Diesel generator cars

So you’ve see those commercials for CSX trains right? No? Well the jist of it is they have a locomotive that get over 400 mpg while hauling a ton of freight. Impressive, right? I thought so too. Then I thought ‘How the heck do they do that?’

First, quick terminology. Most people think ‘motor’ and ‘engine’ are the same thing, and in daily use they are. But technically a ‘motor’ is powered by electricity and an ‘engine’ is powered by fuel.

Steel wheels/steel track is a factor, low friction. But that’s not all of it, and maybe not even the biggest portion of it. The trains have diesel engines, but the engine doesn’t drive the wheels directly as in a car. The engine turns an electric generator, the generator then drives an electric motor which turns the wheels. The result is the diesel engine can turn at a constant RPM. In rocketry there’s a term called ‘Delta V’ which is the velocity change required to get you somewhere. In rocketry, it’s your only fuel cost. On earth, we also have a fuel cost associated with just maintaining speed because of resistance caused by various things. But delta v is still our biggest fuel consumption factor. That’s why you get better mileage on the highway. Theoretically, you should get worse on the highway because of increased aerodynamic drag, but your mileage actually improves because your engine is held at a more constant RPM. Your delta v has decreased. Well, the trains have virtually zero delta v. The diesel engine holds a constant RPM and the electric motor is sped up or slowed down as needed. Diesel engines are inherently more efficient than gas anyways simply because diesel is a more energetic fuel. Then you go designing a diesel to be super efficient in one small RPM range rather than having to compromise for an entire powerband and you’ve got something like one of those little portable generators that run for 2 hours on a gallon of fuel.

The thing that makes this work so well is that an engine driving a generator will only need to work as hard as the power you’re using. You set the engine to hold, say, 1000 RPM and leave it. As you use more power, the load on the generator increases and the engine increases fuel input. You use less power, the generator freewheels and the engine uses very little if any fuel. The whole time, you’re spinning 1000 RPM. It’s just a property of generators.

Electric motors have full torque from zero RPM, which is why they’re capable of moving those big trains. Here’s another advantage over driving the wheels with the engine directly - In your car you have zero torque at zero RPM. To take off from a standstill, you have to ‘rev up’ first to a place where you have enough torque to move your weight. That revving up = wasted energy. With an electric motor you have the torque available without revving the engine.

So here’s the basic idea. You take a small diesel engine, preferably one intended for use with a generator because it will be designed to be very efficient at it’s operating RPM. A 20 hp engine would probably be sufficient for a small car, remember the engine is not driving the car, it’s turning the generator. You mate it to a generator and connect it to a small bank of batteries. You don’t need a lot of batteries because you’re basically using the energy as you make it, storage isn’t necessary. The batteries are the most expensive part of a current generation gas-electric hybrid so here’s another advantage. Anyways, the batteries provide power to an electric motor, which turns the transmission. Everything else works as normal.

Here’s the expanded idea. You add the battery capacity of a current hybrid so that you can store energy. You use a braking-energy-recapture device similar to what current gas hybrids are using. You add a plug in to charge off the power grid anytime you’re near a 120 volt outlet. You can even add a solar panel on the roof for fun. Then you program the engine to only turn on when the battery charge is low (just like what the current gas hybrids do.)

Since it will be a diesel generator, you can run it off biodiesel with no modifications. A non-fossil fuel source that actually helps eliminate waste (used cooking oil.)

Current tech small hybrids can get nearly 80 mpg (Honda Insight) with low friction tires. Plug in versions can reach 130 mpg. With the RPM curve smoothed out by the generator and the inherently more efficient diesel fuel, we’re talking something in the range of 200 mpg. On fuel that costs $0.60 a gallon. And the braking/plug in recharge means as long as you’re only driving in town you’re not using any fuel.

Now accepting offers for funding to build the prototype

Open questions:
Do the math! What is the energy consumption rate of a small car at stop and go 40 mph? What about a constant 70 mph? How much fuel is required for the generator to supply that? What does that translate to in mpg?
What is the price difference between a conventional vs a die-gen (I just made that up ) drivetrain?
Edit: What about a turbine? Smaller, simpler, less lubrication, 40% efficient, virtually any burnable liquid as fuel with very low emissions. Probably not as good, but worth considering.

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04May2008 edit:
I found out a week or so ago about a company called Venture Vehicles with a car to be released in a year or so (hopefully!) that uses a hybrid system where the gas engine powers a generator and the electric motor only provides power to the wheels, just like what I want to do here.  Apparently it’s called a series hybrid and I’m a little late to the party here :/ Anyways, using conservative estimates for vehicle weight and aerodynamics they’ve calculated a 0-60mph of about 7 seconds and fuel consumption rate of above 100mpg (not counting the initial ‘free mileage’ you get from plugging in at home.)  As far as I know they’re not using regenerative braking, and they’re using gas instead of diesel so there’s still more to be gained here and 200mpg looks like a completely achievable goal.  When their prototype hits the road we can probably consider it a proof of concept for the die-gen if nothing else.
Another advantage to this design that I realized from looking at their concept drawings is reduced complexity.  They’re using in-wheel drive, the electric motor is inside the wheel.  There is no transmission, no driveshaft, no transfer cases or differentials, and no axles or cv joints.  We’ve just removed half of the things that cause mechanical problems in a conventional car and replaced them with simple wires to carry current to the in wheel motors.  Plus the reduced cost due to not having to manufacture these components will partially and maybe fully offset the price of the hybrid batteries and electronic control systems, making it no more expensive to build or to buy than a conventional car.

~ by Jason on April 20, 2008.