Hybrid cars are quickly gaining ground in the Indian market. MG has launched the PHEV fray with the Tomahawk while Kia is set to follow up with its strong hybrid Sorento. BYD has also unveiled its DM-i plug-in hybrid technology, but none are on the roads yet. Tomahawk and Sorento will have to contend with the established hybrids from Toyota and Maruti Suzuki, such as the Grand Vitara and Invicto, Hyryder. But what is the difference between them? And what is more advantageous for the owner, a PHEV or a strong hybrid?
Petrol engines are not well suited for stop-start city traffic. The output of an internal combustion engine peaks at a narrow range of RPMs (typically 2,000-3,000 RPM under load) and is relatively low elsewhere. It is also inefficient at light loads, such as when in slow-moving traffic or idling at a traffic light. An electric motor, on the other hand, has a broad power curve, peaks torque at zero RPM, has a high torque at low RPMs, does not consume energy when decelerating, but rather recovers some of it. A hybrid combines both to allow the engine to function when it is most efficient while the motor handles the rest, with the battery storing the recovered energy for later use.
A hybrid system essentially has four key components. A petrol engine tends to be slightly detuned compared to standard internal combustion cars. A traction motor, which powers the wheels, and a generator, linked to the engine and capable of running in both directions, making it possible to charge the battery while decelerating. The battery itself is usually a high-tension lithium polymer or nickel-cadmium unit, voltage-wise rated at 200-300 volts. A control unit decides which power source to use depending on driving conditions several times per second.
Hybrid powertrains can be divided into two broad categories based on their architecture: series and parallel. In a series design, the engine is disconnected from the drivetrain and only charges the battery, while the wheels are always powered by the motor. Series layouts tend to be simpler but have lower power-to-weight ratios. Vehicles using this layout always behave like electric cars, with the engine kicking in whenever the battery charge is low. In a parallel configuration, both the engine and motor can directly power the wheels. Some parallel hybrids have a clutch, allowing the engine to be disengaged when the motor is used for slow-speed or stop-start driving. When the motor and engine work together, the power output is the sum of both. Parallel hybrids can deliver peak power from the motor at lower RPMs while the engine provides a torque boost at higher RPMs.
Mild hybrids are essentially series hybrids with a smaller motor and no generator, which can only supplement the engine and cannot power the car on its own. They are often derided as false hybrids. Strong hybrids and PHEVs, on the other hand, can operate in parallel and have sufficiently powerful traction motors to propel the car on their own.
The primary difference between strong hybrids and PHEVs lies in the battery capacity. Kia has not stated what battery capacity its India-market Sorento will have, but the global version is likely to have around 1,49 kWh. MG Tomahawk, on the other hand, has a 20,5 kWh battery, which is more than an order of magnitude higher. The difference in battery size directly translates into the driving range: while a strong hybrid can manage 1-3 km in electric mode, a PHEV can deliver more than 100. Even more important, the power output of the motor is related to its capacity. A 1 kWh battery can deliver a continuous current of 30-40 kW, which is enough for urban driving but not for passing on the highway. Whenever the driver needs more power, the engine has to be used. In contrast, a 20 kWh battery can provide a continuous current of 100 kW or more, making the motor the primary power source with the engine serving as a supplement. Thus, when the battery is fully charged, the PHEV can use the motor at full capacity and therefore provide maximum power and torque, while a strong hybrid is limited to the power provided by the motor.
Since the battery in a strong hybrid only stores energy generated by the engine, a PHEV can drive significantly longer on electric power before having to use the engine. On the other hand, a PHEV owner has to put up with the charging time and potential inconvenience of installing a charging station at home or work. However, if such a possibility is available and the commute to work is electric, the petrol engine can be driven very rarely, if at all. The owner will be able to enjoy the advantages of an electric vehicle – low noise, low operating costs and high responsiveness – and the petrol engine will only be needed for long trips. With a little practice, an owner of a PHEV can prolong his battery life for years.
Without the opportunity to charge the car frequently, such as when living in an apartment building without a charging station in the parking lot, the large battery of a PHEV becomes an unnecessary liability. In this case, a strong hybrid may well be superior in terms of fuel economy, which is especially important in urban driving. After all, the weight of the battery in a PHEV is several hundred kilograms, which directly affects fuel consumption. In addition, a strong hybrid does not require special charging infrastructure, which is often unavailable in India. Such a hybrid offers maximum efficiency in the stop-start city traffic, where petrol engines are most often used in India.
Despite the obvious advantages, strong hybrids have failed to take off as expected. Their combined market share in the four-wheeler segment is only about 2.3%, compared to nearly 8% for fully electric vehicles. Both hybrids and PHEVs have two powerplants, which increases complexity, weight, and maintenance costs. In addition, mechanics need special training to service high-voltage batteries, which is a limiting factor for many. Finally, battery lifespan remains a contentious issue for potential buyers, given the potential for warranty claims from the second owner.
The choice between the two powertrain types ultimately depends on the owner’s situation and everyday commute. Wherever possible, a PHEV offers the best balance of maximum range, minimal weight and fuel costs. At the same time, strong hybrids do not require a change in driving habits and offer sufficient efficiency in urban driving.