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Engineering

The phrase 'clean-sheet-of-paper car' is much over-used, but in the case of Niro it is totally apt. Niro is a dedicated hybrid car, not an adaptation of a combustion-engined model, and while it employs modified chassis components from other models where feasible, to keep down manufacturing and ownership costs, the vast majority of it is all-new.

That includes the platform on which the car is based and which has been designed purely for electrified vehicles; the combustion engine and the transmission which, unlike those in most other hybrid vehicles, is a dual-clutch automatic (6DCT) rather than a continuously variable transmission (CVT), improving smoothness, efficiency and refinement.

The new Atkinson Cycle combustion engine

The petrol engine is a new version of the normally aspirated 1.6-litre direct-injection (GDi) unit from the Kappa family, specifically engineered for use in hybrid cars. It is a long-stroke 16-valve unit with an aluminium cylinder head and block and a steel oil pan. The engine's dimensions are unchanged, with a 72mm cylinder bore and a 97mm piston stroke, and it develops an identical 104bhp at 5,700rpm and 147Nm of torque at 4,000rpm, but in Niro it marks the first combination of an Atkinson combustion cycle, cooled exhaust gas recirculation, direct petrol injection and a long-stroke specification for maximum efficiency.

The Atkinson cycle engine was invented by James Atkinson, of Hampstead, at the dawn of the motoring age (he first filed for patents in the mid-1880s), but it is now proving to be an ideal solution for emissions-reducing hybrid cars in the 21st century.

It works on the principle of completing all four stages of the combustion process (intake, compression, ignition and exhaust) on just one rotation of the crankshaft, rather than the two required in the more common Otto cycle engine. At the same time the compression ratio is reduced and the intake stroke is shorter than the power stroke, therefore requiring less fuel to turn the engine over.

While this inevitably reduces the amount of power the engine can develop, it is less important in a hybrid car which has the assistance of an electric motor, and which in any case aims to maximise efficiency rather than outright performance. With the benefit of continuously variable valve timing on both the intake and exhaust sides of the engine, the new Kappa unit is easily able to achieve the variations in compression ratio and piston stroke required by the Atkinson Cycle. In Niro, efficiency is further aided by an exhaust heat recovery system which speeds the engine warm-up process.

The new Kappa Atkinson cycle engine features isolated cooling technology which prevents the cylinder head and block from overheating while reducing knocking and friction respectively. Sodium, which has high thermal conductivity, is used in the hollow exhaust valves, and friction losses have been reduced through tapered roller bearings and low viscosity oil. The engine's thermal efficiency of 40 per cent is among the best in the global auto industry.

The power electronics system

In Niro, the combustion engine is paired with a permanent magnet synchronous electric motor developing 32kW (43bhp) from 1,798 to 2,500rpm, and 170Nm of torque from rest to 1,798rpm. Power to the motor comes from a 240-volt lithium-ion polymer battery capable of storing 1.56kWh of electrical energy. Together, the two power units produce a maximum 139bhp at 5,700rpm, with 265Nm of torque available in first gear from 1,000 to 2,400rpm. The result is extremely brisk acceleration from standstill and in typical urban scenarios, where Niro is most likely to be found, with effortless cruising ability at the speeds permitted in the UK. Acceleration from standstill to 60mph is achieved in 11.1 seconds, and the car has a top speed of 101mph.

Niro's battery pack is the lightest and most efficient used by Kia so far – the whole hybrid battery system weighs just 33kg – and has up to 50 per cent greater power density and 13 per cent greater energy efficiency than the systems in rival hybrid cars.

Niro's battery cells have a ceramic coating between the cathode and anode sides, a Voltage Protection Device, a dedicated battery management system and an integrated control system, all to improve safety, fire risk, reliability and ease of maintenance.

Niro is a parallel hybrid, which means that most of the time the petrol and electric power units work together. However, it can operate in all-electric mode for short distances when setting off, under gentle acceleration and when descending slopes or braking. A brake energy recovery system recoups energy normally dissipated as heat and stores it in the batteries for use later. The switch between all-electric, hybrid or petrol engine-only modes is seamless and varies according to the load being placed on the powertrain at any given time.

This advanced powertrain ensures Niro is capable of up to 74.3mpg, with CO2 emissions from only 88g/km, with the 16-inch wheels and tyres fitted to grades ‘1’ and ‘2’. The respective figures for grades ‘3’ and ‘First Edition’, which have 18-inch wheels and wider tyres, are 64.2mpg and 101g/km.

Model
Total power bhp
Total torque Nm
0–60 sec
Max speed mph
Average mpg
C02 g/km
1.6 GDi 6DCT ‘1’ hybrid
139
265
11.1
101
74.3
88
1.6 GDi 6DCT ‘2’ hybrid
139
265
11.1
101
74.3
88
1.6 GDi 6DCT ‘3’ hybrid
139
265
11.1
101
64.2
101
1.6 GDi 6DCT ‘First Edition’ hybrid
139
265
11.1
101
64.2
101

A dual-clutch automatic transmission instead of the more common CVT

Niro employs a dual-clutch automatic transmission rather than the continuously variable automatic (CVT) more commonly found in hybrid cars. The in-house 6DCT gearbox is based on the 7DCT transmission fitted to some other Kia models, and uses a dry dual clutch to keep weight to a minimum and reduce the friction losses through viscosity which occur in wet dual-clutch systems. The gearbox has been specially engineered to work with hybrid powertrains. It is smoother, more efficient and quieter than a CVT.

