The future of hybrid vehicles

Jan. 1, 2019
The electric vehicle was first on the automotive scene but was overrun by the affordability of its gasoline-powered nephew. Is that trend changing today?

At the turn of the 20th century the idea that automobiles, a new technology at the time, had to be powered by gasoline wasn’t a given. Inventors of these vehicles experimented with various ways in which cars could be powered including electricity, fossil fuels, steam and/or combinations of these power sources. In 1898 Jacob Lohner, a coach builder, teamed up with Ferdinand Porsche who had recently invented the electric wheel-hub motor. The motor fit inside the wheel’s hub and was powered by lead-acid batteries. Using one of Lohner’s coaches, at the time a more common term than “car,” Porsche fitted two wheel-hub motors and a battery to create an all-electric vehicle —  the Elektromobil.

(Photo courtesy of Porsche Museum) The Lohner-Porsche "Mixte" touring car from 1903. Ferdinand Porsche is at the wheel. The wheel hub motors can be seen mounted to the front wheels

It suffered from the same problem that electric cars face today — limited range due to battery technology. Porsche added a gasoline-fueled, internal combustion engine that ran a generator to charge the battery making the Elektromobil the first vehicle to combine these power sources and thus the first hybrid. With the batteries fully charged it could reach the blistering speed of 38 miles per hour. This early hybrid was shown to the public at the 1900 Exposition Universelle, held in Paris, which showcased innovations like the Ferris wheel, diesel engines, talking films and escalators.

Within a few decades other hybrid vehicles came into existence—one traveled on land and the other underwater. Invented in the 1930s, diesel/electric locomotives used a diesel engine to drive a generator that provided power for an electric motor connected to the locomotive’s wheels. This technology led to an early from of regenerative braking, called rheostatic braking where the electric motors reversed their function and became generators driven by the weight of the train slowing down. The electricity produced was connected to on-board resistors (braking grid) that dissipated the braking energy as heat. This process is similar to regenerative braking used on modern hybrid cars to charge the battery.

Hybrid cars like the Toyota Prius offer drivers the choice of using gasoline or electricity as a power source. Future hybrid technology will offer increases in efficiency for electric motors, batteries and gas- or diesel-powered engines.

Another early hybrid vehicle was the diesel/electric submarine that came about in 1929. A diesel engine was used to propel the submarine and to charge large batteries. The sub used battery power when submerged and switched back to diesel propulsion when on the surface. Today a nuclear powered submarine can be considered a hybrid in that it uses a nuclear reactor, steam turbine, generator and electric motors to provide propulsion. Today we refer to a car or light truck that uses more than one power source as a hybrid. Typically these vehicles combine gas, or a diesel-fueled internal combustion engine, with a battery-driven electric motor.

Since 1900 a few hybrid cars were created but it wasn’t until 1997 when the Toyota Prius was introduced in Japan that the technology took off. Toyota sold the Prius in the U.S. starting in 2001 and by 2007 they had sold a million worldwide. Currently, many OEMs offer hybrid cars, SUVs, vans and even hybrid trucks. As the U.S. moves toward independence from foreign oil sources (and the climate heats up) the motivation for selling hybrid vehicles is ever increasing. OEMs are spending millions of dollars for research and development of all-electric and hybrid technologies. For at least the next decade hybrids will bridge the gap between fuel-only and all electric vehicles.

Future of hybrid powertrains

Powertrain configurations that will be used for hybrid cars of the future could take many forms. Because the reason for hybrids existing in the first place is energy efficiency it makes sense that the internal combustion engines (gasoline or diesel fueled) and electric motors used for hybrids be as efficient as possible. All internal combustion engines (ICE) are the most efficient when they are operated under a constant load. An ICE running at full power will extract the most heat energy from the fuel it consumed. Vehicles that use only an ICE cannot operate constantly at full power because the engine’s power output must be regulated for slower or faster driving conditions. This throttling of the engine causes it to be less efficient. Conversely, an electric motor is highly efficient under variable loads because maximum torque is available at all speeds. In addition, they can be used to recover lost energy through regenerative braking (see sidebar). A hybrid vehicle combines the advantages of both types of power sources — gasoline (or diesel) and electricity.

Just how much power does it take? More is needed to get a car moving than is needed to keep a car moving.

