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Braking best techniques

Braking techniques fuel economy

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66 replies to this topic

#61 OFFLINE   ScubaDadMiami

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Posted 29 April 2015 - 05:14 PM

 

 

I got a ~15% improvement in mileage when I stopped doing that.

 

When I got the car, the conventional wisdom was to accelerate as fast as would keep the battery charging. Keep that ^ visible, which allowed full 2-bar acceleration if SOC was low.

 

Last year, I tried lower throttle settings, about 1.5-bar, so the car accelerated more slowly, and spent more time charging the battery (and warming coolant in winter). I soon realized that lower throttle settings increased the charge rate as well. I've since set My View up with a tachometer so I could watch RPM. Keeping it under 2,000 RPM most of the time has made a huge difference, as I can still accelerate and climb what pass for hills around here while charging the HVB - win-win-win.

 

You may have to open this is a new browser window to see it, but in the lower chart, "baseline" was using 2-bar, and the "Spring" and "Summer" were more like 1.5-bar acceleration (when prudent, of course)

 
 

Have fun,

Frank

 

This sounds like what I do. Also, things depend on your intended speed and your current SOC.

 

If my HVB is low while my intended speed is city speeds, 30-45 MPH, I tap to engage ICE, bring the pressure down to just enough to keep ICE engaged, and I do a slow burn that keeps me going at the same speed (or slowly speeding up) while charging. Once I get to a good SOC, I do another "soft" tap, engaging HVB and turning off ICE, and I either keep going at the same speed, or I am slowly slowing down somewhat, and I glide until back down to a low SOC. I make changes to technique based upon the road conditions, such as areas with known hills, places where I have to stop for signals, etc.

 

If I have a low SOC and I need to speed up from a dead stop, such as from a red light, I start rolling with EV up to about 15-18 MPH, then I do a brief (as brief as I can get away with, given traffic flow--usually for about 5 to 7 seconds, something like that) two bar burn with ICE. This keeps me moving with the traffic flow, or even better than that. I back off of the throttle pressure but keep ICE engaged, and I try to maintain about 1.125 bar pressure to continue to speed up. After checking with My View many times, this works out to the 2,000 RPM figure.

 

If I have a high SOC, I start rolling in EV up to about 15-18 MPH, then I do a two bar burn to get up to intended speed ASAP. The reason: ICE uses less gas by making a higher burn for a shorter time than it does with a lower burn for a longer time. Once at speed, I glide using EV.









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#62 OFFLINE   fbov

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Posted 30 April 2015 - 09:01 AM

...: ICE uses less gas by making a higher burn for a shorter time than it does with a lower burn for a longer time.

I've come to question this, with very positive results, as this is exactly what I changed.

 

The "higher burn for a shorter time" approach leaves me with lower SOC compared with "lower burn for a longer time." Believe me when I say I was skeptical, but watching the tank average MPG, I realized that at 1.5-2K RPM, the car seems to use much less gas than at 2.5-3K RPM. A low burn that drops the average 0.1MPG is a lot longer than a high burn with the same MPG drop, and you gain SOC and WT (a critical advantage in winter).

 

Conversely, SDM, I'm finally getting the hang of EV'ing from a stop, a technique you once suggested that I had a hard time achieving.

 

BTW, the low burn approach has an interesting affect on following traffic. Invariably, the conventional drivetrain is on my bumber coming out of the intersection, because they're in a low gear putting lots of torque to the road. As their tranny shifts to higher gears, their accelaration rate slows. They invariably end up many car lengths behind because they stop accelerating when they hit high gears. Our little CVT will keep the ICE RPM constant the whole time, so my slow acceleration has continued past their achieved cruising speed. They may catch up, but I've still only been passed once in 21 months driving the car.

 

Have fun,

Frank


Edited by fbov, 30 April 2015 - 09:01 AM.


#63 OFFLINE   scottwood2

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Posted 30 April 2015 - 11:47 AM

I tried the new technique last night.  I did 5 stops (trips) and tried to keep the car from charging the battery while I was accelerating.  It felt really strange and kind of the opposite from what I have been driving.  So this produced much shorter bursts of power keeping ICE off a whole lot more but my battery was not being charged much.  The recharge mainly came from decel and braking.  Once I was up to speed (above speed) I would go to elec and try to just keep the down arrow on.  This works pretty well most of the time and I am always surprised how this little amount of power pushes the car.  If the speed would drop too far down I would use ICE to quickly bring the speed back up.  Usually after 10 MPH drops.   This depends on traffic too. 

 

I almost stopped doing this about half way to my 1st stop as it just did not feel right.  I really thought my FE was going to suck but I decided to stick it out to see what happens.  To my surprise i did almost 56 MPG.  I mainly get upper 40's /  low 50's going home and this was a totally different route so I had no comparison but I did not expect anywhere near this FE. 

 

I continued the other legs of the trips and did even better.  Some in the 60's and even one at 72 MPG (short trip 2 miles). 

 

Coming into work this morning I did 55 MPG.  My old technique would do this and sometime into the 60's but there is not much traffic but a lot of lights so avg speed is way lower. 

 

I really did not focus on the RPM much other than to try to keep the battery from being charged by ICE.  It was hard to do sometime and would still charge at over a 3 bar burn but again at this acceleration ICE is not on very long.  I was also surprised by how much the decel and braking recharged the battery.  My SOC gauge was low on the scale most of the time but it was enough to keep the car moving most of the time without starting ICE to charge. 

