Study of the parameters for a parallel hybrid electric vehicle

. At present, the expansion in vehicle population has contributed to the increase in carbon emissions and has led to an even greater increase in the need for fuel. In this way, the hybrid vehicle (HEV) and the electric vehicle (EV) are being introduced to reduce fuel consumption and exhaust gas emission instead of conventional vehicles. The study of parameters for the hybrid electric vehicle is important to design the powertrain of the vehicle. The parameters of hybrid vehicles consist of the rate of internal combustion engine (ICE) power, transmission gear ratios, motor maximum torque and power, capacity and power of battery. The vehicle model with the regenerative model is built to calculate the regenerative energy for driving on a road condition. The design purposes to match the vehicle with the specified performances, like as speed and acceleration. Many parameters are chosen from analysis of this results. And then, as a result, fuel consumption of test vehicle model reduces 15% than the conventional vehicle in this study.


Introduction
With global gas emissions and rising fuel prices in the world, automotive manufacturers are searching to reduce exhaust emission and fuel consumption, and with these vehicles like this, more efficient powertrain vehicles are needed to produce [1]. The hybrid vehicle is one of the choices for producing this vehicle because its internal combustion engine with the electric motor and alternative powertrain can able to reduce exhaust emission and fuel consumption. Many automakers and powertrains producers are engaged in projects to develop clearer and more economical hybrid vehicles to meet the requirements [2].
When the vehicles drive in the city, they often move at low speed and need to accelerate rapidly. The average speed of vehicle is less than 35 km/hr and the ranges between two bus stops also are less than 500 meters and the maximum speed of city vans is mostly less than 50 km/hr. Generally, the system of hybrid vehicle should be able to emulate the engine driving at low speed, to recover brake energy, and to stop the engine when the vehicle is in bus stops and waiting for traffic lights [3,4].
(i) In EV function, the motor is only used to drive the vehicle from 0 to a certain speed, if then the function is to avoid high emission and high fuel consumption of engine and clutchslipping at the start process. (ii) If the brake regeneration is larger, the fuel consumption can be more reduced. To increase the brake regeneration, the engine must be disengaged from the powertrain to avoid its friction. (iii) The Stop & Go function enables the engine to shut down when the vehicle is waiting and the engine is idling. This function in city conditions is important because there are too many traffic problems.
For the parallel hybrid vehicle, the four schemes are shown in (Fig. 1). In scheme (a) the motor cannot recover the maximum regenerative energy when the vehicle is braking because its clutch cannot be disengaged for the motor. The powertrain systems of the scheme (b) and (c) are complex and difficult to realize in the vehicle. The mechanical construction in the scheme (d) is not difficult for the vehicle. According to this analysis, the powertrain system in the scheme (d) might be suitable for vehicles. The complex mechanical construction for the series-parallel hybrid vehicle [5,6] has prevented it from being used as its main hybrid topology. The configuration as shown in (Fig. 2) is an improvement for the hybrid series vehicle which installs a clutch between the generator and the motor. Therefore, the vehicle can be driven by the engine in a proper speed range and the overall efficiency can be improved compared with the pure series hybrid because the energy conversion process is avoided when the engine drives. There are also several applications for the series-parallel hybrid vehicle. In the past, the focus has often been on developing control strategies for hybrid design [8,9]. In fact, hybrid powertrain parameters include not only motor and battery parameters, such as engine and motor power rating and maximum torque of electric motors but also gear ratio of the transmission and final reduction. According to the vehicle dynamics theory, the gear ratio is related to the optimum performance of a conventional vehicle, but also affects the regenerative power. The parameters related to the motor and battery should be solved by the hybrid vehicle model.
This study tries to prove a matching strategy or methodology for parallel hybrid vehicle which can solve the matching problem. The city van is chosen for the prototype vehicle because many city vans for transport are used a lot in cities. The requirements and limitations of this vehicle are introduced with a detailed illustration of hybrid models. The parameters of the matching strategy are then described to analyse the influence of each parameter and the control strategy for the hybrid system used in this paper is briefly introduced.

