Hybrid-Electric Vehicles

Hannu Jääskeläinen

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In a hybrid drivetrain, two means exist to provide the final drive power for the vehicle. While internal combustion engine/electric motor hybrids are the most common, other possibilities include: internal combustion engine/hydraulic, internal combustion engine/pneumatic, internal combustion engine/flywheel and fuel cell/battery electric. Most hybrid drivetrains also contain a means for energy storage (battery, capacitor, hydraulic reservoir, flywheel, etc.) but this is not essential.

Drivetrain Configuration. Prior to about the year 2000, there were primarily two hybrid drivetrain that were commonly discussed, Figure 5. The first is referred to as a parallel configuration in which the heat engine is the primary power source, while the electric motor is used as a power assist. This assistance can include a variety of functions from operating most on-board systems to providing supplemental power for acceleration, hill climbing, or any combination of these functions. The power split between the heat engine and the electric motor is an important parameter in deciding the total powertrain emission level. For instance, if the heat engine can be downsized, there will be ample opportunity for minimizing fuel consumption and emission levels. The second approach is the series hybrid arrangement. Early series hybrids could be more accurately described as “range extended battery electric vehicles” where the electric motor is used as the primary power source and a small heat engine was used as a generator to recharge the batteries and extend the vehicle range.

[SVG image]
Figure 5. Early hybrid powertrain configurations

After 2000, many hybrid drivetrains did not easily fall into either classification. While the variety of hybrid drivetrains is large, most have their basis in one or both of the configurations shown in Figure 5. The reader is referred to the literature for a more thorough discussion [3087][3101].

Hybrid Functions. Hybrid drivetrains can provide a number of functions that can be used to reduce fuel consumption and/or improve vehicle performance. Not all hybrid drivetrains provide all of these functions. Some of the more important functions include:

Start/stop—stopping the engine when the vehicle is providing no useful function (e.g., stopped at a traffic light) can reduce fuel consumption. The maximum fuel savings can be realized by keeping the engine-off period as long as possible. Since there is a considerable delay in starting an internal combustion engine, a vehicle with start-stop functionality and no other means to propel the vehicle other than the IC engine must start the engine a sufficiently long enough time prior to the need to propel the vehicle forward (e.g., triggered by clutch pedal, brake pedal or gear shifter motion) in order to avoid a negative perception from the vehicle operator. With a hybrid vehicle having supplemental energy storage (such as a battery) and a sufficiently large electric motor, the engine can be kept off even after the vehicle starts moving, potentially increasing the fuel consumption savings. Start-stop technologies are discussed in more detail under Idle Reduction Technologies.

Regenerative braking—regenerative braking allows some of the change in kinetic energy required to slow down a vehicle or vehicle component (e.g., the bucket of an excavator or front-end loader) to be diverted to other useful vehicle functions. In many cases, this energy is diverted to supplemental energy storage for later use. However, this is not always the case. In some nonroad equipment, a number of vehicle functions can occur simultaneously – such as vehicle braking and simultaneous boom and bucket operation – that can still allow regenerative braking energy to be put to immediate use without the need for supplemental energy storage.

Supplementing engine performance—some hybrid vehicles achieve a significant portion of their fuel savings through engine designs that have very low fuel consumption (e.g., Atkinson cycle engine used in the Toyota Prius). With some of these engine designs, using them in a non-hybrid vehicle for propulsion would necessitate added cost to the engine to avoid potential vehicle performance penalties. A hybrid drive train can provide additional torque to provide the required vehicle performance while keeping the engine cost lower.

Internal combustion-engine-off operation—operating a vehicle with the engine off is one means to use energy captured during regenerative braking and further reduce fuel consumption. The extent to which this is possible depends on the capacity of the supplemental energy storage system and the power that can be produced by the second drive motor. Depending on how emissions of GHGs or criteria pollutants that do not originate directly from the vehicle are treated, maximizing the potential duration that IC-engine-off operation that can be sustained by a vehicle by supplementing the vehicle’s fuel supply with other energy currencies such as electricity can be beneficial for the vehicle manufacturer. Plug-in hybrid electric vehicles (PHEV) have a relatively large battery whose full charge can only be obtained by plugging it in when the vehicle has access to an electricity supply. In some cases, this strategy could allow larger and more powerful passenger vehicles to more easily meet regulatory limits for CO2 and fuel consumption without the need to sacrifice vehicle performance.

