The hybrid cars are way better in CO2 emissions (i.e. in fuel economy) in downtown traffic compared to conventional cars of same weight.

In the hybrid car the thermal engine operates only from time to time (at optimum as regards economy and emissions conditions, like 3500 rpm and 80% "open throttle") to charge the battery.

Despite the energy lost in order to change the mechanical energy into electrical energy by the electric generator, the energy lost in order to store the electrical energy into the battery, the energy lost into battery until discharge, the energy lost to change the electrical energy of the battery back into mechanical energy on the electric motor to finally move the car, the total efficiency (Kg of fuel per Km of distance covered) is way better than in the case of the conventional engine running at light load.

Most people admire the low consumption of the hybrid cars and forget that the energy that finally moves the car is generated by the thermal engine.

In other words: the efficiency of the conventional reciprocating engine at heavy load is way better than the efficiency of the same engine at light load.
This "defect" of the thermal engines leaves room for the hybrid cars.

The operation of a big V8 of a SUV in urban cycle resembles more to a CO2 generator than to a mechanical energy generator.

Having a thermal engine that achieves to run at light loads at similar efficiency with the good efficiency it has at heavy load, the hybrid technology becomes, more or less, meaningless (the recovery of a part of the bracking energy cannot justify the cost, weight and complication added by a high-tech hybrid).

Alternative to the hybrid technology is the Variable Compression Ratio (VCR) for both naturally aspirating and turbocharged engines, under the reasonable assumption that the VCR system offers continuously variable compression ratio in a wide range (to keep the engine continuously at optimum compression ratio) without introducing "side effects".
At partial loads the VCR system increases the compression ratio as much as possible (just before knocking) making the engine as efficient as if it were working in heavy load (if necessary combined with a VVA in the cylinder head).
The HCCI combustion is an easy option.

Click for a simple gif animation

The axis of the crankshaft (red) is immovable relative to the engine casing. For a conventional V8 all it needs is 4 slim and lightweight secondary connecting rods (blue), a thin and lightweight secondary crankshaft (light brown) that carries the 1/20 of the piston forces and a lightweight slow-moving control frame (cyan).

For more see the VCRs at