Unlike some other hybrid systems, this is completely modular and very compact. It adds 100kg to the kerb weight of the E250 CDI on which the model is based, and fits an unmodified bodyshell which allows it to be manufactured in two body styles and built on the same production line as the standard car. This also allows the E-Class to gain hybrid drive without losing any boot or interior space, or adding significant cost.

Despite its size, it allows the car to offer the performance of a six-cylinder diesel, but with 15% lower fuel consumption than a four-cylinder. The components are compatible with a petrol engine, which means Mercedes-Benz can offer a petrol-electric E-Class hybrid in other global markets, and the technology is scalable should the carmaker choose to fit it to higher or lower performance models.

Here’s how Mercedes-Benz managed to fit its clever hybrid system into the E-Class.

Components:

-          Engine – Because the Mercedes-Benz hybrid system is modular, the E-Class can be offered with two different drivetrains to suit different buying habits. America, Japan and China will get an E400 Hybrid, with a V6 petrol engine, while the European-market E300 BlueTEC Hybrid uses the same 202bhp four-cylinder diesel engine as the E250 CDI.

-          Electric motor – Integrated into the transmission casing, the disc-shaped 27bhp internal rotor magneto motor is sandwiched between the engine and gearbox. This provides 184lb.ft of torque instantly and is compact enough to fit into a 65mm space, meaning the gearbox position is almost the same as on the conventional car.

-          Transmission – The E300 BlueTEC Hybrid uses the same seven-speed 7G-Tronic Plus gearbox as the conventional E250 CDI, but without the standard car’s hydraulic torque converter. Instead, the hybrid has a wet clutch system, as found on the carmaker’s high performance AMG models, which allows the engine to be completely decoupled for electric driving.

-          Hybrid battery – Most hybrids use a large battery pack under the cabin or boot floor. By using an energy-dense lithium ion unit, the E-Class uses a small hybrid battery installed where the standard car battery would be under the bonnet. This also provides power to the on-board systems via an inverter.

-          Electronics – Unlike the E250 CDI, the E300 BlueTEC Hybrid has electronically-operated steering, brakes and air conditioning systems, which means all are available at full power when the car is driven electrically and when the engine is stopped in traffic.

 How it works:

-          Low speed electric – The hybrid system runs in full electric mode on startup, and is capable of driving up to 0.6 miles at speeds up to 22mph without using the petrol engine. This allows the car to undertake most manoeuvring and pull away at low loads on electric power, provided there is enough charge in the battery.

-          Hybrid – Cruising at a constant speed, the E-Class switches automatically between the electric motor and diesel engine and a combination of both, depending on battery charge, load and gradient. To boost fuel economy, the engine control unit can set the diesel engine to run at its most efficient speeds.

-          Electric boost – If extra power is needed, the E-Class can use the electric motor to provide a boost to the diesel engine. By combining its two power sources, the E300 BlueTEC Hybrid can match the output of the six-cylinder E300 CDI, while offering 35% lower fuel consumption and CO2 emissions.

-          Sailing – At speeds of up to 99mph, the hybrid system can shut off and decouple the diesel engine, coasting or “sailing” silently electric power. This is activated automatically when the driver lifts off the throttle slightly and upshifts in top gear. Because all ancillaries are electronically operated, the sailing mode doesn’t affect the car’s air conditioning.

-          Energy recuperation – When the car is coasting downhill, decelerating or braking, the electric motor acts as a generator, quickly topping up the hybrid battery. This provides enough resistance that the wheel brakes aren’t used until heavy braking is required.