# Automobile Power Generator

Most are familiar with dynamometers that measure torque and horsepower output of the automobile drive train. The auto's drive wheels are driven onto a set of rollers and the output of the system is measured at various speeds. I am wondering if an electric power generator could be driven in this manner? The auto cruise control perhaps could be used to maintain a constant voltage and frequency at various load levels.

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mgb_phys
Homework Helper
It could but it would be a lot of effort for no obvious reason!
Tractors and some 4x4 vehicles have power-take-off shafts ( a drive shaft coming out of the front) that let you connect equipement. such as a generator powered by the engine.

It could but it would be a lot of effort for no obvious reason!
Tractors and some 4x4 vehicles have power-take-off shafts ( a drive shaft coming out of the front) that let you connect equipement. such as a generator powered by the engine.
A lot of effort? Driving your car onto a set of rollers is a lot of effort? For no obvious reason? The very plain obvious reason for this is to generate power for the home. Not everybody has a tractor with a power take off but most everybody has a car. Besides, before I would consider buying a tractor or a 4x4 to power my home I would buy a generator from Home Depot. Much simpler and cheaper. My question was one of engineering curiosity not necessarily something I am advocating to do. By the way, power takeoffs are never on the front of tractors.

russ_watters
Mentor
A lot of effort? Driving your car onto a set of rollers is a lot of effort? For no obvious reason? The very plain obvious reason for this is to generate power for the home. Not everybody has a tractor with a power take off but most everybody has a car. Besides, before I would consider buying a tractor or a 4x4 to power my home I would buy a generator from Home Depot. Much simpler and cheaper. My question was one of engineering curiosity not necessarily something I am advocating to do. By the way, power takeoffs are never on the front of tractors.
He's not talking about the effort of using it he's talking about the effort of building it. The main benefit would be as backup power in the event of an emergency, but the effort (and money) to build it is almost certainly more than it would cost to just buy a stand-alone generator.

In addition, a stand-alone generator would be vastly more efficient.

Danger
Gold Member
By the way, power takeoffs are never on the front of tractors.
I'm not sure about tractors (although I know that they can come off of the side and I think that I recall a front one on a Cockshutt from my youth). Mgb mentioned 4x4's as well. Many of them have PTO winches that can be on the front or the back. Front is favoured.

He's not talking about the effort of using it he's talking about the effort of building it. The main benefit would be as backup power in the event of an emergency, but the effort (and money) to build it is almost certainly more than it would cost to just buy a stand-alone generator.

In addition, a stand-alone generator would be vastly more efficient.

^^This^^

Why build something that has inherent losses through the transmission and contact to a rolling road, making it less efficient and more expensive than a diesel generator?

Yes it can be done, but it shouldnt.

Averagesupernova
Gold Member
...By the way, power takeoffs are never on the front of tractors.
I hate to sound like Yoda, but, wrong you are.

By the way, power takeoffs are never on the front of tractors.

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OK, I'm wrong. Sorry.

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Ranger Mike
Gold Member
no you are not wrong...don't ever give up an idea becuase of what is said
give it up once you do a thorugh analysis and the facts determine the cousre of action

your idea of a back up plan for power generation has merit...roller aparatus can be fabricated relatively cheap..will have effciency less than 100% due to HP loss thru drive train and roller contact..but..beats buying a $4000 Diesel generator used once a year but will take up a lot of floor space..require major reqire to close off power supplied by electric company..the line workes really get miffed whe nthey tap into a power line that is supposed to be dead..ZZZZZZAAAAPPPPPPPP ouch!! hey.. go for it... Thinking about it, its acutally probably not as outlandish as it first sounds. With a diesel engine car (front wheel drive only) and a flange attached straight to the wheel hub (assuming no PTO) there would be only fairly small losses from the transmission. All you'd need to go would be to jack the front of the car up. So it wouldnt be majorly expensive either. I'll never be as good (or as cheap probably) as a generator designed to do this but in a pinch it'd work just fine. It couldnt feasibly be done with a petrol though. EDIT: damn it mike!!! you beat me to it. was looking at diesel generators as you were replying :P Ranger Mike Science Advisor Gold Member two kinds of people . Amigo..the quick and the dead.. old racing habits are hard to break...too many fast pit stops mgb_phys Science Advisor Homework Helper For regions that have frequent power loss then generators are already pretty common - more common than cars. In most developed countries power losses are very rare, enough that having a flashlight is probably the most preparation people need. But when (if) electric cars become popular there is an idea that they will be used to store grid electricity, not so much for power cuts, but also to level out peaks of supply and demand. russ_watters Mentor Here's a 3200W gas generator for$500: http://www.electricgeneratorsdirect.com/Generac-5724/p2681.html

That's plenty to power everything in a home minus air conditioning and electric heating applications (heat, stove, water heater, etc). That's pretty friggin cheap for that much capacity.

