Engine Braking

[wobombat]

so how exactly does it work that the higher electrical load places a greater electrical load on the engine? Considering that the rotor in the alternator will spin at the same speed as the engine regardless of electrical load, how does the electrical load put a load on the rotor? Does it somehow put drag on it with the magnetic field or something? I'm confused as to how this would work.

[kennybobby]

Your understanding of an alternator is correct for a permanent magnet alternator such as is used on motorsickles--they run all the time and dump excess current back into heat. But we are only talking ~30 amps. The magnet drag is probably 1-2 hp in order to make .5 hp.

In most cars the alternator uses a claw-pole rotor that can be controlled as needed to meet the electrical load demand. The higher your electrical load, then a higher current into the rotor and a stronger magnetic field is created, and more mechanical power is required to turn the pulley. And visa versa for lesser electrical loads--the rotor can be turned off and freewheel.

[Uncle Dave]

My dad was also an electrical engineer. Was always saying that there was no FREE LUNCH. Then again he would laugh about it.

[serialk11r]

Originally Posted by kennybobby

Your understanding of an alternator is correct for a permanent magnet alternator such as is used on motorsickles--they run all the time and dump excess current back into heat. But we are only talking ~30 amps. The magnet drag is probably 1-2 hp in order to make .5 hp.

YIKES
Someone hurry and put some voltage converters on those things...1 hp for a few hundred grams is not a bad trade.

[freebeard]

jonEmetro wrote:

Trucks hauling heavy loads employ an exhaust brake. This is a throttle like valve in the exhaust collector that increases backpressure in the cylinders to amplify engine braking.

jake brakes

[Uncle Dave]

Originally Posted by wobombat

so how exactly does it work that the higher electrical load places a greater electrical load on the engine? Considering that the rotor in the alternator will spin at the same speed as the engine regardless of electrical load, how does the electrical load put a load on the rotor? Does it somehow put drag on it with the magnetic field or something? I'm confused as to how this would work.

This is a Good question. Trouble is that Science is not taught in schools any more. Before World War 2, energy could not be created nor destroyed, but changed from one form to another. Mechanical to electrical to heat. If an engine is running, making mechanical, the alternator can change some into electricity, this can run the A/C, radio, lights, chemical, etc. The chemical is the battery, which can go, electrical, mechanical, (Running engine) Which is Chemical; Gas + Oxygen. You sort of have to think about it for a while.

You start with Gas, and you want to use as little as possible to get from here to there. You work at cutting your losses. The extreme is a bicycle, but that, this time a year is sometimes impractical.

Heat is a big waste; However in Boston, you might like to waste some and turn the Heater on. And so it goes.

[bestclimb]

Originally Posted by wobombat

so how exactly does it work that the higher electrical load places a greater electrical load on the engine? Considering that the rotor in the alternator will spin at the same speed as the engine regardless of electrical load, how does the electrical load put a load on the rotor? Does it somehow put drag on it with the magnetic field or something? I'm confused as to how this would work.

Yes the alternator is harder to turn when putting out more power.

amps X volts = watts (a measurement of work/power)

RPM X torque / 5252 = horse power (a measurement of work/power)

So work done with electricity is replaced with work done by spinning the alternator.

If the RPM does not increase the torque has to goes up.

If the output of the alternator was the same all the time (would have to be it's peak rating) the extra energy given off would have to be dumped likely into a heat sink for a car running an 80 amp alternator at 14 volts makes a little over 1000 watts. A quick search for 1000 watt heat sink shows them to be 14 lbs of aluminum inches and finned to cool it for when you were not using all of the alternators capacity. The volume of such a heat sink is about 380 cubic inches, I have never seen such a heat sink on a car.

[mizlplix]

Greetings: Although I am new to this forum, I am far from a Noob at automobile theory.

Engine braking in class A diesel engines employ a hydraulic device in the rocker arm box to open the exhaust valve at the moment of TDC compression to release the energy into the atmosphere.

Engine braking on a Medium duty diesel engine uses an exhaust pipe valve to prevent the exhaust gas flow thereby slowing the engine until released.

Both of these systems shut off the fuel during the braking event.

A gasoline engine is a little more difficult. It uses it's inherent operational friction, but it still pulls fuel from the idle circuits in the carb, or the engine idle function in the ECU, unless it was mapped differently.

It is a myth that automatic transmissions "free wheel" when engine braking. They are always coupled to the engine, albeit less than a manual transmission would be.

Most latter day automatics use a lock-up type converter to prevent the converter turbine from overheating when in an over drive ratio. They usually lock up in mid-third gear.

With the use of an aftermarket shift modifier, the shift points and lock-up can be controlled. Some even resort to a manual switch to lock the converter early to squeeze out better fuel economy.

My EV uses Regenerative motor braking to stop the vehicle. It is so impressive that I rarely use the hydraulic brakes at all. The side benefit is that I recapture about 10% of the energy I used to accelerate.

An early form of this was used in heavy haul trucks in the form of a "retarder". That was a field winding built into the flywheel area of the engine that slowed the truck by electric generation of current, but it was wasted by turning into heat and radiated into the atmosphere.

Miz

[Mark Serva]

Your Dad was right. In fact, in modern consumerism, FREE is about the most expensive option :-)

[Mark Serva]

True that about torque converters. The early one's I think Buick put them on their Roadmasters had a free wheeling TC. Don't know if that was hype or reality. Then, the early 2 speed TorqueFlites (Mopar) had a double pump so they could be push started to overcome the "can't push start" objection from the day. They later had to drop the pump to fit a 3rd gear. I heard, yet don't know, that TESLA was going to use a modified 2spd PowerGlide (GM) without a TC on one or more of their models. The PowerGlide, tho often called a "slushbox" uses, I've read 19HP, making the least power hungry of autos. Unfortunately the limited gear ratios make it less efficient. Don't know what a 3spd manual wastes in power if one includes shifting gears in the mix. They do get hot yet not nearly as hot as an automatic.

The "best" automatic I ever saw was the one VW put on the late run of Buses. It was something of a torque doubler getting going and had an internal fan in the TC. I saw some figures where it got better gas mileage than the average person using a clutch. Of course, the average person, ain't none of us, right?