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Discussion Starter #1
Lately my local-only mileage has been hovering around 66.5 average for the current tankful, which is about halfway down. However, this past week brought a lot of suddenly warmer weather to New England, and the segment mpg's started rising rapidly.

Tonight I took the long way home to see more of this. It's in the 60s now. The engine stayed in LB a lot more than before (on this same route I've driven in colder weather) and now the tankful mpg average is ramping up sharply. It was 66.7 when I got into the car, and 67.9 when I got out, with less than 30 miles between those two points.

Does warmer weather always do this to the Insight's fuel efficiency? If so, this is going to be a fine summer indeed :)

Thanks again to all the people posting their hypermiling tips here so that even us newbies can figure it out! :)
 

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I knew you had it in ya redbug. :)

Remember you've got 4 :shock: different MPG counters :!:

LMPG, Trip A, B and the segment counter (toggled with the FCD button and is "on" when the <=> is shown with the MPG meter).

You could easily monitor each segment of your daily commute using the FCD button (hold to reset the count). If you had you'd have seen some impressive recent MPG numbers. :D

HTH! :)
 

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the best MPGs are obtained with the temps as hot as you can stand it without the a/c running. and the windows up.

have fun in the sun!!!
 

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Discussion Starter #6
OK, here's one for the group: exactly WHY does this behavior occur? Most engines prefer colder (denser) intake air, and the Insight design team didn't take the steps they could have to arrange for the intake air to be pre-warmed to 70F or above.

I guess the real question is why does air above 70F lend itself so much better to lean-burn?
 

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It doesn't have anything to do with the power of the car, it's the general reduction in drag--the bearing grease, the transmission oil, and the air density all work in your favor.
 

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air intake warming

Redbug;
Yes the engineers did take steps to incease the air temperature. There are two small hoses going to the throttle body. (where the throttle wire attaches.) One is the intake hose and the other is the return hose. It contains radiator coolant. This is to heat up the throttle body which will heat up the air passing through the throttle. Unknown what the degree increase will be............Every little bit helps.

Willie
 

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redbug said:
I guess the real question is why does air above 70F lend itself so much better to lean-burn?
A _VERY_ complex question redbug. With several factors all contributing or detracting. Dougie's got one covered. ;)

First off remember more power doesn't necessarily mean better MPG. You'd think that if the same engine can be coaxed to yield more power and you simply don't use it then it would consume less fuel since it doesn't have to "work" as hard. Which is true in part. This is where lean-burn comes in and turns that thinking upside down. ;)

Warmer weather vaporizes fuel better allowing for more complete combustion. Its a basic principal of all (AFAIK) liquid fuels commonly used today. Which effectively rules out anything not petroleum based. Even the fuel stingy Insight _requires_ a richer (greater fuel to air ratio) on a cold (ambient temperature) start to compensate for the inefficiencies. Thats one factor, getting the car warmed-up. And the lower temp you begin from the longer (and more fuel) its gonna take to get it up to operating temp. About 195F (90C) coolant temperature is fully warm.

In _cold_ weather this effect is still evident and more pronounced in an Insight _because_ of its fuel stinginess. Many members have demonstrated that the Insight won't reach operating temperature at idle in _cold_ weather. There's simply not enough fuel being consumed by the car with enough waste heat to compensate for the losses. Part of the reason that auto stop is disabled in temps 40F and below.

Pre-warming the intake air is a system that has been used in cars for _many_ years now. The problem used to be carburetor ice. Ice will form under some conditions due to atmospheric moisture and the evaporative cooling effect of the fuel mist. As you can imagine a car with a plug of ice in the intake will have serious power and performance problems. MPI (Multiport Fuel Injection, 1 injector per cyl usually mounted in the intake manifold and pointed at the intake valve) is not affected by the "old" carb ice condition so adding heat is not commonly used. There is a certain amount of heating the intake air that occurs because of engine and its radiant heat losses and the resulting under hood temperature. In colder weather the under hood temps will be correspondingly lower further reducing intake air and fuel temps (heating of the fuel rail).

The dynamics of lean burn require a good understanding of the chemistry and physics of combustion. To oversimplify; a lean charge tends to burn more slowly. Given x RPM's the exhaust valve is _going_ to open up in a predetermined amount of time regardless of whether combustion is complete. So simply "forcing" a lean mixture past a certain and _highly_ variable point (load, RPM, temperature and several other factors) will fail to produce power.

With a hot air mod in optimum temperature conditions you can partially simulate summer to the engine. Warm the intake air to 110-120F in cool conditions and you'll see a "widening" of the lean burn window. The theory is that when things get cool there's simply not enough time for the fuel to be sufficiently warmed by the intake air to vaporize as effectively and lean-burn doesn't like it. What's happening in reality is still open to some speculation. Honda doesn't publish all the parameters of engine operation that the lean-burn system is designed to work within. There is one, a lean burn speed limit for the Insight, 72 MPH ish ( :?: ) The stratified charge concept is the basics of what allows lean burn and has been used by many Honda engines for almost 25 years. The concept is published in various sales brochures and technical manuals.

Now I know what your thinking. ;) Warm the fuel :!: Its done with diesel because in low temps it begins to congeal (turns to jello) and its not as _HIGHLY_ flammable as gasoline so its relatively safer to do. Ok then, let's warm the intake air even more. Well Rick Reece tried a 150F (HOT) intake air mod and during the coldest part of winter with no apparent MPG improvement. Probably related to your original thinking of colder air = more power. Heat the air too much and it thins too much, correspondingly reducing the available amount of O2 for combustion and resulting in a loss of power.

And then there are the additional factors of more aerodynamic load at speed with colder (more dense) air and less flex in the tires. And the different winter fuel blend and the... and... and...

Anyways there it is in a nutshell.

P.S. Willie

IIRC the coolant flow to the TB is there to normalize its temperature relative to the rest of the engine. Else fuel vapors traveling back up the intake will "freeze" there and more rapidly gunk up the TB. And the ice problem is still an issue. There is a temperature drop from the resulting pressure drop across the throttle plate. Although without the added evaporative cooling effect of fuel, like in a carburetor, I'd suspect ice is a rare occurence.


HTH! :)
 
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