Generally speaking, there are two types of hybrids - mild and full hybrids.
However, there is another third group of hybrids - micro-hybrids, that was not mentioned in the earlier article as there was no such model in the market place at that time.
To recap, full-hybrids can be summarized as cars that can be driven away from stationary using either the electric motor or petrol engine or a combination of both. Mild-hybrids rely on the petrol engine to be driven off the line but receives assistance from the electric motor.
A micro-hybrid is not a hybrid in a strict sense, as its electric motor doesn't provide any propulsion to the vehicle but it does retains several characteristic of a hybrid vehicle, like energy regeneration, which recovers energy that would otherwise be lost when a driver lifts his foot off the throttle and automatic engine start-stop system when idling.
In terms of driving and fuel economy performance, micro-hybrids sit one rung below mild-hybrids. However, micro-hybrids have an advantage in terms of cost and packaging as its components costs less and are smaller.
The Nissan Serena S-Hybrid is an example of a micro-hybrid. It doesn't have a traction battery in the rear of the car like in a mild-hybrid Honda Insight or a full-hybrid Toyota Prius. Instead, it has an additional 'sub-battery' in the engine bay to power the vehicle's air-conditioning, multimedia system and other features when the engine is shut down.
The Serena S-Hybrid uses what Nissan calls an Alternator Regenerative Control Technology. Compared to a standard Serena, the Serena S-Hybrid has a more powerful ECO (Energy Control Motor) motor that doubles as an alternator and an engine starter motor.
In a standard Serena, a regular alternator is constantly running with the engine to generate electricity to power the vehicle's accessories, thus contributing to increased fuel consumption. In the Serena S-Hybrid, fuel consumption is kept low by varying the recharge rate of the alternator, depending on the load and 'sub-battery's' charge level.
For example, when the driver of a Serena S-Hybrid lifts off the throttle, the ECO motor works as an alternator to charge the vehicle's batteries. (Figure 1). Under braking, the ECO motor will operate at a higher resistance to further increase its charging performance.
The charged stored in the sub-battery will be used to power the vehicle's electrical accessories while fuel feed to the engine is kept at a minimum. (Figure 2). This contributes to lower overall fuel consumption in urban driving.
The vehicle's petrol engine will recharge the sub-battery to maintain sufficient charge level (Figure 3).
As the Serena S-Hybrid will experience multiple engine start-stop cycles over the course of the day, it requires a different starter motor. Unlike a conventional car, the Serena S-Hybrid's uses a belt pulley connected to the afore-mentioned ECO motor to crank the engine. Nissan says this solution allows for quieter engine restarts.
Also, the Serena S-Hybrid's engine features two crank position sensors which allows the engine computer to determine which of the four cylinders to be fired up first, thus reducing engine restart time (in about 0.3 second).
As the ECO motor is mounted outside the engine block, it is not able to drive the wheels directly. Thus the motor is not able to assist the vehicle's acceleration like in a regular mild hybrid, which has a motor integrated between the engine and transmission.
However the ECO motor does provide a "torque assist function" by delivering 50 Nm of torque for about 1 second to the crankshaft when driving the vehicle away from standstill (after restarting the engine from idle stop). This reduces the amount of fuel required to move a vehicle from standstill and thus contributes to lower fuel consumption when driving in urban traffic.
The frequent engine start-stop cycle also means that a regular lead acid 12V battery will not suffice. Instead, Nissan has fitted a special high-charging performance sub-battery to complement the standard lead acid 12V battery.
At this point of time, we cannot verify the type of battery technology used in the sub-battery.
BMW Efficient Dynamics models are able to replicate the same energy regeneration and engine start-stop function as the Nissan (although BMW doesn't call it a hybrid) and these models have a special Absorbed Glass Mat (AGM) battery instead of a regular lead-acid unit.
Although the Serena S-Hybrid is not quite a hybrid in a conventional sense, it is still a very practical and cost effective solution. By fitting all these additional components within the engine bay, the Serena S-Hybrid retains all the advantages of a regular Serena.
In Japan, the Serena S-Hybrid boasts of having the largest-in-class cabin space and best-in-class fuel economy of 15.2 km/litre (Japanese JC08 test cycle).
In short, the Nissan Serena S-Hybrid combines the advantages of a regular hybrid but without the higher cost penalty that is usually associated with more sophisticated hybrids.