This is a guess - the question had come up in another thread as to why Toyota uses a bladder instead of a rigid tank, and I believe the answer is safety. In a crash that breaches the fuel tank, there's a risk that the splashed/sprayed fuel will ignite. A bladder reduces the risk of breach because it's flexible, and will simply deform rather than tear open. While there are certainly crash scenarios that would shred the bladder, the overall likelihood of breaching a bladder versus breaching a rigid tank is smaller. Secondly, the bladder reduces the volume of vaporized fuel present, because as it shrinks as fuel is drawn from it, the ratio of fluid volume to vacated space is smaller than it would be with a rigid tank. Hence a Prius aflame after a wreck would be less likely to explode. This is all surmise on my part, as I had asked a fuel cell company exactly the same question regarding my Cessna 180, which has bladders instead of rigid tanks, and the answer was as I set it down above. Bladders have their annoyances, but they also have one major problem, which would probably never happen with a Prius (hopefully) - they can't be left dry for an extended length of time without damaging them - they always have to be kept full. But since we Prius owners will never run out of fuel because the consequences are severe, I don't think this problem is a "problem." Anyone having further information - or CORRECT information - please chime in. Mark Baird Alameda CA
My opinion is that they did it for weight considerations. A bladder probably weighs far less than a more traditional rigid tank would and, therefore, more fuel efficient.
The HCH has solved the emission control problem in another, and perhaps better, way "AT-PZEV Emissions Rating To achieve Advanced Technology Partial Zero-Emission Vehicle (AT-PZEV) status, a vehicle must be a Super-Ultra-Low-Emission Vehicle (SULEV) with zero-evaporative emissions and carry a 15-year/150,000-mile warranty on emissions equipment. Every 2006 Civic Hybrid has a new plastic-resin fuel tank that does not produce any evaporative emissions."
HMMM! Then why put a bladder in a rigid tank? That would add to the weight. It was put in the US version to meet EPA requirements. They don't have it in Europe or Japan.
