If the answer is key, the Sport and Sport+ driving modes ensure that the engine is always running, balancing the two power supplies to deliver solid, effortless performance at all speeds. Its particularly efficient from a standing start, with the electric motor delivering instant torque before the engine coils to its peak, lacks the smoothness of a six-cylinder diesel, but given the efficiency benefits, its an acceptable trade-off.
As usual, you have the choice of keeping the E300 in electric mode, relying on diesel power and saving the battery at a later date or sacrificing some efficiency to charge the battery through combustion and regeneration of the brakes. The latter system also has a clear feature: even in our SE model without adaptive cruise control, it uses the distance sensor to apply regenerative force when the car in front starts to slow down. The subtlety with which it does so makes it much less annoying than it seems.
Commitments? Well, the E-Class platform is not as easy to adapt to hybrid propulsion as the new rivals Audi and BMW, and that is reflected in the considerable weight increase of 360 kg over the equivalent E220d. This makes what was never the most dynamic ox in the class a little heavier and less controlled.
Volume has also had an effect on driving: its still very comfortable and very good at mileage, but a little of the charming flexibility of the regular E-Class has been lost, presumably because the suspension has been reasserted a little to compensate for the weight. The lack of adaptive shock absorbers or full air suspension in our car didnt help at all; both are options you should consider installing. In addition, brakes can be difficult to modulate, due to the numbness of the pedal, as is common in regenerative systems.
As we have become accustomed to PHEVs, there is also a sacrifice of practicality. The luggage compartment is 140 litres lower than the normal 400-litre saloon, with an uncomfortable opening that rises from the floor. Fortunately, unlike the E300e, the E300de is available in state form to combat this.