Now if you wanted to integrate it (or the car-based one) with your home electrical system, you'd need an umbilical and auto or manual transfer switch, plus automatic load shedding or the discipline/knowledge to manually load shed when you turn it on (flipping circuit breakers for large loads). You could probably build/wire something like that yourself for \$1000 or so.

Or you could do no installation at all and just run a bunch of large extension cords to where you need the power.

If I lived in a natural disaster prone area, I'd do the manual approach. If I robbed a bank and retired to a non-extradition country with a beach, I'd do the automatic one. Since I live in a thunderstorm prone area with virtually no natural disaster history, long power outages are exceedingly rare. I've had one power outage that wasn't momentary in the 3 years I've lived in my house. It lasted about 4 hours and during that time, I watched a dvd on my laptop while listening to the news on my radio and using my 17 a-h power tank to keep the laptop charged and run a couple of compact fluorescent lamps. I was putting on my shoes to go find the nearest open Wawa with an ice dispenser or dry ice when the power came back on.

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Conventional dynamometers have two rollers about a foot apart for each wheel, so the car sits easily between them. Be sure to tie the car frame to a tree. The car's engine can easily develop 50 HP at 1500 RPM, so this is probably the best speed to run it. Higher RPMs just wastes gasoline. If possible get a car with a manual transmission- they are more efficient.

Bob the engine is operating at its least efficient at no load and closed throttle. The increase in sfc makes getting a manual a bit of a non issue. For this to be feasable it'd have to be a diesel or nothing.

I was going to have a WTF moment at 50hp at 1500rpm, but I keep having to remind myself that you all have collossal V8's over there.

russ_watters
Mentor
50 HP is 37 kW. The OP didn't say what we're powering here, but you could power an all-electric heat/appliance house on Thanksgiving with 20! Under normal circumstances a house with gas appliances could be powered with 3 if you aren't using the air conditioning.

Averagesupernova
Gold Member
50 HP is 37 kW. The OP didn't say what we're powering here, but you could power an all-electric heat/appliance house on Thanksgiving with 20! Under normal circumstances a house with gas appliances could be powered with 3 if you aren't using the air conditioning.

Been there, done that. A 5Kw ran the oven that baked the turkey and something else stuffed in there, can't remember what it was though. 3Kw lightly loaded ran a crock pot or 2. Coleman camp stove for mashed potatoes. As soon as the oven went off, fridge and freezer got plugged back in and the furnace turned back up and lights were used more liberally. I'm not sure some of the relatives really believed the comercial power was off. Getting by without commercial power isn't that difficult at all for less than a couple of days. Makes you appreciate what you pay for commercial power when you have to generate your own power.

Ranger Mike
Gold Member
We took a 5 k generator to the race track once
Barely ran the small air compressor
And battery charger
Forget it
I think min of 10 k
For house

Averagesupernova
Gold Member
Starting an air compressor and running electric heating such as an oven and crock pots or electric lights are 2 different things. I've used the same 5KW generator to start a small compressor too and it would barely do it. It just depends on what you want to do in your house. Not too likely there will be any hard starting loads other than an automatic washing machine.

FredGarvin
A 5k generator is plenty for running the average house. I use one myself (5kW nominal, 6.5 kW peak). You can't/shouldn't run something like an air compressor or battery charger or air conditioner because the in-rush current is too much for the smaller units.

In reviewing the brake specific fuel consumption (BSFC) maps of automobile engines, maximum efficiency (minimum grams of fuel per kilwatt-hour output) is obtained at about 75% of maximum torque and 35% of redline RPM. I attach a BSFC map for a 2.7-liter engine. The power at max efficiency point (130 N-m, 2100 RPM) is 38 HP. For lower power, plot the constant power hyperbolas on the BSFC map, and reduce the RPM until you reach about 1500 RPM or 75% max torque. Manual transmissions are about 96% efficiency, compared to automatic transmissions at about 85%. Look carefully at a manual transmission and you will not find any water cooling loop. Disconnect the water cooling from an automatic at your own risk. Any energy that goes into heating water does not go into the dynamometer.

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I suppose you'd have to match the engine so that the load fell into the max efficiency range.

Are autos still that bad? Last time I was looking round autos were just about as efficient.

russ_watters
Mentor
I suppose you'd have to match the engine so that the load fell into the max efficiency range.

Are autos still that bad? Last time I was looking round autos were just about as efficient.
But that's just it, you can't match the load unless you can find a good excuse to generate all that extra power.

Lets say a car is it's most efficient when it outputs 35 kW at 30% efficiency and you only need 5 kW and it is 10% efficient. You wouldn't do the 35 kW (117 kW input) if you only needed 5 kW (50 kW input) because you're still wasting a whole lot of power. Not unless you can think of a good reason why you want that extra 30 kW of output power - like maybe microwaving-off your neighbors pool by tomorrow!

You can match the engine to a set load is what I meant, just not an enormous V6 or V8. Admittedly you'd have a hard time finding an automobile engine small enough to match perfectly but it could be done.

This isnt a practical suggestion (obviously buying a new car for a tiny engine is out of the question), just responding to Bob about load matching.