Quote:
Originally Posted by jonr
You definitely want to accelerate in the "250" island in the above graph. I call this moderate acceleration. But you can't cruise in the island because you would go too fast. So the best you could do is cruise at the leftmost red dot which is in the 300 island.
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Now we're coming to the heart of the issue.
In order to cruise at the leftmost dot one would need a crazy high gear for that motor to match the motor speed to the vehicle speed. Since 1000 RPM is barely over idle on most cars (not sure about this Saturn) my gut feeling is that this would not work, I'm thinking that the car would tend to stall in that gear. While it might be possible to actually maintain 65 mph on the flat on a windless day in that gear, I don't think you could actually accelerate to the cruising speed, and the slightest hill or headwind would stall the motor.
As you say, the car cannot cruise at maximum engine efficiency, but it can accelerate at it. There is a relationship between the rate of acceleration and the amount of HP coming out of the motor (a complex one though, involving mass and velocity dependent drag), so at at some (rapid and probably varying by speed) rate of acceleration the engine will be in the sweet spot. Drive the car with pulses of acceleration, shut the motor off the rest of the time, and presto bingo, maximum engine efficiency.
The down side is that PG pretty much sucks for passengers - for a lot of people it is a guaranteed recipe for motion sickness, and at the very least it is really annoying. It's also not very much fun for drivers of nearby cars. In my view PG is only an option for the solo driver on an empty road.
It would be a lot more human friendly if there was an energy storage device like a flywheel between the internal combustion motor and the drive train, so that the motor can be pulsed while the car itself is driven smoothly. This would also resolve the issue of no power steering, no power brakes, no power anything, when the motor is off. Sounds like a Prius, doesn't it? Unfortunately adding this capability to the car sitting in your driveway now is going to be very, very, VERY difficult - this is way past duct tape and coroplast!