This is just a question from a newbie to this forum. I was wondering why nobody is making an electric car with a tiny gas or diesel motor ( say 10cc or something ) that just comes on when needed to charge the battery?

The only issue I see would be having it come on in an enclosed space but perhaps a very low emission motor would change that.

Would such a small engine be incapable of generating enough power to charge a battery?

I would have thought that a super efficient fossil powered engine running at constant speed would equate to pretty good mileage.

I guess you could mod a Prius for put electric running and buy a little Honda gas generator and see what would happen.

Anyway - just wondered why I never hear of such a solution - and yah, I know it still uses fossil fuels.

Thanks for any input on this. I may well be offbase so please let me know if I am.

It basically comes down to 'sure it can be done' but you need a lot of battery capacity (and space!) and an engine sized to replenish them quickly enough in realtime to handle maximum sustained loading. That size engine gets close to a regular engine anyway, and its mechanically more efficient to just hook the engine to the transmission. Remember- electrical generation itself incurs loss, as does battery storage and then the motor itself. Those are often higher than the losses in a regular engine-transmission setup.

Li-On battteries in a few years will extend the electric range of hybrids, and ultimately we probably will see some cars along the lines of what you propose.

Locomotives and newer cruiseships all use diesels to run generators to supply electric drives- but they run for days on end nonstop, and pretty steady loading. The scaling works very well in their favor too.

That is an interesting approach. For example, the engine only has to generate as much electricity as is needed in a 24 hour period. The engine would run nearly continouously, minimizing energy loss from start-stop. A second, less efficient cruise engine could be carried to provide the steady state power needed to cruise at some given highway speed.

The counter argument is the higher efficiency of a mechanical connection. This could be handled with a single gear ratio transmission designed for highway cruise speeds. A clutchless transmission would work very nicely for this application.

Interesting concept.

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Back in the 70's during the oil embargo, either Popular Science or Mechanics mag had DIY plans for just such a vehicle. My friend and I were very interested in building one but just didnt happen. The vehicle was based on a VW bug chassis using a 12-20hp gasoline engine running at a constant rpm while turning a generator that powered an electric motor. Wonder if you can still find the plans in an old magazine archive somewhere??

Good points. I guess I'm just thinking it would be pretty cheap and easy to get one of the Prius 'all battery all the time' mods done and then just add the little generator to see what happens in day to day use. I know the bigger battery in the Prius mod I saw only gives a range of about 30 miles. I don't figure the gas (charger) engine has to actually keep up with the discharge to extend that range to at least double inconstant driving.

It's just that you would never have to worry about it. I suppose it might be more of a user convenience than a technology breakthrough. KISS and all that.

If you just work it in your head it sounds better than if you run it through your slide rule.

Get up - get in car - car is charged. Travel 50 miles to work.
Lunch time - get in car - car is charged. Travel 20 miles to lunch.
Lunch over - car is 90% charged - travel 20 miles to work.
Work done - car is charged - travel 50 miles home.

Maybe put 5 gallons of some fuel in once a week? Dunno.

It just strikes me as an intoxicating (perhaps too intoxicating!) idea.

Someone with an all electric Prius should spend $200 for a little generator and try it out! You know who you are!!

Thanks for the comments!

Never mind. I just did some quick math and using a 2kW 'quiet' Honda generator it would take probably 7 hours and over 1 gallon of gas to fully charge the Prius mod that I looked at. With a range of just over 30 miles on a charge I'm afraid there is no logic to that solution. Oh well....

The problem with retrofitting such a system with an off-the-shelf generator, is that small standalone generators are poor both in terms of fuel-efficiency, and especially emissions controls. Also, the only situations where true series hybrids (constant RPM generator with 100% of the power to the wheels coming from electric motors) are used is in buses and taxis which have a lot of idle time. The small generator simply fills up the battery during the long idle intervals. This method breaks down though for steady-state driving, such as freeway driving.

The Prius already however offers the best of both worlds. At slow speeds, nearly all the power from the engine is transmitted electrically, by spinning a generator then in turn using that to spin a motor that is lower-geared to produce extra torque. At steady highway speed however, most of the power is transmitted through a direct mechanical linkage.

A few mechanical improvements in the drivetrain, along with plug-in batttery capacity, and the ability to draw more horsepower from the battery at a time would be a much better solution IMO.

Break this idea down to examine it's merits. You must consider what type of range you expect and what is then calculate horsepower required to achieve those results. For example, I vaguely remember a few years ago from an Auto Mag that it requires in the order of 13 horsepower to propel an unnamed vehicle, for which I cannot remember the total weight or aerodynamic drag, at a speed of 55 miles per hour. Place a 20 horsepower gas or preferrably diesel engine spinning a generator that would hypothetically produce 12000 watts or 16 hp. (20hp *750 (watts/hp)*.8 (generator eff). Attach 1 each, (10-15 hp peak) pancake electric motor on each rear wheel on IRS or 1 (20-30 hp) to rear solid axle to handle motive power. Considering the advances in battery technology such as A123 or Valence you could supplement electrical power with a minimal weight penalty. Assuming 13 horsepower to propel a vehicle is correct, the 12000 watts to the wheel motor with electric drive efficiency of 80% will produce 12.8 hp. Regenerative braking, boosting battery power, would supply additional power for passing and climbing grades. While not a perfect power plant, the inefficiencies probably are not much worse than the mechanical losses suffered in a transmission and rear of a traditional auto. Varying parameters, such as reducing weight, fuel and aerodynamic drag would could not only provide a local vehicle, but one with extended range.

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For something more exotic, consider an aluminum frame, carbon body in a two seat configuration such as the Honda insight. Attention to detail would yield a vehicle that would easily surpass fuel economy of any vehicle currently on the road.

Another advantage of the hybrids is that they combine the peak power of the electric motors and the engine, for overtaking or climbing situations. With a pure series-electric, you're going to be limited to just the electric capacity alone.

I'd suggest setting up the generator with some sort of simple fixed-gear transmission that is unclutched except during high-speed cruising situations. Let's say 15hp is neeeded for a 60mph cruise, the 20hp engine could be clutched, then 5hp could be siphoned off by the electric generators. This method should supply more spare power to the batteries than feeding all the 20hp to the batteries, then drawing 15hp. Also, you can run the flywheel electric generator as a motor, and have that power plus the engine power plus the regular motors. Your 30hp example is suddently a 70hp car.

I do think the power will need to be a bit higher than what you have listed for a vehicle with modern safety-standard, and to accomodate cargo loads, like people carrying luggage. While it takes minimal horsepower to overcome resistance on flat ground at highway speeds, accelerating or climbing is a completely different story. A lightweight vehicle of 750kg (~1600lb) climbing a 6% grade at 27m/s (~60mph) would require an additional 12 KW, or 16 horsepower in addition to the 16 or so needed to overcome resistance. That figure will skyrocket if you add in passengers and luggage.

A huge advantage of a constant-load generator is that it can be very efficient. A two-cylinder turbodiesel with a good heat recovery system (such as a closed loop steam engine) could be almost twice as efficient as an average gasoline engine.

Why not just have a solid Electric Car?

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