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Old 04-17-2009, 11:23 AM   #1 (permalink)
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Default Looking for clarification on driving tips for maximizing MPG

I've read through many the knowledge base articles and posts, but two suggestions I read seemed mutually exclusive.

Some suggest accelerating very quickly in 1st and 2nd (and tapping a lot of the battery) in order to get into lean burn as quickly as possible (the 1/2/5 method).

Others suggest accelerating gingerly so that you don't tap as much of the battery since any boost you use will eventually have to be paid back.

My instincts say the first suggestion might be better if you are getting on the freeway or similar but the latter would be better for intown driving, but I'd like to get some feedback.
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Old 04-18-2009, 10:20 PM   #2 (permalink)
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Hi Varak,

I can't definitively say which way is best, but here is some theory.

*** Fast Takeoff ***
From internal combustion theory, it pretty clear that the specific fuel consumption for the Insight is lowest at roughly 3500 rpm. Above or below this, and the fuel consumption goes up for a given amount of power. Based on this limited viewpoint, fast takeoffs would be preferred.

*** Slow Takeoff ***
We also know from theory, that any change in motion from rest to a given speed takes a certain amount of power. The quicker this happens, the more work or power is exerted. If you want to accelerate a body (car) to a given speed and use the least amount of energy, then a ssslllooowww takeoff is mandatory.

*** Dilemma ***
So we have two opposing factors, and which do we put emphasis on??? Take off fast to optimize engine fuel consumption OR take off slow and minimize the energy input to achieve a given speed?

If you look at the masters of High Mileage contents, you will see how it is done. Wayne Gerdes and others come to mind.

My preference is taking off and watching the FCD and it tells me how fast to take off. My highest MPG's per tankful have been achieved by the slow method. However as you suggested, there are simply times when this can not be done, like merging into congested areas, and so on.

Hope this helps, Jim.
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Old 04-19-2009, 09:13 PM   #3 (permalink)
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I subscribe to the Gordon Lightfoot method...
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Old 04-20-2009, 09:22 AM   #4 (permalink)
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Geez - I hope it's not the Wreck of the Edmond Fitzgerald!! But, rather enjoying that Carefree Highway!
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Old 04-20-2009, 10:23 AM   #5 (permalink)
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Quote:
Originally Posted by 3-Wheeler View Post
Hi Varak,

I can't definitively say which way is best, but here is some theory.

*** Fast Takeoff ***
From internal combustion theory, it pretty clear that the specific fuel consumption for the Insight is lowest at roughly 3500 rpm. Above or below this, and the fuel consumption goes up for a given amount of power. Based on this limited viewpoint, fast takeoffs would be preferred.

That 3500rpm may well be the lowest BSFC under steady state cruising. Problem with accelerating is that the higher load will reduce AFR. So you could end up with a 13:1 AFR rather than light load of say 14.5:1 so you end up stuff aload of HC's down the exhaust instead of burning them. I guess the insight has that extra factor of really slow acceleration could be done in lean burn which would make a vast improvment.
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Old 10-30-2010, 05:32 PM   #6 (permalink)
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Default "... ssslllooowww takeoff is mandatory"

Let me try to clarify the physics a bit. I am new to the Insight but old at physics.
Power is energy per unit time. So higher power does not necessarily mean more energy or more fuel if you turn it off sooner.
The kinetic energy is 1/2 mv**2 so it depends only on the speed (and weight). The efficiency is the ratio of the increase in kinetic energy to the loss of fuel energy.
So how much fuel it takes to get to a given speed is simply proportional to the efficiency (or its integral to take care of changing conditions).
I think that for most engines that means accelerating at an rpm close the the torque peak and 3/4 to full load, but as mentioned above, that may not be the case for the Insight (maybe it goes in lean burn for a while in each gear). However you accelerate with the least fuel must be the way the engine runs at its greatest efficiency. (with corrections for losses, etc.)
I suspect the way most Americans drive may be left over from flat heads, where the torque peak was around walking speed.

Last edited by David Ingham; 10-30-2010 at 11:13 PM. Reason: typo
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Old 10-30-2010, 08:27 PM   #7 (permalink)
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Quote:
Originally Posted by David Ingham View Post
Let me try to clarify the physics a bit. I am new to the Insight but old at physics.
Power is energy per unit time. So higher power does not necessarily mean more energy or more fuel if you turn it off sooner.
The kinetic energy is 1/2 mv**2 so it depends only on the speed (and weight). The efficiency is the ratio of the increase in kinetic energy to the loss of fuel energy.
So how much fuel it takes to get to a given speed is simply proportional to the efficiency (or its integral to take care of changing conditions).
I think that for most engines that means accelerating at an rpm close the the torque peak and 3/4 to full load, but as mentioned above, that may not be the case for the Insight (maybe it goes in lean burn for a while in each gear). However you accelerate with the least fuel must be the way the engine runs at its greatest efficiency. (with corrections for losses, etc.)
For the Insight-I the analysis is complicated by
(a) lean burn (which reduces fuel usage roughly 50% but cuts power output)
(b) IMA assist (which costs fuel later to regenerate the battery, unless the car has been modified for plug-in - assist can be eliminated if the car has MIMA or an assist cutoff switch)
(c) the very small, low power motor - for P&G, it's better to have more power so that the acceleration phase can be shorter relative to the coasting phase
Quote:
Originally Posted by David Ingham View Post
I suspect the way most Americans drive may be left over from flat heads, where the torque peak was around waling speed.
STM the way most Americans drive has more to do with empty heads, or at least heads that are occupied with other activities.
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Old 11-02-2010, 09:12 PM   #8 (permalink)
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Lets add that one can do opportunistic regen, and assist with MIMA, which under the right conditions can recover charge when a speed reduction using regen rather than brakes is used.

MIMA also allows a mode impossible with the stock system, power charging.
Lets say for this discussion that your average MPG for the trip is 70.

Acceleration at normal rates can drop the MPG to less than 30 MPG, so to maintain the 70 MPG average one must get into lean burn and stay there as much as possible, get into fuel cut on the down side of hills, to balance the low MPG with very high MPG.
With MIMA, one can spend more time in the "zone", with less extreme deviations from the average.
Assist blended with gas powered acceleration can keep the minimum MPG during acceleration above 50, and use power charging on the down slopes.
Power charging is where you maintain or accelerate down hills by applying gas power of ~ 120 - 130MPG.
Without MIMA, one would accelerate to a dangerous speed on most hills with this condition, but with MIMA. one can apply nearly full regen to the pack, while maintaining or even accelerating down the hill.
Stock: <30MPG low end, > 150MPG on the top side
MIMA: >50MPG low end, ~ 130MPG on the top side

I am speaking about the typical New England rolling hills, and 60-70 MPH so this may not apply to the flatlanders, or people in real mountains.

Bottom line, MIMA changes the characteristics of the car so much that many people have commented that the car seems broken without it.


Isuzu actually bought an insight and MIMA and evaluated the system as something they were considering for their small trucks. Time will tell if any automakers give manual hybrid mix control to the driver.
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Last edited by Mike Dabrowski 2000; 11-02-2010 at 09:17 PM.
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Old 11-03-2010, 06:54 AM   #9 (permalink)
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I use power charging with MIMA as Mike describes, but I also use it in another situation. On slight downhills that aren't steep enough to coast, I feed in about 20A of regen with the joystick, easing off when the mpg bar drops to 100mpg. This involves coordination of the joystick and the gas pedal to keep mpg, mph, and regen where I want them, but it can be done.

Moderately steep downhills I coast engine off in FAS; steeper downhills I drive down in fuel cut with regen added as Mike describes.

My target speed is 50mph, which makes more downhills coastable than if I were trying to maintain 60-70mph.

One beauty of MIMA is its flexibility: different people can use it in different ways.
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Old 11-03-2010, 07:12 AM   #10 (permalink)
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Getting back to the OP's thoughts, STM that the Insight-I's unique design makes it unsuitable for pulse and glide.

In a conventional car, the engine consumes gas inefficiently at slight throttle openings. So it's advantageous to accelerate briskly, burning gas efficiently to gain momentum, and then coast while minimizing fuel usage by either shifting to neutral (so the engine is at idle) or turning the engine off entirely (FAS). In this way one avoids long periods of inefficient light-throttle operation.

The Insight-I operates in three modes with the engine on: lean burn w/o assist, normal burn w/o assist, and normal burn w/assist. The idea seems to have been to reduce the size and power output of the engine as much as possible by having assist help out in situations when the engine's max output is insufficient. And then reduce the penalty of low throttle openings at cruise by adding lean burn.

The problems with Pulsing an Insight-I are (a) you go out of lean burn during the Pulse and (b) a brisk Pulse is likely to invoke assist (unless one has MIMA), requiring regen later on. Because the Insight-I has such a small motor and because lean burn (and VTEC) improves its efficiency at small throttle openings, the Insight-I thrives on light throttle applications. So Pulse and Glide isn't as advantageous for the Insight-I as it is for conventional cars (we're talking about >35mph speeds here).
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Last edited by red1dr; 11-03-2010 at 07:15 AM.
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