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Discussion Starter #1 (Edited)
The Volt designers have probably already thought of this method of improving fuel economy in RE mode.

The ICE has a peak efficiency point vs. load. If typical crusing power consumption is significantly below this peak efficiency point, then it would be more efficient to cycle the engine on and off. If the battery starts at 80% SOC, and drains to 35%, then the ICE would kick in and charge (at the peak efficiency point) to 40% and turn off, until the battery again drains to 35%. If the battery SOC ever gets to 30%, then the ICE would go to full power until the SOC is back to 35%, at which point it would throttle back down to the peak efficiency point. If two throttle settings are not enough, then another level could be added at 32.5%

So in summary,
1) The ICE would never be operated below its peak efficiency point vs. load.

2) Only under extreme driving conditions would it need to operate above the peak efficiency point.

In my experience, this mode of control is much simpler (and therefore better) than trying to develop a closed loop controller that maintains SOC. If a simple rectifier is used, then its nonlinearity renders linear control textbooks useless, whereas this hysteric control method always works.
 

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Discussion Starter #2 (Edited)
Peak efficiency point

It's not all that obvious how to define peak efficiency point of the range extender. I think you have to measure it in terms of Liters(of gasoline in)/Joule(of energy out).
 

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Discussion Starter #3
Easy control

Once you know what fuel rate (Liters/s) yields the best efficiency, you just hold that constant fuel rate. The complexities of motor dynamics, rectifier nonlinearities, battery impedance, etc.. all dissapear. The biggest problem is the rectifier - voltage is proportional to speed, but power only flows once the rectifier turns on. To make this speed-power curve linear would require some complicated generator-bus interface power electronics.
 

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Good anyalysis, but it uses logic, and that does not always work. Let me tell you a story about the plug-in conversion that our local electric coop had done to one of their Ford Escapes.

The escape conversion is designed to run 40 miles on electricity only. And it sort of does. But there's a hitch. In order to get EPA smog certification the computer had to be programmed to start the ICE at startup, and warm to operating temperature to warm up the catalic converter. Then it has to start and run every now and then to keep the cat warm after it gets going, even if the battery is capable of suppling 100% of the power needs at the time. The result is a reduction in the MPG of the vehicle.

So as you see, logic and math is great, but the Government requirements have to be factored in also, and they often don't take either logic or math into consideration when they make the rules.:(
 

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Discussion Starter #5
In order to get EPA smog certification the computer had to be programmed to start the ICE at startup, and warm to operating temperature to warm up the catalic converter. Then it has to start and run every now and then to keep the cat warm after it gets going
The proposed control scheme is running at some duty cycle - the RE only charges 5% SOC at a time, so maybe the cat does not cool down during the 5% discharge time. But this sounds like a solvable problem. Doesn't the engine in parallel hybrids (like the Prius) turn on and off regularly in city driving?
 

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Good anyalysis, but it uses logic, and that does not always work. Let me tell you a story about the plug-in conversion that our local electric coop had done to one of their Ford Escapes.

The escape conversion is designed to run 40 miles on electricity only. And it sort of does. But there's a hitch. In order to get EPA smog certification the computer had to be programmed to start the ICE at startup, and warm to operating temperature to warm up the catalic converter. Then it has to start and run every now and then to keep the cat warm after it gets going, even if the battery is capable of suppling 100% of the power needs at the time. The result is a reduction in the MPG of the vehicle.

So as you see, logic and math is great, but the Government requirements have to be factored in also, and they often don't take either logic or math into consideration when they make the rules.:(
Yes, sometimes the government requirements are out of whack. Especially since all these new hybrid technologies are just coming out. It will take some time to get things squared away. As long as it’s simple to just change the programming to pass government requirements then that’s no big deal. A software update can be issued when the government catches up. Here are a few things that don't make much sense but will eventually change.

1) Using diesel engines in plug-in hybrids is currently difficult because the diesel can't pass the emissions tests. With new diesel technology coming out of Europe diesel is a perfect for the Volt’s range extender (as an option). This would allow people to even use biodiesel and just about eliminate bad emissions and net positive CO2 emissions. If you drove the car around compared to a normal diesel with no plug-in technology the emissions would be dramatically reduced. Far less than a normal ICE non-hybrid car. Thus, logically it makes sense to make an exception for diesel hybrids and I'm sure that case will be made and regulations will change. Not doing so is idiotic or in my mind criminal (knowingly not changing the regulations because of other interests).

2) Having a cold plug-in hybrid ICE have to pass emissions tests that a warmed up ICE has to pass. This is just plain silly for the same reasons as number 1.

In short, the government should be kissing peoples’ butts who are using technology that is burning less fossil fuels. That is of course if they really want us to use less fossil fuels. That is still up for debate.
 
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