Car hybrid Systems : REX, IMA, i-MMD, HSD , PHV/PHEV , Mild Hybrid  How They Work And What is Difference?

Over the past 20 years, the word hybrid has become firmly established in the everyday life of motorists. In the late 90s, the term almost became synonymous with the marvel of technology of its time – the Toyota Prius, the world’s first mass-produced hybrid car. This useless, as it seemed to many then, “toy” was sold around the world with a circulation of 10 million pieces. Today, hybrids are produced by all leading automakers, and there are more than 30 hybrid models in the Toyota line! Let’s take a look at what hybrids are on the market and how they differ.

Under the general name “hybrid cars” completely different technical solutions are hidden. One thing unites them – the presence on board of two different sources of energy: not only an internal combustion engine , but also an electric motor with a traction battery. This, by the way, is the fundamental difference between hybrids and electric vehicles – the latest internal combustion engines do not. But hybrid power plants work differently.

Range-Extender hybrid (REX)Parallel hybrid

The very first hybrid design – sequential – was invented at the dawn of the automotive industry by Ferdinand Porsche, and the working prototype of the Lohner-Porsche Hybrid appeared in 1901, a hundred years before the Prius. In fact, a hybrid car is almost the same age as a gasoline one, but it had to wait a whole century in the wings: with cheap fuel, technically complex hybrids simply did not make sense. Yes, and ecology in the XX century, few people cared.
In a series hybrid scheme, the internal combustion engine plays the role of a large generator: it is installed under the hood, but is not connected in any way to the wheels. The engine speed is always optimal, the gearbox is also not required. The only task of the engine is to generate current for the electric motor, which rotates the wheels of the car.

In fact, a series hybrid is an autonomous electric vehicle that does not require recharging from an external network. Such hybrids are effective in conditions of frequent stops, where there are no high speeds, but good traction is needed. First of all, these are urban public transport and mining trucks. The most famous serial hybrid is BelAZ, the largest dump truck in the world: this monster’s huge diesel engine generates current, and electric motors turn the wheels.

belaz Hybrid



In passenger cars, the sequential circuit is used less frequently due to large power losses, but there are such examples: the twins Chevrolet Volt and Opel Ampera. True, they did not become bestsellers, being very “raw”. However, experiments with a sequential hybrid scheme in passenger cars are ongoing. For example, a BMW i3 electric car can be equipped with the REX option (Range Extender, Range Extender), a tiny two-cylinder internal combustion engine to generate power that turns the electric car into a REX series hybrid.

The Koreans at SsangYong are selling an Actyon EV-R in the home market with a sequential hybrid powertrain. And the Karma Revero and Infiniti Emerg-e supercars are REX hybrid attempts to challenge Tesla.

Parallel hybrid

Most modern hybrids use a more versatile parallel circuit. In it, both the internal combustion engine and the electric motor(s) are connected to the wheels, working both separately and together (in parallel).

The first parallel hybrids were quite primitive. In 1999, in response to the Toyota Prius (more on that below), Honda introduced the Integrated Motor Assist (IMA) hybrid system to the Insight, then to the Civic, Accord, Fit/Jazz, CR-Z, and others. The electric motor in it was an electric assistant, and not a full-fledged power plant: such hybrids could not drive on pure electric traction, nor could they charge the battery from the engine while driving. The electric kilowatts were simply added to the gasoline horsepower, allowing for less engine revving and some fuel savings. And the battery was charged mainly from recuperation during braking.

HONDA IMA
Honda IMA: Integrated Motor Assist


Modern parallel hybrids have gone far ahead, and some have even learned to “reincarnate” on the go. The same Honda is actively introducing the i-MMD (Intelligent Multi-Mode Drive) power plant into new models, which operates in a sequential hybrid scheme in urban mode. At low speeds, the gasoline engine is disconnected from the drive wheels, and the car moves only due to the electric motor. As the speed increases, the car turns back into a parallel hybrid, connecting the internal combustion engine with an automatic clutch. Such a system is already being used on the new generations of the Odyssey, Accord and CR-V for some markets. Mitsubishi followed a similar path with its Outlander PHEV hybrid crossover.

Series-parallel hybrid (Toyota HSD)

Formally, the Toyota Prius can be classified as a parallel hybrid. But when developing it, Toyota engineers acted so innovatively and introduced so many bold decisions that the resulting result had to be separated into a separate type. “Prius” is called a series-parallel hybrid or a hybrid of a mixed (combined) type.

In most parallel hybrids, there is a familiar gearbox (most often a continuously variable CVT), and the electric motor is an intermediate link between the gearbox and the internal combustion engine, a kind of superstructure. At Toyota, the gasoline engine, electric motor and drive wheels are interconnected by a planetary mechanism, without a gearbox and clutch. This makes the entire power plant a single entity, allowing the control electronics to distribute power between the nodes in any ratio. The efficiency of such a system is very high.


Toyota called its design Hybrid Synergy Drive (HSD) – a hybrid synergistic drive. It is used in all modern Toyota and Lexus hybrids, and it was also installed under the HSD license on several Nissan and Ford models. Toyota plans to gradually transition to a synergic drive the entire line of vehicles. For some models in the domestic JDM market, there is no longer an alternative to HSD: for example, the Toyota Camry in Japan has been offered only with a hybrid power plant for the past few years.

