Poor cruise performance of Electric Vehicles (EVs) continues to be the primary reason that impends their market penetration. Adding more battery to extend the cruise range is not a viable solution as it increases the structural weight and capital cost of the EV. Simulations identified that a vehicle spends on average 15% of its total time in braking, signifying an immense potential of the utilization of regenerative braking mechanism. Based on the analysis, a 3 kW auxiliary electrical unit coupled with the traction drive during braking events increases the recoverable energy by 8.4%. In addition, the simulation revealed that, on average, the energy drawn from the battery is reduced by 3.2% when traction drive is integrated with the air-conditioning compressor (an auxiliary electrical load). A practical design solution of the integrated unit is also included in the paper. Based on the findings, it is evident that the integration of an auxiliary unit with the traction drive results in enhancing the energy capturing capacity of the regenerative braking mechanism and decreases the power consumed from the battery. Further, the integrated unit boosts other advantages such as reduced material cost, improved reliability, and a compact and lightweight design.
At present, the batteries, which power the Electric Vehicles (EVs), have a poor energy density in comparison to gasoline. Gasoline has specific energy of 12 kWh/kg; in contrast, an energy storage system based on advanced nickel-metal hydride has a specific energy density of 100 Wh/kg [
In a gasoline-powered vehicle, Internal Combustion (IC) engine drives the brake pump, power-steering pump, radiator fan, and so forth through a belt-and-pulley drive. Meanwhile, accessories requiring intermediate power, such as an A/C compressor, are connected with the engine through a clutch mechanism to regulate their speed. This method of delivering power to accessory is well suited as the engine operates continuously even when the vehicle is at a complete standstill. However, a similar method cannot be utilized for EVs as the traction motor does not operate continuously. Therefore, EVs typically employ multiple motor drives to power continuous and intermediate accessories. The dedicated motor drives provide the flexibility to operate the accessories at an independent torque and speed based on the operating requirements and can be switched off when needed.
Each of the motor drives powering the accessories in an EV has its own motor housing, cooling mechanism (if needed), electronic control unit, and a DC-to-DC converter to step down the battery voltage as required. Though these drives provide easy control over the driven system, the added component count contributes to the overall cost of an EV and decreases the net reliability. In addition, these drives could only obtain the electrical power from the battery. Therefore, when an EV operates in recuperation mode, the forward momentum of the vehicle has to undergo a number of energy conversation processes before the energy is made available to the accessory motor drives.
This paper analyses a novel design solution geared towards improving the overall efficiency, reliability, and cost saving by integrating the A/C compressor drive with the traction motor. The integrated unit is expected to operate close to 100% efficiency during recuperation mode. The unprecedented improvement in efficiency is achieved through the direct mechanical coupling of the traction motor with the A/C compressor during braking events. The mechanical configuration of the unit is such that the torque and speed characteristics of traction and compressor motors can be independently controlled during drive mode. In addition to improved efficiency, the integrated unit boosts numerous other advantages such as compact design and weight saving.
However, the key shortfall associated with this method is that the kinetic energy of the vehicle has to be first converted to electrical energy prior to being routed to the battery. As with any energy conversion process, the flow of energy from one domain to the other will incur losses due to inefficiencies. In addition, when the recuperated stored energy is drawn from the battery, it once again has to be converted from electrochemical energy to a suitable form required by the point of utilization. Therefore, the key in improving the efficiency of the regenerative braking mechanism is to eliminate the energy conversions by direct and immediate utilization of the captured energy. The paper first investigates the advantage of integrating an auxiliary unit with the traction drive and then simulates a test vehicle in Simulink® to predict the reduction in battery energy consumption attained by the integrated unit. Further, the paper provides a practical design solution demonstrating the feasibility of integrating an A/C compressor with the traction drive.
On average, an automobile is driven for 250 hours per year [
Farrington and Rugh [
Impact of auxiliary electrical load on EV’s range for various drive cycles [
In another study [
Impact of A/C on i-MiEV’s cruise range [
Su and Hsu [
(a) A conventional configuration of traction and compressor motors controlled by independent inverters [
Apart from electrical integration, the traction and compressor motors can be physically integrated for a number of other reasons. The advantages include the following: Elimination of independent cooling systems for each motor. Minimisation of external electrical cable. Weight and space saving as the motor housing can be shared. Reduced parts count and increased reliability.
