The car industry resides in a state of transition. Like Dylan, it’s going electric, whether you like it or not – but not everyone is ready to embrace this future. Battery-electric vehicle (BEV) sales are distinctly perky, and are forecast to surge to market dominance at some nebulous point in the future, though for the moment their numbers are small. Range anxiety, an infrastructure bottleneck, upfront costs and unhappy resale values frustrate those who would like to go fully electric but foresee too many compromises. Into the void steps the hybrid electric vehicle (HEV), reducing emissions without the angst.
“Hybrid” is a broad definition, including any vehicle using two motive power sources – typically an internal combustion engine (ICE) and a battery connected to an electric motor. Variations cover everything from “ICE-plus”, to cars that, on an everyday basis, function as electric vehicles. As technologies advance, hybrids increasingly don’t fit neatly into categories – the Honda CR-V hybrid, for instance, features not one but two electric motors – but this makes the sector that much more fascinating to follow. Forecourts are filled with fresh ideas and competing technologies, each with pros and cons geared to journey length, congestion levels and driving style. It’s the automotive wild west – but there’s a hybrid out there for everyone.
Mild hybrid
When hybrid vehicles first appeared at the end of the 1990s, they were what the modern taxonomy now calls mild hybrid. The philosophy, as featured in vehicles such as Honda’s original Insight, is to use a motor-generator unit and oversized battery to assist a downsized ICE. Incapable of driving in pure electric mode, the hybrid elements provide stop-start functionality, recover kinetic energy when braking (regeneration) and boost engine power under acceleration. Mild hybrids are enjoying something of a renaissance, sitting in a grey area between conventional ICE-powered vehicles and HEVs. They don’t offer the impressive fuel economy figures of a full hybrid but nor do they carry the price premium.
Series hybrid
Mainstream HEVs use either series or parallel architecture. This refers to the way they feed power to the wheels. A series hybrid is, for want of a better description, an electric vehicle charged by an ICE. The ICE isn’t connected mechanically to the drivetrain (the group of components that power the wheels), but is attached to a generator, which powers an electric motor and charges a battery, (also charged regeneratively). Much as in a domestic generator, the ICE in a series hybrid ignores user inputs and chugs away at an optimum, fuel-efficient speed. When the driver needs to accelerate, that extra power is drawn from the battery, rather than by revving the ICE. They’re very efficient in stop-start traffic – but demand a large and expensive battery.
Parallel hybrid
In parallel architecture, the ICE and electric motor operate in tandem – they’re both mechanically linked to the drivetrain. Having the ICE physically connected to the drivetrain reduces the power conversion losses suffered by a series hybrid, making a parallel layout more efficient when cruising – but less so in heavy traffic. They’re also rather more complex machines.
‘Through the road’ parallels
An offshoot of the parallel hybrid philosophy is the cars that link their ICE and electric motor to different axles, one driving the front wheels, the other the rear (in either combination). In the main, this creates a hybrid with part-time four-wheel drive, but it’s also been popular where power delivery, rather than fuel economy, is the selling point – Honda’s sporty NSX, for example.
Series/parallel hybrid
Inevitably, there’s a hybrid that attempts to be the best of both worlds – a hybrid within a hybrid, if you like. A series/parallel HEV should always be at or near its optimum operating window, switching between series and parallel drive as driver demand requires. It’s the most complex type of HEV, with ICE, batteries, or both, together capable of supplying power, and the battery charged by regenerative braking and/or the ICE. They are wildly popular and often feature dashboard powerflow displays that resemble a London Underground map, as drawn by Dr Emmett Brown.
The interesting thing about a series/parallel HEV is that there are plenty of different ways to do it. Honda’s CR-V hybrid, for example, splits the functionality by using a pair of electric motors: one provides propulsion while the other charges the battery. It allows the car to stay in an economical operating window across the range of driving conditions: automatically choosing electric-only in heavy traffic; hybrid drive as speed picks up; and engine-only on the motorway – but always with ICE or battery available to provide a little extra oomph when demanded.
Plug-in hybrids (PHEVs)
Just to muddy the waters, there’s another group of cars that blend the advantages of BEV and HEV. PHEVs are electric vehicles for people who worry about range. They’re popular with drivers who do a lot of urban motoring or have short, regular commutes – but want something capable of going further. Charged from a socket, they typically offer an all-electric range of 25-35 miles (some less, some a great deal more), which means that those who potter down to the shops, do the school run or drive a short distance to work, rarely need to leave all-electric mode. Less expensive than a BEV, they nevertheless carry a price premium over a standard HEV.
