Interesting Reddit Discussion from r/Prius

Not running? Or not spinning. Not the same thing
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Emphasis added. Why do you believe that one follows from the other?

On ordinary cars, I often didn't pull the parking brake when parked; I just left the car in gear. If you've ever tried to push start a car when the driver with a dead battery hasn't depressed the clutch, you'll notice a lot of resistance. An engine not running provides some considerable force.

 

You don’t need fuel and/or ignition to provide engine braking with the car going downhill with the ignition off (no combustion) but the engine revolving. This is because of the pumping losses in the engine. You are operating the engine as an air compressor without any combustion, which requires energy and slows down the car. I am not sure why this point is not getting across, and the misconception “Scenario: you are coasting down a long slope, your gas engine is not running of course, therefore it's not providing any engine braking (once the hybrid battery is full)” keeps getting repeated.

In the B mode, even when there is no fuel consumption (ignition off, no combustion), the engine is still revolving and spending energy to compress the air. This is why the fuel economy in the B mode is far worse than in the D mode, and using the B mode is never recommended unless you really need engine braking (without ignition/combustion).

 

Full Battery & going downhill - Engine coming on? | Page 2 | PriusChat

 

Everyone here is saying that the engine revolves with ignition off in the B mode, but you keep disputing it.

If the engine does not revolve in the B mode with the ignition off, then how do you explain that the fuel economy is far worse in the B mode than in the D mode?

 

For anyone joining this thread late, there indeed are plenty of links on PriusChat explaining how the engine braking works, but the ones posted by Paul Gregory have had some persistent inaccuracies.

It's not even that elaborate; anyone who has driven any old conventional car and downshifted to slow down has used engine braking, with no need for anything that "manipulates the valves". When your foot is off the conventional car's gas pedal, its throttle is closed, and spinning the engine requires sucking a vacuum past the closed throttle, which is hard work; there's the braking effect. Prius engines do have adjustable intake valve timing, for other purposes, but that plays no essential role in the engine braking; Prius engine-brakes by spinning the engine against a closed throttle, just as other cars without valve manipulations have done for decades.

There are other types of engine—diesel truck engines without throttles—that do use valve manipulation (of the exhaust valves) to achieve engine braking. Those "Jacobs brakes" open the exhaust valves early at the top of the compression stroke (producing the familiar loud jake-braking noise that some areas ban) so the work done by the piston to compress the air does not simply get returned to the piston on the expansion stroke. But that system is unrelated to what the Prius does.

Here on PriusChat, Paul Gregory has repeatedly made two related claims: (1) that the Prius never uses engine braking unless the driver selects B on the shifter, and so (2) if the battery becomes full while the car is slowing by regen, and the driver has not selected B on the shifter, the regen slowing will cease, leaving no slowing effect and the car will careen down the hill.

While Toyota could (in some universe) have designed the car to behave that way, that behavior would be undesirably surprising to the driver: having the car's rate of slowing suddenly drop to free-rolling upon reaching a particular level of battery charge that the driver might not even be paying attention to.

Instead, in this universe, Toyota designed the car so that whatever slowing is currently being applied by regen gets gradually transitioned to a mix of regen and engine braking as the battery charge rises above 77%, and by 80% the regen has been phased out, the car's rate of slowing has not suffered a sudden surprising change, and is now being achieved by engine braking. This transition happens whether the shift position is B or D, but with some changes in the details.

The exact details that change from D to B depend on whether the Prius model is HEV or PHEV, as reflected in the different way the HEV and PHEV owners' manuals describe the use of B.

The small battery in the HEV can quickly reach full charge on a longish downslope. Between 77% and 80% charge, as described above, regen will be phased out for engine braking even in D, but you can select B to have the car begin that transition earlier, charging the battery at a gentler, lower rate. This is a win if the slope is long enough that the battery ends up gaining the same amount of charge either way. In other circumstances, it's just leaving available regen on the table, so that's why the HEV owners' manual describes B as less efficient than D for normal driving but useful on steep downslopes:




In the PHEV, by contrast, there is less concern that any practical amount of regen would fill the pack to capacity, so in fact the PHEV owners' manual plays a cute word trick and makes B stand for regen Boost. This passage appears only in the PHEV owners' manual:

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So, in the PHEV, Toyota suggests you might use B for a more one-pedal-driving feel.

