At one end of the scale, there's a pure ICE--Internal Combustion Engine. This is the normal gasoline or diesel engine everyone knows about. At the other is pure electric--Tesla, Rivian, etc. A hybrid is a mix of the two.
Why add the complexity of a hybrid?
Conventional brakes just turn speed into heat. Hybrids have regenerative braking--this runs the generator, slowing you down as it charges the battery so you can use that charge to get moving again. ICE engines are generally most efficient at a particular speed...but that speed is rarely the one you want to use for normal driving. They are often particularly inefficient at speeds you need to use most often. Atkinson Engines are an extreme version of this--They tend to make less power per liter/cubic inch of engine size but do considarably more work per gallon of gas, and by themselves do not have enough power for normal driving at low engine speeds. Conveniently, electric motors are best where the Adkinson is worst, so a blend of the two works well. Hybrids also make it more practical to power accessories with electricity, so the engine doesn't have to run to keep the AC on for example. They also take advantage of the fact that almost nobody needs full engine power for more than very short periods.
A mild hybrid system has a relatively small electric system. It recovers power from braking, but the hybrid motors and batteries may not be capable of powering the car alone. An early BMW system merely charged the normal 12v battery to reduce the load of the alternator. The first generation Honda Insights had "Integrated Motor Assist" a 13 hp electric motor sandwiched between the 67 hp ICE and the transmission. This allowed regenerative braking and gave a bit of extra power to the ICE--but the engine had to be running if the IMA was running. Honda continued using IMA with bigger electric motors until fairly recently. There isn't a clear line between mild and full hybrid.
A full hybrid will have a bigger electrical power system, generally with the capability of running on pure electric for short periods. Our Ford Maverick has a 162 HP Atkinson cycle ICE, plus 131 hp electric from two motor-generators. All of these are combined in various combinations of charging and generating power through the E-CVT with planetary gears similar to an automatic transmission. The E-CVT doesn't use belts and variable pullies like a standard CVT, it is closer to a conventional automatic...except mechanically simpler. Electric motors have peak torque at low speeds, while the ICE peak is at a higher speed. Since the peaks don't align the total is lower than just adding the two, in this case 191 hp. The ICE will attempt to only run at its most energy efficient speed, and the system will attempt to charge the batteries primarily with regenerative braking. Moderate acceleration from a stop will usually be electric only, switching to ICE after a few seconds. Under hard acceleration both ICE and electric will run until the battery is depleted. Depleting the battery due to acceleration is extremely rare. The end result is that weight and stop and go driving make less difference to MPG, the ICE engine can be designed for efficiency rather than power and drivability so a 4 seat compact pickup truck can get better MPG in real world driving can get better MPG than a subcompact hatchback despite having more interior room and better acceleration.
A plug in hybrid has more batteries than a full hybrid with the ability to run some substantial distance on battery alone. It is intended to be plugged in and run off that charge for most trips, but with the ability to use gasoline for longer trips. This may be similar to a full hybrid with bigger batteries, or closer to a full electric, with a relatively small ICE that only charges the battery with no connection to the wheels. To get any useful range the battery has to be many times larger than a full hybrid, especially since batteries last longer if kept primarily between 20 and 80% charge.
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