B Mode gas mileage observation

In my experiences using B on long grades in the Sierras, I've noticed that it will indeed use fuel at times that you might not when in D. For example, it will tend to not shut the engine off if you're trying to just coast (e.g. you give it a small amount of gas - in D, it would just cancel the regen drag, or use a little bit of electric-only propulsion).

I suppose there are probably times that B can also reduce fuel consumption (e.g. if it reduces the battery charge rate or keeps you from reaching a high SoC, either of which can make the battery temperature rise quickly - and if the battery temperature gets too high, it will run the engine continuously and almost completely avoid using the battery).

Most of the time though, B mode downhill won't use any gas, so it wouldn't surprise me if it was burning gas either to provide heat (if cabin heat was on) or to heat the cat. Running for 5 minutes isn't very long in cool weather, and B mode can cool the engine off quite a bit (I've had the engine at around 96°C going up a hill, then in B on the way down, it can get back down to nearly 70°C before I need power).

Also worth noting that B mode isn't nearly as effective around the 20 mph range as it is above 45 or so; it's not until the higher speed range that the engine can even reach its peak RPMs, though it won't do that until the battery is either full or too hot anyway. (I sometimes notice that after I hit a 100% full - 80% SoC - battery and drain it, then going down the next big hill, it will often only charge to 78% SoC to avoid getting too hot.)

I used B once for a steep, narrow, curvy gravel road where 20 mph was uncomfortably fast - I was closer to 15 most of the time. It didn't really do much, but it definitely burned extra gas as I tried to coast up the occasional incline on the way down.

 

I live at Lake Tahoe and drive over the Mt. Rose highway going to Reno, where I lose 4,500 feet of elevation. I have a scan gauge on the car, and it definitely shows that the engine is using gasoline whether I have the car in D or in B. The dash gauge will be pegged at 150 mpg, but the scan gauge shows gas mileage varying between 50 and 100 mpg. The engine is definitely warmed up at the start of the descent, as I have to climb 2,000 feet to reach the summit of the highway. I also notice very little engine braking effect, even if the engine is screaming at high rpm, once the traction batteries are charged. I fear for the life span of my brake pads. Now I wish I had kept my manual transmission car!

 

There's no reason the braking effect in "B" would be any less than in a hypothetical manual-transmission car in a gear ratio that results in the same engine speed, assuming equal weight, similar engines, etc.. After all, it's essentially doing the same thing, except more automated, and via a more circuitous route of power flow from wheels to engine.

Factors that might give the illusion of less braking effect: 1) The low-friction engine of the Prius, and 2) The Prius's body probably has lower aerodynamic drag than your former car's body, so that isn't helping control your speed on descents as much.

 

Another factor, which is not an illusion:
(3) The Prius has a small engine, so it can't provide as much engine braking as larger non-hybrids. Especially compared to 6- and 8-cylinder models.

 

When descending 5% and 7% grades in my Gen3, switching back and forth between D and B modes reveals that B mode is providing considerably more braking. B can maintain steady downhill speed on 5% grades (at low elevations, below 3000 feet). It isn't quite strong enough for 7%, so some supplemental friction braking is needed. I haven't driven a Gen4 for comparison.

 

Regarding use of fuel in B mode, there is a bug in the scangauge that will report a small amount of fuel usage when the engine is off. I don't know if that applies to when it should be running without injected fuel (I actually think it does not apply in that case), but I would trust the car's indicator (the 0-100 instant indicator to the right of the speedometer) if you were using the scangauge to read this downhill fuel consumption.

 

These steep/sustained downgrades are not the norm, a drop in a bucket for gas consumption. Also, isn't compression braking good for pulling oil higher up the cylinder walls, lubing a zone that's sorta neglected.

 

Hey, fellas, thanks for the insights. I think the fact that the engine uses a modified Atkinson cycle is a factor in reducing compression braking. Below is a brief description of a portion of Atkinson cycle operation:
Modern Atkinson Engine
The key technical aspects are:

• Delayed intake valve closing - this pushes some of the intake stroke charge back into the manifold to be sucked into another cylinder drawing in a charge. Unlike air drawn through a throttle plate, there is no 'pumping loss' during this cylinder-to-cylinder transfer.
• Variable intake closing - this offloads some of the metering function of the throttle plate, again, avoiding 'pumping loss.'

I'm not sure how much pumping losses in a conventional ICE contribute to compression braking, but my guess is that they are substantial. Therefore, I stand by my observation that engine braking in a Prius is not as effective as engine braking in other vehicles. And, as CR94 pointed out, the Prius does have a smaller engine than other cars in its weight class.

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In a conventional Otto-cycle engine, pumping losses contribute substantially to overall losses in part-throttle running. In an over-run condition like descending a mountain at high engine speed with throttle closed, mechanical friction effects are more substantial.

To give credit where due, it was fuzzy-one who pointed out the smaller-engine angle. (However, the Prius engine is considerably larger than that of any car I've had in a long time.) I pointed out that not having a manual transmission is not the reason you perceive less braking effect.

 

I can't speak for the Gen 3, but in the my gen 2 watching the scanguage, the car more easily completely stops injecting fuel when in stage 4 compared to stage 3. If I'm in stage 4, it allows the engine to rev a little higher in B mode, always allows the battery to charge up to 8 bars (versus sometimes only 7 in S3), and goes into fuel cutoff easier. Next time you try the hill, perhaps make sure the car is in stage 4 prior. If you got in and only drove it for 5 mins, it's possible you were still in stage 3. You would need to stop the car with engine idleing for 5 seconds to enter stage 4 once the water temp is high enough.

I've never driven the car in B mode below 40 mph, so perhaps this is just normal behavior? I use it when going up and down mountain roads in Colorado, but these are all 50 mph+.