Diesel generator in homebuilt car
 

I would really like to build a homebuilt car, and for a powerplant I would like to use a small diesel engine. It seems that there are a number of small, lightweight diesel generators available, and I'm thinking rather than trying to mate the engine up to a car transmission and disposing of the generator, why not just use the generator as a transmission and use electric motors to drive the wheels? Similar to a diesel locomotive, or a Chevy Volt without the battery pack and a diesel engine instead of gasoline. My thinking is that since the transmission and drive motors are electric, you could run the diesel engine at it's best BSFC rpm all or most of the time and just vary the load rather than rpm.

Does anyone see any problems with this idea? Does it seem feasible? Has this been tried before (and met with miserable failure)?

What you are describing is a serial hybrid vehicle. They aren't really known to be the most efficient since you always have to have that engine -> generator -> batteries -> electric motor -> wheels energy conversion going on. The more steps, the more losses. This is one reason the Volt gets unimpressive mileage when the engine kicks on. Running an engine at peak BSFC definitely helps negate that penalty, but it is still there. So, engine -> wheels is normally going to be more efficient, especially if the engine isn't too oversized. However, it does make for a relatively easy plugin hybrid setup.

Sure it will work, mileage of course is the question.

I think it would be worth while to see if you could find a clutch that would hold up so you could have the engine power the wheels at cruising speed, a light weight motorcycle chain could even be used as your engine shouldn't need to be over 15hp or 20hp, you might even be able to get away with not using a clutch and instead shifting it "in to gear" once the speeds match up, or use a centrifugal clutch on the tail shaft of a motor that has a tail shaft so once you hit 60mph say, the motor generator lock together powering the wheels directly.

You'll also need a battery bank to buffer, unless of course you over size the generator or are willing to wait for train like acceleration, 0-60 time of 20 minutes?

You need to explain how to do these at the same time:
Best bsfc is for a certain load range. Diesels tend to be less efficient at full power. But a diesel engine also tends to be heavy, comparatively, especially if you only run it really close to best bsfc:
http://ecomodder.com/wiki/images/b/b8/Vw15d.png
Volkswagen 1.5L diesel
Here best bsfc occurs at about 1/2 full power. And full power bsfc is about 75% as good as best bsfc, like the difference between 40 mpg and 30 mpg.
What I mean is that for the power range you want, the engine is twice as heavy as you need. I don't think this leads to the best economy.

My father and I did this project long ago when parts were really hard to get and batteries were crap. Our car was a 1970 Vega chassis. We had a 30 KW diesel generator set and a 45 HP AC motor from an elevator. And 300 pounds of lead batteries. We could just maintain 50 mph on the Grapevine grade. 0 to 60 in about 30 sec. On a flat course the diesel would run a few minutes until the batteries were charged (also boiling) and then shut down for 20 or 30 seconds. Big pulses small glides. Fuel economy was a little worse as a hybrid than it has been as a gasser.
But you can do much better today. If I could do it again I'd use an Atkinson engine and a PM alternator, and lithium batteries, natch. Built from the ground up, a series hybrid is the best route to high fuel economy.
-mort

I guess I wasn't clear with my intentions. I don't want to build a hybrid. One battery, just to start the engine. I just want to use the generator in place of a conventional transmission since the small diesels available tend to be used in generator applications. Rather than just throwing the generator away or selling it, I figure why not use it? I really just want to see if I can build a car myself and power it with a small diesel engine; and the idea of using a generator as a transmission came when I was searching for small diesel engines and all the ones I was finding were coupled to generators.

Would the power loss when converting energy forms (mechanical kinetic to electricity) be greater than the kinds of losses seen in an automatic transmission?

Maybe a gas engine would actually be more efficient because of the weight advantage?

Your answer is in post 2.

Despite everything you just said, I would love to have something like a Volt with a diesel generator running DIY B100.

ah, so the engine speed is going to control the vehicle speed... huh, you should get that nice smooth 0-60 time of several minutes then!

Ryland, why would the engine speed dictate vehicle speed? If the engine is not mechanically coupled to the drive wheels, how would there be a correlation between engine speed and vehicle speed? Diesel-electric locomotives and diesel-electric buses don't operate that way, why would a car be any different? And don't tell me that diesel electric buses take eons to accelerate; I've rode several and they were perfectly adequate in terms of acceleration. As far as I'm aware, such buses don't operate on freeways, but the kind of car I want to build wouldn't be well suited to freeway travel anyway. Just something to commute through the city to work and back.

