Improvements+to+Hybrid+Electric+Vehicles

toc In the 20th century, vehicles were powered almost exclusively by combustion engines which consumed either diesel or gasoline fuels. These vehicles generate exhaust gases and air pollution which have been negatively impacting the environment since their inception. A proposed solution has been the invention of various types of more efficiently powered vehicles. A hybrid electric vehicle (HEV) is a vehicle which “replace(s) the conventional internal combustion engine with a combination of an electric motor assisting a smaller, more fuel efficient internal combustion engine” [14]. However, the success of hybrids on the market today has been limited. Many consumers feel that hybrids are not worth the [|money they cost]; a recent article proposed that it would take an average owner eight years for their gas savings to equal the extra amount spent on buying a hybrid [20]. Today’s engineers are eager to make hybrids more fuel efficient and increase their popularity. = Methods for Improvement =

The most pertinent methods for improving HEVs are the ones being tested today. Researchers are carrying out simulations and experiments to test the viability of their proposed changes. By comparing their results to those associated with current HEVs, the scientific community is able to form arguments based on the resulting data. Plans for improving the efficiency of hybrid vehicles focus mainly on optimizing braking systems [1,10], making changes to the powertrain [2,6,9,11,12], and finding the most efficient battery type [3,4,5,8].

Braking System
Within vehicles which rely solely on internal combustion engines (ICE), [|antilock braking systems] (ABS) are used in the case of reduced traction between tires and road [15]. Because conventional vehicles only rely on ICEs, the ABS traditionally only had to control the usage of the hydraulic braking system. However, HEVs rely interchangeably on both hydraulic braking and regenerative braking. In many HEVs the lack of an updated ABS means that the regenerative braking system is switched off when the ABS is in control and remains deactivated for a predetermined amount of time after the ABS is switched off [1]. By adjusting the control logic of the ABS so that there is a coordination between regenerative and antiskid braking, hybrids can increase their efficiency. Zhang et al. created their own hybrid ABS and tested it through simulation and experimentation; their results confirmed the effectiveness of changes to the braking control system. With their control logic they were able to prevent wheel locking and increase the state of charge of the battery simultaneously (from 45% to 45.5%) [1]. This research is one of many which demonstrate the possibility of counteracting the battery drain of the ABS without sacrificing the function of the braking system. The main drawback of this method is the complexity of creating the best hybrid ABS. Zhang notes that “research and development of these technologies must proceed together with the continuous collaboration between automotive engineers and control engineers [1]”. While researchers have found that changes to braking systems will yield a valuable improvement to HEVs, there is no established control plan which maximizes the efficiency gain. And until a multitude of control plans are thoroughly tested and compared, no consensus can be formed about the best possible option to be implemented in HEVs.

Powertrain
The [|powertrain] of any vehicle is a term used to describe the key components which generate and transmit power [18]. In hybrids, this includes the electric portion of the power generation such as the electric motor and the battery. The HEV powertrain is widely considered to be one of the most eco-friendly alternatives to ICE powertrains in the short term [2], but researchers are confident there are further improvements which can be made. There are two main types [22] of [|drivetrains] (a subgroup of the powertrain) which are utilized in today’s HEVs [16]. These are the series and parallel drivetrains. Series hybrids power the wheels exclusively through the electric motor, whereas in the parallel the engine and motor work together to generate the power [13]. Many parallel hybrids today have adopted a continuously variable transmission (CVT) in order to compensate for the extra usage of fuel in comparison to the series. CVTs ensure that the engine is constantly operating at an optimal level for the current velocity [2]. However there exists a response lag between when the driver presses on the accelerator and when the input reaches the wheels which decreases the CVTs effectiveness and ultimately the overall efficiency of the vehicle. When Lee et al. tested their own modified CVT which anticipated and sought to correct the lag, they found the result to be a 6.9% experimental increase in fuel economy from the old CVT algorithm during mild acceleration. The fuel economy improvement in a typical urban driving cycle was not nearly as high, with the best result yielding a 2.6% increase [2]. Lee notes that “In actual applications, a compromise between the fuel economy and the acceleration performance is required [2]”. While the CVT improvements are effective, there is again a need for further research in finding the best modification for optimal fuel efficiency. Other control improvements revolve around determining optimal power allocation between the multiple sources within the powertrain [6]. Engineers have called for a global effort to create the best supervisory control system (SCS) but current optimization-based solutions fall short due to requiring an enormous amount of computation and an inability to be applied in real-time. Researchers today seek to investigate the efficiency of the powertrain by producing control maps for overall maximized efficiency which are easy to implement and are read in real-time [6]. A recent evolution of a control map by Shabbir et al. yielded a similar fuel economy to the previous recorded iterations but was “significantly less aggressive” on the battery. While perhaps these control maps won’t do much to lessen the gas consumed when the vehicle is running on the engine, they can increase the life of the battery substantially. And with a possible increase of electric range, newer HEVs can be adjusted so that they rely more heavily on the motor than the engine.

Battery
The selection of an optimal battery in HEVs is imperative but complex. Batteries and ultracapacitors are the most common options for energy-storage systems (ESS) in HEVs today. Fuel cells are also clean energy sources but have limited performance capabilities in HEVs. The main types of batteries being used are Lead-Acid (LABs), NiMH, and Lithium-Ion. LABs are established batteries which are in largescale production today. They are relatively cheap but have a low energy density compared to other lighter battery materials [4]. Many researchers favor the usage of LABs and predict that in the future new developments will make them even better for HEV usage. Garche notes that “the addition of certain types of carbon in a variety of configurations has been shown to provide a very significant improvement in the performance of LABs that are intended for use in the high-rate PSoC duty that is the characteristic of HEVs” [8]. NiMH batteries have an energy density twice that of LABs, are not harmful to the environment, and are recyclable [4]. However, the batteries lose their effectiveness with repeated charge and discharge which makes them less popular. Lithium-Ion batteries have twice the energy density of NiMH [4] and are popular though some suggest their reign is [|soon to be over] [17]. It is clear that there is no perfect battery type for all HEVs, though many researchers have their favorites. Khaligh et al. argue that future hybrids will likely not rely on a single ESS device because they cannot meet all the requirements of an advanced hybrid drivetrain. Instead they suggest that better efficiency lies with a form of a hybrid ESS, one which incorporates a combination of various batteries and capacitors, which will collectively meet the needs of the driver [4]. Newer research suggests that combining energy sources, such as a [|lead-acid and a supercapacitor], is already garnering interest [19]. = Conclusion =

The improvements to HEVs tend to fall into two basic classifications: improvements to the software and improvements to the hardware. The software improvements, i.e. the innovative new control plans and algorithms being suggested, promise small increases in efficiency but have a high complexity which makes them difficult to design. Hardware improvements such as battery and motor changes are more straightforward but their effectiveness tends to be limited to specific driving types. In fact, the effectiveness of the majority of proposed optimizations to vehicles are dependent on the driving cycles the vehicles will face. When HEVs were just being created the technology needed to account for the various driving cycles wasn’t widely available and wasn’t practical. Now, with an increase in technological capacity, researchers are able to generate data for improvements for a multitude of driving patters [7]. And with real-time control capabilities vehicles have the capacity to analyze their own driving conditions and make choices to optimize their performance no matter what driving condition is encountered. The research demonstrates the success of the proposed improvements. However, these improvements need further studying for a consensus to be formed on exactly which new controls will produce the greatest increase in efficiency. These agreements are difficult to form when studies are conducted in isolation. In order to expedite these improvements and make them ready for HEVs as quickly as possible, researchers need to work together and improve upon each other’s designs. = Bibliography =

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