The graphs shown illustrate what amount of horse power is required for constant speed driving and acceleration. The upper graph shows how much horse power is required for acceleration of a 3050 lb. car with a frontal area of 22 square feet (Toyota Prius numbers). To go from 0-to-60 in 10 seconds requires 140 horse power. 0-to-40 mph in 10 seconds takes about 85 H.P. The faster the car is accelerated from a stop the more power is required. For example, 0-to-60 in 4 seconds takes 330 horse power (Tesla model S numbers). The lower graph shows that the same vehicle only needs a fraction of the power used for acceleration to maintain constant speeds. In this example just 12 horse power can maintain a steady speed of 60 mph.

Because ICE engines and electric motors have very different power-to-energy characteristics each will be used where it is most efficient. The engine will provide constant power source for charging batteries and/or providing power to an electric motor. The electric motor will provide short bursts of power for vehicle acceleration and climbing hills. As shown by the graphs the hybrid of the future will not be required to have a high-horsepower ICE engine. About 50 horsepower would be sufficient for a standard car. The small output ICE engine will be complemented by a 100+ horsepower electric motor, and battery with enough capacity for high energy driving requirements. 

Series hybrid

Two basic types of hybrid configurations are series and parallel. A series hybrid configuration is like a series electrical circuit, i.e., battery positive, light bulb one, light bulb two and battery ground. In a series hybrid layout energy flows from the fuel tank, to the engine, that drives a generator. From the generator power is directed to either the electric motor, battery or both.

A series hybrid configuration shows how the engine is used to generate power for the electric motor and charging the battery. Because the engine is connected to the generator it can be operated at full throttle providing the best fuel efficiency. Under many driving conditions it would be shut off and only started when needed to run the generator.

The end of the hybrid series circuit are the drive wheels of the vehicle. The power bus controller is a computer that determines where power is needed depending on driving conditions. During steady-state driving the engine could be turned off to conserve fuel and only the battery used to power the motor. When acceleration is required, and the battery’s energy is used at a high rate, the engine would be started to recharge the battery and power the electric motor. When the brakes are applied the motor would act like a generator and recharge the battery through regenerative braking. For maximum efficiency, energy for the vehicle will be constantly switched between the battery and on-board fuel. When the vehicle is not in use both energy sources are replenished. The battery charged from a stationary power source and the fuel tank filled.

Regenerative braking

Einstein once said, “Energy cannot be created or destroyed, it can only be changed from one form to another.” and that is the premise of regenerative braking. The average car weighs around 3200 pounds with light trucks adding another 2000 pounds to that figure. To accelerate this mass to 60 miles per hour takes heat energy that comes from fuel or a battery in a hybrid. Conversely to slow down the same amount of mass also takes an equal amount of energy. The braking systems on cars and trucks convert the energy of motion into heat through friction. The heat is dissipated and its potential energy is lost. Regenerative braking can recover much of this heat loss and turn it into work to power a vehicle.

Many current hybrid cars like the Tesla, Toyota Prius, Ford Escape and others use regenerative braking to convert wasted brake heat into electrical energy. The concept is simple. The electric motor used to propel the car can also be used as a generator to charge the battery using the energy of the vehicle slowing down though braking. Using an electric motor as a generator is not a new concept. The starter/generator was installed in Cadillac models in 1912 where it was used to start the engine (a vast improvement over cranking by hand) then once the engine was running it was used as a generator to charge the battery.

Ford Motor Company and the Eaton Corporation are developing Hydraulic Powered Assist (HPA) regenerative braking systems. When the brakes are applied the vehicle’s kinetic energy is used to power a hydraulic pump that forces fluid into a high-pressure accumulator. Pressure (1300 psi or higher) is created by compressing nitrogen gas in the accumulator. When the vehicle is accelerated the high-pressure fluid is used to drive a hydraulic motor that is connected to the powertrain. This translates the kinetic energy of the car into mechanical energy that assists with vehicle acceleration. It is estimated that an HPA system could store up to 80 percent of the vehicle’s inertia lost during braking and turn it back into energy for acceleration. This could reduce fuel consumption by 25 percent or more as this technology matures.

Because of the size of the accumulator, current designs of HPA systems are targeted for large trucks over 10,000 pounds. As the technology is improved HPA designs will be made smaller and can be applied to cars and light trucks. Regenerative braking on hybrid vehicles allows on-board batteries to be used longer without external charging thus extending the range of both hybrid and all-electric vehicles.

