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Discussion Starter · #1 ·
We live in the mountains of NC and have now had the chance to do several mountain passes in the new van. Interesting observation: when cruise control is on and going down a long incline, once the traction battery is fully charged, the van will engine brake to maintain speed as opposed to engage the brakes. It really revs high to maintain speed and is quite noisy. Wonder why they would do this? Would seem to use more gas unless the Atkinson cycle motor has some way to cut fuel while engine braking.
 

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Just found another thread discussing this! Sounds like it’s not unique to the sienna hybrid and mostly to prevent brake overheating. I guess it makes sense to do that, though the system integrates the brakes into the regen/hybrid system so seamless and uses them with the following function on the cruise control, it seems they could prevent such high rpm engine braking. Interesting…
 

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Wonder why they would do this? Would seem to use more gas unless the Atkinson cycle motor has some way to cut fuel while engine braking.

The gasoline engine (ICE) is not burning fuel when this occurs; rather, it is being used as resistance by making the wheels turn it. The sound is just air being pumped through it. The oil and coolant pumps are mechanically connected so they are running too.

A plug-in hybrid (PHEV) with a much greater sized battery can (usually) absorb most of the energy being regenerated but if a PHEV starts with a full or nearly full battery (or sometimes too hot battery), it may engage the ICE also.

A BEV doesn’t have an ICE so what do they do in this case? They have to use the friction brakes or...... they use a menu setting that sets the vehicle to charge to say.... 80% of full so it will have some open battery capacity to absorb the downhill regen. Owners that live at high elevations use it. Often referred to as “hilltop mode”.
 

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Cool. That makes sense, thanks for the reply. In some googling it appears most modern fuel injected vehicles cut fuel when coasting in gear (ie engine braking) and will actually burn more fuel while idling. Never knew that. All those times I would put a car in neutral downhill to “save gas” probably ended up burning more!
 

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I think the oil pump in the sienna is electrically run as it will kick on BEFORE the ICE turns over to help prevent wear in frequent start/stops.

As others said, the fuel injectors are not putting any fuel into the engine when this happens.

It is really interesting all of the historic "best practices" handed down are not correct at all for modern fuel injected vehicles. Another one is "warming up" your car/engine. ICE engines made in the last 20+ years do better if started and immediately driven (don't floor it). They are efficient enough at idle that they don't really warm up very fast just idling and the emissions are TERRIBLE as the cat never gets hot.
 

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The gasoline engine (ICE) is not burning fuel when this occurs; rather, it is being used as resistance by making the wheels turn it. The sound is just air being pumped through it. The oil and coolant pumps are mechanically connected so they are running too.
the hybrids have no mechanical water pump, its an electric water pump. the oil pump is driven from the crankshaft tho, so if the engine's turning, its pumping oil.
 

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The way the hybrid system does engine braking is actually really interesting. MG2 is used to generate electrical power, but when the batteries are full, it then dumps the electrical power to MG1 which spins the ICE electrically to get rid of the excess energy. So it's not burning any fuel at all, it's just using engine resistance as an energy sink. Car Care Nut on Youtube has an interesting video on "B-mode" that explains how this works.
 

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... Another one is "warming up" your car/engine. ICE engines made in the last 20+ years do better if started and immediately driven (don't floor it). They are efficient enough at idle that they don't really warm up very fast just idling and the emissions are TERRIBLE as the cat never gets hot.
For sure. When I acquired my 2016 Tacoma I would let it "warm up" in the driveway during the winter, but after about two months realized that the temp needle never moves until I'd actually driven about 6 miles. I think this is why heated seats have become so pervasive.
 

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I think the oil pump in the sienna is electrically run as it will kick on BEFORE the ICE turns over to help prevent wear in frequent start/stops.
No it is a VARIABLE oil pump that is connected to the timing chain.

The original official Toyota press release materials may be the cause of the confusion: “The gas engine employs Variable Valve Timing-intelligent system by Electric motor (VVT-iE) on the intake camshaft, and VVT-i on the exhaust camshaft. A variable cooling system (electric water pump and electric thermostat) and a fully variable oil pump further improve engine efficiency.”

 

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Cool. That makes sense, thanks for the reply. In some googling it appears most modern fuel injected vehicles cut fuel when coasting in gear (ie engine braking) and will actually burn more fuel while idling. Never knew that. All those times I would put a car in neutral downhill to “save gas” probably ended up burning more!
I know if I idle my Subaru I only get 18mpg city/hwy mix, it's rated for 19 city, 27hwy, on average I get 21 easily. So remote start and idle kills mpg.

As for engine braking, I use it all the time so I don't burn or warp my rotors. It's like driving manual with an automatic. Definitely slows the car down without too much braking. Also, on my Sienna I only notice it going to 3000rpm. That's not too high and I only do it for a short time.
 

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The way the hybrid system does engine braking is actually really interesting. MG2 is used to generate electrical power, but when the batteries are full, it then dumps the electrical power to MG1 which spins the ICE electrically to get rid of the excess energy. So it's not burning any fuel at all, it's just using engine resistance as an energy sink. Car Care Nut on Youtube has an interesting video on "B-mode" that explains how this works.
Very interesting! I subscribe to the Car Care Nut so I'll have to check out that video.

I do notice that when driving in manual mode, downshifting too quickly results in the engine activating, but I always assumed that when the engine is activated and EV indicator light is off, that fuel was being consumed. While driving in manual mode, you can downshift at the following speeds from the following gears WITHOUT the ICE activating. These are the highest speeds I've been successful at keeping EV indicator on and maximizing regen braking.:
6-5 30 MPH
5-4 26 MPH
4-3 19 MPH
3-2 16 MPH
2-1 >10 MPH
 

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I do notice that when driving in manual mode, downshifting too quickly results in the engine activating, but I always assumed that when the engine is activated and EV indicator light is off, that fuel was being consumed. While driving in manual mode, you can downshift at the following speeds from the following gears WITHOUT the ICE activating. These are the highest speeds I've been successful at keeping EV indicator on and maximizing regen braking.:
6-5 30 MPH
5-4 26 MPH
4-3 19 MPH
3-2 16 MPH
2-1 >10 MPH
Just curious - are you driving in manual mode so that you can maximize regen braking or for another reason ? I ask that because using automatic mode already does a fantastic job of regen braking. Or perhaps you might be correct that manual mode is more efficient and captures more energy.
 

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I can't imagine the gain from constantly engine braking with manual downshift is worth remembering when you can and can't do it without triggering ICE. I'm sure the current battery level also comes into play too. Seems like a lot of factors to juggle for what is probably marginal increase in additional battery charge.
 

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I’m unclear on what manual mode on a CVT is doing.
i was supposed to hold/fix the ratio (ICE vs final drive). i.e. controls the MG1 so that you get more torque during acceleration (at the expense of fuel savings). But for deceleration - i dont know what is gained.
 
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