Many of us would have started out on our personal mobility journey by driving cars with 3-cylinder engines. We would have experienced the gruff, unrefined and under-powered 3-pot powertrains of the past. They were high on vibration and noise, while delivering uninspiring on-road performance. That frugally-engineered, built-to-a-price 3-cylinder engine meant to offer the maximum bang for the buck in the small car segment is slowly fading away. In its place a new breed of turbocharged, direct-injected 3-cylinder petrol mill is taking over and bringing the fun back into motoring even while it manages to meet the more stringent emission norms that are currently in place globally.

Emission reduction is just one reason why 3-cylinders are finding their way into Indian B segment cars. The petrol-diesel fuel price differential has all but vaporised and that has led to a marked shift in preference in favour of the former. Disincentivising diesel has taken it out of the mix for many prospective buyers, but they will still miss the torquey, inherently more fuel efficient nature of oil burners. On the other hand, regular 4-cylinder petrol engines just can’t seem to match the tractability of diesels and can’t seem to offer a convincing balance between performance and efficiency. The focus is always skewed towards improving mileage. This is where turbocharged, direct-injected petrol 3-cylinders have come in to fill that performance gap.

Drab to drool-worthy

Driving the Kia Sonet 1.0 T-GDi earlier this week was a sort of reminder of how satisfying this engine can be for younger buyers. That buyer is looking for a peppier powertrain, that doesn’t lack in refinement and yet can also not be a drain on his purse. These are customers who are still buying a sub-4-metre vehicle and so their big picture expectations include mileage and cost of maintenance as parameters to consider. But, the sports utility vehicle is the more aspirational body style today and that also changes the mindset of the buyer.

The generic problem for 3-cylinder engines has always been lack of refinement arising from the basic construction and the characteristics of the combustion cycle. For long, they have been assumed to be straight-6 or inline-6 cylinder engines that have been cut in half. Some of that must have been due to the firing sequence of the odd number of cylinders. So, when the first cylinder starts the firing sequence, there are no reciprocating cylinders (unlike in a 4 or 6 cylinder engine), making the engine’s rotational torque uneven and thereby prone to higher primary and secondary vibrations. Of course, today’s more refined 3-cylinders adopt a more controlled 1-3-2 cylinder sequence that enables lowering torque imbalances and crankshaft vibration.

In most 3-cylinder engines the occurrence of vibration is higher at slower speeds due largely to the fewer number of power strokes (number of times the cylinders are fired). This tends to even out at higher speeds and at higher engine rpm levels. To contain the rotational forces that literally shake the 3-cylinder engine, multiple measures are adopted. Last week’s experience with the new Kia Sonet’s G 1.0 T-GDi engine reveals how it manages to deliver on its promise of performance and efficiency.

Sonnet in the bonnet

Opening the engine bay in the Sonet reveals more of the inherent advantages of 3-cylinders. These engines are compact, lighter overall in weight and can consequently be easily fitted into smaller engine bays typically found in sub-compacts and hatchbacks. The Sonet’s 1.0 T-GDi is a 998cc, turbocharged, 3-cylinder that delivers 120hp of peak power and 172Nm of torque. Those are already impressive performance figures for an engine in this size class. But the key aspect of this output is the way peak torque is available from a low 1,500rpm and stays available to exploit all through the mid-band till the needle touches 4,000rpm. I was driving the GTX+ variant that features the 6-speed iMT (intelligent manual transmission), the innovative clutchless manual gearbox, which frankly needs a bit of experiencing to fully get comfortable with. But audible warnings and suggestions, on the MID within the instrument cluster, to shift gears and optimise performance in accordance with the engine speed will make you a quick learner.

So, how have the Sonet’s engineers at Kia managed to cut vibrations? First, the combination of turbocharging and direct petrol injection ensures much improved combustion, increasing the power and torque output and at the same time cutting down the incidence of primary vibrations. The fuel is also fed into the engine at a constant pressure of 250 bar ensuring complete combustion and lower emission generation. The turbo in use in the Sonet is an electronically controlled wastegate actuator turbocharger, which enables better charge pressure control, improved throttle response and also helps in light-weighting of the engine. The engine’s exhaust manifold is also integrated within the cylinder-head and can therefore be cooled efficiently using the cylinder-head water cooling system.

On the road, the powertrain’s behaviour is predictably peppy and the iMT that I was driving was actually a little better than the DCT thanks to taller gear ratios. There is a bit of turbo lag, but once the forced induction starts its magic, the power is delivered in dollops. It was easy to push the Sonet to make quick overtakes while continuing to stay in third gear or to simply floor the pedal and watch the needle climb (well past the 100kmph mark when tested in closed sections of the road). There is an initial bit of throaty idling and even some mild thrumming vibration at the wheel, but once the engine settles, there is barely any difference in refinement levels between this and the other 4-cylinder, 1.2-litre petrol engine that is also offered as an option. Cabin isolation is also good and the NVH packaging ensures that other than at very high engine rpm levels, there is not much of a hint that it is a 3-pot in the bonnet. Other measures to counter engine vibration include external dampers, counter balance weights and special engine mounts.

There is a flip side to turbo petrol 3-cylinders; they are not the most frugal when it comes to fuel consumption. The rated mileage for the two petrol engines on offer in the Sonet is set next to each other at about 18kmpl. The best I managed was about 11kmpl; but being more judicious with your driving style will help get more mileage out of this engine. The other gearbox option with the 1.0 T-GDi is of course the 7-speed dual clutch (DCT). More than 35 per cent of all Sonets sold till now are of the G 1.0 T-GDi, and of this 60 per cent are of the iMT variants and the remaining 40 per cent are of the DCT gearbox.

The coming of age of the turbo 3-cylinder is clearly upon us and unless there is going to be a significant new advance in the technology of the internal combustion engine, the lower manufacturing costs and the ability to handle emission regulation is going to drive an increase in the adoption of 3-cylinder mills. And it is not just mass market brands, but also luxury brands like BMW that are looking to reap the benefits of this engine class. And more innovations like cylinder-deactivation are slowly being introduced to further improve the efficiency of this engine type.

2021 will definitely see more vehicles that feature this engine.

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