Electronics > Mechanical & Automation Engineering

I’m toying with the idea of a car engine + generator for power backup

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Ed.Kloonk:

--- Quote from: james_s on August 07, 2022, 12:26:10 am ---That sounds like a very inefficient way to make a generator. You've got all those different coupling stages between the engine and the generator, surely it would be more efficient to just use a portable generator.

--- End quote ---

I know. It'd be fun to tease the neighbors with it though..

Berni:

--- Quote from: james_s on August 07, 2022, 12:26:10 am ---That sounds like a very inefficient way to make a generator. You've got all those different coupling stages between the engine and the generator, surely it would be more efficient to just use a portable generator.

--- End quote ---

Well it is not that bad of an idea really.

Sure you might use some more fuel to generate the same amount of power, but a car already has a powerful engine that you regularly maintain anyway, while you rarely need a backup generator.

There is no need to adjust the throttle because the the cruise control does the job on its own. It will keep the wheels turning at exactly the same speed no matter the load put on it, so the more load you put on the generator the more the throttle opens to keep it going. You also don't need to hit exactly 50/60 Hz, going +/- 10Hz is going to work just fine.

You also don't need to run it all the time. If you are this serious about power then you probably would also want to own a sizable battery bank to make it so you don't need to run the generator on any short few minute power outage and give you time to get the generator up during a long outage. This way you can run things from battery backup for light loads to keep fridges running and lights on. If you have a central heating system it it also plenty of power to run the furnace and pump hot water around.

That being said for a lot of cases people can get by using a little 2kW generator just fine. You just avoid turning on any large loads. Using a car makes more sense when you want like 20kW of backup power.

AaronD:

--- Quote from: Berni on August 07, 2022, 09:30:24 am ---There is no need to adjust the throttle because the the cruise control does the job on its own. It will keep the wheels turning at exactly the same speed no matter the load put on it, so the more load you put on the generator the more the throttle opens to keep it going. You also don't need to hit exactly 50/60 Hz, going +/- 10Hz is going to work just fine.

--- End quote ---

The cruise controls that I've used aren't exactly "precision".  In terms of PID control (not the only way to make a control system, but very common), I wouldn't be surprised if they were purely proportional (throttle = gain x error), with low enough gain to guarantee that it doesn't oscillate ever.  That goes for both vacuum and electric ones.  The speed does droop with increasing load (non-zero throttle requires non-zero error), though it might manage to keep +/-10% at highway speed.

For comparison, the mechanical governors on small engines are almost always purely proportional (flyweights (or a paddle in the fan) control the throttle directly, while the speed control pulls on a spring), and some of them do have high enough gain to oscillate.

james_s:
I think a cruise control is going to be precise enough for this application, there is quite a lot of mechanical inertia in such an arrangement and the governors on conventional portable generators aren't exactly precision devices. It's normal for the frequency to be around 64 Hz at light load and 56 Hz under load. For most things this doesn't really matter.

brucehoult:

--- Quote from: AaronD on August 07, 2022, 11:56:28 am ---
--- Quote from: Berni on August 07, 2022, 09:30:24 am ---There is no need to adjust the throttle because the the cruise control does the job on its own. It will keep the wheels turning at exactly the same speed no matter the load put on it, so the more load you put on the generator the more the throttle opens to keep it going. You also don't need to hit exactly 50/60 Hz, going +/- 10Hz is going to work just fine.

--- End quote ---

The cruise controls that I've used aren't exactly "precision".  In terms of PID control (not the only way to make a control system, but very common), I wouldn't be surprised if they were purely proportional (throttle = gain x error), with low enough gain to guarantee that it doesn't oscillate ever.  That goes for both vacuum and electric ones.  The speed does droop with increasing load (non-zero throttle requires non-zero error), though it might manage to keep +/-10% at highway speed.

--- End quote ---

The cruise control on my 2008 Subaru (Outback 2.5xt) droops a maximum of about 5 km/h from the set speed at the start of a steep hill, but if the hill continues at the same steepness long enough (a km maybe) then it does get back to zero error.

It's obviously deliberately tuned for smoothness/comfort rather than precision at all costs. When following another vehicle it maintains a very steady 2 seconds behind (that's the setting I use) with gradual speed changes, but it only does what can be adjusted with coasting or with acceleration in top gear (and torque converter locked up) unless the following distance changes quite a lot.  I've never actually measure it, but at a guess it starts actually braking when the following distance drops to about 1 second, or will change down if it drops maybe 4 seconds behind (it's quite a bit). Once speeds are again roughly matched it takes maybe 10-15 seconds to adjust the following distance back to 2 seconds.

I had an almost ten years newer Outback (2017) for the year I lived in California and the cruise control on that was probably a bit more proactive on making bigger adjustments, both in braking earlier if the following distance started to rapidly drop, and in accelerating harder to stay with the vehicle in front.

But the older 2008 model works very acceptably and a heck of a lot better than not having the system!

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