Originally Posted By: JHZR2
Originally Posted By: rcy
Originally Posted By: GMBoy
True - Hybrids shine in city driving. Highway driving is all motor. I routinely get anywhere between 21-23mpg with mostly city miles with Tahoe Hybrids that I drive for work. These are right off the line not fully broken in units, too.
You'd be surprised at how often (as per the graphic display in my NAV screen of what the hybrid system is up to) the electric motor is assisting the ICE in powering the front wheels during highway driving. Going uphill, the traction motor will assist the ICE as it will if I need sudden acceleration. The other nice thing is complete shutdown of the ICE when travelling downhill (if the hill is steep enough to maintain the speed of the vehicle).
It's not all engine at highway speeds by any means.
Well that is kind of a bummer then. I would have hoped that the MPG situation was really effected by the fact that the massive carriage of the vehicle was being powered mostly really by the ICE, and there wasnt much assist or regeneration.
After the electrical system offers some assist, does the engine get loaded heavier to recharge, or does it sit at a low SOC to wait for free energy to charge it back (regenerative breaking)? If youre going down the hill and the engine turns off but you dont touch the brake, does the battery recharge? A generator action would act like a brake, slowing you down, so I can see scenarios where the battery could get dropped to a low SOC with no chance to regenerate it without loading the engine (hurts MPG) or slowing the vehicle (which has to be made back up by burning gasoline).
Any time you are coasting in this vehicle, the electrical motors (front - and rear if you have AWD) regenerate electricity. So going down a hill, if it's steep enough, you can maintain speed and regenerate. If it's not steep enough you will slow down (very slowly, mind you, it's not like 60 - 0 in 10 seconds are anything).
Also, with the hybrid, there is no alternator, power steering pump, belt driven A/C (or drive belts for that matter). There is a 12 volt battery up front that is used to power up the computers when the vehicle is first started and the odd accesory, otherwise the traction battery is powering a lot more than the front and rear wheels.
So as you drive, engine power is needed to keep the battery charged as it depletes from the A/C, electric power steering, Mark Levinson stereo, kids watching DVDs etc. etc. The traction battery also recharges the 12 volt battery as there is no alternator.
Also, to answer one of your questions, the battery does not sit at a low SOC after offering assist, it is constantly being charged in an attempt to keep it around 80% fully charged (Toyota having figured out that between 20 and 80 is best for the life of the battery). At highway speeds, it pretty much stays at 80% (like I mention below, MG1 can charge the battery at the same time as powering MG2). In the city, I can keep it in full electric quite easily and deplete it to 20% at which time the ICE will start to charge it back up.
It's actually interesting to watch the energy monitor when driving. Sometimes MG1 ( supplies energy exlusively to the battery, sometimes exclusively to MG2 (which drives the front wheels), sometimes to both, and sometimes I see energy flowing from both MG1 and the battery to the wheels (MG2). Interesting fact - in reverse, it's pure electric. The ICE does zero assist when driving backwards.
If you're really interested
http://www.autoshop101.com/forms/Hybrid01.pdf
is a decent write up on the Toyota hybrid system (for the Prius, but the technology is the same in the RX)
or
http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive
