Effects of Energy Storage Systems on Fuel Economy of Hybrid-Electric Vehicles

Three energy storage systems, namely Nickel Zinc, Nickel Metal Hydride and Lithium ion batteries were simulated on ADVISOR (Advanced vehicle simulator) to determine their impact on fuel economy. ADVISOR, a drivetrain analysis tool developed in MATLAB/Simulink for comparing fuel economy and emissions performance and designed by the National Renewable Energy Laboratory by Ford, GM, and Chrysler was used for the simulations. In choosing the batteries for simulations, only the latest technological advanced batteries of NiZn, Li ion and NiMH were used. The results showed that NiZn battery influence in fuel economy and system efficiency far exceeds the other batteries especially for the combined Powertrain. While a lithium ion battery is seen to be well suited for Parallel and Series powertrains at higher speeds, average values for all drive cycle singles out NiZn as a better performing battery. NiMH showed the worst performance. This confirms NiMH, which is the predominant energy storage system today in the HEV industry, is deficient in advancing the growth of HEV’s.


Introduction
Ever rising crude oil prices and stricter standard emission regulations have put a lot of pressure on automotive manufacturers to produce more fuel efficient and zero emission cars (Cao andEmadi, 2012, Dagci et al., 2015). Developing Powertrain systems for automotive vehicles with higher fuel efficiency and lesser emissions without sacrificing high performance level is an enormous challenge to the automotive industry (Bayindir et al., 2011). By combining benefits of electric vehicles and conventional vehicles, Hybrid electric vehicles are known to produce almost zero emissions, low noise, and faster responses hence are more reliable (Ayetor et al., 2013, Chung andHung, 2015). A hybrid electric vehicle (HEV) is described as one with two energy storage systems both of which must provide propulsion together or independently (Tate et al., 2008, Cuma andKoroglu, 2015). The sources of propulsion have both conventional IC engine or fuel cells and electric motors. There are approximately 40 various viable hybrid topologies each having specific advantages and drawbacks (Yilmaz and Krein, 2013).
All these topologies are somewhat variants of Series and Parallel hybrids.
Generally, parts of HEV include IC engine, electric motor, battery, power control unit, and reduction gear. For Series and Combined types, additional generator is required. Combined types have a Power Split device to split traction power between the engine and electric motor.

Motivation
Presently hybrid electric vehicles have mainly been advertised for their ability to minimize fuel consumption and eliminate emissions (Lee and Hogt, 2010). This has primarily been achieved through addition or improvement of a traction motor/generator which supplements the IC engine. Fuel savings have been recorded mainly where more of the electrical energy is used instead of the IC engine (Carlson et al., 2010). Useful attributes of fuel savings and fewer emissions are contradicted by the overall cost of a hybrid electric vehicle (Parks et al., 2007, Chan, 2007. Even though over the entire life of an HEV the running cost will be lesser, most consumers might not even need a car for that long (Egbue and Long, 2012). If HEV's are to become competitive then drastic reduction in cost must be considered. Batteries contribute significantly to overall cost of the hybrid vehicle; therefore the need for a lower cost yet efficient battery cannot be overemphasized (Offer et al., 2010, Sulaiman et al., 2015. It is confirmed that traction battery is the most critical component of the vehicle and will be the most expensive component in most cases (Sabri et al., 2016, Dai et al., 2015. To further increase the range of electric motor operation thereby minimizing engine use, batteries play a very significant role (Hwang et al., 2015).
Lithium batteries (especially lithium polymer) have been researched extensively and are being considered as the future batteries of HEV. The world record for longest distance travelled on a single battery charge was with Lithium-ion Batteries (Dong et al., 2014). A special 1999 Mitsubishi coupe using Li-ion batteries covered 2124 km (1330 mi) on a single charge. However, Lithium batteries boast of higher specific energies but very high cost (50% greater cost than NiMH) coupled with inability to be recycled (Walker, 2015). It is incumbent alternative batteries for HEV's are considered. This research attempts to assess performance of alternative HEV batteries. Nickel Zinc battery is simulated against Lithium and NiMH. Performance is analysed using the criteria of fuel consumption, energy usage, system efficiency and emissions.

Battery specifications
HEV require batteries that can be recharged as secondary batteries (Castaings et al., 2016). To increase the output voltage, its cells are placed in series. Battery capacity (Ah) gives indication of how long a battery can give a certain amount of current. For a rating of 3Ah and assuming current is 0.5A, it implies such a battery will be able to deliver 0.5A continuously for 6 hours (360mins). Power Density (kW/L) and Energy Density (kWh/L) are used to answer the question of how much a battery weighs (Farmann et al., 2015, Arbizzani et al., 2015. The higher the values the smaller the batteries will be in volume to deliver energy or power (Song et al., 2015). Specific Energy (kWh/kg) and Specific Power (kW/kg) measure the respective values in relation to the weight. A battery of 2kWh/kg will deliver the same energy as 1kWh/kg but with half the weight of the latter. That is, if specific energy is doubled the weight of battery is cut by half. In selecting a battery, the following must be considered: Energy Density, Power Density, Specific Energy, Specific Power, long life, safety, cost, temperature range, Memory Effect and Recycling (Sun et al., 2016).

