The time of the hybrid vehicle is upon us. A quick google search will reveal that almost every manufacturer today has a hybrid vehicle or plans for producing one. However, this same search will show that there is not much offering in a diesel-powered hybrid as compared to the gasoline-powered equivalent. This is truly mystifying given the long history of the diesel engine as an efficient prime-mover. It is my aim in this document to explain the various aspects of a potential diesel-electric hybrid, and suggest that this particular configuration neatly sidesteps the drawback of an all-electric vehicle, namely extremely limited range. The diesel-electric system offers flexibility of fuel and as will be shown, could be driven coast-to-coast, nonstop, with an efficiency of around 75 miles per gallon.
The electric motor is an extremely efficient device which can be used as the source of propulsion in any land-based vehicle. One must only look to the success of the electric train (and the diesel-electric locomotive) to realize that such a vehicle is indeed capable of great efficiency and power. Modern motors of all sizes are rated in terms of efficiency against standards set by the National Electrical Manufacturers Association. Their ratings stipulate minimum and nominal efficiencies based on continuous rated output, and to reach the "Premium" efficiency, a mark of excellence, the motor must posess around 90% efficiency or greater.  A continuous output at 90% efficiency is indeed far better than the internal combustion engine could ever hope to equal, and this is the main drive to produce all-electric vehicles. Unfortunately, electrical storage systems are not on par with the convenience and space and weight efficiency of petroleum-based fuels, and so it proves a difficult task to supplant the gasoline engine. One other great advantage of the electric motor is that it can produce maximal torque at standstill, and posesses a rather flat torque curve all along its range of operation.  Torque is the characteristic of an engine which provides acceleration, thus the electric motor can operate efficiently over a broad range of speeds, reducing or eliminating the need for a transmission.
The diesel generator has long been a means to produce local power for areas with frequent outages or sensitive equipment which require constant power. Efficient generators come in both internal-combustion and turbine-powered varieties, but here I will focus on turbines owing to their well-known advantage with respect to power-to-weight ratio, a very important parameter for any moving vehicle. Capstone Turbine Corporation manufactures gas turbines in a variety of sizes, designed to operate on a variety of fuels. Their smaller models produce around 65 kW, have only one moving part (making them theoretically quite reliable) and are small enough to be placed in an area roughly the size of the engine bay of a car.  This turbine, according to their published literature, can run on just about any fuel, and when run on diesel fuel, consumes approximately 22.7 L/h (6 gallons per hour). 
Herein I will use the 2001 Honda Civic EX as an example vehicle, because I have already done the calculations required.  First, we must examine the drivetrain in a little more detail. Let us say that the engine has been replaced with an Azure Dynamics AC55 motor, controlled by a DMOC445 variable frequency motor controller. The peak charging potential is 450 V, the peak rms current is 250 A, and the peak efficiency is 87% for the whole system.  I will assume that the unit requires a similar controller to convert the three-phase AC output of the Capstone generator to DC to be stored in the battery bank. Thus we can approximate efficiency with an average 85% loss at the motor and an 85% loss at the generator, for a combined electrical efficiency of 72%. The generator, running at 450 V and its peak 127 A of current, assuming the above losses, nets out 41.1 kW of power, as per the equation 
here P is electrical power in Watts, I is current in Amps, and E is the potential, in Volts. It is important to note that this figure is the net power obtainable from the generator, inclusive of all losses in the system. I will assume the use of #2 diesel as fuel, as it is readily available at pumps around the country, although the Capstone generator may also run on jet-A, kerosene, or biodiesel.  The Civic needs about 11 kW of power to overcome air resistance at 29 m/s (65 mph) , and the Capstone unit is capable of providing a total of 41.1 kW at any given time to the drive wheels. The generator can be run at its maximal efficiency point to charge the battery while simultaneously providing ample power to the motors, should the battery pack be completely drained. If we further assume that any surplus power produced by the generator goes to charge the battery, then we find that the generator is burning about 22.7 L at 29 m/s, while still being capable of recharging the battery pack. Since only around 27% of the power produced by the generator is going to the motors, the fuel economy of the setup works out to be around 40.5 miles per gallon, with the remaining power going to charge the batteries. The effect is even more striking at city speeds, where electric motors truly shine. If one runs the generator for an hour, it consumes 22.7 L of fuel, and generates 41.1 kW of energy. At 15.6 m/s (35 mph), the Civic's chief source of resistance is rolling resistance, given by the following equation 
where P is the rolling resistance, C is the coefficient of rolling resistance, m is the mass of the vehicle in kg, g is the acceleration due to gravity, and v is the velocity, in m/s. A coefficient of rolling resistance of 0.018 was assumed, as this is a typical value for a passenger vehicle driving on concrete, and the mass was obtained from the data gathered for the previous calculation. [8, 5]. The power required to propel the Civic at this speed is 3.22 kW. The generator consumed 22.7 L of fuel, meaning that operating the generator for one hour enables a driver to travel whatever distance requires a total of 41.1 kW to traverse. Dividing these two numbers reveals that for every hour the generator is run, the vehicle can travel 12.75 hours at 15.64 m/s. In the more familiar english units, this works out to be a distance of about 447 miles, which consumed the 6 gallons stated above, for a fuel economy of 74.5 miles per gallon.
