Mercedes-Benz E-Class E300de 2019 UK first drive

With a healthy level of charge in the battery, you’ll usher in the engine only up really steep gradients or under full throttle. Doing so exposes perhaps the biggest flaw with the diesel-electric set-up: refinement. This is one of the less intrusive four-pot diesels in its class, but against the silky near-silence of electric mode, its presence at lower speeds is noticeable enough to have you back off to try and convince the system to shut it down again.

There’s also another flaw with the powertrain in Comfort driving mode. Decide you need a burst of power to pull smartly out of a junction and it sometimes dithers for a split-second, seemingly unable to decide whether to switch on the engine or not before firing away. 

Nevertheless, once above 50mph, where executive cruisers like this are most at home, the transition between diesel and electric power is relatively seamless. When the engine does kick into life, it balances driving the wheels and releasing charge into the battery. 

The result is impressive real-world efficiency: a 110-mile round trip on mixed motorway and A-roads with a full charge at each stop yielded nearly 130mpg, with the electric motor managing to match its official range. But the figures impress even when you’ve run the battery down; more than 50mpg is easily achieveable if you’re the type to buy a plug-in hybrid and never plug it in. 

If response is key, Sport and Sport+ driving modes ensure the engine is always on, balancing the two power sources to deliver strong, effortless performance at all speeds. It’s particularly efficient from a standing start, with the electric motor delivering instant torque before the engine spools up to its peak. It lacks the smoothness of a six-cylinder diesel, but given the efficiency benefits, it's an acceptable trade-off.

As usual, you have the option to hold the E300de in electric mode, rely on diesel power and save the battery at a later date or sacrifice some efficiency to charge up the battery via combustion and brake regeneration. The latter system also has a neat feature: even in our SE model without adaptive cruise control, it uses the distance sensor to apply regenerative force when the car in front begins to slow. The subtlety with which it does this makes it far less annoying than it sounds.

Compromises? Well, the E-Class’s platform isn’t as easy to adapt to hybrid propulsion as newer rivals from Audi and BMW, and that shows in the substantial 360kg weight increase over the equivalent E220d. It makes what was never the class’s most dynamic steer that bit more ponderous and less controlled.

The bulk has also had an effect on the ride. It’s still very comfortable and extremely adept at mile-munching, but a little of the regular E-Class’s lovely suppleness has been lost, presumably because the suspension has been firmed up a touch to compensate for the weight. The lack of adaptive dampers or all-round air suspension on our car didn't help matters; both are options you’ll want to consider fitting. What's more, the brakes can be difficult to modulate, due to a numb-feeling pedal, as is common with regenerative systems.

As we’ve become used to with PHEVs, there’s also a practicality sacrifice. The boot drops 140 litres from the regular saloon's to 400 litres, with an awkward cut-out rising from the floor. Happily, unlike the E300e, the E300de is available in estate form to combat this.