Hi,

My commute to work ends with going down a hill. The hill is 35mph and I usually just step on the brake ever so gently to engage the regeneration without actually using up the brakes. I keep my speed below 42mph so that the ICE stays off.

Lately however, I've been getting lazy and downshifting to the "b" gear. The ICE is off as I start my descend down the hill. When I start to hit 40mph, I shift to the "b" gear. As expected, the MPG meter goes to 60mpg instead of staying in the E-mode. For a while, I assumed that the ICE was spinning but without gasoline running it. I thought it just spun to provide engine braking.

What I noticed however, was that when I approached the intersection at the bottom of the hill and I start to slow down (using brakes) to make a left turn, as I approach 0 mph, my mpg meter starts to ride up to the 0 mpg, which to me indicates that the ICE is actually running, using up gasoline.

Is that how the "b" gear works?

My apologies if this has been discussed before. I tried searching the forums, but I think the search mechanism gets confused when I type in B gear.

Thanks!

 

"B" gear increases "drag", slowing the car by 2 methods, a) increasing battery regeneration b) if the speed is > 17 mph, running the ICE as a vacuum brake.

In addition, "B" changes the programming a bit, preventing the ICE from shutting off when coasting or stopped ( if in EV mode the ICE starts if going > 17 mph ), and running the ICE in a "power" setting. I really notice the ICE rpms if I'm accelerating in "B" past 40 mph. I believe it prevents the ICE valve timing from changing to economy timings. Try it - shift into "B" at 45 or 50 mph and notice the increase in rpms and the Instant MPG gauge jump to ~ 30mpg, shift back to "D" and it drops back to 50-60 mpg

 

I wrote an article about B mode on my blog.

Essentially for maximum MPG you never want to use B mode. The only reason you would want to run B mode are for long down-mountain runs so you avoid brake fade failure, or possibly braking in the winter on icy roads to slow down without applying friction (therefore avoiding some slips!)

Check the article out.

http://geekswithblogs.net/gaijin42/a...x?Pending=true

 

Thanks for the replies. I guess my question was .. when in B mode, does the ICE run with gas or is it basically just an air pump? If no gas is used up, why does the Instant MPG meter go to 0 as I slow to a stop?

 

It depends

When stopped, the ICE runs with gas, and if needed will charge the traction battery. When coasting, the ICE runs a big ole air pump to help slow you down.

 

Geckoboy — Have a look at my comments in Posts #5, 7, 13 and 15 (so far!) in the thread "Fuel Injector Shut-Off" in the Ford Escape Hybrid section of this Newsgroup. This whole matter is discussed in quite some detail there. Yes, whenever the TCH's FE gauge reads precisely 0 L/100 km (Canadian version) or 60 mpg (US version), the ICE is using no fuel at all, even though it is being spun by MG1 (powered by MG2) to protect MG1 from over-revving. This only occurs at speeds above 64 km/h (~40 mph). As soon as the ICE starts using fuel again, you'll see the FE gauge rise above the 0 (or 60) tick mark.

Stan

 

Hi SPL,

I've read the posts, good detail. It still hasn't answered my question though, so let me rephrase it.

When I go down the hill and stay below 42MPH in "D" mode, the car stays at EV only mode. Halfway down the hill, I shift to "B" mode (still staying under 42MPH) I notice the instant MPG goes to 60MPG (from e-mode) which is normal, and according to your details, it uses 0 fuel consumption, which is what I expect. Approaching the bottom of the hill to make a left, I slow down to a stop. As my speed reaches 0 mph (complete stop), my instant mpg rises to 0, telling me that the ICE is actually on and consuming fuel. A few seconds after the complete stop, the ICE shuts off and the instant mpg goes down to e-mode.

if the "B" mode is using vacuum braking (air pump), why does my instant mpg go to 0 as I come to a complete stop (all the while never going above 42mph).

 

Regen/engine braking ceases at speeds under ~7mph. Toyota made design choice for "B" to idle the engine using fuel when speeds drop under ~7mph.

They could have chosen to let the engine stop. Why? Good question for Toyota. I don't know.

 

Geckoboy — If you're coasting at under 64 km/h (~40 mph), either the vehicle goes into pure-EV mode (0 rpm), or else the ICE will be idling around 1000 rpm and using fuel. [If you started your coasting above 64 km/h in fuel-cut mode (FE reading precisely 0 L/100 km = 60 mpg), the ICE will have been spinning at ~1000 rpm but using no fuel. Then as the car slows down, it will switch into one of the two modes given above. If it decides to start powering the ICE with gasoline, the FE gauge will start to rise above the 60-mpg tick mark, and will approach 0 mpg as the car slows to a stop if the ICE remains "on." This is correct, as the FE plunges if you're not moving but the ICE is idling. Toyota presumably chose to spin the ICE in fuel-cut mode at the same speed (~1000 rpm) as the idle speed, so that they could "start the ICE up" again at idle speed without the driver being able to detect this fact! It's a very smooth transition indeed. I don't see any contradictions with your data. Why did they re-start the ICE? Only the ECU knows! Sometimes they do, and sometimes they don't.

By the way, there's no vacuum brake booster. The power brake pump is purely electric, so that it can operate even when the ICE is "off." The engine braking effect that Toyota use in fuel-cut mode is due to the fact that the ICE's expansion stroke is longer than its effective compression stroke. This means that, with no energy input (no gasoline), it's actually doing work cooling the air that it breathes in! It's this work that constitutes the engine braking effect. [Had the two strokes been effectively equal, no net work would have been done, and the ICE would (surprisingly, perhaps) be quite easy to turn over in fuel-cut mode.]

Stan