Engineering

Stonic is sold in the UK with a choice of three powertrains which conform with Kia's downsizing policy while employing advanced technologies like turbocharging, direct injection and stop/start systems to minimise fuel consumption and emissions while ensuring outstanding performance. All versions have a six-speed manual gearbox.

The car is based on the platform and running gear of the latest Rio supermini, but there has been extensive tuning work to take into account its greater ground clearance, higher centre of gravity and increased weight. The suspension features independent MacPherson struts at the front and a torsion beam axle at the rear. Ride and handling benefit from the stiff body shell, which is made of 51 per cent advanced high-strength steel.

Refinement has been enhanced through detailed work on the aerodynamics, body structure and insulation, while active safety benefits from a number of advanced driver assistance features.

The 1.0-litre T-GDi engine

Kia has embarked on an ambitious policy of reducing average fuel consumption and CO2 emissions of its model range by 25 per cent by 2020, based on 2014 levels. It aims to do this through engine downsizing, more efficient combustion systems and the addition of alternative-fuel vehicles where appropriate.

The 1.0-litre T-GDi engine Stonic fits in perfectly with this strategy by delivering exceptional power and torque, despite its small capacity, through turbocharging and direct injection. The T-GDi unit sprays a fine mist of fuel directly into the cylinders through a high-pressure injection system which ensures it uses only as much fuel as necessary for the load being put on it. The result is highly efficient combustion with excellent performance. Direct injection with turbocharging also helps to boost low-speed response and driveability.

In Stonic the 1.0-litre T-GDi three-cylinder 998cc engine with four valves per cylinder develops 118bhp at 6,000rpm and 171Nm of torque continuously from 1,500rpm to 4,000rpm.

The key targets were instantaneous response, highly efficient combustion and exemplary torque across a wide portion of the rev band. These goals have been more than met with the aid of innovative technical solutions. The T-GDi engine features laser-drilled injectors with six holes laid out in a pyramid shape so that the fine mist of fuel is spread more evenly throughout the cylinders than if it was being consistently sprayed into certain points. Thanks to a high-pressure pump, injection is up to 200 bar.

The T-GDi engine has a straight intake port which ends in a sharp throat, reducing air resistance at all stages of the process. This improves cylinder tumble flow for faster, more efficient combustion while suppressing engine knocking.

There is a single-scroll turbocharger paired with an electric wastegate motor. This improves turbocharger performance while scavenging clean air for the engine to re-use for combustion. At the same time it allows the wastegate to open to improve the flow of spent exhaust gases. It is an innovative system which allows higher low-end torque, more immediate response at any throttle opening and improved fuel economy at high engine loads.

The engine is fitted with an integrated exhaust manifold which reduces exhaust gas temperatures, bringing the benefits of higher speeds with greater fuel efficiency. Lower temperatures also result in cleaner emissions by allowing the catalytic converter to operate more effectively. Engine temperatures are closely regulated by a dual-thermostat split cooling system, which allows the block and cylinder heads to be cooled independently. The main thermostat controls the flow of coolant to the cylinder heads above 88º C to reduce knocking, while the engine block thermostat shuts off coolant flow above 105º C to reduce friction and improve efficiency.

A number of factors contribute towards the exemplary driveability and efficiency of the all-aluminium T-GDi unit. There is continuously variable valve timing on both the inlet and exhaust sides, electronic throttle control and light, low-friction moving parts. The crankshaft is offset from the centre-line to aid smoothness.

The turbocharger is integrated within the exhaust manifold in a one-piece casting, improving sealing while reducing weight. A number of detailed engineering solutions minimise throttle lag – the delay between the driver pressing the accelerator and the turbocharger delivering boost – and reduce internal friction.

For added durability, the cylinder block has been heat-treated and the crankshaft, pistons and connecting rods have been strengthened.

The T-GDi engine accelerates Stonic from standstill to 60mph in 9.9 seconds on the way to a top speed of 115mph. Combined fuel consumption is 56.5mpg, with CO2 emissions of 115g/km.

The 1.4-litre multi-point injection engine

The entry-level 1.4-litre MPi (multi-point injection) petrol engine in Stonic grade ‘2’ is an all-aluminium 16-valve 1,368cc unit from the Kappa family.

It features performance-enhancing continuously variable valve timing on both the inlet and exhaust camshafts. This ensures high torque at low engine revs for good driveability in cities, and excellent power for the fast motorway speeds common in Europe.