Advanced transmission capability—some hybrid drivetrains can provide significant flexibility in selecting the engine operating condition for a given vehicle power demand. For example, the engine can be operated at minimum BSFC for a given power output, Figure 6 [3086].

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Figure 6. Basic engine operating line for the Toyota THS II hybrid drive

Hybridness. The various functions that can be performed by a hybrid drivetrain depend very strongly on a number of factors including the relative power of the secondary drive motor compared to the internal combustion engine.

One way of classifying electric hybrid vehicles is by the relative power of the electric machine(s) and the internal combustion engine. This is sometimes referred to as hybridness, H where:

H=Sum of power of all electric machinesSum of electric machine power + Engine power (1)

Hybridness thus reflects the ability of the electric machine to propel the vehicle without the internal combustion engine and to recover regenerative braking power. Table 1 provides one break-down of this classification and Figure 7 illustrates the various values H where the different hybrid functions realize their full potential [3087]. In Figure 7, the concept of a plug-in hybrid seems to primarily encompass plug-in hybrids with the IC engine operating as a range extender and may not fully encompass the range of plug-in hybrid options available that have appeared after 2008.

Table 1 also shows a classification of hybrid functionality used by one component manufacturer [3088]. While such classifications are useful in discussing various hybrid powertrains, the reader is cautioned against applying such divisions too strictly. The evolution of hybrid drive technology means that such classifications can quickly become obsolete.

Table 1
Classification of hybrid electric vehicles according to relative electric machine performance
ClassificationHybridnessHybrid FunctionalityVoltage
Micro hybrid<10%Stop/start function
Regenerative braking up to 2 kW
12 V
Micro-mild hybridStop/start function
Regenerative braking 4 to 8 kW
Torque assistance 3 to 8 kW
48 V
Mild hybrid10-15%Stop/start function
Regenerative braking 8 to 15 kW
Torque assistance 6 to 15 kW
110 V+
Full hybrid40-50%Stop/start function
Regenerative braking 20 to 100 kW
Torque assistance 20 to 100 kW
Vehicle capable of electric-only operation for limited range
330 V+
[SVG image]
Figure 7. Available hybrid functions related to drivetrain hybridness

Micro hybrid drivetrains appeared around 2007 and initially amounted to little more than a conventional internal combustion engine with start/stop capability. According to the earlier definition of hybrid drivetrains, they were not in fact true hybrids since there was no second motor to provide power to the vehicle. Early micro hybrids were primarily designed around a 12 V electrical system and even in those cases where the electric machine could be used for regenerative braking; energy recuperation was limited to about 2 kW. An important driver for adopting these 12 V start/stop systems was that they provided an import CO2 reduction over the NEDC cycle for relatively little cost. However, with Europe’s adoption of the WLTP cycle, the benefit of these systems is greatly diminished [3347].

A mild hybrid has a relatively small electric motor to provide traction power and a small capacity battery. It is designed to provide a start-stop function along with a small amount of acceleration power and a small amount of regenerative braking. Start/stop events can be more frequent than with 12 V start/stop systems because the engine can be shut off while the vehicle is in motion and engine restarting is more seamless. However, early mild hybrid drivetrains were based on an electrical system operating around 110 V and the extra cost due to the high voltages did not always justify the relative fuel savings and some manufacturers have abandoned this approach.

In 2011, a number of German car manufacturers announced plans to develop dual voltage 12V/48 V electrical systems for passenger cars. A 48 V electrical system enhances a number of hybrid functions that could be incorporated into passenger cars while avoiding the need to go to higher voltages typical of mild hybrids. This is below the threshold of 60 V, above which special precautions must be built into the electrical system and special training provided to ensure the safety of service personal. The higher voltage allows up to about 12 kW of power assist and enhanced regenerative braking. These and other factors add up to a system that is capable of providing the fuel economy/CO2 benefits of a mild hybrid but at lower cost. For some vehicle manufacturers, these 48 V systems seem to be replacing the previous generation of higher voltage mild hybrid systems. The European switch to the WLTP means that these 48 V hybrids will likely replace 12 V start/stop systems in many applications [3347]. Confusingly, these 48 V hybrid systems are sometimes referred to by a variety of terms including micro-, micro/mild- and mild-hybrids.

