Sector: Supermini Price: £14,539 (est) Fuel: 65.7mpg CO2: 99g/km

With the tide of opinion starting to turn against diesels, driven by air quality concerns and the rising cost of complying with emissions regulations, it’s easy to foresee heavy oil disappearing from some segments of the market in the near future. Equally, it’s not difficult to predict which manufacturers will make that move first.

Suzuki dropped the diesel engine from the Swift last year and, with its substantial product line-up mostly consisting of small cars, it’s efficient petrol engines which are likely to yield the biggest economies of scale. What’s unusual with the range’s only sub-100g/km model is, it’s done without the complexity of downsizing and turbocharging which has become the norm.

Instead, the 1.2-litre DualJet engine, which joins the range early in 2015, achieves a 10mpg improvement in fuel economy and 16g/km reduction in CO2 emissions through detailed engineering of its four-cylinder petrol unit. It’ll be offered as a £500 upgrade over the current 1.2-litre petrol engine, and only offered as a two-wheel drive model in the top SZ4 trim.

The goal was improving the engine’s thermal efficiency. This is a measure of how much energy from a given amount of fuel actually ends up turning the wheels, rather than being lost as heat. The higher the thermal efficiency, the less fuel you need to drive the car and the more efficient it becomes.

So Suzuki reshaped and reduced the size of the combustion chambers, designing them to mix the air and fuel more effectively for improved ignition, in turn exerting a more powerful force on the piston and improving performance.

To offset the extra heat this produces, the DualJet uses two fuel injectors in each cylinder, one of which sprays a fine mist to bring temperatures down, while exhaust gases are recirculated back to the engine to further improve cooling. The engine also has optimised water and oil cooling, and a low-friction coating on moving parts.

The net result is a slight reduction in power, from the 94bhp produced by the other 1.2-litre engine, though torque has increased slightly with it. This is a still a typical Suzuki small petrol engine, delivering most of its performance right at the top of the rev range, but it offers plenty of performance for overtaking if you’re prepared to hang onto gears.

Drivers uses to the peppy performance of a small turbocharged petrol engine might find it an adjustment, though. The DualJet will happily cruise at low revs, but any increase in speed normally means dipping into a lower gear. On the upside, fuel economy of more than 55mpg is easy to achieve on a combined route.

It’s a good fit for the Swift, which is a good reliable and honest small car. Interior space is generous, though the boot is small in this class and Suzuki uses harder plastics than most. It’s priced keenly, even with the DualJet upgrade, and although the engine needs to be worked hard it’s quite good fun to drive as well – sure-footed, if a little short of reassuring steering feedback.

Suzuki may be a predominantly retail brand, but with a reduction in running costs – even if it’s just viewed as a tax advantage – this new petrol engine gives a good reason for fleets to take an interest in the Swift.

Verdict:

Quiet, smooth and fuel efficient, the DualJet is a great addition to the engine line-up offered in the Swift. But the need to explore the top end of the rev range is an adjustment for drivers used to the turbocharged petrol or diesel engines fitted to most rivals.