thermostatic regulated battery modules, converters, mechanical-electric transmission/gear boxes, automatics systems, etc., which are produced by dictating the conditions, monopolistic producers. This solution is expensive in production and difficult in exploiting.
The recuperative system was built by PhD Leon M. Kołodziejczyk thanks to Tadeusz Szewczyk’s sponsorship. It was constructed on the base of the project developed by the author with usage of elements produced by the world-wide, rival, professional firms. The pre-prototype has been built by skilled workers. The pre-prototype has met all requirements – in spite of using the excessive simple solutions in automatics. The prototype and then serial product will required specialized alternative designs of devices, which are presently produced on a mass scale. The recuperative drive is the solution which meets all constructive assumptions and it is relatively cheap and easy in exploitation.
Leon M. Kołodziejczyk, PhD
