Hybrid Cars: Parallel vs. Series

New variations on a very old idea

The hybrid car, powered by both an internal combustion engine (ICE) and a battery-driven electric motor, is not a new concept. The first hybrid was built in 1901 by Ferdinand Porsche, then a young engineer at the Lohner Carriage Works in Vienna, Austria. Porsche’s idea was to replace two tons of lead acid batteries in the company’s electric cars with a relatively lightweight gasoline engine. The gasoline engine drove a generator to recharge a smaller battery feeding electric motors in the wheel hubs. By 1905, the Belgian engineer Henri Pieper combined the motor and generator into a single unit and invented a single-lever control system that took care of all the drive and recharging functions, including regenerative braking. He got a U.S. patent on his system in 1909. In 1914, Hermann Lemp at General Electric figured out how to scale up the DC power controller to run a diesel-electric locomotive, and since then, hybrid drive has been common for railroad, ship and submarine propulsion.

These are all series hybrids, so called because the ICE drives the generator, which drives the electric motor, which drives the wheels. This arrangement works well in low- speed, high-torque applications because the electric motor produces maximum torque at zero revolutions per minute (rpm). Maximum cranking power is available to get the locomotive or the submarine moving, without the use of a complex, heavy clutch and transmission.

The series drive is less useful in high- speed cruise, because there’s an efficiency loss between the generator and motor— and a double efficiency loss if a battery sits in the electric flow between the generator and motor. In automobiles, that’s always the case. The driver may need more power, for quick acceleration or hill climbing, than the engine-generator combination can provide instantaneously — so you want some power in reserve, stored in the battery.

A car, however, is light and agile compared to a loco- motive. A sophisticated lightweight transmission can be designed to bypass the electric motor at cruising speed and connect the ICE directly to the wheels. The result is a parallel hybrid system, where the ICE and the electric motor work one at a time or in tandem, depending on what’s most efficient for the driving situation. Excess power from the ICE is routed through the generator to keep the battery charged and ready to pitch in for bursts of acceleration. This is the system that runs most of today’s hybrid and plug-in hybrid cars, including all the hybrids from Toyota, Honda, Ford, Hyundai and BYD.

The exception is the Chevy Volt. In principle it’s a simple series hybrid like Pieper’s 1905 car — the battery runs the electric motor to drive the wheels through a slick continuously-variable transmission. The ICE never drives the wheels but rides along until needed to recharge the battery. The Volt, to be sure, uses 21st century integrated-circuit controls to make smooth, quiet power throughout the speed range.

Any proper hybrid does better in stop-and-go driving than a pure ICE, because it doesn’t idle when stopped and because it recovers energy through regenerative braking. So do you want a series or a parallel hybrid? In theory, the series hybrid should be less complex and more easily adapt- able to a wide variety of auxiliary power plants, from diesels to Stirling heat engines. The Volt drive train will thus be the base for dozens of car and truck applications. But parallel hybrids are here right now and here to stay. So we’ll see.

 

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2 Responses to Hybrid Cars: Parallel vs. Series

  1. José DeSouza Reply

    August 21, 2014 at 5:34 am

    An even more clean and highly advanced form of passenger hybrid propulsion would be the combination of a series hydraulic hybrid bus with a system of retractable electricity collection, so as to conceptualize a sort of lean-infrastructure trolleybus system (main advantage: no need to string overhead wires all along the buses’ paths). Only a matter of replacing the ICE on the HH bus with an electric motor of equivalent output. All mature technologies. Just a matter of putting together what’s begging to be put on the market on a massive scale:

    http://www.altairbusolutions.com/
    and
    http://www.mobility.siemens.com/mobility/global/en/interurban-mobility/road-solutions/electric-powered-hgv-traffic-eHighway/the-ehighway-concept/Pages/the-ehighway-concept.aspx .

  2. Larry Jenkins Reply

    September 13, 2015 at 10:09 am

    I’m looking for the ease of conversion, therefore the ‘in series ICE’ is more appealing, let me explain. I have a 98 Outback I dearly love, but with the low manual trans gearing 3500 rpm equals 70 mph. The vehicle is solid and runs smooth, but I can’t help but think the ability to shift from 2nd to 5th gear would give me higher mph with less rpm’s. My thought was, would that help with the problem with the ‘in series’ ICE option? The reason I think my Subaru would make an ideal host is that this vehicle rolls forever on a straight away. At 70- mph it will go for a mile before dropping to 60 mph. From the top of the Cajon Pass at 60 mph, it will gain speed to over 80 with enough momentum to go back up a slight incline for a short climb and make it down to the 210 Fwy in neutral, which is over 20 miles.

    I believe I could fit several batteries in a modified spare tire area and remove the back seats to put several batteries between the rear axles and the front seats. I want to maintain the existing cargo area and keep my center of gravity low. My Outback has overloads that gives me off-road clearance, so I already look like a 4- wheel drive vehicle. Right now its locating the right ICE configuration, so my desired option was for a 6 – 12 HP diesel power plant to run a low rpm generator that would kick in once it dropped below the minimum battery operating level.

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