There is a wide spectrum of hybrid systems, using different types of energy storage, traction motor and controls. All have an internal combustion engine as the primary source of power, electric batteries or other systems to store and release energy harvested through regenerative braking, and a supplementary drive. Most are 'parallel hybrids', which use the IC engine and other sources together, with a control system to balance them. But whatever the structure, the net effect should be improved overall fuel efficiency and often better performance and driveability.
For Luca Zai, chief engineer at Ricardo's Hybrid and Electronic Systems Product Group, take-up is about whether they can hit a payback target. "If calibrated for a specific mission then yes. This is particularly true in the case of city buses or refuse trucks where they have high stop density and a predictable route."
The simplest systems are 'microhybrids' – such as used in the Citroen Berlingo e-HDi van – which combine a stop-start function with regenerative braking. A reversible starter-alternator harvests kinetic energy as the vehicle slows. This is stored briefly in a capacitor and used to start the vehicle again. Citroen claims this reduces fuel consumption by 15% in urban driving.
The other extreme of hybrid development is demonstrated by US firm Oshkosh with its ProPulse military truck, an off-road-capable 8x8 'series hybrid', with no direct connection between the IC engine and the final drive. Individual electric motors on each axle are powered by a diesel-driven generator via a bank of ultra-capacitors. At its destination, the truck acts as a mobile 120kW ac power station for a field hospital or airstrip.
In the middle are 'mild hybrids', which again use a combined starter-alternator but have more substantial energy storage and traction systems that can contribute to acceleration. The energy storage can take the form of batteries, capacitors (super-capacitors or ultra-capacitors – there is no clear distinction) or a kinetic energy recovery system (KERS). The latter flywheel-based systems have been used in buses: GKN's Gyrodrive, for example, with a carbon-fibre flywheel running at 36,000rpm, has resulted in fuel savings of 25%, and the firm claims the payback period for bus operators is three to four years. It adds that the system is smaller than battery or super-capacitor units, and can be retro-fitted.
UK firms are at the forefront of kinetic battery technology: Torotrak has the Flybrid, while Ricardo's TorqStor is proposed for use in off-highway trucks and trains. The TorqStor's flywheel is mounted in a chamber having a permanent vacuum (rivals use vacuum pumps to reduce air resistance), with the drive transferred via a magnetic coupling. Zai points out, however, that flywheel technology is not suitable for pickup and delivery, "because the energy decay is too much".
Further 'mild hybrid' advantages are possible if the vehicle uses 48V electrical architecture. Taylor Hansen, vice president of CPT, says: "Compared with 200—600V full hybrid and battery-electric vehicles, the low-voltage approach avoids the need for high-cost safety features and large battery packs." And Zai adds that for vehicles up to 5 tonnes gvw, benefits include: "Enabling engine downsizing; filling the 'torque hole' during initial acceleration with the motor/generator and electric supercharger; and electrification of ancillaries, such as the oil and water pumps". This is 'intelligent electrification' or 'smart hybridisation'.
Meanwhile, electrically-driven superchargers, such as CPT's COBRA, can benefit heavier vehicles too. Air is delivered rapidly to the ICE, improving combustion in transient conditions and at low speeds, while also increasing fuel economy, reducing the need for exhaust after-treatment and allowing engine downsizing.
That said, bBattery technology is key to the future of general purpose 'full hybrids'. Lithium-ion (Li-ion) or Lithium-Polymer (LiPo) batteries currently yield an energy density up to 240Wh/kg – and some claim up to 800Wh/kg. "Li-ion chemistry will be the mainstream for years to come," comments Zai. Certainly, it is the most weight-efficient, but other chemistries based on lithium and sodium are waiting in the wings.
Indeed, lithium-sulphur (LiS) batteries could be next in line, with a possible energy density of 500Wh/kg, and a demonstrated life of more than 1,500 charging cycles. Lithium-air batteries theoretically offer energy densities as high as 12,000Wh/kg – roughly the same as petrol – but are perhaps 20 years from commercial application.
Older designs of hybrid vehicles tend to use nickel metal hydride (NiMH) traction batteries, which do not have the energy density of Li-ion but are stable and reliable: reportedly, just 0.003% of Toyota Prius batteries have failed. But recently, BASF demonstrated NiMH batteries with a capacity of 140Wh/kg, and is aiming for half the cost of Li-ion.
The established, low-cost lead-acid battery still has a future: in hybrids and EVs it is generally used for SLI (starting, lighting and ignition), and is being developed further with lead-carbon chemistry. For 48V mild hybrid, however, an 'Ultrabattery' has been tested. This has a super-capacitor in the negative plate itself, improving service life by keeping the battery in the optimum state of charge during the frequent charge/discharge cycles of regenerative braking and starting.
What about electric motors? Well design is moving quickly here, too. While conventional traction motors use powerful permanent magnets incorporating rare-earth metals, switched reluctance (SR) motors avoid them. Instead, a steel rotor is attracted to electromagnet stators, switched by the motor controller. These units offer good power density, and both Ricardo and CPT have demonstrated SR motor-generators for 48V mild-hybrid applications. GKN's EVO motor is a similarly compact 'axial flux' permanent magnet design, used in the firm's two-speed eAxle and as a starter-generator.
Whatever the architecture, a hybrid must be properly specified. "At the end of the day, it's down to the specific calibration," advises Zai, adding that this is easier for a large fleet. But he says reliability is not a concern: "Systems on the market have been certified by OEMs or Tier 1 suppliers… The reliability of additional components, such as power electronics, is high. But control software and energy management is key to ensure consistency of performance."
Potter Logistics has been using a pair of Mitsubishi Canter eco-Hybrid trucks for 18 months. These 7.5 tonne gvw curtainsiders operate on collections and deliveries for the Palletline network, covering the YO postcode, and are each covering 30,000 miles a year. "There have been no issues at all," says fleet manager Colin Bamford. "When we first bought them, we heard a few scare stories, but they've been fine."
The trucks do around 10 stops a day – not a high stop density – but Bamford is pleased with their economy. "Fuel consumption was predicted at about 17—18mpg, and it's doing it."
The Canter has a 150bhp Euro 6 diesel with a six-speed automated transmission, a 40kW (54bhp) electric motor-generator and a 270V, 2kWh Li-ion battery pack. There is no load space penalty and, even with a tail-lift, usable payload is 3.2 tonnes.
The trucks are no more difficult to service than conventional vehicles, states Bamford: Staff had some training at Northside Mercedes, but the fluids and lubricants are all standard spec. There have been no issues with low temperatures either.
The only problem? The safety stop switch mounted on the chassis was knocked to the 'off' position during a delivery in York once, and it took a while to work out what was wrong.