[bwilson4web]

Originally Posted by Arragonis

Again I wonder about the benefits of Diesel + Hybrid vs smaller and more powerful Diesel.

I'll try to give an answer with these assumptions whose ratios somewhat match the parameters of the 1.5L/1.8L Prius:

1. smaller diesel 50 kW, ordinary transmission 0 kW
2. smaller diesel 50 kW, hybrid transmission 50 kW for 5 minutes
3. larger diesel 100 kW, ordinary transmission 0 kW

Using a standard transmission, the smaller diesel will have significantly lower acceleration needed to deal with traffic. As such, it will slow down going up grades and otherwise be seen as a traffic barrier like a small, commercial utility vehicle. But it will have lower engine overhead from internal friction needed to keep the engine spinning and the slower, overall speeds will reduce rolling and aerodynamic drag. The brakes will convert kinetic energy into wasted heat every time they are used.

The larger diesel with standard transmission will accelerate faster and maintain speed going up and down ordinary hills and mountains. However, the engine overhead fuel consumption will be higher. This is the fuel needed to keep larger pistons, valves, engine oil pump and water pump functioning. The brakes will convert kinetic energy into wasted heat every time they are used as is common in urban traffic.

The smaller diesel with a hybrid transmission will have the power to perform like the larger diesel with some limitations. It will run out of steam going up significantly large mountains but hills in range of 100-200 m. will not be a problem. The lower overhead of the smaller engine will reduce fuel consumption at all speeds and the ability to recover a significant portion of the kinetic energy when braking minimizes waste heat. Best of all, the smaller engine can be cycled OFF when running at low speeds and cycled ON to generate power to maintain speed and store the energy needed for the next engine OFF cycle.

When the smaller diesel is in power generation mode, the somewhat fixed energy loss of engine overhead is a smaller percentage of the power generated to sustain speed and charge for the next engine OFF cycle. For example:

• 65 km/h (40 mph) -> 6 kW (8 hp) for an NHW11 sized, compact sedan
• small diesel: 6.5 kW - 0.5 kW (overhead) -> 6 kW needed :: 6 / 6.5 ~= 92% net efficiency
• small diesel + hybrid: 12.5 kW - 0.5 kW (overhead) -> 50% to wheels, 50% to hybrid storage :: 12 / 12.5 ~= 96% net efficiency

Now I've omitted a lot of details to emphasize the engine efficiency gain because its internal friction load remains relatively fixed relative to power generated. If we factor in the thermal dynamic efficiency, that 4% difference triples to nearly 12%, a significant performance improvement. But even this 4% difference increases at slower, urban speeds when the energy demands are much, much lower.

Now I've assumed the same diesel cycle in all cases. If we run the same analysis and include the efficiency gains of an Atkinson versus Otto cycle, the differences in the gas-powered world are even more substantial due Otto cycle, partial power, throttle plate losses.

This is a simple model built to emphasize how engine overhead becomes a relatively smaller part of the net power demand. No fancy cycle changes are needed to demonstrate the hybrid advantage.

Bob Wilson