When Does Gas Kick In?

Top-speed of electric-only driving is based upon kW draw. If demand exceeds capacity, the engine will simply start up to provide that additional power. This is why you get a wide variety of answers when asking about maximums. People make anecdotal observations rather than us an aftermarket gauge to figure out what's really happening.

Long ago, it was a 10 kW draw. That would allow a decent amount of power up to 25 mph. Faster, power dropped off significantly until the max of 46 mph. But with the larger battery for PHV, despite also being a gen-3, max was increased to 62 mph (that's 38 kW or 51 hp).

With the gen-4 Prius, you can see even faster electric-only speed. That is also dependent upon power demand, but is a little more squishy than the hard limitations we saw with prior generations. Basically, the engine will shut on & off during the course of ordinary driving. It's among the methods used to deliver outstanding efficiency without plugging in.

With Prime, you've got the much larger battery along with the clutch to include the second motor for quite a bit more overall power... enough to accelerate all the way to 84 mph (that's 68 kW or 91 hp) using only electricity.

 

Agreed that the top speed when the electric motor is providing power is a function of power available but even downhill where no tractive power is required the Toyota HSD system is limited in the speed that it can allow without starting the ICE.

The limit is the rotational speed of the MG1 - in order for the ICE to remain stationery MG1 has to rotate backwards at a speed proportional to the road speed. The max RPM of the motor MG1 has increased over the generations from ~10,000RPM to 13,500 to the current 17,000 RPM. There has been a corresponding increase in maximum EV speed from ~42 to the gen4 where it is in the region of 68-84mph (uncertain as I have seen differing values).

Also when providing tractive effort the maximum power from the battery has increased from Gen2's 20kW to 25kW in Gen3 to 30kW in Gen4. The Prime has a larger value that is probably limited by the motor's combined 76kW limit (53+23) rather than the battery.

kevin

 

As observed many times. On my Gen 4 package 4 with ATP (not Prime).

When accelerating, the EV indicator(Green EV light on Muli Information Display) will stay on (under certain conditions) until 74 MPH. The EV light will not come on above 74 MPH.
When decelerating the EV indicator will not come on (under any condition) until below 72 MPH.

note: The EV light is an indicator for ICE(internal combustion engine) stopped see page 120 of owners manual.

 

It should be pointed out that even when the car's speed and SOC cause the the ICE to spin up there is no fuel burned and the rotation is purely to remove excess energy.

 

I can go all electric at 70, did it this morning on the drive in to work. The electric propulsion is amazingly efficient, even at higher speeds I can keep the car on battery for a long time w/o killing battery immediately if I am careful with accelerator. Have to modulate with right foot to minimize current draw. Here are a couple from last year, just watched speed and tried to time lights so I never came to a complete stop.

 

Nice.

 

I've found claims of different gearing for the Prime, though. If the gearing numbers that I have (IIRC somewhat based on EPA numbers) are correct, then the Liftback could actually go faster than the Prime with the engine off.

The documented maximum engine off speed in the Liftback is 110 km/h, or 68 mph, as @kevinwhite said. However, I've personally seen the EV indicator come on as high as 73 mph indicated. And, based on documentation on the MG1 RPM limits and my understanding of the gear ratios, the Liftback should be able to mechanically, without overrevving MG1, shut the engine off as high as 98 mph.

There's a catch, though, and it's lubrication/cooling. The Liftback's transmission fluid pump is driven off of the engine, so without the engine running, you don't get cooling or lubrication. The programming seems to have ways to compensate for this, and I've actually noticed times occasionally where extended running at speed in EV (say, downhill) will result in the engine starting - this isn't to absorb regen power (as I've sometimes noticed it even during positive power flow to the wheels), this is to circulate transmission fluid to cool down the transmission and lubricate it.

The Prime, on the other hand, has an electric transmission fluid pump, and therefore can do this without firing the engine.

My guess is that this is the real reason why Toyota decided to limit the max EV speed on the Liftback - they didn't want heat buildup and lubrication failure at high speeds.

 

How is that pump connected? There are no engine belts.

 

The oil pump's on the driver's (for the US) side of the transmission case, connected to a driveshaft that goes through MG1 and the sun gear, and connects into the planet carrier (which is connected to the transmission input shaft and therefore the engine).

 

Thanks. I am just here to learn.

 

Thanks for that. I've noticed it happen a couple of times in the cooler weather recently when the HV battery is not fully charged up. I didn't worry too much, but now I have a good idea why it does.

 

In cooler weather, it could also be due to coolant temperature being too low for adequate cabin heating.

 

ICE stays on until coolant is up to 103 degrees. I watch my SG as I get closer to work, so if I'm on battery I'll switch the heat off so ICE won't kick on when I'm a half block from work. I'd rather glide in and be a little chilled when I get there.

 

I believe the difference in speeds might be from the “EV only” button on the console. In pure EV mode, it seems to cut out around the lower specified speeds.

That’s different from the EV indicator, which when in normal/power/eco modes will come on all over the drive range.


iPad ? Pro

 

That's correct and I certainly have never tried to select EV Mode while driving at speed.