A better way to compare the real world efficiency of the series hybrid with the parallel is to compare the Tesla with the Prius. Although the Tesla is a pure electric vehicle, it can be evaluated as if it had an on-board generator.
The Prius has a combined 55 mpg vs the Tesla’s 7.85 km per kwh of utility energy. The Tesla is slightly lighter than the Prius but the Prius was designed to maximize efficiency, whereas the Tesla was designed to maximize performance.

7.85 km / kwh from a 35% efficient Diesel Generator, assuming an equivalent conversion efficiency as the Tesla’s battery charger, yields 68 mpg for the Tesla vs. 55 mpg for the Prius. Reasonable to assume weight of Diesel Generator plus fuel vs removed battery size would be similar.

There is an additional efficiency gain with the series hybrid, however, since a portion of the generator output goes directly to the PEM (power electronics module) without charging the battery, so avoids the Tesla’s 86% charging efficiency. For instance at a typical driving speed of 60 miles per hour, the Tesla would be using about 11 hp, so for a 15 hp generator, 73% would be sent directly to the PEM and Motor. Also charging efficiency could be improved in a series hybrid since less than 1 kwh of energy storage would be used for normal driving (i.e. acceleration & modest hill climbing), the balance would be for the (normally) very unusual climbing mountains at speed also sustained travel at greater speeds then the generator output to the PEM DC Bus. You could therefore optimize the efficiency of the 1 kwh storage to ~95% with various means, at a higher cost per kwh, but an efficiency gain that might warrant the increased cost.

Another factor is the fact that the Tesla was optimized for performance, i.e. 0-60 in 3.9 sec, and top speed of 130 miles per hour. This feature required the use of a 2 speed (forward only) transmission, since I believe 1st gear gets you to 80 miles per hour. There would be somewhere between 5-10% loss of energy (motor shaft hp) in the transmission, and a further loss of 10% in the rear differential. Both of these losses could be eliminated with the use of one or two rear wheel motors for propulsion.

These factors could push the Tesla series hybrid mileage up to about 83 mpg. In addition the scale economies of a standard generator module and much smaller generator size – 15 hp vs 76 hp for the Prius, would probably lead to much more efficient generators like LNG gas turbines or the StarRotor Brayton cycle which is claiming 45-60% efficiency. This could easily push the Tesla series hybrid mileage up to 107-142 mpg. Also it should be noted that this is the first generation Tesla vs the 9 year design advantage of the Prius, tweaking always improves performance significantly. The Tesla was also designed with an R&D budget that Toyota would consider Pocket Change, and is, much more than the Prius, an entirely virgin technology with virtually no scale economies.

Telsa is a two seater roadstar and Prius is a mid-size sedan. It's not apple to apple.
So, your Diesel Generator weighs nothing and the 7.85 km / kwh fuel economy stays unchanged.

Ken@Japan

With a plug-in hybrid you don't need as many batteries. The Tesla has about 280 kg of batteries for a 250 mile range, you could remove 170 kg of the batteries and still have a 100 mile range. The 170 kg saved is a lot heavier than a small gen-set.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm

As Ken pointed out, not really because the vehicle payloads and configurations are radically different. A better experiment would compare a Prius in parallel mode to a Prius in series mode. This is entirely practical with at least two different approaches.

The Prius has two motor generators connected by a planetary gear, the Power Split Device (PSD) in the transaxle. Simply fix the planetary gears so they do not rotate and grind off their ring-gear teeth. In an NHW11 and you have a series hybrid with these characteristics:

1) 24 hp generator (MG1)
2) 40 hp motor (MG2)



Now there remains a control problem, substituting control computers to operate the ICE, MG1 and MG2. But this is a solvable problem as Toyota has brilliantly shown. All you have to do is buy a used, NHW11 Prius and conduct the experiment. However, this simple change, welding the planetary gears and removing their teeth from the ring gear also allows or proposes a simpler model.

