Mitsubishi Outlander PHEV Forum

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 Post subject: PHEV's virtual electric overdrive.
PostPosted: Wed Apr 22, 2015 10:15 pm 
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Hi,

Just so I do not take another discussion off topic I will start a new thread. Back in the topic : "Re: Research into Wear & Tear of a hybrid (Start-Stop) engine" Page 2 this came up:

Trex wrote:
anko wrote:

BTW: I beieve we don't get anywhere near the sweetspot. If the engine was operating near the sweetspot (which would require a longer gear ratio),


Hi anko,

So you think we need an overdrive gear ratio?

But you also say: Otherwise, why would the car alternate between parallel and EV mode when the battery is ”empty" or Save mode is engaged?

That could be considered something just as good as overdrive ie shut the petrol motor off after charging the hv battery for a period of time could it not? An electric overdrive? Yep, I like that name.

Regards Trex.


Now it is a follow on to this I want to discuss. Why does Mitsubishi use such a short gearing ratio for Parallel mode driving? I would think it fairly obvious in that we have a small 2lt petrol motor pushing along a 1.8 tonne 4WD and that is not talking about aerodynamics (bad). Then we have the requirements of any excess power to charge the hv drive battery. All in all not much chance of a longer gear ratio which as anko stated would make the petrol engine operate more efficiently. Now anko did not say we needed a overdrive just a longer gear ratio but lets see why cars have overdrive which Wikipedia state:

"Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine speed, leading to better fuel consumption, lower noise and lower wear."

Just what we want. Now for a example of overdrive. My diesel Mitsubishi Pajero which I use for towing duties has a 5th gear overdrive that when driving at around 90kph the diesel motor is spinning about 1950rpm. Now lets bring in one of my favourite graphs:

Image

Now as can be seen in the image the PHEV's petrol motor at 92.75kph is spinning at 2454rpm. That's not good compared to the Pajero's 1950rpm. But the PHEV has a trick up its sleeve in that the petrol engine revolutions are going to zero after charging the drive battery and going into Ev mode and discharging drive battery (then restarting petrol motor and charging the drive battery etc etc). This happens as we drive along in parallel mode automatically if the loads on the PHEV are low enough ie 2454rpm 0rpm, 2454rpm 0rpm etc at 92.75kph.

Now the good thing about these graphs in the image is there is enough information to work out what the average rpm are of the petrol motor over time in this trip Mitsubishi did using a MUT-III.

The first thing to do is get all the quantities into minutes for such a short trip of 9.16Kms.
92.75kph = 1.546 km/min
10.03Lts/h = 0.167Lts/min
Now the trip of 9.16km distance divided by our 1.546km/min = 5.925 min
If the petrol motor had run all the time on this trip at 0.167Lts/min X 5.925 min = 0.989Lts consumed
But it did not run all the time and used 15.32km/Lt so 9.16km divided by 15.32km/L = 0.598Lts for 9.16km
Now the fun bit 0.598Lts used divided by 0.989Lts if motor ran all the time = 0.605 X 100 = 60.5% of the time motor actually running!
So now our badly revving petrol motor running at 2454rpm has actually been running at a average of 2454rpm X 0.605 = 1484.67rpm

1485rpm PHEV versus 1950rpm Pajero. Starting to look like a overdrive to me. Go the PHEV. :lol: I hope my math is right.

I am calling this a virtual electric overdrive.

Regards Trex.

image from anko


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Wed Apr 22, 2015 11:33 pm 
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Location: Netherlands, Utrecht area
Trex wrote:
Now anko did not say we needed a overdrive just a longer gear ratio but lets see why cars have overdrive which Wikipedia state:

"Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine speed, leading to better fuel consumption, lower noise and lower wear."
With this definition in mind, I would agree. When I wrote my response, I was thinking more in terms of (what I found in same Wiki article):
Quote:
Achieving an overdriven ratio for cruising thus required a gearbox ratio even higher than this, i.e. the gearbox output shaft rotating faster than the original engine speed.
And that is something we may not necessarily need. Until somebody does the math that proves we do :lol:


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Wed Apr 22, 2015 11:47 pm 
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Location: Near Port Macquarie Australia
anko wrote:

And that is something we may not necessarily need. Until somebody does the math that proves we do :lol:


Well don't look at me cause I'm not going there. :lol: :lol:

Regards Trex.