A full hybrid provides many of the same functions as a mild hybrid, but to a larger degree. Since it uses a larger electric motor and battery, it can provide a greater amount of acceleration and regenerative braking power. In addition, a full hybrid provides an electric launch, whereby the electric motor can accelerate the vehicle without the combustion engine for small distances. The electric motor can be used to accelerate the vehicle by itself (in pure electric mode) or in combination with the internal combustion engine (for greater acceleration than that provided by the engine alone).

While earlier plug-in hybrid vehicles (PHEV) were primarily range extended battery electric vehicles, they have evolved considerably. Many modern PHEVs have all of the functions and capabilities of a full hybrid but with a larger battery that gives them greater electric-only driving range. In addition, plug-in hybrids have a charge port which can be used to charge the battery externally from electric mains to allow them to have full electric range without having to run the combustion engine. Favorable treatment by regulatory agencies such as the US EPA of upstream GHG emissions associated with the production of electricity used to charge the battery in PHEVs has encouraged a number of vehicle manufactures to include PHEVs in their model line-up. They are particularly appealing for high powered premium vehicles where the extra cost is more easily absorbed and the potential reduction in certification CO2 emissions large. In 2010, the US EPA estimated that the upstream GHG emissions for a midsize electric vehicle were about 180 grams/mile. For a typical midsize gasoline car, the upstream GHG emissions were about 60 grams/mile leaving the electric vehicle with a net upstream GHG emissions value of about 120 grams/mile.

Fuel Economy. Figure 9 shows the CO2 benefit (fuel efficiency) of modern gasoline fueled hybrid vehicles over conventional gasoline and diesel vehicles. In both urban and highway driving, conventional diesel drivetrains provide a 17-20% improvement over conventional gasoline drivetrains. In highway driving, the gasoline hybrid benefit is marginally better than the conventional diesel while in urban driving the hybrid benefit is clearly superior at 40-50%.

[chart] [chart] [chart]
Figure 8. Comparison of CO2 emissions for model year 2013 gasoline- and diesel-powered cars and light trucks over the US FTP and HWFET cycles

Diesel Hybrids. In North America, hybrid vehicle drivetrains received a significant boost in the 1990s through the government-supported Partnership for a New Generation of Vehicles (PNGV). Interestingly, GM, Ford, and Chrysler all developed diesel electric hybrids in pursuit of the 80 mpg goal of PNGV. In Japan, Honda and Toyota developed hybrids based on gasoline engines.

While there are yet no production diesel hybrids in North America, there were a number in Europe. Over the NEDC driving cycle, modern diesel hybrids provide about 18% CO2 benefit compared to the average gasoline hybrid. However, a number of gasoline hybrids appear to be able to match the benefit of the diesel, Figure 9. Given the higher price of modern diesel engines, diesel hybrids appear to be more suited to the premium car market.
Figure 9. Comparison of CO2 emissions for model year 2013 gasoline- and diesel-hybrid electric vehicles over the NEDC cycle

Emissions. While emissions from hybrid vehicles are often assumed to be lower than from comparable non-hybrid vehicles, this assumption is not always supported by evidence. While fuel consumption and GHG emissions are the primary benefits of hybrid vehicles, these benefits can sometimes be overstated and/or come at a significantly higher price than anticipated. Many of the certification procedures used to certify vehicle fuel efficiency were developed for non-hybrid vehicles and may not capture many of the intricacies of hybrid vehicle operation. Some examples include:

With regard to criteria emissions, tests with urban buses have shown that hybrid buses can significantly increase NOx emissions [3089]. One possible reason for this discrepancy is that the certification procedure used to certify heavy-duty hybrid vehicles does not certify the entire drivetrain. In the US for example, the engine used in a hybrid drivetrain is typically certified over the same set of FTP speed and load conditions as an engine used in a non-hybrid vehicle thus making the hybrid certification process poorly reflective of the actual in-use operating cycle.

Applications. Hybrid drivetrains are not limited to light-duty vehicles. They are an attractive option for medium and heavy-duty vehicles and some types of nonroad equipment. They are also being adopted in marine applications where they are best suited for vessels with large variations in power demand, coastal trades and operations within emission control areas. The combination of diesel and electric drives can allow a number of benefits including: engine operation at its most efficient point regardless of power requirement or load, engine-off operation (required in some ports for specific maneuvers) and enabling the use of smaller engines to save fuel (a smaller diesel engine can be used in applications such as ferries where full power may only be required for relatively short periods of time and where shore power can be frequently accessed to charge batteries). The technology was also becoming applicable for some deep-sea shipping segments such as crane operations.