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To improve fuel economy, and in turn cut CO2 emissions, CV manufacturers have been turning their attention to hybrid technology – supplementing their diesel engines with electric power.
This arrangement can work in several ways. By combining the two power sources, the hybrid engine can bolster flat spots – for example at lower revs – without significantly increasing consumption, as the combustion engine remains at its optimum operating level. Or the electric source can be used on its own when the vehicle is operating at low speeds and only needs a low level of power for propulsion.
There are currently two broad types of this hybrid diesel/electric configuration on the market – mild and parallel – with each set-up assisting to a different degree in overall vehicle propulsion.
The resulting fuel savings depend on the level of electric assistance and vehicle application, but tend to be between 10 and 30%.
However there are currently developments in an area called series hybrid technology, which takes the responsibility of driving the wheels away from the combustion engine and on to an electric motor, enabling it to operate more efficiently.
Designed to assist propulsion when the engine is engaged, usually from static or at lower revs, the mild hybrid takes the form of an electric motor working in conjunction with the combustion engine to achieve more efficient propulsion.
Power used by the electric motor is recovered when the combustion engine is on overrun and when the vehicle is decelerating by using regenerative braking.
This recovered energy may also be used to drive the power steering and other electrical auxiliaries in some cases, which require a significant degree of power from the engine.
The arrangement tends to cost and weigh less than parallel hybrid technology, as well as be easier to fit. However, fuel savings tend to be lower – around the 15% mark.
Connaught Engineering's Mild Hybrid+ system,
which can be retrofitted, has returned
fuel savings of 25% in independent tests
and is on trial in two RAC Ford Transits
Parallel uses both a (larger) electric motor and combustion engine unlike mild hybrid technology, either can individually power the vehicle when required, although they can also work in unison.
Electric power tends to operate at low speeds and low throttle, where less power is required.
As the vehicle speed or acceleration demand increases, the combustion engine kicks in and both power units work together in parallel on the open road the combustion side alone provides the propulsion.
Again, power used by the electric motor is recovered when the combustion engine is on overrun or when the vehicle is decelerating by using regenerative braking.
Already offered by the likes of Mercedes-Benz on its Axor, parallel hybrid technology is currently being developed by several truck manufacturers.
Mitsubishi Fuso has 10 of its Canter Eco Hybrids
on trial with the likes of Tesco and Royal Mail.
Incorporating parallel hybrid technology, they
are expected to boost fuel economy by 20%
Seen as the next stage in hybrid technology, this set-up removes the combustion engine from the process of turning the wheels, instead relying solely on an electric motor to do the job.
The motor gains its power from either a battery pack or generator, which is run by a combustion engine.
The engine used is usually smaller in a series hybrid as it only has to meet average driving power demands, operating constantly at its most efficient level instead of constantly moving up and down the rev range as with stop/start driving.
Again, regenerative braking recharges the battery pack.
This arrangement eliminates the need for a complicated multi-speed transmission and clutch. However the large battery pack and motor, along with the generator, add to the cost – making series hybrids more expensive than parallel hybrids.
Many hybrids use a regenerative braking system that recovers some of the energy lost through stopping.
This energy is channelled back into the main battery (or in a hybrid electric/combustion engine set-up, into a storage battery) to be used for propulsion – it is estimated that up to 30% of energy is lost through heat energy dissipation when braking.
This system is particularly useful in an urban delivery role, when constant stop/starting is common.
As the driver applies the brakes, the electric motors reverse direction the torque created by this reversal counteracts the forward momentum and eventually stops the vehicle.
Whenever the electric motor begins to reverse direction, it becomes an electric generator or dynamo and power is fed back into the battery.
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