As reviewed just now, and amply covered before in this and other threads, engine braking is in fact phased in automatically, and is not limited to when the driver has selected B, but the driver can select B to tailor such details as how early it is phased in.

This latest quote sounds as if Paul Gregory has actually experienced the "once the battery is full, there is no resistance at all" behavior in testing in his own car. That would be new information, if true. He promised such testing on two different occasions earlier this year. That history should be part of the conversation, and can be reviewed starting here.

Edit: originally left out the owners' manual footnote for the HEV case

 

The same way it does in any other car. That the Prius planetary gear set doesn't use a clutch doesn't mean that the engine is disconnected from the wheels.

If you understand that the Prius can employ engine braking as well as regen and friction brakes, why do you think a scenario in which the battery at full charge would leave engine braking unavailable?

I appreciate that you have questions, but note that you didn't answer the one I posed to you.

Why do you think that an engine not running provides no engine braking?

 

Yep, when I used to adjust the valve clearance on my 1985 Toyota Corolla LE, I had to bring the cylinder No. 1 to TDC by turning the crankshaft manually with a wrench (then the same thing for No. 4). That requires a considerable torque when you are passing through certain strokes as you can feel the engine sucking and compressing the air! Imagine how much engine braking that would provide when you had to. If you do the math, it comes to a lot of hp of engine braking (power = torque times angular speed).

 

Jeez - why'd I bother posting a link. It's already been discussed at nauseam over the years

 

That is because the engine is not stopped—it is revolving.

When you are saying the opposite of everyone else is saying, the burden of proof lies on you, not the others.

 

Paul, I am not seeking any admission from you that you've been educated. I am posing questions to you to what you think about this and for what reasons.

Exactly. Since they are connected, the lack of a clutch isn't pertinent.

Or the engine must turn forward, right?

If the engine is being turned forward by force coming from the drive wheels, that's engine braking.

By being a drag on movement and being attached to the drive wheels. Have you ever had a car with a manual transmission?

If so, you've surely felt a stopped engine inhibiting movement of the car.

Why do you think that an engine not running provides no engine braking? "How could it?" doesn't explain why you believe an engine could be turned forward without any force being applied to it.

 

The trick is to prove it to Paul without RPM on a tachometer being proof. We've been here before, as a curious reader can see in this post et seq.

I measured about 12.6 kW (or around 17 HP) in a gen 3. No regen at the time, so that was predominantly engine braking, though a fun Prius fact is even when regen into the battery isn't happening, the car's electrical system can still be using the captured power, accounting for a small but meaningful couple HP of slowing.

The engine was turning 3936 RPM on that occasion, and that ratio to the road speed would pretty much match 2nd gear in a familiar stick-shift car.

 

In fact, I estimated it about just that much by assuming 50 ft·lb of torque on my wrench. The average torque would actually be less, as you don’t feel the full torque at every stroke; so, if you divide that by four, convert to metric units, and multiply it with the angular frequency, that’s roughly what you get.

 

Granted, you have not been physically present in any of the other members' cars, like mine, when tachometer datalogs from downhill coasting runs have been reported to you. To give you the benefit of doubt, I am trying to think of any sane way to explain your dogged repetition that it hasn't been proven to you yet, and the best explanation available seems to be simply that no tachometer is enough for you unless it is yours and you are there at the time. So ok, if that's how you operate, that's how you operate.

Of course, using the same standard, there certainly isn't anyone else here who has seen any evidence of your claims either. You may think you've proven something already, but no one here has seen it. You had two opportunities in the summer and even were advised on how you could gather and present the most credible evidence; you squandered both opportunities, never reached full charge of your PHEV's battery either time, and thus produced no evidence either way on what does or doesn't happen at full charge. That history's all still around, links are here.

So if no tachometer that you haven't personally seen can ever be 'evidence' to you, then the only way you ever will "be a bit smarter" is if you pick one up yourself, and then make sure you follow the steps needed to make an effective test of your claims.

We agree that one of two things must happen when the battery becomes too full to accept more regen: either the resistance to forward motion from regen gives way to an uncontrolled downhill roll (as you expect), or the engine's RPM rises from zero to a level that supplies roughly equivalent resistance from engine braking (as so many Prius drivers here routinely experience).

If you do a test, like your two last summer, where neither of those two things happens, and the car just continues with the same resistance to motion and the engine stays at zero RPM, then you simply failed to test the claim, as the battery didn't charge to the needed point. That'll require trying again, making sure the battery does reach full charge.