The car is stopped but the engine is running. Which is efficient if it is used as an electric generator?

The car starts on its battery. A discharged battery will be placed. It was waiting to charge this battery. And then the batteries changed again.
(When the engine is running, the battery can be changed)


What is the efficiency of this process?
I want to calculate if it is worth buying an electric generator.

Data:
otto, benzine 30% eff benzine =>work
50HP
1300cm cub
100W alternator
12V 50Ah Battery
1000 rotation/minute

 

The parasitic load at idle of an engine capable of moving a car at 100 km/hr is going to be very high relative to the output of the alternator, so no, it is not a very efficient way to charge batteries.

 

Depends on what you're working with. The 400 HP Volvo Semi truck I drove for Sugar Beet harvest this year had a message center readout that showed fuel use and it showed that that huge 400 HP Cummins engine only burned .5 gallons per hour at idle yet at idle it could easily move the 80,000+ # truck across a dirt field in low gear.

Odds are at idle your 50 HP engine maybe burns 1/4 gallons of fuel an hour unless it's a emission compliant engine of which then it probably burns more fuel an hour at idle than my 400 HP semi truck does.

 

Odds are at idle your 50 HP engine maybe burns 1/4 gallons of fuel an hour unless it's a emission compliant engine of which then it probably burns more fuel an hour at idle than my 400 HP semi truck does

Unless that Volvo was real old, it probably is also "emission compliant".

 

If you really want to get into this the following is a very good read on the subject: Improving Alternator Efficiency Measurably Reduces Fuel Costs.
The link is a long read but covers all the key things of importance as to the efficiency of an automotive alternator. I believe in another thread you had I mentioned the power expressed in watts and mentioned 1.0 Horsepower in a perfect world was 746 Watts. Good luck delivering 1.0 shaft HP and getting that. Anyway, the link covers about everything you could want to know and things you never wanted to know.

Ron

 

The parasitic load at idle of an engine capable of moving a car at 100 km/hr is going to be very high relative to the output of the alternator, so no, it is not a very efficient way to charge batteries.

I agree, but it is an interesting question. If your task is to charge batteries and you already have a car with an alternator that can obviously do the job, how long would it take to payback the expense of buying a regular generator instead, which we assume is more efficient. My hunch is that you'd have to charge a lot of batteries before you come out ahead.

 

When I bought my first generator circa 1990 I made several mistakes. The generator suited my needs but my first mistake was the 1.0 US gallon fuel tank. The generator was a 4,000 watt continuous power, 5,000 watt surge generator delivering 240 VAC 60 Hz. running at 3600 RPM driven by an 8.0 HP engine. I should have looked at a 7.5 US gallon fuel tank version. My run time at 1/2 power, 2,000 watts was around 1.5 to 2.0 hours. Now consider under full load at 4,000 watts I was burning about 1.0 gallon of fuel per hour. The newer Honda engines can do much better. What we had was a generator which under full load required 8.0 horsepower shaft at 3600 RPM. Doing the math 8.0 HP * 746 Watts should have been about 6,000 watts so my generator efficiency was about 66% give or take. Then we get into the fuel consumption which on a more efficient engine would be less fuel burned for my power delivered.

There is much more to this than just the alternator efficiency and alternator efficiency alone has many factors which need considered. You also need to consider the efficiency of the engine driving the alternator. Using an automotive engine simply to run an alternator to charge a battery seems rather inefficient.

Ron

 

Unless that Volvo was real old, it probably is also "emission compliant".

2001 model so minimal emissions regs beyond the smoke reduction ones of which it had been reprogrammed to improve power and fuel efficiency thus making it smoke a bit under high load.

~425 HP retune of a stock 400 HP Cummins with 10 speed transmission. Still averaged a rather impressive 6.5 - 8 MPG from worst days to best days for the harvest!