Hybrid cars

Hybrid vehicles obtain their motive power from two sources: a conventional internal combustion engine (primarily petrol) and an electric motor.

There are basically three types of hybrid cars:

Series hybrids: the vehicle’s wheels are driven only by the electric motor. The role of the internal combustion engine is to generate electricity which powers the electric motor which, in turn, drives the wheels.

Parallel hybrids: the vehicle’s wheels are powered by either the engine or the electric motor. In the current Toyota Prius and the Honda Insight, for example, the original source of the energy is petrol and the vehicle can then run on the engine or the motor.

With both series and parallel hybrids, the electric motor provides the power at low speeds, for example in urban traffic, and the power source switches to the combustion engine at higher speeds, for example on motorways. The switchover from one to the other is managed by a sophisticated computer system.

Energy is stored in an on-board battery which, while the hybrid is in motion, is in a constant state of flux. At lower speeds, below about 15 mph, the battery provides the energy to power the electric motor, while at higher speeds the batteries are recharged and energy is stored. During acceleration, the engine and motor operate together to provide the extra energy required. This means that smaller, more efficient and more economical combustion engines can be used, and emissions are correspondingly lower.

Plug-in hybrid electric vehicles (PHEVs): petrol-electric hybrids with a battery that can be recharged from the mains supply as well as, in most PHEVs, by the engine. PHEVs run on the electric motor for up to 40 miles and then revert to the petrol engine when the battery is almost depleted. As such, they are intended for commuters whose daily journeys are within the range of electric-only operation. The batteries are then recharged overnight at home.

Because PHEVs can be recharged from the electric mains supply, they are classified as ‘electric cars’. PHEVs will qualify as electric cars when the UK government subsidy of up to £5000 per electric car comes into operation as all-electric cars and PHEVs become available from 2011. The other two types defined above cannot be charged from the mains and will not be eligible for the government subsidy.
In all three types of hybrid, the batteries are topped-up by ‘regenerative braking’. This process captures the kinetic energy, or momentum, which is generated when the driver takes his foot off the accelerator and coasts, or when the driver applies the brakes. The motor converts the energy to electricity which is stored in the battery. Regenerative braking reduces fuel consumption by about 20%.

 ‘Mild’ and ‘full’ hybrids

‘Mild’ hybrids include regenerative braking and also a stop-start system. This system saves fuel and reduces emissions by automatically shutting down the combustion engine at idling speeds when the brake pedal is depressed. When the brake is released, the engine re-starts immediately. ‘Full’ hybrids mean that  the vehicle can operate on the electric motor alone – both the combustion engine and electric motor can be used either independently or integrated with each other. Compared with ‘mild’ hybrids, ‘full’ hybrids offer greater fuel economy and lower emissions – making them highly suitable for urban operation.

The Toyota Prius

The Toyota Prius website provides an excellent description of the key features of a full parallel hybrid. With thanks, it is reproduced here.

Starting
From start-up, the Prius can drive under electric motor power alone up to 31 mph, when the petrol engine kicks in. Using the high torque immediately available ensures a fast and smooth start. When the battery is low, the petrol engine powers the generator to recharge it.

Driving

Above medium speed, the Prius uses the 1.8 L petrol engine and its two electric motors in synergy. Using the Atkinson cycle, the petrol engine is maximised for operating efficiency providing only 89g of CO2 at a combined fuel consumption of 72.4 mpg. The battery can also be recharged by the electric motor which then acts as a generator. Sensors constantly monitor the driving conditions to ensure the required amount of power is instantly produced on demand, to maintain a steady drive.

Overtaking

When overtaking or during sudden acceleration, seamless power comes from both energy sources. The 1.8 L petrol engine is supported by the 60 kW electric motor providing all the additional boosting power while offering an impressive seamless acceleration. The Prius can accelerate from 0 – 62 mph in 10.4 seconds.

Braking

When braking and during deceleration, the electric motor acts as a high-output generator and recharges the battery. This kinetic energy is normally wasted. During braking the Prius consumes no fuel, the use of the electric motor as a generator makes negligible noise and the Prius produces zero emissions.

Electric mode

The driver can activate Electric Vehicle mode (EV) to drive in pure electric mode for speeds up to 31 mph. The electric motor is remarkably quiet compared to a petrol engine.

Stopping

When stopping the petrol engine is switched off by the Stop and Start system to reduce fuel consumption. All other systems, including electric air conditioning, continue to function.

The next generation of batteries

Batteries are playing a key role in the development of hybrids and electric vehicles. The current nickel metal-hydride batteries are being replaced by lithium-ion (The G-Wiz, for example, now offers them in the new G-Wiz L-ion). Having evolved from watch, mobile phone and laptop technology, Li-on batteries are lighter, potentially cheaper, longer lasting and can store twice as much energy as the NiMH batteries.

Lithium-ion batteries also power the Smart Fortwo Electric Drive, MINI E, Mitsubishi i-MiEV, the plug-in Toyota Prius amongst other new models trialling in the UK in 2010.

Hybrids’ potential

The great advantage of hybrids is that they can provide total fuel savings of up to 30% compared with their conventional counterparts. This translates directly into reduced emissions so hybrids are environmentally friendly, particularly during urban driving when the hybrid is working on the electric motor alone and emissions are zero. Hybrids can also be fuelled using standard petrol and diesel – both of which are readily available and require no unique infrastructure.
Hybrids represent the way ahead for the next few years but, in the long term, they are a step along the path towards fuel cell electric cars with permanent zero emissions.