These advantages are attractive for the automotive industry as they enable the EVs to operate at higher efficiency, improved reliability, and decreased maintenance and manufacturing cost.
When a vehicle brakes, the kinetic energy of the vehicle has to be absorbed to bring the vehicle to a stop. In a conventional vehicle, the brake pads, frictional resistance (traction), and aerodynamic losses help in dissipating the kinetic energy. However, in a vehicle fitted with a regenerative braking mechanism, the bulk of the kinetic energy is captured and directed through a number of devices before being stored in the battery. Figure
Energy flow diagram during regenerative braking.
With reference to the energy flow diagram (Figure
This section models the velocity of the EV and the forces at work during a braking event. The specifications of the vehicle to be simulated are tabulated in Table
Simulated vehicle parameters.
Category | Parameter | Value |
---|---|---|
Vehicle | Vehicle type | Midsize EV |
Vehicle mass | 1450 kg | |
Tyre radius | 0.20 m | |
Coefficient of friction between tyre and road | 0.0015 | |
Coefficient of drag ( |
0.30 | |
Projected frontal area | 0.60 m2 | |
|
||
Traction motor and transmission | Traction motor rated power | 35 kW |
Motor based speed ( |
2000 RPM | |
Transmission ratio ( |
6 | |
Transmission type | One-speed fixed gear | |
Transmission efficiency ( |
0.90 | |
|
||
Electrical accessory load | A/C compressor motor | 3 kW |
Figure
FBD of a vehicle in a regenerative braking mode under constant acceleration.
The vehicle speed can be described by
Adopting a noninertial frame of reference, the forward inertial force (
The four forces countering the forward inertia of the car are the traction, aerodynamic drag, frictional braking, and the regenerative braking force created by the motor functioning as a generator. The aerodynamic drag and the regenerative braking force are functions of vehicle’s instantaneous velocity while the traction force remains constant.
Assuming the vehicle’s weight is evenly distributed among the four wheels, the net rolling resistance due to the friction between the tyre and road is given by
Motor torque-speed curves, where
The regenerative torque acting on the vehicle’s wheel is given by
The motor speed can be calculated from the vehicle speed using
Variation of inertial and regenerative braking, braking, and tractive and drag forces as a function of time.
Analysis of Figure
An alternative method to increase the absorption of the recuperated energy while using a less power motor is addition of an auxiliary (AUX) regenerative load. The auxiliary unit can be clutched with the traction motor during braking events to increase the regenerative load. Literature review uncovered that the A/C unit requires substantial amount power (about 2-3 kW) and operates nearly 50% of vehicle utilisation time [
Two independent simulations were performed to study the implications of adding an auxiliary regenerative load during a braking event. Figures
Comparison of regenerative braking with and without the AUX load of 3 kW.
|
Regenerative load |
|
|
| |
---|---|---|---|---|---|
Without AUX | −0.66 g | 35 kW | 118 kJ | 425 kJ | 0.278 |
With AUX | −0.66 g | 38 kW | 128 kJ | 415 kJ | 0.308 |
Variation in the braking forces (
Figure
Decreasing the regenerative braking force by varying the transmission ratio.
Simulink has been used to study the reduction in battery energy consumption achieved by integrating the A/C compressor (or any auxiliary unit) with the traction drive. For the study, a MATLAB® based computer program, Advanced Vehicle Simulator (ADVISOR) [
The graphical setup in Simulink for predicting the efficiency improvement.
With reference to Figure
When the energy flows through path 2 (regenerative braking), it does not undergo any form of transformation as the mechanical energy arriving at the motor shaft can be channelled directly to drive the A/C compressor. Therefore, this energy flow path is assumed to be free from any losses. Any excess power generated during the regenerative braking will be delivered to battery through energy flow path 2 for later utilization. However, in this case, the mechanical energy arriving at the motor shaft has to be first converted to electrical energy by the motor and subsequently to electrochemical energy to be stored in the battery. These energy transformations will incur losses, which has been accounted for by the introduction of
An electric generator converts the mechanical energy to electrical energy with a typical value of 95.0% [
A drive cycle is a schedule that governs the vehicle’s speed as a function of time, which is essentially a mathematical representation of a road. It is a key input for vehicle simulation as it facilitates producing a repeatable experiment for performance comparison. The paper simulated the test vehicle using five EU legislative cycles listed in Table
EU legislative cycle analysis.