Operation modes of hybrid vehicle system
The operation modes of the parallel hybrid vehicle are described below: (i) EV: normally the conventional vehicle has clutch-slipping effect when it accelerates from 0 to a certain speed, but hybrid vehicle is only driven by the electric motor to a certain speed for avoiding slipping effect.
(ii) Boost: when the gas pedal is fully pressed by the driver, the motor will assist the engine to reduce engine load and keep it running at low load engine speed to improve the fuel economy.
(iii) Pure engine drive: the gearbox often shifts to high gear for medium and high speed and the effective way is to drive the engine directly. In this case, the motor will not work even if battery energy is high enough. However, if the battery energy is too low, the engine should provide to charge it in this case.
(iv) Regeneration: when the driver presses brake pedal, the motor will produce the negative torque to charge the battery. The greater of the motor power is, the more regenerative energy is recovered.

Fig. 3. Operation modes of the hybrid vehicle system
A typical driving pattern of the hybrid vehicle system is shown in (Fig. 3). In real road, the distance of EV mode can be very long in heavy traffic conditions and mode of the pure engine drive can be short because the maximum speed in the urban cycle is often lower than 50 km/h. The electric motor consumes the battery energy in EV and boost modes and this energy is replenished by the regeneration; however, it might cause over discharge of the battery at low velocity as well as small regeneration conditions. The control strategy of the hybrid system should be to avoid using the engine to charge the battery directly due to low efficiency. This means that the engine of a hybrid vehicle should be downsized to avoid excessive fuel consumption to charge the battery in boost mode. However, if this situation happens, the engine will be able to drive the vehicle and charge the battery at the same time. Therefore, the purpose of parameter matching is described as follows: The requirement of drivability in boosting and the potential regeneration energy in braking are the main concern for motor design.
(ii) Engine power rating should meet the power requirements of pure engine drive mode and achieve acceptable dynamics in acceleration without the assistance of the motor.

Engine Matching
Usually, the engine power capacity should meet the power requirement in the maximum velocity. The dynamics factor in the high gear should be greater than 0.04 to overcome road unevenness while driving at high speeds.

Engine Capacity
The power requirement of the hybrid vehicle can be calculated by where -car velocity, km/h; -total mass of car, kg which is the sum of curb mass and cargo mass; -percentage of slope; -acceleration due to gravity, 9.81 kg⋅m/s; -car front area; D -wind coefficient; 0 and f1 -zero and first coefficients of road rolling resistance. The values of the parameters used in equation (1) are shown in (Table 1).
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Average traction power
The average traction power output of the engine will be greater or at least not smaller than the road resistance when the vehicle drives in urban cycles which has many stop & go situations. Otherwise, it cannot able to charge the battery in regeneration mode and eventually, the charging drive will not be avoided. The average traction power is calculated by where -equivalent factor to transform the inertia to traction and other parameters are the same as equation (1). The average traction power is highly related to the average velocity, but it is much smaller than the engine power capacity selected in the previous section. Therefore, the engine power design is natural to meet the requirements of the urban cycle.

Engine torque capacity
If the battery or motor fails, the dynamic characteristics of the mode of pure engine drive should also be considered. The dynamics factor that indicates the acceleration ability and slope-climbing ability is expressed in where ICE -maximum engine torque, Acc -accessories torque, -gear ratio of the transmission, 0 -ratio of final reducer, and -wheel radius.

Electric motor matching
The main function of motor is to drive the vehicle to a certain speed and to assist the engine when the gas pedal is pressed fully, and to generate electric power to charge the battery if brake pedal is pressed. The maximum regeneration power is applied to recover the electricity as much as possible when the vehicle is braking; thus the turning point and maximal power are decided. Finally the electric power should be balanced for the sustainable drive.

