Why Does the Engine Turn on at +62 MPH?

It has to do with battery amperage (either regen or just discharge) When the battery amperage reaches a critical point the engine kicks in. Going 60 or so miles per hour uses a high amount of amperage, more than at lower speeds. And 62 miles per hour is the limit of EV only under ideal conditions

 

Could it be related to this fact? 62 miles/hr is about 100 km/hr.

 

The ICE comes on at 62 mph because that's how they designed it.
There is a lot of answers here at PriusChat.
Newbies need to read the manual or search this site for answers before posting.

 

could be.

 

At just a little over 62.1 mph (100 km/h), the engine may not have actually started, it may simply have only been pumping to balance out the carrier speed within the power-split-device. That's a normal part of the design.

At faster speeds, the engine will begin consuming fuel, but the RPM will remain unusually low. That's due to the battery-pack continuing to feed the traction motor with electricity. The result is extremely high efficiency, over 100 MPG. We call that: EV-BOOST

For example, watch this commute with the first section of highway at 70 mph...

 

what is the significance of 100kph?

 

On the Gen 1 and Gen 2 Prius, the design of the HSD forces the engine to turn on at about 42 MPH.
Toyota Prius - Power Split Device
To not do so would over rev M/G1.

With Gen 3, Toyota added a second planetary gearset so it was harder to over rev M/G1, I suspect that allows you to go all the way to 62 MPH without over reving M/G1.

 

At 62 mph, the generator would be spinning close to 10,000 rpm (rated Max). I think it was geared that way so that 72% of torque from gas engine can turn the wheels, when it comes on.

If it was changed to allow higher speed (as in Ford Energi), less ICE torque will get to the wheels and reduce gas efficiency.

The system was finely balanced to achieve synergy from both fuels working as one.

Going above 62 mph is also the speed when aero drag gets significantly higher, making less sense to operate with electricity.

 

Thanks guys for clarifying the 62 mph/100km/h question. It makes sense now.

 

They (sort of) picked the 100 kph number as the number they would test at to insure all parts would not exceed any design limits and longevity limits. They could have just as easily picked 99 or 101, but didn't.

But to more answer the OP question...once the various parts are rotating at the RPM they need to rotate at to be going 62 mph, even if you are demanding very little electrical power (i.e. going downhill) you could reach the bottom of the hill and soon be going uphill at 62+ mph and suddenly press on the pedal and need to, essentially over torque some component for it to quickly come up to the correct RPM (see the complexity of the planetary gear power split device). The car could/should be able to handle this...a few times even many times...but not necessarily everyday for the life of the car without over stressing something.

As a side note. I have found myself going 63-65 mph a few times and the ICE did not come on. I let up on the pedal and dropped to below 62 mph. So the 62 mph is not a absolute limit. There is a bit of leeway in power demand and/or time above 62 before the ICE triggers.

Mike

 

I thought it was to prevent max RPM of Electric motors.

 

Yes, it is to prevent over-revving the max RPM of MG1. But, why was the power-split device (PSD) gear ratio chosen that way?

To make my point more clear, let's look at it from gas engine point of view. First, let's look at BSFC of the engine to see the efficiency mapping in various rpm and torque combination. The lower BSFC number, the less gas you need to generate the same amount of power.



If you want max efficiency (220 g/kWh), the engine will need to rev around 1,200 rpm making about 80 NM (59 lbs-ft) of torque. That makes 13.5 hp. That's the lowest power engine can produce before taking some efficiency hit.

The amount of power you need to maintain speed around 62 mph (100 kph) is about 13.5 hp. That's why the gas engine was designed to run. For any constant speed below, ICE efficiency will not be at peak. This is why pulse and glide works wonder. You want to accelerate (with peak BSFC) and then glide with engine off. However, if you go above 62 mph, there is no use with pulse and glide trick.

Another reason is what I mentioned in my previous post. When the ICE is running, you want the most torque split into the wheels. This way, you get the direct mechanical drive without conversion loss. Therefore, PSD was geared with 72% of the torque goes to the wheel and 28% to the generator (MG1).

If 75% is split to the wheels, MG1 would rev above 10,000 rpm at 62 mph.

If PSD were to gear so MG1 rpm reaches max at 85 mph, ICE torque would need to split around 60% (guesstimate) to the wheels -- the rest goes through conversion (generator -> traction motor -> wheels).

 

Sticky !

 

The max EV speed is not 100km/h because of drivetrain limitations; the drivetrain limitations are because of the 100km/h max EV speed.

100km/h is the highway speed limit in Japan so they built it to go 100km/h.

Don't be surprised if the next generation, through an amazing feat of engineering, has just enough range to satisfy CARB's increased ZEV range requirements and the ability to complete the EPA testing purely in EV, with a n EV mode limit of 84mph (fast enough for EU highways.)

 

That may be a reason to reach *at least* 62 kph, but it does not explain why not *more* than 62.

See USB's post.

 

But he's not answering the question. The answer is "because they make more money this way". It turns on above 100km/h because they built it to turn on, because it's a low volume PHEV and uses the same hardware as the Prius. They aimed for highway speed in Japan and hit the target as cheaply as the could. The graphs don't explain it, they show the result of the engineering. Gen 4 will have new targets and new graphs, people will ask the same question and we'll get the same bull about efficiency.

 

It is somehow comforting that someone who can't remember his name in Maine knows more about how to build a PHEV, than Toyota engineers do after 14 years of practice with Hybrids and 4 years with PHEVs.

 

Not the same hardware as a G3 Prius -- an additional gear is added.

Do you really think Toyota saved money (let alone gobs of money) by spec'g the gear with a few less teeth ? LOL

You always look for the money, but a better understanding is benefit/cost, particularly in an engineering company like Toyota.