Lately, I've been working on evaluating the condition of my PHEV kit's battery pack. More info on that
over here.
The main reason for this post is because earlier this week I ordered the parts required to make an
arduino automatic coolant temp hack. Its a device that tricks the car into thinking its warm before it actually is. This in turn allows you to get into S4 faster, and do more engine off coasting. It will be especially handy in cold weather where the engine temperature can fluxuate like crazy and the engine stays on unless you're above ~155F. With the PHEV kit working and more engine off driving being done this will augment the PHEV too.
In the past I had been hesitant to try this modification even with bwilson's nice analysis that it in fact doesn't create more emissions. I himmed and hawed over it for a while. The final straw was to hear that the 2014 PHEV Prius uses the engine for hard acceleration, and shuts off as soon as the extra power is no longer required. There is no warm up time or anything, just on/off. Hearing that finally convinced me that there is no downside to this mod. Let me engine off as soon as possible and I'll save gas and emissions.