Actually, they have--and you're looking at the very first one.
The Volvo V60 Plug-In Hybrid is the first diesel plug-in hybrid vehicle. It will make its first U.S. appearance at next week's New York Auto Show, though as a display model only--it's not expected to be sold in the U.S. in its current form.
Instead, the U.S. iteration of Volvo's plug-in hybrid technology will likely be offered in a couple of years in the next-generation XC60 crossover.
Volvo showed an XC60 Plug-In Hybrid concept last year, at the 2012 Detroit Auto Show.
First diesel hybrid with a plug
The concept of a diesel hybrid has already been explored by Peugeot, Citroen and Mercedes-Benz, but Volvo is the first company to take the concept and add a plug-in battery pack that can recharge on grid power.
Volvo saw the potential in matching its 2.4-liter 'D5' five-cylinder turbodiesel engine with a rear-mounted electric motor and plug-in rechargeable battery. In theory, this could be the ideal mix for real-world economy.
The V60 is, like the recent Peugeot and Citroen diesel hybrids (also not sold in the U.S.), known technically as a "through-the-road hybrid."
A 215-horsepower, five-cylinder turbodiesel sits up front, and powers the front wheels through a 'Geartronic' torque-converter automatic transmission.
On that diesel engine, an 'Integrated Starter Generator' replaces the usual alternator, and as the name suggests can be used to start the engine (it's directly linked to the crankshaft) or used to top up the batteries--quite similar to the General Motors "eAssist" mild-hybrid system.
The large lithium-ion battery pack has a total capacity of 11.2 kilowatt-hours, though only 8 kWh of the pack is used--which Volvo says improves battery life significantly, as the pack is never fully charged or discharged.
The battery sends power to a rear-mounted electric motor, offering peak power of 68 horsepower and sustained output of 27 hp.
Most of the battery's energy comes from plugging in to recharge, but like any hybrid, the car boosts efficiency and regains a handful of miles via regenerative braking. The diesel engine's overrun can also put energy back in the battery if it's not needed to propel the car.
More numbers: This Volvo is quick, sprinting to 62 mph in as little as 6.1 seconds in 'Power' mode (maximum engine and electric outputs combined), with a top speed of 143 mph. Electric-only top speed is 78 mph.
Volvo V60 Plug-In Hybrid
Combined fuel consumption on the European cycle is 129 mpg. Volvo freely admits you're unlikely to see this number in daily driving, and it's simply what the car achieves in testing.
Like Chevy Volt or Toyota Prius Plug-In drivers, you will see either much less or much more, depending on how frequently you recharge, how long your journeys are and what you've had for breakfast. More useful, though equally variable, is the quoted 31 miles of electric range in 'Pure' mode.