Recharge an RV battery by hand cranking a generator

[dlgoff]

After I earned some money mowing lawns by hand, ...

I think I've seen your mower somewhere?

 

[sophiecentaur]

I think I've seen your mower somewhere?

View attachment 233071

One of those cut me up in heavy traffic the other day.

 

[russ_watters]

What RPM would be possible by hand (let' say there is no compression)?

No more than about 120rpm before torque drops off rapidly. Optimal is around 90-110.

 

[russ_watters]

To me, that means the same as pulling on the start cord.

Portable generators have the same recoil start mechanism as a lawn mower. If you repeatedly pull the start cord on your lawn mower with the kill switch on (so it doesn't start), what average RPM do you think you could maintain?

If there's a threshold rpm above which you are generating enough voltage to charge and below which you aren't, using the average rpm would give the wrong answer. For example you might calculate the average never exceeds the threshold when in reality you exceed it half the time and are therefore be charging half the time. I don't want to dwell on this though, since it isn't the OP's scenario and would be horribly inefficient physiologically: most people couldn't do it for more than a minute.

To sum up though:
1. Physiologically, a human is capable of putting out enough energy in a reasonable time to start an engine.

but:

2. The particular scenario doesn't work because they aren't spinning the alternator at its required rpm to generate enough voltage to charge the battery.

Incidentally, there are commercial products for exactly this purpose:
https://www.k-tor.com/pedal-powered-generator/

That's rated for 3A at 14V. Thinking about this I remembered that in some yachting races they take the human and wind power thing to the extreme where all their sophisticated electronics are human powered; when they aren't trimming sails, crewmen are charging batteries.

 

[sophiecentaur]

There is a vast difference between starting an engine manually and charging the vehicle battery. The picture that's way up the thread of an aeroplane being started by hand is an example of what you can do with a single compression. Aero engines of that sort have fancy magnetos that produce a really meaty spark (Modern electronic ignition will do the same for you) and the engine is very high compression for work at altitude. There are stories of aero engines starting when someone leans on the prop, when it's been primed and at the right point in the cycle. (They chop off arms apparently- or so a pilot was telling me when we were standing near the prop and chatting.) Wartime engines were started using a blank rifle cartridge - a one off impulse.
But an RV engine is not an aero engine and the OP was basically about a bit of poetic license in a film.

 

[Rive]

Wartime engines were started using a blank rifle cartridge

I've just come across this video today...

If there's a threshold rpm above which you are generating enough voltage to charge and below which you aren't

Well, as it seems there is...

 

[russ_watters]

Well, as it seems there is...

What confused me about the comments previously on this is that they (and the graphs in that link) are focused on current, when it appears to me that what matters most for this discussion is voltage.

E.G., all of those graphs/ratings are at 12V. So if the graph says at XXX rpm the charging current is zero, that is only true if your battery is at 12V. Earlier in the thread it was speculated you could start a car with 11 Volts; a level where the battery is considered "dead" and the charger is in fact providing substantial charging current.

For the purpose of answering the question by the OP the threshold rpm is so far above what a hand crank can generate it doesn't change the yes/no answer, but the explanations were not making sense to me for that reason.

 

[Rive]

If we are free to patch the plot holes, then we have ways around the RPM threshold (for example with something based on these), but we actually have a whole system what got abused by that hand cranker

 

[jim hardy]

This was a fun thread to read . One can watch the question come into focus.

At first i thought they'd taken off the fanbelt and were spinning the generator by hand which is laughable.

The picture was worth a thousand words.

Starting that generator's engine is one thing
cranking it even to idle speed is quite another.

Is there a thread on physics blunders in movies ?
Like in :The Martian" where duct tape and plastic sheet over a five foot hatch holds against an atmosphere of pressure ?

 

[Spinnor]

Some WWII era aircraft were started with inertia starters, one or two guys would input energy for a minute or so via a hand crank to spin up a rotating mass and then the pilot would engage the starter. Since the conversion of rotational energy to electrical energy is fairly efficient I am guessing an inertial energy storage system could be coupled to a specially designed generator that could put out large currents for short periods but long enough to start the engine?

 

[jim hardy]

an inertial energy storage system could be coupled to a specially designed generator

That'd be an interesting idea to tinker with.

Flywheel stores kinetic energy ½Iω² ,
where I = moment of inertia and ω is radians per second.

good refresher for units. (I still use slugs , feet and RPM)

old jim

 

[anorlunda]

Volvo used to make diesel engines with a wind-up starter. It was like a clock mainspring.

