Engineering 1-0-1: Designing your own remote washdown station for 4WDs

QUESTION

Hi John,

Love your work and respect to a fellow engineer. I’m a 1st year engineering student doing a group project on designing a low-impact washdown station (for Cape York - hence proper off grid).

We were ‘'considering’ running pumps etc. directly off the battery/alternator with lovely low-tech DC electricity. I can already see the problems; electrocution/short circuit risk when plugging in/unplugging/washing, small safe capacity of non-deep-cycle batteries.

The other risk is how alternators are not only imperfectly efficient in their own right, let alone when a V8 engine is turning and burning, just to keep them going.

What would be the overall efficiency of, say, idling an engine in a LandCruiser at 1500 RPM to produce electrical energy?

Also, yes, we are meant to consider all ideas in our design process, including stupid ones.

Our other crazy idea is putting cars on a dynamometer/rolling road to produce torque in the roller to convert into electrical energy.

Your thoughts are appreciated.

Lindsay


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ANSWER

Lindsay,

Thanks for the kind words mate.

It’s not ‘low impact’ to use individual vehicles to power the wash station (because of the second law of thermodynamics and the inefficiency of conversion).

Engines are spectacularly inefficient at idle, obviously (because they’re designed to be volumetrically efficient when they’re working). And turning petrol into rotation of a pulley at 1200-1500rpm, and then to electricity via a belt, is very inefficient. And because of Thermos II, there’s an obvious loss of exergy every time you add a process. And it’s hardly ‘green’ (whatever that term actually means). (Greenest option: leave the 4WD filthy.)

Just spit-balling it, I’d be surprised if engines were 15% efficient at idle, and I’d be surprised if there wasn’t a further massive reduction from the driven pulley to electricity coming out of the alternator (though there’s plenty of power at idle to run it). A bright young chap like you can research the precise efficiencies, with Google at his side, I’m sure.

Also, 12V x 150 amps (maxxing out the alternator, which might exceed its duty cycle) = 1.8kW, which (I guess) is (just) sufficient to operate a pressure washer after you invert it (the DC, not the washer).

Roller dyno: same problems, essentially. Plus overkill, and even more potential for dickhead abuse. You don’t need a 100kW wash station...

What’s wrong with mounting a 2.4 kVa solar array and an inverter on a roof, and plugging in a Karcher from Bunnings? (I’m a bit ‘ghetto’ when it comes to solutions.) ‘Ghetto’ is like Occam’s Razor for engineers: “In the absence of a compelling reason to do anything properly complex and/or nutty for its own sake, the ‘ghetto’ solution is usually the best solution.” ("Cadogan’s Razor”)

Plenty of sunshine up there, and if it’s dark, you’re typically too busy to wash the fourby, because you’re blending in by getting shitfaced, like everyone else at Coen, or wherever.

Also, you have to concede that some 4X4 users will fail the ‘idiot’ test, and you will need to ‘idiot proof’ any setup you implement. Much easier to do that with a unit that doesn’t require the potential idiot to interface with his or her own electrical system and your onsite hardware. (Universal connectivity is also going to be a problem for you.)

It’s far better to have a standalone unit with all the protections taken care of (core balance relay) using commercially available hardware that’s easily available and somewhat modular, in my view.

Put the array and inverter out of reach and all the other hardware in a (watertight) box, and just have an on/off switch plus a hose coming out of a hole in the wall. (Clean, simple, tidy.)

Respect back at you. (Mainly because I appreciate the brain bleeds you have had and are yet to have: solid mechanics, thermofluids, Laplace, Fourier, multivariable calculus… It’s a living hell, much of it you will never be able to forget.

But the world has never needed engineers more than it does today. And that’s the price of admission.

Best regards,

John Cadogan


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