Diesel generator in homebuilt car

[Frank Lee]

just do it then

[Daox]

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.

[Ryland]

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.

[jared1970]

Originally Posted by Ryland

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?

[Ryland]

Originally Posted by jared1970

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.

[jared1970]

Originally Posted by Ryland

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.

[mort]

Originally Posted by Ryland

Battery buffer!

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

[ksa8907]

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.

[Cobb]

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.

Originally Posted by jared1970

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)?

[unheardofinstruments]

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.