IS THE GRASS GREENERS?
Vehicle emission standards have become progressively more stringent since their introduction in the US in 1968. At the November 2015 UN Climate Change Conference in Paris, 197 countries agreed to limit global warming to less than 2°C versus pre-industrial levels. Many governments have since introduced stricter vehicle emission limits and some have committed to future bans of the internal combustion engine (ICE). Vehicle manufacturers responded in kind with the introduction of electrification programmes to achieve these targets. The following provides an overview of the most
common electrification solutions.
HYBRID-ELECTRIC VEHICLE (HEV)
HEVS use both an ICE and at least one electric motor as a means of propulsion. Within this description there are myriad drivetrain architectures, each with different objectives regarding cost, performance and packaging, utilising varying proportions of electric energy to drive the vehicle. Irrespective of drivetrain differences, HEVS all provide improvements in fuel consumption, vehicle emissions and driving performance.
Combining an electric motor, which develops maximum torque from standstill and maintains this at low motor speeds, with the rising torque curve of an ICE provides many advantages.these include the possibility of downsizing the ICE without a loss of vehicle performance, utilising higher gearing to predominantly operate the ICE within its most efficient range, and regenerative braking – the recuperation of electrical energy when decelerating or braking – with the motor operating as a generator to convert kinetic energy into electrical energy that is stored in the battery.
HEVS can be grouped into three categories depending on the configuration of the ICE, electric motor and transmission. Parallel hybrids use both the ICE and electric motor independently to contribute to driving the vehicle and this system is typically used in mild hybrids and some full hybrids. In a series hybrid, there is no mechanical connection between the ICE and the vehicle drive; the ICE drives a generator and charges the battery, both of which supply the electric motor that propels the vehicle. It is also used as a range-extender in some EVS and is commonly used in city buses and diesel-electric locomotives. A combination of the two, known as the series/parallel or power-split hybrid is explained in more detail in the description of full hybrids.
More importantly, HEVS are classified according to the proportion of electrical power provided to assist the ICE.THE most basic HEV is the now widely used start/stop system that switches off the engine when the vehicle is at standstill, restarting when the driver lifts off the brake and depresses the accelerator. It utilises an upgraded starter motor to support the frequent starting and a deep-cycle battery to cope with the constant cycling and power draw from accessories when the ICE is switched off. On more sophisticated versions, the alternator is programmed to recharge the battery only when needed but primarily during deceleration and braking. Some of these systems are thus categorised as micro hybrids owing to their use of regenerative braking.
MILD HYBRID (MHEV)
MHEVS are a cost-effective approach to reducing emissions and improving driveability as they provide manufacturers with the opportunity to electrify existing powertrains without making major changes. MHEVS will typically use a shoebox-sized 48V lithium-ion battery to provide sufficient power to run higher load ancillaries normally driven by the engine: typically steering, water pump, air conditioner and electrically powered supercharger or electrically driven turbocharger. A belt-driven starter generator (BSG) or an integrated starter generator (ISG) is coupled to the petrol or diesel ICE, acting as a powerful starter motor with sufficient capacity for frequent start-stop restarts, as the engine can switch off when coasting or braking. It also provides additional power directly to the ICE under high-load conditions and supports regenerative braking to charge the battery. However, as a mild hybrid, it cannot propel the car on electric power alone. The vehicle’s normal 12V electrical system and battery are retained to power instrumentation, lighting and other systems, being charged from the 48V battery via a 48V/12V DC/DC converter.
The additional power provided by the system reduces the load on the ICE, improving its fuel consumption and emissions, and allows a reduction in engine size while maintaining efficient performance.
FULL HYBRID
Also known as self-charging hybrids, full hybrids feature larger batteries and more powerful electric motors than mild hybrids and can drive short distances on the electric motor alone. The majority are classified as series/parallel hybrids, as the transmission mechanically routes ICE power directly to the final drive or generator. The generator charges the traction battery and provides power directly to the electric motor that drives the vehicle. A power controller determines what combination of ICE, Ice-electric, battery-electric drive or battery charging is required, changing this according to driving conditions. For example, gentle standing-start acceleration can be handled by the electric motor drawing power from the battery, but the ICE will also engage if stronger acceleration is required. In steady-state cruising, when the ICE is at its most efficient, it will drive the vehicle – either directly or via the generator and electric motor – while recharging the battery when required. Regenerative braking is used to charge the battery when decelerating. Atkinson cycle and other high expansion ratio ICES are often used, with the power controller keeping them within their most efficient operating range to reduce fuel consumption and emissions.
PLUG-IN HYBRID (PHEV)
Plug-in hybrids are effectively an extension of the system used in full-hybrid, with the additional feature that the traction battery can be charged via an onboard charger from the mains supply network, not only from regenerative braking or the ICE. In many key markets, the CO2 certi cation test cycle includes running in electric mode and for this reason, both the traction battery and electric motors are usually increased in size to extend the electric mode range. However, as many PHEVS are electri ed versions of vehicles originally designed with ICE drivetrains, battery storage space is often limited and electric mode range is typically 40 to 70 km, which is more than adequate for most daily commutes.
As with full hybrids, a power controller determines the optimum drive modes for the prevailing driving conditions. In some applications, it even incorporates input from the vehicle navigation system to minimise emissions and optimise range over a planned route. Driver selection of operating mode is also possible should the driver wish to preserve battery charge for later use.
BATTERY-ELECTRIC VEHICLE (BEV)
Battery-electric drivetrains are simpler than any of the hybrid drivetrains.the most basic designs feature an electric motor powered by a battery charged via an onboard charger from the mains supply network as well as from regenerative braking … all managed by a power controller.two or more drive motors are utilised on some SUVS and high-performance vehicles, while a motor on each axle effectively provides four-wheel drive. With electric motors developing their maximum torque from standstill and being effective over a full “rev” range, a single xed-gear ratio between the motor and drive wheels is used in most applications. However, high-performance vehicles will typically have two gear ratios to support high-speed acceleration and top speed.
While electric motors are compact, the batteries required to provide an acceptable driving range are large and heavy.these can be packaged into modi ed versions of vehicle platforms originally designed for ICE drivetrains, but dedicated BEV platforms are becoming more common. With motor(s) positioned at the front, rear or both axles, the batteries are packaged under the oor, between the axles, lowering the centre of gravity to minimise the impact on vehicle dynamics. Sometimes referred to as “skateboard platforms”, they are designed to be easily adapted to different sized vehicles.
Claimed driving ranges vary from 200 km for city EVS to more than 500 km in larger vehicles. Some BEVS may utilise a range extender; a small auxiliary ICE power unit that drives the generator to charge the battery and extend electric range.
FUEL-CELL ELECTRIC VEHICLE (FCEV)
An FCEV is an electric vehicle in which the electricity is generated onboard by a fuel cell stack instead of being supplied by a battery charged by the mains supply network. The fuel cell stack generates electricity through a chemical reaction between hydrogen and oxygen; the hydrogen is stored onboard the vehicle in high-pressure tanks and the oxygen is drawn from ambient air. Managed by a power controller, the generated electricity is supplied directly to the drive motor and to a compact battery, which also stores energy from regenerative braking and supports the fuel cell when additional performance is required. Suf cient hydrogen can be stored to provide a driving range comparable to ICE vehicles.
The fuel stack emits no pollutants, only water, and if the hydrogen can be sourced from renewable sources, such as hydro, wind or solar power, it can be created without the release of CO2.
It is clear motoring is moving to a purely electric future, possibly complemented by hydrogen power.timelines will vary between markets for a variety of reasons and electri ed hybrids will provide an interim solution.