In normal operation the 6DCT transmission changes gear automatically, with no need for intervention from the driver. One clutch operates the odd-numbered gears (first, third and fifth) and the other clutch is responsible for the even-numbered gears (second, fourth and sixth). The 6DCT is able to work automatically in both Eco and Sport modes, with the latter delaying up changes until the power unit has attained higher revs for greater performance. But there is also a Manual mode, accessed by sliding the lever to the side when in the Drive position. The driver is then able to up- or down change at will by rocking the gear selector forwards or backwards.

The layout and gear train have been designed for light weight and compact size, while new low-friction bearings and low-viscosity oil reduce friction, promoting greater efficiency, lowering reaction times and reducing noise. A new Transmission-Mounted Electric Device (TMED) allows the full output of both power units to be transferred in parallel to the drive wheels with minimal energy losses. This differs from the power-split system typical of CVT transmissions, which convert a portion of engine output for delivery through the electric motor, resulting in a loss of energy. TMED allows the hybrid system to access battery power directly at higher speeds for swifter response.

An all-new platform exclusively for electrified vehicles

There will be no combustion engine-only version of Niro. Its all-new platform has been engineered exclusively for electrified vehicles, and a plug-in hybrid version (PHEV) will be introduced for the 2017 model year.

The platform has been designed to accommodate present and future electrical components within a compact vehicle footprint without intrusion into the spacious interior and luggage area. A slightly raised crossover-style seating position makes getting in and out easier and provides a superior view out.

The structure is particularly strong and includes 53 per cent advanced high-strength steel, which ensures that rigidity and safety do not compromise weight. This is aided by extensive use of aluminium for the bonnet, tailgate panel, front bumper back beam and several chassis elements, including the front knuckles, front lower arms and brake callipers. The aluminium bonnet and tailgate alone save 11kg when compared with comparable steel components.

Even the parking brake pedal, which is made of fibre-reinforced plastic, contributes to the weight savings. Newly designed front seats save 1.3kg each thanks to their high-strength steel frames, yet they are still able to cushion occupants against bumps and vibrations as a result of high-density cushions, while specially profiled springs and pads provide high levels of thigh, hip and side support.

As a result of all this, Niro's kerbside weight is from 1,500 to 1,587kg, depending on model specification. To ensure body integrity and rigidity in critical areas, hot-stamped steel is used to reinforce the A- and B-pillars, roof rails and wheel arches.

The battery pack is housed beneath the rear seat, with no intrusion into the boot, as is the 45-litre fuel tank.

No compromises in safety, comfort and refinement

By re-engineering certain existing components, Kia has been able to ensure that Niro has a unique chassis specifically designed for a hybrid while being the equal of the brand's other compact models for ride comfort, handling and refinement.

Niro's road behaviour begins with fully independent suspension all round, based on MacPherson struts at the front and a multi-link arrangement with dual lower arms at the rear. It has been tuned to deliver maximum on-centre steering feel, high-speed stability and immediate response to steering inputs. Because each rear wheel is able to absorb bumps independently, shocks are less likely to be transmitted to the cabin than with a cheaper torsion beam rear axle, while the wide stance and low centre of gravity enhance stability.

Capitalising on the experience gained through the development of two generations of Optima hybrid (neither sold in the UK) plus the Soul EV, Kia has been able to give Niro consistent and linear deceleration characteristics and feel unusual in an electrified car.

All-disc brakes are used, ventilated at the front, and there is a foot-operated parking brake. The anti-lock braking system includes Brake Assist, which ensures maximum retardation in an emergency stop, regardless of the force applied to the pedal by the driver.

Steering is via Kia's proven column-mounted motor-driven power-assisted system (C-MDPS), which gives the car a compact turning circle of 10.6 metres and requires less than 2.7 turns of the wheel between the extremes of lock. The steering gear has been mounted well forward and features an increased gear ratio and a larger electric motor for greater self-centring and responsiveness. Niro's agility is also helped by aluminium front suspension knuckles, lower arms and brake callipers, all of which save weight and counter inertia.

Niro ‘1’ and ‘2’ have 16-inch alloy wheels with 205/60 R16 tyres, while grades ‘3’ and First Edition are fitted with 18-inch alloys and 225/45 R 18 tyres.

Electronic Stability Control and Vehicle Stability Management, which collectively control skids when cornering or accelerating on surfaces with uneven levels of grip from left to right, are standard.

Passive safety provisions start with the rigidity of the body structure, which is reinforced in critical areas and features crumple zones front and rear and anti-intrusion beams at the sides. There are seven airbags, including one to protect the driver's knees; all-round height-adjustable headrests, height-adjustable front seat belts with an automatic tightening device in the event of emergency braking; ISOFIX child seat top tethers and anchors; and doors which unlock automatically after an impact.

Niro's exceptional refinement proved particularly challenging to perfect because of the hybrid powertrain. Contributing towards this are asymmetric engine mounts to minimise powertrain movements at the attachment points in the subframe, high-density under-bonnet insulation, an acoustic shield with a special support structure to improve the sound of the combustion engine when accelerating in the most commonly used speed ranges, and the adoption of a 6DCT gearbox in place of a CVT.

Road noise is minimised by high-insulation rigid bushes in the front subframe, high-strength steel in the rear wheel arches and additional insulation in the A- and B-pillars. The low drag co-efficient (Cd) of 0.29 contributes towards the suppression of wind noise, aided by laminated windscreen glass, a cowl over the front windscreen wipers, specially profiled door mirror casings and – as an example of the attention to detail of Kia's engineers – covers over the holes in the roof rails, which sit flush with the bodywork.