Chevrolet Volt

An American Revolution

Figure 1: Chevrolet Volt

Deemed an “extended-range electric vehicle,” the Chevrolet Volt debuted at the 2007 Detroit auto show. Although the Volt’s initial cost of around $40,000 is significantly higher than a fuel efficient compact car like the Honda Civic one has to compare the cost of fuel to value the real savings associated with the Volt. Fuel for a Honda Civic would cost the owner almost $2,000 a year in fuel for 15,000 miles; a Volt owner could theoretically drive the same amount of miles for $180-$300 assuming current electricity rates [1]. GM will be able to reduce development and production cost of the Chevy Volt because it shares the same delta platform as the 2010 Chevrolet Cruze and the Saturn Astra. The Chevy Volt is scheduled to go on sale in November 10, 2010.

Peak Oil

Originally suggested by M. King Hubbert in 1956 peak oil is the point in time when the maximum rate of global petroleum extraction is reached, after which the rate of production enters terminal decline. When the decline in the amount of barrels begins we will see prices begin to skyrocket for the price paid for gas at the pump. This will be particularly harmful in America because 77% of people prefer to drive to work alone and our cities are not set up with a good public transportation system to get people to work [2]. All of the major car companies have started to take a greater interest in the rise of gas prices and the inevitable need to look into alternative sources of fuel to power cars. The Chevy Volt is a beginning step in the right direction to having cars that are not dependent on oil.

Figure 2: Hubbert’s Peak Oil Theory Curve

E-Flex Platform

The E-Flex drive system was introduced as an attempt to standardize many components of possible future electrically-propelled vehicles, and to allow multiple interchangeable electricity-generating systems. The initial design as envisioned in the Volt combines an electric motor and 16 kWh (58 MJ) lithium-ion battery plug-in system with a small engine (1 liter) powered by gasoline linked to a 53 kW (71 hp) generator. The initial production Volt will be propelled by an electric motor electric motor with a peak output of 120 kW (160 hp). Ordinarily, the Volt would be charged while at home overnight through one of its two charging ports, one on each side of the vehicle. A full charge reportedly takes 10 hours from a standard North American 120 V, 15 A household outlet.
Since the electrical drive train is not affected by the method used to charge the batteries, several options could be available for acting as an electrical power source. The primary configuration specified in the Volt promotional literature uses a turbocharged 1.0-liter engine with three cylinders, a flex-fuel engine capable of running gasoline or E85 (85% ethanol, 15% gasoline) [3]. Another engine-driven power option would be a pure ethanol (E100) engine, a diesel engine capable of running biodiesel fuel, or even a hydrogen fuel cell, once that technology becomes practical. These other options could possibly be introduced in as little time as two years after the Volt’s debut.

Figure 3: E-Flex Platform

Series Hybrid

Series hybrid vehicles, like the Chevy Volt, are much more similar in design to a battery electric vehicle than an internal combustion. In a series hybrid system, the combustion engine drives an electric generator instead of directly driving the wheels. The Chevy Volt is capable of 40 miles of cruising range on battery power alone, a distance capable of satisfying the daily commute of 75% of Americans [4]. Assuming some form of outlet at work the typical American could get to and from work without using any gas. Regenerative braking improves efficiency by minimizing the losses in the battery. After 40 miles (64 km), the range of the Volt will need to be extended through the use of a small 3-cyl internal combustion engine drives a 53 kW generator. This arrangement creates a sustaining charge current to the HV batteries and permits them to continue powering the 111 kW electric drive motor.

An advantage of a series hybrid is the lack of a mechanical link between the combustion engine and the wheels. The combustion engine can be run at a constant and efficient rate, even as the car changes speed; closer to the theoretical limit of 37%, rather than the current average of 20%. At low or mixed speeds this could result in ~50% increase in overall efficiency (19% vs 29%.) The requirements for the engine are not directly linked to vehicle speed any more, which gives more freedom in engineering.
The power from the combustion engine must run through both the generator and electric motor, so the engine-to-transmission efficiency is 70%-80%, which is less than a conventional mechanical clutch having an engine-to-transmission efficiency of 98%. During long-distance highway driving, the combustion engine will need to supply the majority of the energy, in which case a series hybrid will be 20%-30% less efficient than a parallel hybrid which is the system used in Toyota hybrids.

Battery Selection

The battery in the Chevrolet Volt is one of the most crucial design aspects. The T-shaped, 400 lb battery is designed to sit where the drive train exists in other cars. The battery consists of 300 individual 3 volt lithium-ion batteries wired in series [5]. Lithium-ion technology is currently the best choice for this type of application. These batteries offer outstanding energy density, which means more energy and less weight. Pound for pound, lithium ion batteries can store roughly 6 times the amount of energy as typical lead-acid batteries. Lithium-ion batteries can also handle many charge/discharge cycles, and they do not require a full discharge before recharging can occur. Also, they hold their charge well over extended periods of time, losing only 5% of their charge over one month’s time [6]. And finally, lithium-ion batteries can be easily manufactured in various shapes, making it easy to efficiently fill the T-shaped space.

However, there are a few drawbacks associated with lithium-ion batteries. Immediately after manufacturing, the battery begins to experience a form of degradation. This means that over time, they will slowly begin to hold less and less of a charge. Also, in rare cases, failure has caused batteries to burst into flames [6]. Since the battery is one of the most critical (and expensive) components, a poor life span could deter possible customers. As well, the risk of combustion will not sit well with anyone. These are merely two examples of important battery considerations which Chevrolet must thoroughly investigate, and hopefully remedy.

Lithium-Ion Electrochemistry

The basic form of a lithium-ion battery involves an anode (positive electrode), cathode (negative electrode), and a separator. Each of these three components are ‘sandwiched’ together with the separator in the middle, as its name implies. These components are then submerged in some organic solvent, which acts as an electrolyte. Simply put, the separator keeps the anode and cathode separated, while the electrolyte allows ions to be transferred. Figure 4 shows a simplistic diagram of both the charge and discharge mechanisms.

Figure 4a: Discharge Mechanism Figure 4b: Charge Mechansim

The anode is constructed of Lithium Cobalt Oxide, or LiCoO2, and the cathode is constructed of carbon [6]. During the discharge reaction, positively charged Lithium Ions are transferred from the anode, through the electrolyte, and then bond with the negatively charged carbon. The anode half reaction is seen below as Equation 1, and the cathode half reaction is seen as Equation 2 [7]. During the charging stage the processes are reversed and the lithium ions return to the anode.


Environmental Impacts

Why then, is there so much hoopla about the Volt when hybrids such as the Toyota Prius are already on the market? The key is that the Volt can operate 100% independently of gasoline. All of the internal workings have been modified to function primarily on electricity, with gasoline power as a backup or safety net. This shift of primary energy is a huge step in the right direction, since we can now focus on methods of providing electricity instead of gasoline.

Currently, one might argue that even though the car operates on electricity, we may still rely on coal or other fossil fuels to provide this energy. That means that even though the car’s emissions are significantly improved, the environment still feels the effects through the creation of electricity via fossil fuels. This is entirely true. The problem is not yet solved. However, we are only beginning our endeavor into the renewable energy crisis. By removing automobiles dependence on fossil fuels we open many more doors into alternative fueling methods. Since electricity can be provided in a number of ways, automotive companies will be ready for the day when our primary energy supply is no longer from fossil fuels. The flexibility to run on electricity, which can be provided via solar, wind, geothermal, etc., is a huge accomplishment. So while a fully electric car does not solve our renewable energy crisis, it is a huge and necessary step in the beginning of our journey.



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