Methodology
ADVISOR is an advanced vehicle simulation software developed by the National Renewable Energy Laboratory to allow studies of advanced vehicles. Three different HEV powertrains have been modelled on ADVISOR in this work. These powertrains include SERIES, PARALLEL and COMBINED (POWER-SPLIT). Each powertrain is tested with each of the batteries: NiZn, NiMH and Lithium ion. The specifications of these batteries were modelled based on their highest performance and current technological advancement.

Powertrain Specifications
The Toyota Prius vehicle was modelled for each of the three Powertrains. Actual Body Weight=2783 pounds (1398kg)

Parallel Powertrain Architecture
The parallel powertrain architecture used for the modelling on ADVISOR/Matlab is shown in Fig. 2.

Combined Powertrain Architecture
A model of the combined hybrid electrical vehicle is shown in Fig. 4

Parallel Powertrain (UDDS and NEDC)
At an average speed of 19.6mph (31.54kph) the NiZn energy storage system was the most fuel efficient (75mpg) as seen from Fig. 5. The NiZn control system was such that battery output power was more than the other batteries at 3511kJ. As battery power output increases fuel economy is improved hence the results.
Depending on the power demands of an HEV and the State of Charge of the battery, power output must be varied for peak performance and fuel economy. In order to operate in a peak condition, the NiMH gave the least power output resulting in it having the lowest fuel economy (54mpg). Same fuel economy patterns were recorded for the New European Drive Cycle (NEDC).

Parallel Powertrain Fuel Economy for Highway Fuel Economy Cycle (HWFEC)
The pattern for the HWFEC differs considerably from UDDS and NEDC. This time best fuel economy (57mpg) was recorded in the Lithium energy storage system whose control system allows it to give the highest battery power output of 1896kJ ( Fig.6). Thereby easing the power output of the engine and reducing fuel consumption. The control system regulates power delivery and energy storage based upon the state-of-charges (SoC) of the battery. The lowest energy output for the NiZn system show the battery was recharged more often during the cycle compared to the other systems.

Combined Powertrain (UDDS and NEDC)
Fuel economy of the NiZn storage system is 71mpg and it is by far the highest of the energy storage systems. Its high power output (3556kJ) ensures that mostly electrical power is used, thereby lessening the fuel consumption. 48mpg corresponds to the lowest output of 2215kJ for the NiMH storage system (Fig.7). Again, patterns of fuel consumption were the same as for NEDC.

Combined Powertrain Fuel Economy Results for High Way Fuel Economy Cycle (HWFEC)
The patterns remained the same as for the UDDS and NEDC cycle with NiZN again having the highest fuel economy of 77 mpg (Fig.8). Even with the least energy input of 101kJ, it produced the highest indicating a depleting state of charge for the battery.

Series Powertrain Fuel Economy Results for UDDS and NEDC
The best fuel economy favours NiZn storage system whose SoC shows its discharges energy throughout the entire cycle. This is followed by Lithium ion and NiMH respectively. It is also noticed that Nickel Zinc had the least input, but highest output (Fig.9).

Series Powertrain Results for HWFEC
For its high battery power contribution to propel the vehicle, Li ion had the highest fuel economy of 55mpg (Fig.10). The control system is such that the battery state of charge has to be maintained within a limit for optimum performance. The SoC for Lithium shows that throughout the cycle, the battery was discharged to as low as 0.4 state of charge (Fig.11). NiMH on the other hand operated between 0.76 and 0.68 while recharging for the very first 300s hence the low output and low fuel economy (Fig.12). This is because during recharging the engine powers the wheels as well as charging the battery consuming a lot of fuel.

Average Values
For each Powertrain, it is shown from figures 14, 15, 16 that NiZn is the best for fuel economy and overall system efficiency. For the combined type alone, it achieved an average of 68mpg compared to 54mpg and 53mpg for Li and NiMH respectively. Also for almost all the cycles and for all Powertrains, NiMH showed the worst performance.

Conclusion
It is quite obvious from the re sults that the type of energy storage system used on an HEV has a tremendous effect on the fuel economy. It can also be deduced that the type of HEV owertrain and control strategy also affects the fuel economy significantly. Under a highway fuel economy cycle, Lithium showed the best fuel economy and system efficiency. However, average results show NiZn is most preferable for all cycles in terms of fuel economy and overall system efficiency. Also for almost all the cycles and for all Powertrains, NiMH showed the worst performance. It can be concluded that NiMH which is the most used energy storage systems today in the HEV industry is not the best. Today, automotive industries are almost convinced that Lithium batteries are the way to go. This research also showed that the future of HEV