"B" Range...What's Really Going On...

wow. thanks for running these tests!

 

Hi Matt,

Thanks for doing all these tests and the recordings! I've a few observations as below:

1. Traction Motor Torque (Actual) is higher in D (-110 Nm @ 51 kmh) than in B (-21.25 Nm @ 53 kmh). Without engine braking to assist, MG2 has to do more work and the higher torque is expected.

2. Battery Current is higher in D (-89.93A @ 51 kmh) than B (-30.3A @ 53 kmh). In B, some of the regenerated power by MG2 is being consumed by MG1 (function as motor) to spin ICE, the balance power will be used to charge the battery so lesser current to battery is also expected.

3. Brake Master Cylinder Torque only shows pedal pressure and not be as useful if you want to know whether does it save the brake pad. I would rather monitor Wheel Cylinder Pressure Sensor instead which can give you some indication that brake pad is used. In cases where battery current max. out, you should see an increase in this sensor because hydraulic braking has to make up the shortfall of regen. braking.

4. Stopping time is longer in B (~22 secs.) than in D (~11 secs.). It is not clear whether you used brake in B. It seems to me that you're only shifting to B and not using brake, otherwise I can't think of why it takes longer for B to come to a complete stop.

5. Your test distance is relatively short (I think less than 200m). B is usually used for long and steep descent, where MG2 max. out regen. or SOC reaches near upper limit.

6. The hill grade is unknown. The higher the hill grade, MG2 has to regen. more to maintain speed and is more likely to max. out sooner than a gentle slope or level ground.

As your tests are biased towards short distances, I would caution against "one size fits all" comment to use D instead of B.

Vincent

 

ehh.. whatever the case.. these are tests that prove the claims that informed Prius drivers have known forever.

yes, "B" does not add any additional regen and wastes energy to slow the vehicle down.. but now we know that normal regen braking in "D" is not just better, but provides 3 times more!? say that again... 3 TIMES more amperage to charge the traction battery.. impressive.. i know that the amperage reaches about 90 or so from my own monitoring, but never really bothered checking it while in "B".... because I never shift into it unless it's ACTUALLY required... which is rare for me.

but yes.. as mentioned above... were there any tests made while applying the brake pedal while in "B"? i do not know.. great info nonetheless.

 

I used B yesterday when I was descending a long hill, but only after my battery had fully charged and I was getting no deceleration anymore because it was full. At that point I didn't want to stress my brakes, so I enabled B mode to have the engine turn over and slow the veh. I guess this begs the question though, what's more important, wear and tear on the ICE or on the front brakes?

 

brakes are fairly cheaper than an engine?

 

It is not about wear and tear, it is about safety. When the brakes overheat to a great degree, the metal in the brake rotors or drums develops hard spots. These are known as hot spots. The hot spots resist the friction from the brake shoes and pads. Because the shoes or pads have nothing they can grasp, there's no friction. Consequently, braking power is lost.

The ICE is lubricated by engine oil anyway so wear and tear is minimal.

Vincent

 

I believe I read that Hobbit tested braking in B and found that the regen amperage was no greater than braking in D.

There shouldn't be any more wear on the engine than if it were spinning with fuel. There are many cars that do DFCO (deceleration fuel cutoff), which is essentially the same thing.

 

Doesn't B act as an engine brake, therefore actually using the engine and not just shutting it off?

 

My mind doesn't do well with graphs.

So I prefer to simply put the Prius in B when going down steep inclines and then simply whisper "It's Full Of Stars".

 

Engine braking is done by not injecting fuel. The throttle plate is closed, which causes the pistons to have to work hard to suck in air, but without any combustion cycle to make any power. The air pretty much acts like a spring in the cylinder, so the compression doesn't use a whole lot of power as it will just push the piston back down. However, with the variable valve timing, the valves can open and close at different times making the engine braking even more powerful. The engine is essentially turned into an air pump. The same thing happens in most cars when you downshift or put them in L. Here's the wikipedia article about the subject if you want a little more info.

 

Learn something new everyday. I know how it works in older cars with gear setting, but didn't know the new engines were different.

 

It's been said but it's worth emphasis - you've done the right thing and should always do this for safety reasons. If you're using your brakes all the way down a long hill, the phenomenon you could encounter is called Brake Fade.

Brake fade - Wikipedia, the free encyclopedia

Using B mode will reduce the pressure on your friction brakes and lower your chances of encountering this.

If I know the descent will fill my battery anyway, or if I'm going to need more than just regen braking, I will use B mode from the top, but it's a judgement call based on the conditions. On many descents I come in and out of B mode anyway, adjusting to the grades, curves and traffic encountered.

 

I always love a informative and technically accurate post. GREAT POST

 

I'm not sure that deceleration changes because the battery is fully charged. If the battery is fully charged, then the additional charging is disposed of, I am not sure how. The Brake position is to slow the car down faster than in drive. There is no wear and tear either on the engine or the brakes. The brakes do not resemble regular friction brakes until 8 mph or less. My Gen 2 2005 brakes looked new and unused at 105,000 miles. Riding the brakes is my norm on the C and PIP and 2005.

 

The engine revs to use up the charge.

Well, the engine is still "on", just without fuel. So I'd imagine the wear & tear is similar to when the engine is on normally.

Or when braking past the regen range on the eco meter, or when braking suddenly because the car thinks you want to come to a stop as soon as possible.

 

Someone done the similar test before, and D with brake generated more electric than B. Also, I read somewhere before, B doesn't brake by ICE engine. Just the simulation of regenarate brake and real brake to act like engine brake.

 

That's totally wrong - see post #12 above for how it works on the Prius.

 

From what I understand 'B' is low range/gear. ( I know it is not really a gear.) Similar to the 1st or 2nd gears on a conventional automatic that most people never use.