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Old 12-24-2010, 04:35 PM   #30 (permalink)
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Acceleration requires multiple times more energy than cruising, we can agree on that.

What Frank is trying to say is the IC engine is operating at higher BSFC when accelerating.

It may be better understood by adjusting the road conditions to the ideal state.
You climb a grade using the highest gear and then coast downhill with the engine off.

This is essentially pulse&glide (engine off glide). You can see this when you have an instant fuel economy reading. Slight grades reduce instant mileage fractionally, while the potential energy state is increased by elevating the vehicle. Then that same energy is used to coast downhill with no fuel consumed.

In climbing a grade, your ideal would be at best BSFC in highest gear. The increased efficiency translates into increased elevation, which is stored energy. Like the roller coaster which travels most of its track without power the benefit is distance travelled without any additional energy requirements.

Understanding this, it should be easy to understand that constant speeds where lower power requirements are normal, is precisely where the throttled IC engine is least efficient, even though the mileage may seem to be better (and it is) the load on the engine is so low that efficiency is terrible.

Build a vehicle with 4 separate engines that are each 25% of the size of the original engine. Use all 4 for acceleration and only one for cruising and you mileage would be much better, because you could optimize the BSFC for the conditions by using 1,2,3 or 4 depending on the loads applied.

If your 60 MPH sustained load is 15 HP, you need a max 25 HP engine for best BSFC, but then you would not have any reserve power for sustained grades or strong acceleration.

Car engines are designed for the worst case scenario so they have the power reserves to provide acceleration or grade climbing capabilities, but that same over sized engine operates at less than half its maximum efficiency when it only needs to produce 10% of its maximum power, because the air intake is restricted and the actual compression before combustion is less than half of the same compression when there is no manifold vacuum.

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