It develops 98bhp at 6,000rpm and 133Nm of torque at 4,000rpm. Acceleration from 0 to 60mph takes 12.2 seconds on the way to a top seed of 107mph, with average fuel economy of 51.4mpg and CO2 emissions of 125g/km.

The entire engine and transmission weighs only 82.4kg. The main block features a ladder frame for structural stiffness, while the cylinders are fitted with cast-iron liners for improved durability. Weight was further reduced by integrating the engine support bracket with the timing chain cover. The shape of the piston skirt was optimized to reduce its size, while the compression height of the piston has also been reduced, resulting in weight savings.

An accessory drive belt which does not require mechanical auto-tensioning further lowers weight and cost. Because it is designed to maintain an ideal tension setting, the belt runs quieter. There are long-reach spark plugs creating space for a larger water jacket to promote more efficient engine cooling while permitting an increased valve diameter for better airflow and combustion efficiency. Lightweight, heat-resistant plastic is used for the intake manifold.

The valvetrain features friction-reducing hydraulic lash adjusters which ensure proper clearance between the valve stem and roller swing arm to reduce valve tapping noise. The valve springs have a beehive shape. The valvetrain is driven by a silent-type, maintenance free steel timing chain.

The centre line of the cylinder bore is slightly offset to minimise the lateral force created by the rotating piston and rod assembly. The net effect is an improvement in fuel consumption and a reduction in noise, vibration and harshness.

The 1.6-litre CRDi diesel engine

The CRDi turbodiesel engine in Stonic is a 1.6-litre unit from the European-designed and European-built U2 range. It features a high-pressure injection system and a continuously controlled variable oil pump to reduce friction losses. Power is 108bhp, while the maximum torque is 260Nm, available from 1,500 to 2,750rpm.

It allows Stonic to accelerate from 0-60mph in 10.9 seconds on the way to a top speed of 112mph. Fuel economy is up to 67.3mpg, with CO2 emissions of 109g/km.

The engine capacity is 1,582cc and there are double overhead camshafts operating 16 valves. The block and bedplate are formed from cast iron while the oil pan is made from aluminium.

The U2 engine employs latest-generation common rail diesel injection, a variable geometry turbocharger which adjusts the flow of exhaust gases by altering the angle of the turbo vanes according to engine requirements, and variable swirl control to increase swirl at low-to-mid loads to reduce emissions and decrease it at high loads to maximise power.

There is an electronically controlled exhaust gas recirculation (EGR) cooling system and a 1,600-bar injection system capable of delivering up to five injections per cycle, permitting precise control of the amount of fuel being delivered directly to each cylinder. Other refinements include chain drive for improved reliability and a bed plate–type lower crankcase for better block rigidity and lower noise, vibration and harshness. The auxiliary belt is a serpentine–type for improved reliability and compact packaging.

UK line-up

Model
Power bhp
Torque
Nm
0-60 sec
Max speed mph
Comb. mpg
CO2 g/km
'2' 1.4 MPi 98bhp 6-speed manual ISG
98
133
12.2
107
51.4
125
'2' 1.0 T-GDi 118bhp 6-speed manual ISG
118
171
9.9
115
56.5
115
'2' 1.6 CRDi 108bhp 6-speed manual ISG
108
260
10.9
112
67.3
109
'First Edition' 1.0 T-GDi 118bhp 6-speed manual ISG
118
171
9.9
115
56.5
115
'First Edition' 1.6 CRDi 108bhp 6-speed manual ISG
108
260
10.9
112
67.3
109

All versions of Stonic are fitted with Kia's Intelligent Stop & Go (ISG) engine technology which turns off the engine when the car is stationary in traffic and the driver puts the gear lever into neutral and releases the clutch pedal. The engine restarts as soon as the clutch pedal is pushed.

ISG consists of crankshaft position, battery and vacuum sensors plus neutral, on-off and clutch switches that feed into an electronic control unit. This operates the ISG starter, intelligent alternator and cluster. The air conditioning units and bonnet switch also feed into the ISG ECU.

The crankshaft position sensor measures the crank angle during engine run-out and monitors it while the vehicle is stopped, ensuring the starter is activated for as short a time as possible by optimising cranking and combustion. The battery sensor monitors the battery condition and temperature, while the clutch and neutral switches recognise when drivers wish to continue driving and ensure the engine is started. There is a brake booster pressure sensor to make sure the engine continues to operate if brake boost falls too low.