 

Thx for the tip and my other half appreciates the speedier drive as well  :)



#64 OFFLINE   SnowStorm

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Posted 30 April 2015 - 08:27 PM

This discussion sent me back to one of my OBDII data logs.  I found an extended period cruising along at 54 mph with several cycles of EV versus ICE.  First, when ICE is on it goes to about 85 ft-lbs and 2000 rpm which is 32 HP or 24 kW.  Looking back through the graph you can see an obvious "preference" to run the ICE near these values so, having no BSFC map :sad: , I'll take that as the most efficient point.  Second, the limit for ICE charging of the HVB is 35 amps (it never goes higher) which is about 10 kW.  So for peak efficiency with a low SOC (HVB charging at 10kW) you need to use 14kW (17 HP) to run the car.  If you don't need that much power, the ICE is forced to run below the peak - no way out.  As SOC goes up, the charge rate starts to drop off.  I can see an earlier point where, after an extended ICE run at 55 mph, the HVB current was near 0 and the ICE was at 60 ft-lbs and 1550 rpm (13kW).  So much for steady speed.

 

When accelerating, we still want to stay on top of the magical BSFC mountain!  But its quite a balancing act between SOC, car speed, traffic, hills and fun!  Forgetting traffic and hills for the moment, below are some observations/suggestions (all subject to peer review!).

  1. If you accelerate too slowly with ICE on, you may not use all 24kW of peak efficiency power.  Not good.
  2. Accelerate too hard and you will push the ICE past peak and/or pull battery power to help.  Keeping RPM below 2500 certainly seems advisable.
  3. Keeping SOC low in stop/go driving seems good since it gives a place for any excess ICE power to go (you can't always use 24kW) and helps prevent negative split mode.
  4. It seems good (as several have suggested) to start in EV for a bit.  Helps keep SOC low.
  5. Once ICE comes on, push it to the "no charge" point but no further.  Its more fun!  Might be more efficient as well.

We don't have any regular daily commute to do comparisons on so must leave that to others.

 

(P.S. I think this post about "braking techniques" is feeling hijacked!)



#65 OFFLINE   ScubaDadMiami

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Posted 01 May 2015 - 01:49 AM

The "higher burn for a shorter time" approach leaves me with lower SOC compared with "lower burn for a longer time." Believe me when I say I was skeptical, but watching the tank average MPG, I realized that at 1.5-2K RPM, the car seems to use much less gas than at 2.5-3K RPM. A low burn that drops the average 0.1MPG is a lot longer than a high burn with the same MPG drop, and you gain SOC and WT (a critical advantage in winter).

 

Conversely, SDM, I'm finally getting the hang of EV'ing from a stop, a technique you once suggested that I had a hard time achieving.

As to the first point, that is correct. I only use that technique (2 bar burn) when I already have a high SOC. If I have a low SOC, I get rolling in EV, then I tap to engage ICE but at a low RPM, usually well below 2K RPM (I never checked, but it corresponds with about .5 bar), charging up while driving from between 20 MPH to about 35 or so. The long, slow burn is usually for more than a minute, whereas the 2 bar burn is for usually only about 5 to maybe 9 seconds, depending on upper speed.

 

Glad to hear that you have learned to chase the bar! ;)



#66 OFFLINE   jestevens

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Posted 01 May 2015 - 07:24 AM

I never bother with trying to control whether the ICE is charging the HV battery or not..the battery NEEDS to be charged and eventually it will be regardless of how "ginger" you try to be with the throttle.  Trying to monitor the SoC all the time using just the factory gauges seems like it takes too much time away from concentrating on actually driving the car.  Even if the car is in warm up I still get to use that energy later on in the trip driving around the 25MPH area at work on pure EV.

 

Sure, it's exciting to try to max out the MPG but you still can't change the laws of physics..or the patterns of other drivers..I have noticed my coach score go way down as I react appropriately to sudden changes in traffic - it's not worth an extra 1 or 2 MPG to me to not treat this thing like a car..  The beauty is with both this car and the prius sedan, you can get very good fuel economy with only a few modest changes in driving style.


Edited by jestevens, 01 May 2015 - 07:25 AM.


#67 OFFLINE   fbov

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Posted 01 May 2015 - 09:19 AM

...The long, slow burn is usually for more than a minute, whereas the 2 bar burn is for usually only about 5 to maybe 9 seconds,...

Clearly, in any tradeoff scenario, there are points of diminishing returns. I believe a 10x different in burn duration is well past that point. I'm suggesting more like 1.5x duration, 8 to 12 second burns instead of 5 to 9 seconds. The goal is lower RPM, not the lowest.

 

To Snow's point, I see the 2K-2.2K RPM sweet spot a lot, but also one at 1.5K RPM, and a couple scenarios that run more like 1-1.1K RPM. Of course, I'm drving mostly with a 35 MPH speed limit, so I'm not surprised I see lower ICE RPM modes.

 

And you've got the BSFC map link, right?

http://ecomodder.com...ion_(BSFC)_Maps

 

Viewing several BSFC maps, I see a common thread... peak efficiency is a plateau of high load in the 1.5K-3KRPM range for Atkinson (Toyota) and many conventional gas engines. In Snow's example, I expect the 24kW@2000RPM running point would shift lower as road energy needs, or charging needs, diminished without leaving the optimum efficiency plateau region. At 1.5K PRM, you might get 18kW, which assuming 10kW charge, leaves only 8kW for the road, which is at the low range of the power needed to maintain speed.

 

If nothing else, it points to the benefit of using ICE uphill, where gravity adds load, so you crest it with high SOC, and extend the EV glide even farther with some downhill slope. If you live in hilly country, where you fill the battery going downhill, the reverse may be true. There's a lot to be said for knowing your route!

 

Have fun,

Frank








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