Also, regarding my original post and the intent of my idea, I do NOT want to build a hybrid. There will be NO battery bank, just one battery to start the engine. The generator will simply act as a transmission, and the electric motor(s) would act as a differential. In effect, I would be duplicating the drive system of a typical car only using electricity. Replace the gearbox with a generator, and replace the differential with electric motor. The engine would operate, for the most part, the same way car with a CVT does.

I'm just trying to simplify the design process of a homebuilt car. If I ditch the gearbox, I don't have to try to adapt the engine to it. I just have to fabricate some mounts for the engine and generator. I'm not sure how big sufficient motors would be to drive the wheels. Ideally I would like to drive two wheels with an internal electric motor, but if motor size is a constraint, maybe I could use one large motor as a differential and drive one wheel with a shaft.

just do it then

Sorry, but this idea simply isn't going to work well for a road vehicle. The power has to be modulated somewhere, and you're bringing it right back to the engine where it would be in a normal car. So, you can't run the engine at peak BSFC (unless you add a battery pack). You've now eliminated the only benefit a series hybrid really has. You still have all the negative sides of it. Also, since you're eliminating the gearing, you're also going to require incredibly massive electric motors to handle the torque that you'll need for reasonable acceleration.

But you said you want to use a small generator and that you don't want to use a battery buffer, so how are you going to get decent acceleration?

A 7,000 watt generator is a decent size, diesel ones look to weigh nearly 400 pounds and can put out a steady 6,300 watts, have you ever driven an electric car while keeping the watt draw under 7,000 watts? my little 1,400 pound electric car can draw 24,000 watts or more to get up to speed, once it's up to speed the draw drops down to around 5,000 to 8,000 watts but if I had to keep it's power draw that low it would take me close to two blocks to get up to 35mph.

Thank you! That is exactly the kind of insight I was looking for. I'm not an electrical engineer or even terribly knowledgeable about electricity, but when you explain the wattage requirements that's something I can understand. So, it sounds like the power (watts) to weight ratio would be absolutely horrid if such a system were used in a car.

One question I have, though, is why does it work so well in buses that seem to accelerate pretty well? How do they get around this problem?

Battery buffer! it doesn't take a huge battery pack if you just need it to get up to speed, I personally wouldn't use lead acid batteries, but if you did want to I would use regular starting batteries because they can handle quicker discharge the small group 51 battery in my car is 500 cold cranking amps and 40 amp hours, it's about as small as you will find in a car, drain it down half way and you have 240 watt hours, but you can pull over 5,000 watts out of that single battery for about 2 minutes, string a number of them together for higher voltage and you have enough power to get you up to highway speed! coasting down a slight hill and your generator is producing extra power, that gets dumped in to the battery bank, sitting at a stop sign and you can dump a lot of energy in to that battery bank instead of throttling back your engine.
Lithium batteries of course would be lighter and handle the quick charge and discharging much better.

So, even though my original idea was to avoid the extra cost, weight, and complexity of having a batter bank, it sounds like it's the only feasible way to make such a drivetrain practical. You would have to build it as a hybrid, in other words.

But that is only part of the answer. The question is why diesel-electric buses have good performance. The answer is that the engine isn't running at best economy when they are accelerating. The cruise power may be around 40 hp and the full power needed to get up to speed could be 5 times higher. At 40 hp the engine might be in the 30% efficient range. But at 200 hp it may be 20% or worse. Really no different than any other car truck or bus.
-mort

for most non-hybrid cars the most inefficient driving is stop and go, for electrics, the most inefficient is highway. that is why hybrids are so popular, drive it anywhere anytime and get pretty much the same mileage.

part of the problem of your idea, though i like serial hybrids, is that it all comes down to the efficiency of the conversion. PHEV that have a manual trans and start stop ability are going to be the most efficient.

I did the math on this once a few years ago and used the generators in the northern tool catalog as reference Cheap gas, honda gas and the kubota diesel of that time. Turns out fuel economy was between 20-40 mpg depending on generator used. :turtle:

A friend ran a small diesel engine to run a tiny hydraulic drive from a fork lift lifter welded to the input shaft of a subaru gearbox/cut down drivetrain. It was a very cool cutup build mad max style for around the farm using quad wheels. It has incredible torque and being 4wd it can tow the tractor out of a bog. More like a digger or bulldozer setup in miniature. Regen with hydraulics is simple as the motor can be a pump and a few litres of accumulator can store and reuse 80% of the energy needed to stop instead of 40% efficiency with electric regen. Definitely a lighter option as the drive is a few kilos and for low revs so no reduction needed. The cost is doing the hoses and fittings.