Evaluation of regeneration energy
The electric motor will generate the maximum negative torque to recover the kinetic energy during braking. The brake force is calculated by where brake -total brake force, wheel -mechanical brake force, and -negative torque of motor. To achieve the maximum regenerative energy, wheel is set to zero. Therefore, the brake force can be written by The total electric energy can be calculated by = ∫ 0 1145.88 , (6) and the efficiency of motor and battery is set to 90%. Thus the regeneration rate regen, which is the total regenerative energy versus the motive from a certain velocity to zero, is an indicator to value how the power and the basic point affect the brake recovering, derived by

Dynamics Evaluation
The design of electric motor torque is to satisfy mode of the pure motor drive and to assist the engine when the pedal is pressed fully. To overcome the slopes, the factor of the first gear and second gear should be large enough. The factor is shown in where the propelling torque is the motor torque only compared with equation (8). Also as the EV mode will not be used too many in this parallel hybrid vehicle configuration because the motor will only drive the vehicle to a certain speed until the motor speed is greater than idle speed, the acceleration time is not mainly decided by the motor but by the motor and engine together.

Gear ratio of transmission
According to (Fig. 1), the transmission affects the dynamics and fuel economy of traditional vehicles, and it also relates to the hybrid vehicle because the route of power in this parallel hybrid configuration is the same as the traditional vehicle [10].
However for the powertrain of hybrid vehicle as to the analysis before, the dynamics would be improved greatly with the assist of the electric motor. Thus, the main factor in gear ratio design is the only fuel consumption. Usually, the equivalent fuel consumption is calculated by it is used to evaluate the fuel economy of different gear ratios to compare the fuel economy of different gear ratio groups. Where, -equivalent fuel consumption in g/(kw⋅h),density of diesel fuel in g/mm3. For the same diesel engine, is the same and is influenced by and 0 greatly. The relationship of the speed and can be written by and the equation can be rewritten as From the above equation (11), for the vehicles, the first road resistance 1 is very small, so it has been ignored in this equation; therefore equation (11) is rewritten as where 1 and 2 -constant values. From this equation it can be found that the smaller and 0 are, the lower we get for the same velocity.

Battery matching
According to (Fig. 1), the transmission affects the dynamics and fuel economy of traditional vehicles, and it also relates

Electric power matching
The power selection of battery is determined by the maximum power of electric motor as shown in where -battery charging and discharging efficiency, -battery power and The lithiumion battery is widely used in hybrid electric vehicles due to its high power density ( , W/kg) [11]. In general, the charging and discharging power density of the battery are different. In HEV application, the battery pack is often a set of many cells, and for the first, several cells are paralleled together to form a cell module that has greater capacity, and then the cell modules are connected in series to battery pack so as to meet the voltage and energy requirement [12].
The number of cell modules is decided by the motor maximum voltage demand max of the motor as shown in where cell max -maximum voltage of battery cell. The power demand of each cell can be written as = . (15) The number of parallel cells in cell module can be calculated by -motor power. where cell -nominal energy of each cell. Since the charging power density and discharging power density of the battery are different, the charging power density should be used in this equation.

Electric energy matching
The requirement of electric energy will be confirmed by simulation equipment and test on real road. For the hybrid vehicle, the typical European city cycle (EUDC) is used to simulate the electric energy demand. The electric energy is consumed when the vehicle accelerates at high speed. According to previous calculation, the total energy BP of the LMO battery pack is calculated as = .
(17) The SOC using range has been set from 30% to 70%. Thus, the energy of battery pack can satisfy the energy requirement of hybrid electric vehicle.

Comparison of experimental results
For this study, a prototype car is built and then the experiment is conducted. According to the European city cycle (EUDC), the test results of this experiment are shown in (Fig. 4) and (Table 2). The fuel-saving is about 15% compared to the same size vehicle with the conventional engine after adjusting fuel consumption for electric energy consumption.

Conclusions
The methodology of parameter design is effectively proven for the postposition parallel hybrid powertrain. The keys to this methodology are summarized as follows.
(i) Power requirement determines the engine power rating when the vehicle is driving at high speed on normal road, the highest gear ratio must correspond to the maximum speed at high engine speed, and the dynamic requirement should be met by the lowest gear ratio. In order to realize the low engine speed in the urban cycle, the / selection must be smaller.
(ii) The power and torque of the electric motor are related to the potential of regenerative brake, and the proper regenerative rate should be achieved.
(iii) The size of battery should satisfy the requirements of electric power and energy during the specified driving cycle. At last, this parameter matching methodology for a post-positioned parallel hybrid configuration can be easily extended to other HEV types.