[ELI5] Why do hybrid vehicles charge the battery using the engine?


In hybrid vehicles, I know the battery can be recharged using regenerative braking, but I also see them sometimes recharge the battery using the engine directly. Why would they do this? It seems like it would be better to use the motor just to move the vehicle. Why go gas -> engine -> battery -> wheels?

In: 5

It depends on how efficient the gasoline engine is at moving the vehicle versus driving the alternator. If the alternator + electric motor is more efficient than the internal combustion engine + mechanical transmission, then it’s better to use the gasoline as an electrical energy source rather than as the prime mover for the vehicle.

The the most part, electric motors are more efficient at “moving” the vehicle itself. And a very small gasoline/diesel engine with a generator attached to it can easily generate the amount of electricity needed to keep the battery charged for the electric motor.

The size of the gas engine is very small, for example, the Chevy Volt used only a 1.5L gas engine. That is a very low performing engine, and if you tried to push the car with that it would be gutless. But, it’s enough to produce electricity. And the electric motor gives the car a very good amount of power.

Some vehicles though, like the Toyota Prius, actually use a different hybrid system, and both the gas and electric motors push at the same time.

So there are basically 2 kinds of hybrid vehicle, “series” and “parallel” layouts.

In a parallel layout both the ICE (Gas) engine and the electric motor can drive the car.

In a series layout only the electric motor actually drives the car and the gas engine just charges the batteries, it’s basically a generator.

The reason being that gas engines aren’t “universally” efficient, meaning if Honda says the Civic gets 30 miles per gallon, that’s not 100% true all the time. Cars burn more fuel driving up hill, or when loaded up with people, or when driving super fast (air resistance) or just do to a literal thousand factors of physics involving the engine.

The point being an engine’s actual efficiency varies tremendously from moment to moment depending on the exact specific conditions of the car at that moment. Engine manufactures will do their best to “Target” the most common situation the engine will be driving in and “optimize” the engine to that specific condition. A civic engine might get 30 miles per gallon driving at 60 miles per hour on a flat highway with 2 people inside. The same civic might get 10 miles per gallon driving at 20 miles per hour in the same conditions.

Electric motors do not have this issue, they are much closer to being what you might think of as being “universally efficient”.

So the logic is, use the gas engine to charge a battery and optimize the engine’s efficiency for the exact “work” required to charge the battery. The ICE is now always operating right in it’s efficiency sweet-spot, getting theoretically best fuel mileage possible.

Because if they didn’t the battery would always be drained and there would be no benefit to driving a hybrid.

Gasoline engines aren’t very efficient, so sacrificing a little bit of efficiency to keep the battery charged doesn’t cost very much, and it enables the overall – much more efficient – hybrid system to function.

Depends on how exactly it’s set up, but often a hybrid needs both the gas engine combined with the electric engine at the same time to work. In order to increase fuel efficiency, the gas engine will be running on the Atkinson cycle as opposed to the Otto cycle, (which is the one pure gas engines normally run.) The benefit is that the Atkinson cycle can extract more energy from a given quantity of fuel. The downside of the Atkinson cycle is that it doesn’t generate as much power in the moment and worse, it creates inconsistent power throughout the revolution of the crankshaft. Basically, instead of smooth acceleration your car would jerk like the transmission is shifting every revolution of the crankshaft. In order to get around this, an electric motor is integrated into the block and computer controlled to feed power at exactly the point in the revolution that the power delivery in the Atkinson cycle would drop off, with the effect of smoothing it out. An electric motor needs power so some of the energy from running is stored. On balance the energy saved by running the Atkinson cycle more than makes up for any losses associated with charging a battery and running an electric motor. This sort of the system that Honda uses.

The Ford Maverick and Toyota Prius, OTOH, use an ECVT transmission. It’s a super simple transmission that’s centered around a planetary gearset but requires a gasoline engine as well as 2 electric motors that all input power into the transmission at different speeds. A computer calculates and controls the speed and direction that each motor needs to turn at to get the proper wheel speed.

In every case, the power to run the electric motors must come from somewhere and if the only source of energy going into your vehicle is gasoline then some of that energy must be scavenged from the gas engine at some point or form. If you can recapture otherwise wasted energy (regenerative braking) or simply run the engine in the most optimum (efficient) range for longer then overall otherwise lost energy gan be used and your overall efficiency goes up.

Other hybrid systems are just electric cars with a range extender. A small gas engine to charge a battery, but the drivetrain is otherwise all electric. Like the Chevy Volt. Most power is needed to do things like accelerate. Actually cruising down the highway takes a surprisingly little amount of power. We have ‘big’ engines in cars sized to meet the huge power demands of a few moments. Why not let the electric motor do all the heavy lifting and have a tiny, efficient, gas motor just keep the battery topped up?

From the earliest days of EV’s, getting a battery that was big enough was known to be a problem. A big battery was heavy and expensive, but a big battery was needed to provide the kind of range that customers said they wanted.

Also, it has taken time for enough factories to be built to produce enough of the batteries. Even though people wanted a battery with 300 miles of range, the vast majority of people drive less than 80 miles a day (40 miles to work, and 40 miles back).

Making an EV a hybrid helps in several ways. The battery only has to be big enough to travel 100 miles or so. This makes the battery smaller, lighter, and cheaper than a 300 mile battery. However, once in awhile a customer may want to drive much farther than 100 miles. That is what the engine is for. Its rarely used, and only on longer trips.

My son had a Chevy Volt hybrid. He was the perfect candidate. He drove 12 miles to work and 12 miles back, on all electric mode. Once a month he would take it farther on the weekend in order tp force the engine to fire up and run for a short while.

I have a Prius (parallel hybrid). Electric excels at low-speed stop-and-go driving. ICE is efficient at constant speed on the highway. Each engine is most efficient at different types of driving. Parallel hybrid with the battery recharged by regenerative braking or the ICE allows the car to continuously choose the most efficient method of propulsion at the moment based on the type of driving.

When the engine is charging the battery, it can do so while constantly staying in a high efficiency zone because it can run at certain torque and speed regardless of the vehicle’s velocity because it’s not connected to the wheels. And the motor is very efficient at moving the vehicle. Let’s say the engine charges at 35% efficiency and the motor works at 90% efficiency, that’s 35%*90%=31.5% efficiency.

Where as if you use the engine to drive the vehicle directly, depending on the speed and torque, its efficiency varies wildly from let’s say 5% to 35%. In most cases it’s worse than the series hybrid option.