Conditions often allow me to choose between moderate to high current draw in stealth mode, or a lightly loaded ICE (<1300 RPM). How much power do you find it's best to allow in stealth mode before you decide to give the throttle a slap and fire up the engine?
From what I have gathered here, ultimately all the energy is derived from the engine. Knowing that using electricity that has losses from conversion from gas to battery back to electric propulsion is less efficient than just gas to propulsion alone. I vote fire up the engine! Travis
Just try both ways to see which gives you better mpgs. Theoretically, the ICE is probably the way to go for a constant 40 mph on a perfectly flat stretch of road. Where I live there are constantly little hills, some traffic lights, etc and it will be better to use stealth whenever you can.
Keeping the battery full and using the engine is probably not the most efficient use of the car's energy. The Synergy firmware will keep the battery at whatever level it thinks is best, so you are going to be charging it regardless. Most of the charging energy will come from the regen systems. There is a lot of discussion about the cost of the regen energy, but it is the least expensive energy you have in the car. Toyota estimates that as much as 1/3 of the energy savings comes from the regen systems. The only real cost is in the extent that the engine runs just to charge the battery. By using the battery to extend your glides, you can use the regen energy most effciently.
STEALTH is the way to go for constant 40 MPH cruising. That's the speed limit on a stretch of flat road through the city I live and have been driving my Prius on for the past almost 13 years. Doing that with the engine off has overwhelming proven the more efficient choice. Of course, I'm no sure how you'd actually force the engine to stay on. It's remarkably easy to just allow the system to switch to electric-only then. The catch is replenishing the battery afterward. In my case, the speed limit climbed to 45, so it was a no-brainer just letting the system do what it does best then... power split.
The gas engine fires up if you request more than about 15 hp. Below that power, it is better to use the battery. I believe this is why 46 mph stealth limit was chosen. It is even better if you don't use the battery or gas engine and let it glide. Then use the gas engine to bring it up to speed. However, this is not possible if you need to cruise at 40 mph constant speed.
Yes, the catch of replenishing the battery is what I took into account. You can't stay in stealth too long at a high current draw, and the next thing you know your battery is low, and the engine has to both charge the battery and maintain 40 mph. I've tried it and it works great at 30 mph on flat road, but 40 mph is too high and your battery will be run down in about 1-2 miles. Obviously it would be best to use both stealth and ICE (pulse and glide). Slight decline or more is great for staying in stealth at 40 mph for a long amount of time.... and you still need to use the electric motor to maintain 40 mph even on a slight decline. If you are maintaining 40 mph on perfectly flat road, the car does not automatically go to stealth. I tried it myself yesterday after reading this thread. No need to force the engine to stay on. It is where it wants to be, without getting fancy with P&G.
Thanks for the feedback. I've been P&G'ing when conditions permit, trying to synch the P and G with the gentle rise and fall of the road. But it's too hard to tell for sure what works and what doesn't, especially when the MPG gauge doesn't take into account changes in your state of charge. It sorta feels like 20A (about 1/2 of the EV part of the green bar) is the right cut-off point, above which I shouldn't try to stay in stealth. 20A is 3kW, which is probably about 3HP after you account for losses in the inverter and motor. That makes sense, because (according to a BSFC map I found on The Internet) the least amount of power the engine can economically produce is around 2.2BHP at 1300RPM.
Per the red operating line, engine has to make about 75 Nm (55lbs-ft) at 1,300 rpm. That's about 14 hp. The idea to boost efficiency is to use the battery when ICE is not efficient, bringing out synergy. The energy for the battery usually come from regen brake that would be lost in a non-hybrid.