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 12:28 am 
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When we compare a PHEV drivetrain to a typical car, overdrive is really meaningless. The overdrive in a normal car is just a shortcut way a manufacturer can supply a more powerful engine and utilize an existing differential to achieve lower engine rpm and better fuel economy. Adding a few more cogs to a gearbox is easier than to reduce the ratio in a diff. Differential design has an optimum ratio for the hypiod gears.

I had a HONDA Prelude and 3rd gear was straight through the gearbox. 4th and 5th gears were both overdrive.

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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 5:06 am 
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Location: Lyon, France
I think that :

- the lack of overdrive (or of gearbox) is a choice for simplicity and cost, but NOT a choice for efficiency (quite the contrary !)

- the choice of the ratio speed / RPM is simply a compromise between the lower possible speed in parallel mode (about 65 km/h with this ratio but it could have been for example 55 km/h with a lower ratio, or 75 with an higher ...), and the consumption at high speeds i.e. almost 11 L/100 km at 130 km/h but I think it could have been less with a higher ratio ...

And just an observation : at 130 km/h the ICE is permanently running (even in EV mode).


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 5:46 am 
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Location: Netherlands, Utrecht area
gwatpe wrote:
When we compare a PHEV drivetrain to a typical car, overdrive is really meaningless.
Why would it be meaningless? In normal day-by-day driving it is great to have a little ICE power reserve available for short moments of acceleration or climbing or .... Thanks to that, we are less depending on E-support and the battery is not drained (as much) while the engine is running. But there are situation in which you may find yourself driving at a constant speed on a flat and open road. In those situations you don't need the power reserve and a second gear could result in the engine being used more efficiently. Whether that advantage outweighs the disadvantage of additional weight, complexity, moving parts and what not? Perhaps not. Especially if you take into account cost, like Grigou said.


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 6:21 am 
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The lack of a gearbox and reduced dependence on a clutch was a major factor in our choice of the car. I seem to have very bad luck with clutches and gearboxes!


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 7:17 am 
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Overdrive is a ratio more that 1:1 in a conventional gearbox. As this car has no gearbox at all, there cannot be an overdrive, so the question is meaningless.
The ratio in the reduction gear of the drivetrain has been chosen to make it possible to engage the engine in parallel mode from 60-170 KpH. Any other ratio would limit this span unacceptably.
Anybody who has ever driven a CVT hybrid must be aware of the nasty noise those things produce - I think Mitsubishi's choice was the right one

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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 8:01 am 
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jaapv wrote:
Overdrive is a ratio more that 1:1 in a conventional gearbox.
Not necessarily true. Trex pointed us to a very sensible definition that says that an overdrive gear is a gear that is so long that it will not allow the car to reach top speed: To few revs to generate the amount of power needed to reach top speed.
jaapv wrote:
As this car has no gearbox at all, ...
Not true. The car does have a box with gears in them. In fact, it is a single-speed gearbox, just like the one found in the Koenigsegg Regera is a single-speed gearbox. The one thing they cannot do is switch gears. But after adding an overdrive gear (or more in general, a 2nd gear), they could.
http://www.caranddriver.com/news/2016-k ... -info-news
jaapv wrote:
..., there cannot be an overdrive
So many cars have one. So, why could ours not have one? I mean, not now of course. But from the drawing board?
jaapv wrote:
..., so the question is meaningless.
To some, bending their heads around this stuff may be fun. Or even educational. To me it is. Wouldn't call that meaningless. ;)


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 Post subject: Re: PHEV's virtual electric overdrive.
PostPosted: Thu Apr 23, 2015 9:47 am 
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jaapv wrote:
The ratio in the reduction gear of the drivetrain has been chosen to make it possible to engage the engine in parallel mode from 60-170 KpH. Any other ratio would limit this span unacceptably.


I of course agree with "would limit this span" (but would prefer "would change this span") but not with "unacceptably".

Remark : the real span seems to be 65-170 rather than 60-170.

So for my average utilization, I would prefer 70-183 km/h (183/70 = 170/65). Not for the 13 km/h above 170, but for the consumption at 130 km/h ... and also to stay in the "sweet spot" of the ICE at 130 km/h, wich is the limit of authorized speed in my country ;)


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