I've had my Prius for about 2 weeks, and I've learned that the reason the gas engine starts when the car is cold is to heat up the catalytic converter per EPA requirements. Is that true? I’ve also had the gas engine turn on while crawling through a neighborhood (with plenty of electric power to spare) and it wouldn't turn off, even at a dead stop. So I assume that was because the catalytic converter had cooled off. That seems like an awful waste. Why not let the catalytic converter heat up naturally when the gas engine actually needs to be run. What could the difference possibly be? Toyota has gone to great lengths to make the Prius a gas saving machine, with the aerodynamics, the coolant thermos, even lowering the middle of the cabin and inch or so. It got me thinking, if this is a real issue with the EPA, why not put an electric heating system in the catalytic converter to keep it warm that way? Would that work, or just offset the payoff by draining the battery and causing the gas engine to kick-on more often?
Originally, emissions were actually the highest priority. So the thought of sacrificing a little gas for the sake of being cleaner makes perfect sense. After all, look at how dirty certain metro areas are and how cheap gas was back in 1997 when Prius sales first began. The emission ratings of SULEV & PZEV are what the hybrids still strive to achieve. Heat is still required for the cleansing process. But now, 11 years later, many more people care a great deal about MPG. So the temptation to disregard emissions is high. In fact, there are some hybrids that do exactly that. Prius is most definitely not one of them. .
Offset. Toyota engineers spent years optimizing the design; if the engine runs a little to heat the converter, then that is almost certainly the most economical way to do it.
Hi ShimpyII, Heating the converter by running the engine is probably more efficient than using electricity. At idle, 70 percent of the fuel energy goes out the exhaust valves as heat. Even in the best situation, the effiiciency of generating electricity in the Prius is something like 30 percent of fuel energy. Its only because partial power operation is so poorly efficient in gasoline engines that this low electricity generation efficiency, and reuse efficiency, actually improve mileage. Most likely what you are seeing, though, is the engine warm-up cycle, not the cat rewarming. If your going in slow streets, even when you get warmed up to be able to glide / EV, the engine will cool down below the limit. And then its run again to get up over the limit. Its actually more complicated than that (several temp limits in several stages that have other things that need to happen to get into and out of).
Using electricity to preheat the engine makes a lot of sense, as long as the electricity comes from somewhere else. That's why people use electric block heaters. As previous posters have stated, if you are going to burn fuel to make electricity just to use it for heat, it's better to make heat by directly burning the fuel. Tom