A heavy-duty maintenance-free AGM (Absorbent Glass Mat) battery delivers the power necessary to run the system, and intelligent alternator management reduces drain on the battery while accelerating and recharges the battery when coasting and braking.

All the driver has to do is stop, put the car into neutral and lift his or her foot off the clutch. After a brief pause, the engine cuts out. It restarts as soon as the driver pushes the clutch. The system has been engineered not to stop the engine during warm-up from a cold start or if the air conditioning system is working hard. Drivers can also manually turn off the ISG system through a switch on the dashboard.

Sporty driving characteristics

Stonic is based on the platform and running gear of the latest Rio supermini and has an identical 2,580mm wheelbase, but has been tuned to take into account its greater ground clearance (up by 42mm), higher centre of gravity and increased weight, and to give it slightly more sporty driving characteristics. The suspension features independent MacPherson struts at the front and a torsion beam axle at the rear. Stonic's responsive steering is aided considerably by a stiff body shell which is composed of 51 per cent lightweight high-strength steels.

Spring and damper settings take advantage of the stiff body, which allows the suspension to do its work without having to compensate for flexing of the car's structure. The rear dampers are almost vertical, while those at the front benefit from pre-loaded linear valve technology, delivering more consistent handling and suspension response over broken surfaces.

Pre-loaded linear valve technology introduces a completely new piston design which not only increases driving comfort but also ensures excellent isolation of vibrations in the vehicle body. Rapid opening and closing processes in the valve ensure outstanding wheel damping, which adds to safety by improving handling precision. In addition, innovative piston geometries further optimise the damper's noise emissions.

The gearbox for the motor-driven power steering (MDPS) is mounted as far forwards as possible, improving feel for the driver, while the number of teeth on the steering's serration column shaft enhances off-centre feel while contributing towards an improvement in noise, vibration and harshness. This all results in faster steering responses with greater feedback and increased driver confidence.

The column-mounted MDPS requires just 2.52 turns between the extremes of lock for a tight turning circle of 5.2 metres. Ventilated front disc brakes and solid rear discs are fitted across the range and supported by anti-lock (ABS), electronic brake force distribution (EBD) and a Brake Assist system (BAS). Collectively, these allow Stonic to be steered and braked at the same time without risk of going out of control, ensure that most braking effort goes to the wheels which are best able to take advantage of it, and automatically deliver maximum stopping power in emergency braking, regardless of the pressure applied to the pedal by the driver.

Alloy wheels of 17 inches in diameter are standard, with 205/55R17 tyres. All models have a tyre repair kit in place of a spare wheel.   

Anything for a quiet life

Stonic is not only an engaging car to drive; it is also a quiet one to travel in as a result of a comprehensive package of NVH counter-measures

The insulation of the cabin from engine noise has been improved by the fitment of an elongated upper cowl panel and thermoplastic elastomer materials on the dash. What engine noise does enter the cabin is more sporty sounding thanks to ducting material on the intake side of the engine, while on the exhaust side there is a three-baffle system with a dual tube and increased duct ratios.

Road noise has been reduced by the addition of a reinforcing bracket to increase the structural rigidity of the front subframe. The cowl top panel and kick panel between the rear seats have been shaped to reduce noise radiation, while five rubber seals in the pillars on each side of the car provide further insulation against outside noise.

The best of active and passive safety

The ultra-stiff body shell has beneficial effects beyond handling and comfort. It provides a greater barrier against injury in the event of an accident.

Structural additions include a partitioned inner assembly of the front strut mount, strengthened connections in the C-pillar cross-member and the application of structural adhesives on major chassis components. Advanced high-strength steels reinforce all major chassis parts, and there are multiple load paths to dissipate crash energy in the front of the car.

Electronic Stability Control (ESC) and Vehicle Stability Management (VSM), which work together to stabilise the car on slippery road surfaces or when simultaneously cornering and braking, are joined by Straight Line Stability, which senses any difference in applied brake pressure between the right and left of the car and intervenes to keep it straight. Stonic also features Cornering Brake Control, which delivers asymmetrical brake pressure when braking in tight curves to counter loss of traction, and Torque Vectoring, which brakes an inner wheel if the car is in danger of running wide in corners, bringing it back onto the driver's desired line.  All versions have Hill-start Assist to prevent the car from rolling backwards when setting off on steep inclines.

There are six airbags with pre-tensioners and load limiters to brace occupants in their seats in extreme braking or if an accident is about to happen, and to help prevent injury to chests. A visual and audible seat belt reminder warning is fitted, and there are ISOFIX child-seat mounting points.