Parallel hybrid

Like the series hybrid configuration a parallel layout uses two power sources in separate paths to the vehicle’s drive wheels. The battery powers an electric motor connected to the vehicle’s transmission. The other path of power is the ICE engine, also connected to the transmission. One advantage over series hybrid systems is that no generator required. The electric motor can still act as a generator to charge the battery during braking. The ICE engine could be made smaller as it is directly connected to the transmission. Because the electric motor would have two times, or more the power of the ICE, it could drive the rear wheels during max power requirements while the ICE could drive the front wheels for low-speed driving. With a low-power ICE, the transmission would be less expensive to manufacturer and could be smaller than a standard transmission.

In the parallel hybrid configuration there is no need for a generator. The battery is charged form an external power source or through regenerative braking. The vehicle’s computer will switch between using the electric motor and engine to power the vehicle.

Additional benefits

Owning a hybrid of the future will offer several benefits. A hybrid with a large, powerful electric motor will be more fun to drive than a car equipped with only a small fueled engine. All-electrical power accelerates the Tesla Model S from 0-to-60 mph in 3.2 seconds and only a few ICE powered cars can match that acceleration. This is a long way from the image of the “not-a-thrill-to-drive” Toyota Prius with a 0-to-60 time of around 10 seconds. While many future hybrid cars will not reach the acceleration performance of the Tesla S, future hybrids that offer owners a high-performance driving experience will be more common. Consumers will buy efficient vehicles, not to only help the environment, but because they will be fun to drive, and less expensive to own and operate.

An additional benefit of future hybrid car designs is a moderately sized battery, probably around 5 kWh, which will be able to be charged overnight from a standard household plug. In addition, the widespread use of hybrids will have a dramatic effect on consumer fuel purchases in the future. If gasoline prices start to increase, owners of hybrids can instantly start using more electricity and less gas reducing the demand for oil and lowering fuel prices overall. It will take a high percentage of hybrid vehicles to affect fuel prices but consider that through 2016, about 11 million hybrid cars have been sold worldwide with 36 percent of that total sold in the U.S. As the cost of hybrid vehicles comes down, and performance and efficiency goes up, sales will increase affecting fuel prices.

(Image courtesy of NetGain Motors, Inc.) The combination of a super-efficient gas or diesel engine, plus the power of a large electric motor will offer drivers of hybrids the best characteristics of both power sources.

Because power for acceleration will be provided by an electric motor, the engines used in hybrids will have small displacements and be more fuel efficient. On average, an ICE wastes around 50 to 70 percent of the heat energy stored in gasoline or diesel. Instead of providing power to turn the wheels, wasted heat energy is used by the radiator/cooling system, engine components like pistons, cylinder blocks and heads, and the exhaust system. Engines used in hybrids can run at steady speeds and will have higher thermal efficiencies that could reach 50 percent or more.

Small displacement engines that use turbocharging and Atkinson cycle designs will power the hybrids of the future. Diesel engines could be revisited for automotive applications because emissions can be more easily controlled during constant power output. A diesel hybrid could use all electric power for city driving and diesel power for rural areas where pollution is less of an issue. Atkinson cycle types of engines provide good fuel efficiency as a tradeoff for lower power-per-displacement, when compared to traditional four-stroke engines. Variants of this engine design were used in 1997 in the Toyota Prius. Currently there are over 40 OEMs that use Atkinson-cycle engine designs like variable valve timing. The combination of the Atkinson-cycle engine with a large electric motor provides the most efficient means of producing power for hybrid vehicles.

This drawing shows how hydraulic assist braking could be used in a hybrid car. During braking the hydraulic pump sends high pressure fluid to the accumulator. When accelerating the accumulator sends the high-pressure fluid to the hydraulic motor to drive the rear wheels. A computer would control hydraulic valves to operate the system (not shown).

Conclusion

Internal combustion engines operate best when under a constant load. Engines operating at their most efficient configuration will have increased longevity, be less complex, cost less to manufacturer and emit lower emissions. Hybrid drivetrains will combine the ICE and electric motor to take advantage of the best characteristics of both. The hybrids of the future will circumvent the trade-off between power and efficiency that current internal combustion engine powered cars are subject to. Hybrids have been sold in the U.S. for over 20 years and there is opportunity for many future refinements. Hybrid vehicles will bridge the gap between fuel-powered only to all-electric power. The hybrid of the future will be less expensive to own and operate, plus provide consumers with a fun, environmentally friendly driving experience.

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