Drive cycle | Distance | Total time | Braking time | % braking time |
---|---|---|---|---|
ECE 15 | 1.0 km | 195 s | 41.2 s | 21.1% |
EUDC | 7.0 km | 400 s | 48.0 s | 12.0% |
EUDC, low | 10.6 km | 1224 s | 103.0 s | 8.4% |
ECE 15 + EUDC | 10.9 km | 1225 s | 201.8 s | 16.5% |
NEDC | 10.9 km | 1184 s | 204.3 s | 17.3% |
For the purpose of discussion, consider the test vehicle to be operated under ECE 15 + EUDC drive cycle. The scheduled velocity versus time for the stated drive cycle is shown in Figure
Simulated vehicle’s (a) velocity and (b) regenerative power on ECE 15 + EDUC drive cycle.
The A/C compressor load has been superimposed over Figure
Figures
Excess regenerative power directed to the battery.
The reduction in battery energy consumption attained by integrating the A/C compressor with the traction motor was accessed by simulating the test vehicle in the EU legislative drive cycles. During the simulation, the vehicle dynamics block was driven by the selected drive cycle and the energy consumed from the battery was measured for the two distinct cases:
Percentage saving in battery energy for a test vehicle simulated in EU drive cycles with the A/C compressor operated using a combination of compressor motor and clutch.
With reference to Figure
Figure
Concept of the integration of an air-conditioning compressor into the drive motor of an Electric Vehicle.
Cabin air conditioning and long cruise range are the key features required to increase the market penetration of EV. However, studies reveal that cabin air conditioning has an immense impact on the range performance of the EV. Adding battery capacity to compensate for the poor range performance is not a practical solution as it inadvertently increases both the structural weight and the capital cost of the EV. Literature review identified that the cruise range of an EV can decrease by 22 to 48% depending on the A/C usage. The review coveys the necessities to identify means to enhance the efficiency of A/C to mitigate its impact on the cruise range of the EV. The review also identified that integrating the compressor and traction drive decreases component count and material cost and improves the reliability.
When a conventional car brakes, the forward momentum of the car is primarily dissipated as heat energy in the brake pads. Undoubtedly, this not only results in energy wastage, but also expedites the wear rate of the brake pads. To counter this, hybrid vehicles and EVs are fitted with regenerative braking mechanism. This paper set out to improve the regenerative braking mechanism by integrating an A/C compressor with the traction drive. The objective of such an integration is to improve the energy recovering capacity of regenerative braking mechanism and reduce the energy consumed from the battery. Based on the analysis performed on a midsize EV, it was shown that the energy recovering capacity of the regenerative braking mechanism improves by 8% when a 3 kW A/C compressor acts in unison with a 35 kW traction motor during braking events. In addition, subsequent simulations showed that the power consumed from the battery reduces by an average of 3.2% due to the elimination of energy conversion losses.
A design solution fulfilling the requirements of the regenerative braking mechanism explored by the paper was discussed. The design integrates the A/C compressor, the compressor drive motor, and the traction motor into single housing. This setup facilitates the traction motor to physical clutch with the A/C compressor during braking events and facilitates the immediate utilisation of the captured energy without the necessity for any energy conversion processes. Though the compressor and traction motors are contained in single housing, the interference between these motors is eliminated through the usage of coaxial shafts. This mechanical configuration permits independent speed control over the drives during normal operating modes of the vehicle. The paper also identified the rotary piston compressor as a viable choice for the proposed motor design.
Acceleration (m/s2)
Frontal area (m2)
Coefficient of drag
Energy (kJ)
Force (N)
Gravitational constant (m/s2)
Mass (kg)
Revolutions per minute (RPM)
Power (kW)
Transmission ratio
Radius (m)
Time (s)
Torque (Nm)
Velocity (m/s).
Efficiency
Coefficient of friction
Air density (kg/m3).
Base speed
Generation
Max
Initial
Inertial force
Drag force
Tractive force
Weight
Regenerative braking force
Transmission
Brake force.
The author declares that there is no conflict of interests regarding the publication of this paper.