In this configuration, the battery pack acts as an efficiency buffer, allowing the system to operate at near-optimal efficiencies under all conditions. Furthermore, this configuration allows an end-user to plug the battery pack into the power grid (assuming a suitable interface) and charge the batteries using off-peak power, which is certainly produced by a more efficient power generation source. Depending on the source of electricity, it may produce little emmisions, further strengthening its appeal. In local driving, the diesel-electric hybrid performs as an all-electric vehicle, which is where the all-electric vehicle works best, but if a driver needed to travel 300 miles in a few hours, they would find this vehicle to have exceptionally good highway fuel economy.
When comparing prospective power plants, it is worthwhile to reduce all calculations to the fundamental unit of energy, the Joule. Based on the above resistance calculation, published EPA fuel efficiencies (32 mpg city, 37 mpg highway) , EPA test schedules , and the energy content of gasoline, one can determine the energy budget of the Civic under the test conditions. Test schedules were converted to numerical data, then integrated to determine total number of joules consumed during the test. Test distances were divided by published EPA fuel efficiencies to determine amount of fuel consumed, and therefore total energy burned . Efficiency is the ratio of energy used to energy consumed. Under the city conditions, the Civic requires about 5.7 MJ to operate, while the engine consumes approximately 42.26 MJ, for a total energy efficiency of 13.5%. The Civic operating under highway conditions requires 7.3 MJ while the engine consumes 34 MJ, for a total efficiency of 21.4%. While the generator in the hypothetical Civic is running, it is consuming 888 MJ/h, but it is producing 148 MJ/h of usable power (either on the spot or for future use in the battery pack), for a continuous efficiency of 16.7%. Note that diesel fuel is more energy-dense, so even though the generator is less energy efficienct, it is more efficient in terms of gallons of fuel. Although the generator-motor configuration does not seem significantly better than the gasoline engine, I have thus far avoided talking about regenerative braking. Three-phase electric motors (like the AC55) are capable of such braking without modification, and energy savings have been reported between 15 and 30 percent.  The exact impact of regenerative braking is difficult to quantify because it depends heavily on the amount of braking (and consequently acceleration) that a driver is doing to maintain or alter speed. However, if we take this range into account, we find that the overall efficiency of the system increases to somewhere between 19.2 and 21.7 percent, which is much closer to the peak efficiency of the Civic on the highway. Of course, the effect of regenerative braking will likely be larger in stop-and-go city-type driving, so the difference between the two powerplants will be even more noticable in the city. As further evidence of the potential efficiency of the system, I would point to the efficiency of large-scale diesel-electric hybrids of this configuration as used as the workhorse of the railroad industry. In those systems, efficiencies ranging from 18-26% are achieved, averaging 23.5% . While there are economies of scale at work there, it doesn't seem unreasonable at all that the automotive power plant could achieve 17% efficiency.
I have tried to present what I believe to be a realitic, conservative evaluation of the diesel-electric hybrid drivetrain. I suspect that if automotive and electrical engineers were gathered to work out the details, they could improve the efficiency numbers by some margin (perhaps more in line with the locomotive numbers), but I prefer to offer an estimate which I believe is readily attainable with off-the-shelf components. Nevertheless, in every method of fuel economy evaluation, the diesel-electric is on-par or better than the current state-of-the-art technology, and so it remains curious that the diesel-electric hybrid platform is not more readily adopted.
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