Which has the best engine?

Diesel engines dominate this sector of the car market, and will account for no less than 90 per cent of new 5-Series sales in the UK. The mainstay of the firm’s fleet will be the four-cylinder 520d, which arrives this summer, but the six-cylinder car offers a tantalising combination of power, low emissions, refinement and economy.

The 525d and 530d models use the same 3.0-litre all-aluminium straight-six, yet in the more expensive variant tested here power output is rated at 242bhp. The unit also produces 540Nm torque, making it the most potent car in our line-up on paper.

In comparison, the Mercedes’ 3.0-litre V6 diesel puts out only 228bhp, yet it matches the BMW’s punch with an identical 540Nm of torque. The Audi slots between the two, with 237bhp, although its 500Nm output trails those of its rivals.

At start-up none of these silky smooth six-cylinder units lacks refinement, but on the move the BMW is the most impressive. The CDI unit in the E-Class sounds ever-so-slightly gruff after a stint driving the 5-Series, while Audi’s TDI can’t quite match the BMW for refinement, either.

The A6 loses out when it comes to power delivery, too, as it lacks the linear nature of its rivals. While the BMW serves up an almost seamless wave of torque, the Audi’s muscle arrives in a more noticeable surge. This is accentuated by our test car’s six-speed Tiptronic gearbox, which is too keen to kick down. With fewer ratios than its rivals, the Audi seems to hunt for gears more.

The BMW’s smooth-shifting eight-speed automatic is a £1,495 option, and it’s a delight to use. Spend an extra £110 and you can specify steering wheel-mounted paddles to control manual changes, but even without them the wand-like gear selector gives excellent transmission control.

A self-shifter is standard on the E350, and the 7G-Tronic set-up is keener to shift down a gear under heavy acceleration. It cleverly skips gears when appropriate, too. However, without complete manual control, we could record performance figures for the Mercedes only in kickdown.

While all three of our contenders are closely matched, the heavier E-Class was outpaced at the track. It was four-tenths-of-a-second slower than the 530d from 30-50mph, and took six-tenths longer to accelerate from 50-70mph. The 5-Series covered 0-60mph in 5.8 seconds, and it feels the fastest and most responsive choice.

It is effortlessly rapid with strong mid-range responses, and with so many ratios to choose from it was considerably quicker than the Audi in all our in-gear tests. The eight-speed box aids refinement and economy, too. At 70mph, the newcomer cruises at just 1,500rpm, while the six-speed Audi registers 2,200rpm. The BMW is also the most efficient car, emitting 160g/km of CO2 – considerably less than the Mercedes and Audi, which both put out over 180g/km.

Plus, during the course of our test, the 530d delivered the best economy, so it’s a clean sweep for pace, emissions and efficiency...

BMW: 5 stars Faster, cleaner and more fuel-efficient than its rivals, the 37.4mpg BMW has a clear advantage in this company. Performance is superb – and that’s before you consider its CO2 emissions. A figure of only 160g/km is amazing for an executive saloon. Unusually, the auto model is greener than the manual variant; go for the standard gearbox and output rises to 166g/km.

Mercedes: 4 stars While the BMW has a clear lead in the engine department, the Mercedes’ 3.0-litre V6 CDI delivers punchy performance. It gives the E-Class the edge over the Audi in the sprint from 0-60mph. However, at the pumps we averaged only 32.2mpg during our time with the E350. Unlike its rivals there’s no manual gearbox, so the standard 7G Tronic unit is the only choice.

Audi: 4 stars The tried-and-tested 3.0 TDI delivers decent refinement and strong performance, yet it’s not class-leading in either category. The £1,400 optional six-speed Tiptronic box raises CO2 emissions by 10g/km, to 189g/km. That makes this the dirtiest engine on test. It’s also the least frugal on the combined cycle, although we averaged 33.9mpg – which was better than the E350 managed.