Mazda working on high-tech rotary hybrid

Mazda has been caught patenting a top-secret next-generation hybrid powertrain that employs a small rotary engine to drive the rear axle.

The drawings, unearthed by

The patent filings show a traditional front-engine, rear-wheel drive car but differ from, say, a BMW 3 Series in the use of two in-wheel electric hub motors for the front axle and a third motor at the back of the engine.

Powering all three electric motors are both a capacitor and a small lithium-ion battery.

As well as a rotary engine, Mazda's patent also suggests the use of a more conventional inline four-cylinder and what's thought to be a V6, perhaps for a high-performance variant. Presumably this could also be switched for Mazda's new inline six.

Until now, in-wheel electric motors have been ruled out by the majority of car-makers because of the unhelpful amount of weight they add to unsprung mass – which impacts both handling and ride comfort – but Mazda appears to have solved this problem by using compact ultra-lightweight motors.

Saving yet more weight overall, Mazda appears to have avoided plenty of unwanted kilos by replacing a large battery pack with a far smaller lithium-ion battery and a capacitor that is capable of rapidly discharging its current to those front motors when needed, while clawing back energy more efficiently under braking.

Those tiny in-hub motors, meanwhile, deliver plenty of power because they work at higher voltage than regular motors.

There's is catch though; high voltage means the small motors will not offer the low-down torque that conventional motors do.

The other disadvantage is the added complexity of a set-up that uses different voltages, requiring up to three inverters including one between the engine and the driveshaft.

The electric motor that supports the petrol engine produces 25kW of power and is fed electricity via a 48-volt 3.5kWh lithium-ion battery.

Unusually, instead of bolting the transmission directly onto the engine, a lack of space sees Mazda use a rear transaxle.

Splitting the engine and the gearbox is said to liberate space for better interior packaging.

There's been no official confirmation but the advanced hybrid system could have been created for packaging into the new rear-wheel drive Mazda6 and/or Mazda's born-again flagship RX coupe.