# Hybrid Rail Buggy - for fun



## judebert (Apr 16, 2008)

The idea seems sound to me, although extremely unorthodox. I like the idea that the car is using all-electric power to get started, and eventually winds up using all-gas power to cruise, with the diff handling the tradeoff automatically.

The two weak links I can see are the controls and the electric motor. (From now on, "motor" is the electric motor, and "engine" is the conventional gasoline engine.

You haven't mentioned the motor mounting, but since the engine is connected to the diff, it sounds like you want the motor connected directly to the wheels. This is a challenge; you need a lot of torque to start moving, but you need to keep the motor below its max RPM even when you're moving at your top speed.

You could insert a clutch or something to disconnect the motor from its wheel(s), but that requires either an extra pedal (ick!), a centrifugal clutch, or extra electronics.

Additionally, the big torque you need to start is rough on batteries. Most controllers don't supply that kind of power, either; you'll need one of the expensive 1000+A controllers. (Then again, we're talking about a rail buggy; maybe you _could_ get away with it.)

The control problem is the more complex issue. Let's start with the basics: the generator can't provide more total energy than it's being fed. We'll also assume that the motor and the engine are connected to the same pedal.

When you just want to start moving, you barely press the accelerator. The engine is running almost at idle, turning the generator with a little bit of energy. Most of the energy required for the motor is coming from the batteries.

As you press the pedal farther down, the engine starts revving. The motor also starts picking up the speed. Some of the engine's output is driving the wheel, and some the generator. But the motor is pulling even _more_ energy than before. The batteries are probably still being drained.

Eventually you get up to cruising speed. Most of the engine's output is driving the wheel; a very little is driving the generator. And the motor is still trying to drive its wheels, too. If you've got a transmission, there's weird stuff going on between the motor and the engine; the engine might be running at 1.5K, while the motor is running near its top end (probably 5K or less). 

All in all, there are a lot of interactions that are difficult to predict. You'd be blazing new trails. If you're comfortable with that, we'd all love to see where it leads you!

However, I'd like to propose an alternative: connect the motor directly to the engine's crankshaft. If this is a VW engine, I'm talking about attaching it through the fan pulley bolt (somehow; you'd have to figure it out in any case). Include an electrical switch to turn the motor on or off, and treat it like an electric turbo.

This solution is nice because the engine speed and motor speed are always identical. When your motor is on, it's pushing the engine and the wheels together. When it's off, it's providing very little resistance to the motor; it's like an additional flywheel. You could wire it to allow you to run the turbo by itself, although it would have to overcome the engine resistance, and some raw fuel would probably be exhausted.

You could figure something out for recharge, but even if the batteries run dead, you've still got the entire gasoline range of the engine. 

You'd still be trailblazing, though. Nobody's attempted a hybrid conversion here, mostly because of the control issues. 

Good luck!


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## codedysfunction (Jan 22, 2009)

Thanks for the quick reply and detailed info. Your thoughts have made me rethink the overall drive train idea. I never gave much thought to the RPM limitations of the electric motor, not to mention the unnecessary wear on the bearings and or brushes when cruising on ICE and the motor shaft being engaged.

I think that I'm going to do version 1 on a small scale. Something like those enclosed cage-type go karts. Hopefully I can find differentials that small and maybe a gearbox to increase the gear ratio of the elec motor and or ICE.

On the subject of elec motors, is there a handy selection guide that you know of? In the past, any time I've used motors (usually small) I've just purchased one over-sized and decreased the voltage to make it work for my purpose. In this case I think selection needs to be pretty close since I don't want to oversize and can't afford to under-size (at least too often). 

Also, do you have any thoughts on whether I should use a brushless motor? I realize that the design will have to incorporate some sort of controller (if not included in the motor housing). It seems that this would be the best way as it would use less energy. I believe that initial cost is higher than a conventional dc motor, but the fact that they have lower mainenance requirements may make the investment worth while.

Thanks again,

CD


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## judebert (Apr 16, 2008)

I'm afraid I don't have a motor selection guide. My EV is -- by forum standards, anyway -- bog standard. I just used well-known batteries, motor, and controller.

I have done some research on motor types, though. For a small project, brushless DC should be available in the required size and power band. The more torque you need, the more scarce they get. Controllers are also a bit more complicated, although that's become less and less of a problem since the RC modelers got into it.

I'm all about low maintenance. I want an AC induction motor (also brushless) for my next conversion (if any).


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