Automotive Engineer

Small cars have always had small engines but these days their displacements are falling – and so too is the number of cylinders. An inline four gasoline engine has long been the default low-cost configuration but the need to cut fleet average carbon emissions is making it increasingly unattractive.

Thermal efficiency and friction losses are relatively high. And engine weight is coming under greater scrutiny as OEMs look to make every possible saving to cut fuel consumption.

Diesels are inherently more efficient of course – even more so with only three cylinders – but the higher cost compared to gasoline engines makes them less well suited to A-segment applications, and the packaging requirements of the exhaust aftertreatment system can be a challenge for small vehicles.

So when Kia started work on its third-generation Picanto model, a new three-cylinder gasoline engine was developed in parallel as part of the Kappa engine family. It’s a modular design, with bore and pitch common with the four-cylinder variants, but the focus for the triple was the lowest possible fuel consumption.

“We’re convinced that for a one-litre naturally aspirated engine a three-cylinder is the best compromise,” says Dr Joachim Hahn, powertrain engineering manager at Kia’s European technical centre. “We had a clear target on CO2 emissions, which makes a three-cylinder more attractive than a four. We also investigated a two-cylinder but we couldn’t offer one without some measures to overcome the specific NVH challenges. “Our engine doesn’t have a balancer shaft – we don’t think this would be possible with two cylinders, which would limit the potential for cost reduction and, even harder for us, friction reduction,” says Hahn.

Fiat has invested heavily in its 900cc two-cylinder gasoline engine family, available in naturally-aspirated and turbocharged variants, both with electrohydraulically controlled inlet valves.

But the Fiat 500 is a premium A-segment model – the additional cost of its powertrain is easier to build into its margin than it is for Kia with the Picanto. So the Kappa engine is conventional in its design, but features technologies selected to provide consumption reductions at affordable cost.

Each of its two camshafts is fitted with a cam phaser – that’s a first-in-class feature, says Hahn. The extra freedom this gives to valve operating strategy delivers higher ratings and lower emissions.

“It enables us to have high low-end torque and high peak power but we also use the phasers across the whole speed range for internal exhaust gas recirculation,” says Hahn. “This is an effective measure for dethrottling the engine, with direct influence on fuel consumption not only on the NEDC but also in customer use.

“Another reason is it helps us to retard the intake and advance the exhaust valve timing at idle – this is good for comfort because the NVH is a challenge with three cylinders.”Further improvements in refinement come from the weight-optimised pistons and conrods. Reducing the moving masses in the cranktrain helps to lower the three-cylinder’s unbalanced first-order moments. A stiff cylinder block also contributes to the low vibration levels. Hahn says that Kia’s benchmarking studies showed that the Kappa engine was better than the competitor engines here.A further benefit was that a lower inertia flywheel could be used to damp the torsional vibration. This contributes to the engine’s low weight and also helps it rev very easily.