Sector: Lower medium Price: £20,640–£26,700 Fuel: 3.7–3.9l/100km CO2: 97–104g/km

Opel has been subtly but effectively updating its core models for over a year now, and the Astra is the latest to get a small but important upgrade for its fleet presence, in the form of a new Euro 6 compliant 1.6-litre diesel engine.

The 1.6 CDTi replaces GM’s long-running, hard-wearing 1.7-litre diesel range. So this single engine, which is already available in the Zafira Tourer and Meriva, and is likely to be powering the Insignia in the near future, stands to be a staple of Opel’s future fleet presence.

It’s a thoroughly modern unit, too. Opel has used a variable vane turbocharger to offer smooth and progressive power delivery, a new multiple injection common rail system for a more efficient combustion cycle and fitted a NOx trap in the exhaust to meet Euro 6 requirements. The six-speed gearbox is also new.

Like its predecessor, the 1.6 CDTi is available in two power outputs – with 110hp and 136hp – and headline fuel economy figures have improved with it. Likely to be the fleet engine of choice, the 110PS version emits 97g/km CO2 and consumes 3.7l/100km, marginally less than the 99g/km of the equivalent 1.7 CDTi.

Unfortunately, this wasn’t available to test. Instead, we spent some time with the 136hp unit which, in its most efficient form, consumes 3.9l/100km and emits 104g/km CO2. That’s actually slightly higher than the nearest equivalent 1.7 CDTi, at 3.7l/100km and 99g/km, though Opel has hinted at further improvements in the near future.

CO2 emissions of less than 95g/km, at least for the 110PS version, would certainly be helpful with PSA’s Euro 6 compliant BlueHDI 120 offering more power (120hp) and CO2 emissions from 82g/km in the Astra-rivalling Peugeot 308. Even the larger Insignia isn’t far off the Astra, with 2.0 CDTi versions now down to 98g/km.

But refinement is impressive for a small engine. GM calls this the ‘Whisper Diesel’ and, even from cold, it lives up to its nickname. There’s a barely audible whirr, but almost no rattle even when idling and the engine is practically silent while driving. It’s a huge change from the 1.7 CDTi, which was far from a noisy engine, but was starting to fall behind rivals on NVH.

Power delivery is strong and, even in the more powerful version, it lacks a mid-range spike which some downsized engines can suffer from. While the claimed economy isn’t the best in this class, Opel has form for coming close to its published figures in real-world use and a 1.6-litre engine is neither overpowered nor overwhelmed in a car of this size, which should help.

It’s also worth remembering that this generation of Astra is close to retirement. Spy shots have already shown its replacement being tested under heavy camouflage, rumoured to launch during 2015. So while improvements to fuel economy are minor in this car, expect the Whisper Diesel to have fuel economy to really shout about when it makes its way under the bonnet of this car’s replacement.

Verdict:

Refined and powerful, the 1.6 CDTi is a delight to use, but Opel needs to target further reductions in CO2 and fuel consumption to stay ahead.