Disconnecting the plantetary gears from the ring gear eliminates their mechanical loss. It might be easier to simply compare the planetary gear mechanical power losses versus the electrical losses of power from MG1 to MG2. Fortunately, the measurements of many of these losses are readily available.

I would recommend starting by reading the Dept. of Energy final Prius report and associated SAE and Toyota papers on Prius performance. Then we can construct a credible model and have something to discuss.

The second approach uses the Electric Vehicle (EV) mode switch on some Prius. Using EV mode, travel over some given course measuring the distance and power used. Then stop the Prius and disable EV so the engine recharges the battery and measure the fuel burn. Then repeat the same test in hybrid mode with the engine. I would probably have the vehicle creep up a small hill since the change in altitude provides an easily calculated change in potential energy.

A third approach would be to put a small deisel generator in the rear of an EV switch equipped Prius rigged to provide battery power. Then using EV mode, repeat the above test over a longer distance using the deisel generator for battery power.

BTW, it is popular to speculate about advanced engine-generator systems with very high efficiencies. For example, a floating piston, deisel generator that eliminates the crankshaft and uses a linear generator to extract power. But without a practical example, such devices remain myths. All of the proposed experiments are well within anyone's home budget. Do the experiment.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

If you reduce battery 110kg/280kg, your battery power becomes 110/280=40%.
The Tesla 185 kW motor is now utilized only 73 kW.
The same weight with 60% less power means we see completely different animal.

Ken@Japan

Nice try, but that's not how it works. A battery stores energy, not power.
The peak load that the motor draws is a design concern, with reduced battery size, more load per cell, but this is easy to accomadate by storing peak load in a capacitor bank on the PEM DC bus - that peak load is only used on extreme accelerations (i.e. 0-60 in 3.9 secs), so the total energy the capacitor bank must store is quite small.

Actually a 5 kwh battery bank should be sufficient, to supply the energy needed to climb mountains, unless you also want the plugin option, and that would be 5/44 = 11% of the Teslas Battery size, quite lighter than the replacement generator / fuel tank. So actually increased fuel mileage could be expected above my figures due to weight reduction.

The difference in style is not significant as both have similar drag coefficients, and wind resistance is not a big factor, at most a few percent difference in fuel economy between the two. Weight is a more significant factor, the Tesla is 600 lbs lighter than the Prius, but I ignored the weight loss also in converting the Tesla to a series hybrid, and as a series hybrid the weight gain in going to the Prius sedan size would be less significant, since the series hybrid is much more efficient at recovering braking energy, which accounts for most of the energy price that is payed for by increased weight.

Also, a fully mature series hybrid design, would likely mean a common chassis, so the change from sedan to a roadster would only mean a different body, made of composites, not a big weight factor.

You just don't know the W/kg battery power specification.

Ken@Japan

Again, the utility space shoud be compared.
Prius is 5 passengers plus 16 ft3 luggage volume and Tesla is 2 passengers with almost no luggage space. It's apple to cherry.

Ken@Japan

I've seen that at the everspring site - chinese bad translation - it should be W-HR/KG.
A flashlight battery can supply a 100 kw - just run it through a DC-DC convertor to a High Voltage Capacitor - and rapidly discharge the capacitor into a load - a 7.5 W-HR flashlight battery would supply 100 kw for 7.5 X 3600 / 100,000 = .27 secs assuming 100% conversion & discharge efficiency. With 16 flashlight batteries you could power the Tesla from 0-60 in 3.6 sec!

Luggage space, volume, passengers are irrelevant to the calculation, it is strictly a matter of rolling / air resistance ( not very significant as they are both aerodynamic small cars - a few percent difference at most ). Main difference is weight - weight means that more energy is needed to accelerate the vehicle and I believe the Tesla returns about 75% of that energy to the battery on deceleration. A significant factor for the 600 lbs weight difference, but not enough to reduce the Tesla series hybrid mileage to that of the Prius's.