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Originally Posted by Arragonis
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Not sure about the Jetta = Compact Car vs Prius full size argument. . . .
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It has to do with how the USA measures and classifies different cars based upon body volume:
- Prius, NHW11 sedan body (2001-03), compact car
- Jetta TDI, compact car
- Prius, NHW20 (2004-09), full size car
- Prius, ZVW30 (2010-current), full size car
- Jetta Sports Wagon TDI, station wagon
http://www.fueleconomy.gov/
Quote:
Originally Posted by Arragonis
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A couple of questions if I may. Do you know how the Atkinson engine compares to the Diesel in terms of efficiency ? Nobody as far as I know is following Toyota down that lonely path. And secondly is the 3rd Gen Prius an Atkinson unit ?
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Ford and Toyota use the Atkinson cycle in their hybrids. From the Toyota SAE papers, the 1.8L brake specific fuel consumption overlaps earlier diesel engines but this can be misleading without the 'operational line.' The operational line is the rpm-torque curve the engine follows through the graph of the brake specific fuel consumption:
One of the little understood aspects of the Ford and Toyota hybrid transmissions is the role of the 'generator' (aka., MG1.) It has two modes that either takes or feeds a fixed percentage of the power from or to the engine, 28% (more about this later.) Since we can easily measure MG1 torque and speed to calculate the power, we can divide it by 0.28 to get the operating engine power. Then we can calculate the fuel consumption using either injector timing and rpm or the mass air flow data. I've used this technique to plot relative, fuel consumption from various trips:
This plot shows real world measurement of our 1.5L Prius along the operational line:
These charts give a clue as to the peak efficiency range of the engine based upon rpm. Efficient driving means staying within the sweet range and avoiding, if possible, rpm ranges that are less efficient:
- 1,300-2,600 rpm - efficient
- 2,600-3,200 - power and less efficient
- 3,200-4,000 - required when climbing hills
- above 4,000 - let's burn a lot of gas!
I mentioned power flows either to or from the engine side of the power split device at a fixed rate by the generator, MG1. In both modes, the generator is providing counter torque. At low speed, the power flows from MG1 around the power split device and merges in the big motor, MG2 to power the wheels. At higher speeds, power flows from MG2 back to MG1 which provides counter torque to keep the engine running at a lower, more efficient rpm. It took me a long time to understand this flow but this earliest plot shows these two modes:
One last data point, these metrics explain how my vehicle mileage improved as I learned how to keep the vehicle maintained for efficiency and efficient operational modes:
But this just illustrates our human ability to learn and maximize results. The far left points came from when I bought the car and drove it 1,300 km back home. It was in the Fall and the first dip came from cold weather operation and lessons learned about tire pressure and transmission lubrication. So now, I just drive it and get a solid 4.52 l/100km (52 MPG) over a combination of urban and extra-urban driving. This is our NHW11 model, 1.5L, sedan style Prius.
Bob Wilson