The E300de is good enough to make us reconsider the whole diesel PHEV format, which conveniently disappeared just as the public attitudes towards the fuel started to change.
Under the bonnet is the familiar 'OM654' 191bhp 2.0-litre four-cylinder diesel engine that does thoroughly decent service in the E220d. It's mated to an electric motor offering up 120bhp and, more importantly, 325lb ft of torque. The combined system output is a healthy 302bhp and 516lb ft - 14bhp less than the petrol-hybrid E300e but with the same torque output.
This powertrain resolves one of the big stigmas of oil-burners: its healthy 34-mile electric-only range from a 13.5kWh lithium ion battery means any concern over pollutants in cities are quashed. That range is obtained from a 1.25-hour charge from a 7.2kWh charger or five hours via a three-pin household plug.
Bring it into life - silently, of course - and it defaults to Comfort and Hybrid modes. This is a happy medium for the most part, as the powertrain will opt for electric power as much as it can. Measure your throttle inputs (i.e. don’t just clog it everywhere) and it allows a decent turn of pace in electric mode – easily enough to get up to motorway speeds without struggle.
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.