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Originally Posted by drivesafe
Well this has taken an unexpected turn. I took some physics in university, and I understand energy better than most people. My career turned out to be IT so I also have a very strong handle on computerised systems. You're making a big claim to say that I don't know what I'm talking about. I'm quite sure there are some things I don't understand, but there's a lot I know in detail. We are probably just miscommunicating on some things.
Kinetic energy might not be what you think it is. It is specifically the energy of moving mass, and to avoid relativistic physics we'll just stick with motion relative to the Earth.
An idling engine does have kinetic energy, because parts are rotating or reciprocating.
But as long as it remains idling, it's not kinetic energy that the alternator is extracting from the system. Here's why:
An engine converts stored chemical energy into heat through combustion.
The heat creates pressure which exerts a force on the piston, and that force does what physics calls "work" by moving the piston some distance.
Work done (J) = force applied (N) times distance moved (m)
Not all the heat does work – some of it escapes by making the engine hot and being transferred to the air via the radiator etc. That energy is basically lost to us.
While in neutral, all of the work done either accelerates the engine itself (temporarily storing energy as kinetic energy in the moving parts) or is also lost, via friction, as heat.
The alternator is connected by the belt to the crank. Through the pistons, conrods, crank and belt, the force exerted by the hot gas in the cylinder is transferred mechanically to the alternator.
Inside the alternator is a magnetic field that's created by a current flowing in the rotor.
Rotating that magnetic field inside the coils creates an electric field within the coils themselves.
The electric field results in current flowing through the vehicle's circuits and doing various kinds of work there.
Chemical energy -> heat and pressure -> mechanical transfer of force -> rotation of magnetic field -> electric field -> electrical energy.
Kinetic energy is not part of that process.
When we engage the transmission the mechanical forces are *transmitted* through the gears and shafts to become a rotational torque in the wheels and then a tangential friction force against the road.
This force does the real work, reacting back through the tyre, wheel, axle and suspension to accelerate the vehicle in the direction you want to go.
Some force is always required to overcome friction losses, so that portion of energy is again lost as heat.
But the driving forces which are greater than friction do work and transfer energy from the heat in the engine into... kinetic energy.
Kinetic energy (J) = mass (kg) times velocity (m/s) squared. If I accelerate my 3,000kg vehicle to 50km/h I have given it 578kJ of kinetic energy.
Now I want to slow down.
I have too much kinetic energy.
What will I do with it?
Well, I can apply the brakes. Brakes use friction to exert a counter-acting force against the motion of the wheel. That force is transferred through the axle, suspension, body etc and even to yourself (you feel it against the steering wheel and seat belt etc) and does *work*. Doing work means transferring energy. What energy?
Kinetic energy is extracted from the vehicle (and the driver's body) and converted to heat energy in the brakes, which again is lost to us.
We got energy from fuel and through the engine and transmission converted it to kinetic energy of the vehicle and its contents so that we could move at 50km/h.
When it's time to slow down, we have some kinetic energy we don't want any more. We have to remove kinetic energy from the system. With brakes our only option is to discard it.
And it's nearly the same thing going down hill. In that case we have a lot of gravitational potential energy (well, *we* don't but the gravitational field between the car and the centre of the earth does) and gravity is doing its thing by converting some of that potential into kinetic energy ie we're getting faster as we roll. Now we can use our brakes to extract that excess kinetic energy from the vehicle and throw it into the air as heat.
Smart alternators (actually it's the computers which are smart, the alternator is just a tool) are used to extract some of that UNWANTED kinetic energy and store it in a form we can later use instead of just losing it. It works just like I posted previously.
I am *not* saying that this is the only time the alternator generates current, or the only reason it does. The car needs electrical power all the time it's operating (and these days even when it's switched "off"). Ultimately all of that electrical energy will be provided by the alternator as it has work done on it by the serpentine belt (force times distance moved just in a circle this time) and that energy is transferred to an electro-magnetic field that causes electrons to move through the circuits in the car. By far the majority of that energy is coming directly from the combustion pushing the pistons turning the crank and driving the belt.
But sometimes the flow of fuel stops. Sometimes the force that's turning the crank isn't coming directly from combustion. Sometimes we're doing *engine braking* where forces coming *from* the wheels through the transmission are doing all the work of compressing air and overcoming friction in the engine etc. AND because the alternator is still on that belt, some of the work done by those forces is to turn the alternator. Kinetic energy from the vehicle becomes electrical energy via the alternator. Come to think of it, in this moment you actually *are* sucking out some of the engine's own kinetic energy too because it's also getting slower.
How much kinetic energy is extracted by the alternator? It depends on the load. The computer knows that if the cranking battery is not full (which it knows because it's been monitoring the current flows in and out of the battery and controlling the alternator voltage so that the battery does *not* get fully charged while cruising) it can increase the alternator voltage causing a current flow into the battery. The battery becomes a load that wasn't there a moment ago. Charging a battery is another form of doing work, of energy conversion. We convert electrical energy into chemical energy.
Let me repeat: when the computer senses engine braking being used it deliberately ups the alternator voltage and extracts some excess kinetic energy from the vehicle's motion and causes it to be stored in the deliberately-not-fully-charged battery.
One of the things you said above I do need to agree with. The computer is also smart enough to know that when you are asking the engine for more power to go to the wheels, it would be helpful (a bit) to reduce the amount of energy being extracted by the alternator. So it can drop the alternator voltage, reducing the currents being generated, and that means less work being done by the engine to turn the alternator. Once you ease off the pedal the computer can bump up the alternator power again.
I don't think it's wise to use the term "free" to describe energy. Thermodynamics is a zero-sum game where everybody loses eventually. Unless you've got some form of external electrical charging, all the energy in a vehicle comes from the fuel. Braking is literally discarding energy. Smart alternators and all the BMS complexity associated with them let you indirectly extract a tiny bit of that excess kinetic energy and use it for your electrical needs instead.
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