But to help everyone stay on topic to the OP, I will rephrase the the scenario the OP refers to in #1

Two men are in the desert. They have an RV with a dead battery. They have a portable generator that won't start. Their solution is to wire the DC output of the generator to the RV battery, then pull on the portable generator recoil starter cord to spin it. They used that rig to recharge the RV battery enough to start the RV and then drove home. Is that realistic? How long would it take?

 

[jim hardy]

Is that realistic?

I vote no.

 

[Rive]

No way. That RPM difference (apparently any engine-paired alternator would require 1000+RPM*) is just too much.

* at least I could find nothing what would work below this threshold. I mean nothing, what could be present in a common RV... Any wind based battery charger would be cheating I think.

 

[arydberg]

There are two parts to this question. One is there enough energy available given vigorous arm motion for perhaps 15 minutes to start the engine. ( the answer is yes) Two can someone efficiently couple this energy to the engine. A generator and battery is one way to do this but the generator must be rotated at a high RPM. I saw the show but forgot how the generator was turned.

On a side note inerta starters were used to start large aircraft engines. I saw one where as a person cranked the flywheel spun faster and faster . At some point it was coupled to a reduction gear to start the engine.

 

[CWatters]

There are two parts to this question. One is there enough energy available given vigorous arm motion for perhaps 15 minutes to start the engine.

I'm old enough to remember when vigorous arm motion for about half a turn was enough to start the engine on a warm day at least. I guess the remaining 14 mins 59 seconds of cranking are lost in the generator, charging process and the starter motor :-)

 

[Tom.G]

I'm old enough to remember when vigorous arm motion for about half a turn was enough to start the engine on a warm day at least. I guess the remaining 14 mins 59 seconds of cranking are lost in the generator, charging process and the starter motor :-)

Fantastic efficiency: 0.1% or less

 

[ZMacZ]

I think a spring engine much like in a watch would work better..
If the engine is in a well kept state it would need to rotate but a few times..
So storing the manual labour into a spring, which then gets used to apply all labour performed in a much shorter while,
will get the engine started..
I'd say about 200 rotations for one engine rotation, possibly 300 or 400..
So, you'd crank the engine by hand, but the actual force applied, would be compressed into a single rotation..
Did you know that the old 2CV of the Citroen brand did have a hand crank for it's engine ?
(One could also use a two stage engine, using a small 1 cilinder engine to crank the RV engine..)
(now you would only need to have a 25 CC diesel engine, that starts with a handcrank,
or kickstarter..diesel works so nice and simple, air+fuel+compression = running..)

But hey, whatever works and gets the engine going..

 

[Rive]

So storing the manual labour into a spring, which then gets used to apply all labour performed in a much shorter while,
will get the engine started..

I have a feeling that it is not exactly a new idea... I think these guys here are doing exactly just that

Ps.: Well, not exactly. As it seems for those engines it was an inertial starter - a flywheel...

 

[Rx7man]

The way I see the problem is that you need MUCH more torque to bring the first cylinder up through the compression stroke, after that it'll help you start to regain the energy as it decompresses.. so you go from needing something like 500-800 amps to 200-400 amps, depending on engine size of course.

Also most alternators require a certain amount of power going into the exciter coil.. I've found most of them need about 5 amps at 12V... = 60W... That's already more than you can produce for any extended time.

Most generator units are the same, with a small permanent magnet to get it going.. key word: SMALL.. Our 5kw generator needs to rev to nearly full speed (3600 RPM) until it comes online, Once it's alive, it can rev down to 1400 RPM or so and continue making power.

Based on those two facts alone, their method of generating the power isn't going to work AT ALL. If they had a small permanent magnet DC motor they might have a chance of getting some useful power out of it.

Here's a Tiger tank getting started with a flywheel

 

[arydberg]

I'm old enough to remember when vigorous arm motion for about half a turn was enough to start the engine on a warm day at least. I guess the remaining 14 mins 59 seconds of cranking are lost in the generator, charging process and the starter motor :-)

Except that was a 4 cylinder engine as opposed to a V8 in most RV's

 

[jim hardy]

Except that was a 4 cylinder engine as opposed to a V8 in most RV's

Yep model A's 4::1 compression lent itself to direct handcrank start

 

[CWatters]

Yes gets harder with more cylinders...



Yes I know it's got an inertial starter.

 

[jim hardy]

The sound of those things is unforgettable.
Dad worked at the airport before jets, used to take us kids out there to watch the airplanes come and go.
Guess i was imprinted - "Real engines are Round"

 

[CWatters]

I'm afraid you can't really best the sound of a Merlin.

 

[jim hardy]

I'm afraid you can't really best the sound of a Merlin.

They are distinctive. I got to walk among them at EAA show in Lakeland.
'I love the small of Avgas in the morning.'