PHEV's virtual electric overdrive.

[Trex]

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:

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:



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

 

[anko]

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):

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:

 

[Trex]

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.

 

[gwatpe]

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.

 

[Grigou]

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).

 

[anko]

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.

 

[maby]

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!

 

[jaapv]

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

 

[anko]

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.

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-koenigsegg-regera-photos-and-info-news

..., 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?

..., 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.

 

[Grigou]

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

 

[anko]

... 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

You are saying you would prefer a longer gear, because apparently at 130 kph we have to much revs / not enough torque to get to the sweet spot. I would tend to agree (although I do not see that as an issue), but if this is true, why does it stop charging at 130?

 

[Grigou]

Not quite that.

I remembered this graph http://www.myoutlanderphev.com/forum/viewtopic.php?p=13993#p13993 (blue area).

But when I see it again, I am puzzled ... Finally I don't really understand this blue area. Why is it an area, not just a line (at least in the context of this graph) ?

 

[Trex]

Hi,

I started this discussion just to show the PHEV with its single ratio gearbox (that is not overdriven) has a "virtual" overdrive which if you think about it is pretty incredible. Just look at the average revs it did on that 9.12km highly monitored trip. 1485rpm . That's lower than any car I can think of at 92kph. It covers everything IMO in that definition which I will repeat. 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."
Lets run through them.

Reduced engine speed. The instantaneous revs are higher but over time are lower than just about any car I have driven in 41 licensed years (driving around farm way before that).

Leading to better fuel consumption. For its size I think it has great fuel economy. Like I have said before I can get 6.5Lts/100kms (by tweaking the modes) on the same trip as the Prius (which got 5lts/100 ) and I do not drive slow (but stay legal) as my wife and kids can attest. BTW that is not counting the grid charge on the PHEV.

Lower noise. Beats the noise when the petrol motor is switched off and when on they have put in extra sound deadening in to keep it quiet.
I cannot even hear it unless it goes back to series mode (very seldom) on the highway at the speeds we travel here.

Lower wear. Again over time the PHEV wins again IMO by having a way lower average RPM and this is not counting grid ev.

I know I am bring up the Prius a lot but it is the king over here for combined average fuel economy (urban/highway) of the non plug in cars (including small diesel cars) but the PHEV is way more comfortable, smoother and quieter on trips IMO. All in all a great car.

As to some posters that say that the PHEV is so different that it is not possible to compare them with "normal" cars or words to that effect I say BS as there are plenty of quantifiable measurements we have obtained to do so.

Regards Trex.

 

[anko]

I remembered this graph http://www.myoutlanderphev.com/forum/viewtopic.php?p=13993#p13993 (blue area).

But when I see it again, I am puzzled ... Finally I don't really understand this blue area. Why is it an area, not just a line (at least in the context of this graph) ?

Have a look at http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption. It has this graph showing the area (revs and torque) where a particular engine has its best performance:

In the example, you could say the sweet spot is the tiny circular area labeled 206, but that is not very helpful, as it is extremely hard to achieve, as it for instance requires to keep RPM between 2400 and 2450 RPM. By flagging the zone labeled 215 or even 220 as the sweet spot, you give people a realistic target.

 

[Grigou]

Thank U anko, it's very clear now

And finally, even if MMC had put another ratio to increase the span of speed at 70 - 183 km/h, the sweet spot would have remained far above of the green curve (except when charging the battery). No huge benefit finally ...

 

[anko]

Thank U anko, it's very clear now

And finally, even if MMC had put another ratio to increase the span of speed at 70 - 183 km/h, the sweet spot would have remained far above of the green curve (except when charging the battery). No huge benefit finally ...

Maybe if we could do 160 km/h at 2200 RPM or so ..... :mrgreen:

Even during driving + charging we are still well below the sweetspot. To get to the sweetspot would probably require a charge current so big, that it would damage the battery.

 

[Trex]

Even during driving + charging we are still well below the sweetspot.

Text under this graph translated with onenote says:
"Above image (ei-diagram) shows an area (blue circle) where the 4B11 petrol engine the most efficient functions. This area is also called sweetspot , here is the specific consumption is lowest."

It seems Mitsubishi might not agree with you for the 4B11 petrol engine if we stay in that blue area.

Regards Trex.

material from anko

 

[anko]

Indeed, that is what the document says. But, if 11 liters of fuel per 100 km (@ 100 km/h) gets us into the bleu circle, where will 14 or 15 liters of fuel per 100 km get us? Well above the red line and that should not be possible. And 14 or 15 liters/100 km @ 100 km/h are easily achieved when towing. Especially when you take into consideration that the the engine would be more efficient under higher load, 25% more fuel consumption should result in more than 25% more power.

My interpretation: the distance between the green line and the bottom of the blue circle (ei-diagram should be interpreted as egg-shaped area ) is more than the maximum charge current (I believe that is approx. 8 kW). You could add an imaginary yellow line, which is the green line + 8 kW (how big the gap between the green and the yellow line is, depends on RPM / speed, as the y-axis shows torque, not kW). That yellow line will, in my opion mis the sweet spot completely.

For the remainder of my post, let's assume the x-axis was about rpm, not speed. The graph can easily be transformed that way, as we have a single speed gearbox.

Making the existing gear longer would shift the green line to the left, making the gap between the green line and the sweet spot smaller, demonstrating a more efficient use of the engine. But also would it cause the green and the red line to meet at a speed lower than 170 km/h, resulting in a limited top speed.

Adding a second longer gear would instead, would introduce a second green line, a bit above the existing one. The lower line (first gear) would be great for dynamic and / or fast driving, the second for optimal performance.

For towing, you would in theory want a third gear, but that had to be shorter. It would introduce yet another green line. Because of the shorter gear, it would be shifted to the right, but because of the higher weight and air resistance it would be shifted to the left. So, bottom line it could end up somewhere in the same place as the current one.

I reality we don't have a shorter gear available. So, towing moves the green line up towards the sweet spot. So would it move the imaginary yellow line into the sweet spot! But I believe that the imaginary yellow line is fixed (by programming) and the output of the engine will only exceed the yellow line when the green line does.

 

[Grigou]

Why do you believe that the max charge power is approx. 8 kW, anko ?
I presume that it comes from a calculation ?

A few months ago I've made an estimation on a french forum, wich was :

- time for full charge in Charge mode (at 0 km/h) : 40 ' (not sure of this time, I think I've read it somewhere but not remember where)
- "full charge" (in Charge mode) = from 30% to 80 % SOC = 6 kWh

So the result is 6 * 3/2 = 9 kWh. Not far of your 8 ... but is this number a limit ? After all, my calculation is ONLY for 0 km/h ...

 

[anko]

Mainly by looking at fuel consumption in liters per hour when charging at 0 km/h and comparing this with the fuel consumption when truly idling at 0 km/h.