While other alternatives have emerged, including plug-in hybrids (PHEV) and the extended range electric vehicle (EREV) subset, full electrics (EVs), hydraulic hybrids in the commercial vehicle sector, and other alternative fuel combustion engine vehicles; hybrids remain the predominant alternative option with the Prius selling more than 100,000 vehicles per year over the last seven years, reaching a cumulative total of more than 1,000,000 vehicles.

As in previous years, several news reports have questioned and examined the value and future of HEVs. Now, as the U.S. enters the second decade of HEV sales, the question is still examined:

“Why do people buy a hybrid vehicle?”

According to the New York Times research compiled by TrueCars.com, only two HEV models (and one diesel) have a fuel and maintenance savings payback for the incremental cost of the alternative powertrain technologies within a five- to ten-year period at current and recent historical high gas prices. Furthermore, payback is dependent on incentives including the federal tax credit up to $7,500.

These analyses are simplified by assumptions about the conventional vehicle purchase choice that consumers are comparing, which is often not the same vehicle in a non-hybridized option (e.g., your purchase choice may not be between a Chevy Cruze and Volt). But there is some evidence that with the current offerings, the market will begin to level off as early adopters, technology enthusiasts, green purchasers, and those with more complex return on investment assumptions become saturated as indicated by recent market studies suggesting only 35 percent of HEV owners are purchasing another HEV for their next vehicle.

Moving forward
To continue growth of the alternative vehicle market some of the strategies that may be considered include:

• Attempting to increase the appeal of the “alternative fuel” segment (e.g. who really needed thousands of songs in their pocket a decade ago, but there’s no denying how much people want that now)
• Driving down the cost of the incremental technology
• Focusing on sectors and technology solutions that maximize the financial payback.

The rest of this article will focus on the financial payback aspect, and the technology and application forces that are driving financial payback in some HEV sectors.

Financial payback and baseline MPG
Financial payback for the incremental cost of an HEV is primarily driven by a reduction in fuel costs. In the case of HEVs, the fuel costs are just the gasoline costs, while electricity costs should be factored in if using a PHEV, EREV or EV. So obviously payback will be function of miles traveled per year and miles per gallon fuel economy of the conventional comparison vehicle.

Consider an mpg-versus-fuel-used-per-year graph at 20,000 miles/year representing a high-mileage application. It is evident that to save an equivalent 500 gallons on a 10-mpg vehicle the HEV fuel economy must be 13.3 mpg, whereas to save the same 500 gallons versus a 20-mpg conventional vehicle the fuel economy must be doubled to 40 mpg.

Relevance to fleets
This type of comparison is particularly relevant to fleet operations. In fleet operations several drivers are at work: specific services need to be performed by the fleet, fuel costs are a bottom line profit margin driver for the business, and interruptions or changes to fleet operations can negatively impact costs. For these reasons, fleet managers typically focus on the purchase of the minimum number of different vehicle models to satisfy the business needs.

Increasing the volume purchases of each vehicle type and reducing the variety typically improves the cost efficiency of maintenance, operation and purchasing for fleets. Naturally when considering developing a “greener” fleet, managers prefer to look for a cleaner, more fuel-efficient version of the vehicles they are already using that serves business needs, that the field personnel are familiar with, and that can provide a reasonable financial payback.

Taking into account the previous discussion, one strategy of fleet managers is to begin by focusing on their lower-mpg vehicles with the highest annual mileage usage. These will have larger fuel cost reductions for the same percentage mpg improvement (or conversely for the same fuel cost reduction they will need a lower percentage mpg improvement).

Right battery pack size
From a technology perspective, the electric powertrain components that enable fuel usage reduction in an HEV are, at their most basic, an energy storage battery, a traction motor and a motor drive that controls the power flow from the battery to the motor and subsequently the vehicle driveshaft during acceleration, and then controls power flow back to the battery during deceleration. There exists now, as the industry has expanded, a wide array of HEV powertrain solutions with multiple motor/generators, clutches, gears and belts, and electrical power converters but in the end, they are primarily putting energy towards the driven wheels to reduce the energy required from the combustion engine.

Given the basic HEV elements, there are two key modes of operation with reference to the battery pack and the energy stored in it: charge depleting and charge sustaining. In a charge depleting controller, the battery energy is incrementally drained during operation with the expectation that the battery will be recharged externally. This is the primary operating scenario for PHEVs, EREVs and of course, EVs. HEVs, by contrast, without the external charging plug, operate in charge sustaining mode where the electric energy depleted from the battery to contribute to propulsion is managed so that regenerative energy returned to the battery during braking is sufficient to maintain the battery state of charge (SOC) within an acceptable band dictated by battery life concerns.

The follow-on design question is then how to size the battery pack for fuel consumption payback. Cost studies including a 2012 report⁶ from the U.C. Davis, Institute for Transportation Studies, support previous works that the initial battery energy transfer during hybrid operation is the best utilized in terms of fuel consumption reduction.

Increasing battery size does reduce fuel consumption but at a lower rate increasing payback. In comparing HEV, PHEV-20 (PHEVs with a 20-mile electric range), PHEV-40, and EVs the study showed the lowest breakeven gasoline fuel price was achieved with the HEV utilizing only a 1 kWh battery pack.

Today, at these current battery costs and with careful design-to-value minimizing complexity and battery ratings, there is real opportunity to introduce HEVs to replace conventional high-mileage, low-mpg, commercial-class vehicles and achieve payback that are not dependent on government incentives. In addition to the fuel consumption reduction simply from the HEV power transfer to and from the driveshaft, payback may also be reduced by other secondary factors such as reduced brake wear and the potential for downsized gasoline engine selection.

There are of course many other design considerations in HEV powertrain design and matching with the best vehicle applications, but fuel consumption payback is a significant criterion with some of the key issues and trends discussed above. These payback trends will all certainly reduce as battery energy costs continue to decrease from above $1000 per kWh price point (for small HEV packs) to below $300 per kWh (for larger EV packs in future years).