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Testing today's electric and hybrid electric cars

13 Jun 2014  | Peter Allen

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It may be too much to claim that the era of the internal combustion engine is over, but it's certainly fair to say that we're entering the age of the electric vehicle. Not only are all the major manufacturers developing electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), several new manufacturers, such as Tesla and Detroit Electric, have gotten into the game.

Rising fuel prices and government incentives are perhaps the biggest reasons for the move to electric vehicles. In the United States, gasoline costs an average of $3.37 per gallon and is forecast to increase steadily. Electricity prices, on the other hand, are rising less quickly as the U.S. increases natural gas production and utilities switch over to natural gas for generating electricity.


Different architectures mean different power sources
EVs, HEVs, and PHEVs are much different from internal combustion engine vehicles. In a purely electric vehicle, batteries provide the energy and electric motors provide the propulsion. In an HEV or PHEV, the vehicle includes an internal combustion engine, but depending on the vehicle architecture, the engine plays more an auxiliary role.

There are several different HEV architectures:

Series. In a series hybrid, the internal combustion engine charges the battery, which powers the electric motors. The battery can also be charged by connection to the grid and by regenerative braking.

Parallel. In a vehicle with a parallel architecture, both the internal combustion engine and the electric motor can propel the vehicle, sometimes alone or sometimes simultaneously. While the internal combustion engine is propelling the vehicle, the electric motor/generator is charging the battery.

Combined. Combined hybrid vehicles have characteristics of both series and parallel hybrids. They use power-split mechanisms that allow the internal combustion engine to either drive the transmission directly or charge the vehicle's battery. This architecture combines the advantages of the series and parallel architectures, and allows for more efficient use of the vehicle's internal combustion engine. This added efficiency does have a cost, however. Combined hybrids are more complex and priced higher.

One of the biggest differences between vehicles powered by internal combustion engines and EVs and HEVs are the components.

Batteries. In a vehicle powered by an internal combustion engine, the battery is typically a lead-acid battery with a nominal 12 V output. The batteries in an EV or HEV are very different. A typical EV or HEV battery uses nickel-metal hydride (NiMH) or lithium-ion (Li-ion) technology and has an output voltage of 400 V or more. The viability of EVs and HEVs is very much dependent on improvements in battery technology, and there are many companies working on improving the energy density of the batteries used in EVs and HEVs. This will be a very dynamic area as EV and HEV vehicle development moves forward.

Electric motor/generator. The electric motor/generator in an HEV or PHEV both propels the vehicle and helps to recharge the battery while braking. These motors are three-phase AC induction or permanent magnet motors.

Inverters. To power the motors in an EV or HEV, the vehicle requires an inverter to convert the high-voltage DC power provided by the battery to the multi-phase AC power required by the vehicles motor/generator. The inverter may be required to operate over a wide input voltage range and provide multiple outputs. Inverter efficiency is a key specification.

Battery charger. EVs and PHEVs will have an on-board battery charger that converts AC to DC to charge the vehicle's battery quickly and efficiently.

DC-DC converter. The DC-DC converter creates the DC voltages needed to power auxiliary components and the HVAC system. They may have an output power of 2 kW or more.

Other components. In a vehicle with an internal combustion engine, components such as coolant pumps, air-conditioning compressors, and power steering systems have traditionally been mechanically powered. A serpentine belt mechanically couples power from the engine to the pump or compressor. In an EV or HEV, these components are electrically powered. This technology has proven to be so successful that mechanically powered pumps and serpentine belts are now the exception rather than the rule.


Battery tests
Batteries are at the heart of today's EVs and HEVs, which is why testing them is so crucial. These aren't your typical car batteries—they may have an output voltage of 400V or more and must supply up to 5o kW or more.

To ensure that batteries will reliably power a vehicle, manufacturers and suppliers must run many different tests, including:

Life-cycle testing. During life-cycle testing, you charge and discharge the batteries or modules to determine how the battery's charge and discharge capacity and the battery's DC internal resistance changes over time.

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