Rechargeable Plug-in Hybrid (PHV/PHEV)

Even the most modern hybrid loses in efficiency to a fully electric car, because it does not consume gasoline at all. Yes, for some time the hybrid can only drive on electric traction, but in this mode the battery will quickly run out, and for charging you will have to use the internal combustion engine and waste fuel. But why not replenish energy reserves from an external power grid, as electric cars do? This is the idea behind the PHEV (Plug-in Hybrid Electric Vehicle) – a hybrid car with the ability to connect to external charging.

outlander PHEV


Like a traditional hybrid, PHEV can charge the traction battery on the go: from the operation of the internal combustion engine and with the help of recuperation – the conversion of braking energy. But when you arrive at home or at work, the Plug-in Hybrid can be charged from a stationary power grid, like any electric car, and then drive with a full battery. This is much more profitable than self-loading while driving.

Compared to a fully electric car, a PHEV hybrid also has tangible advantages:

  1. Large power reserve. Even top-end electric cars do not yet travel more than 500 km on a single battery charge, and budget city electric cars have a hard time even a modest 200 km, especially in winter. A rechargeable hybrid on a full tank of fuel will travel more than 800 km.
  2. Independence from external sources of electricity is something that electric vehicles lack so much. If you do not charge the electric car in time, it will simply turn off. And PHEV has a classic internal combustion engine under the hood, and charging from an external network is a nice option for it, not a necessity. As long as there is fuel in the tank, it will go, and very economically. This factor is critically important when traveling between cities: there are many times more conventional gasoline filling stations on the roads than electric charging stations.

Plug-in hybrids are the main automotive trend of recent years. There are already a lot of them on the market, and a hybrid of any design can be rechargeable: both serial, and parallel, and a hybrid of a mixed type – after all, everyone has a traction battery. Examples: Toyota Prius PHV and Prius Prime, Mitsubishi Outlander PHEV, Ford Fusion Energi, Hyundai Ioniq, Chevrolet Volt, Volvo V60, Audi e-tron, Chrysler Pacifica, Range Rover PHEV and others. Even Chinese automakers are already making their own versions of plug-in hybrids. It’s not just about fuel economy and environmental friendliness: it’s important for car manufacturers to be at the forefront of modern technology.

In the future, PHEV hybrids, like electric cars, will get rid of charging wires. There are already small-scale cars that can charge the battery of the power plant in special induction parking spaces, by analogy with wireless charging of smartphones. That’s just the infrastructure of cities has not yet kept pace with the progress of transport.

Moderate “soft” hybrid (Mild Hybrid)

Surely you have seen cars in the Mild Hybrid configuration – the people called them a mild hybrid, although it would be more correct to call such a power plant a “moderate hybrid”. What is Mild Hybrid, and how “soft” is it?

Mild Hybrid is the “germ” of a hybrid: a standard car, greened up with energy-saving technologies. Behind the beautiful hybrid term is the Start-Stop system, familiar to many, which turns off the engine during stops, but is a little more advanced. Mild-hybrid internal combustion engine is equipped with a special motor-generator; while driving, it works like a standard generator, generating current. When the car stops and the Start-Stop system turns off the engine, the motor-generator comes into play, ensuring the operation of all car systems: electrics, heater, air conditioning … The driver does not even notice that the engine is turned off. When you press the gas, the engine instantly starts directly from the motor-generator and the car starts off.

D4 Mild Hybrid


Each manufacturer has its own implementation of a “mild” hybrid. Sometimes the motor-generator can generate energy during braking, storing it in a separate battery. The 48-volt Audi MHEV motor-generator even knows how to help the engine during acceleration. But we are not talking about a full-fledged hybrid power plant with the ability to drive on electric traction. The mild hybrid is just a helper for the internal combustion engine, not a separate traction motor.

Features of the operation of the hybrid

What is the difference between a hybrid and a conventional car in everyday use? After all, not all drivers are ardent enthusiasts of new technologies; many just need a reliable and well-functioning solution without unnecessary complications. And the hybrid fits that description perfectly.

The strength of the hybrid is the absence of any special requirements and procedures, like electric vehicles or cars with gas equipment. The hybrid system is fully managed by smart electronics without driver intervention. Periodic maintenance and replacement of consumables does not require a special “hybrid” car service: any qualified service station can handle typical work. And some features of the winter operation of hybrids cannot be compared with the winter problems of electric vehicles, the range of which in the cold almost halves.

The internal combustion engines of most hybrids operate on the economical Atkinson cycle, which allows you to even out the load on the power plant, removing dangerous peaks. In the engines of conventional cars, it is not used due to a drop in power, but in hybrids, this drop is compensated by an electric motor. The result is an optimal balance of qualities: power, efficiency, reliability and low exhaust toxicity.
In addition to direct fuel savings, the hybrid reduces the cost of some parts. For example, brake pads hardly wear due to regenerative braking. Most hybrids do not have the usual starter and alternator – in this, the hybrid power plant is even simpler than the classic one. And in Toyota hybrids there is not even a gearbox – its role is played by the “planetary”.

There are exactly two cons and hybrids: high price and gradual wear of the high-voltage traction battery (VVB). Moreover, these disadvantages exclude each other: a new (and expensive) hybrid has no problems with the battery, but a car with a worn-out VVB can be bought with a good discount for its replacement.

Technology advances incredibly fast. Until recently, hybrid cars seemed like something alien, but today, against the backdrop of fully electric cars, they even look slightly outdated. But the lack of a developed infrastructure for charging electric cars and their short range will be arguments in favor of hybrids for a long time to come.

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