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Old 06-06-2008, 10:32 PM   #1
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Why would constantly being lean mean poorer fuel economy?

This doesn't make sense to me. If the fuel mixture is lean, why would you get poorer fuel economy? Is this assuming that the ECU is not advancing the timing in response to the lean mixture? That the only way to get the ECU to do this is to basically have an ECU that can properly respond to a lean condition such as on honda civics?

I was thinking about pumping losses and how people do pulse and glide in order to "overcome it". Well what if you could set it up in such as way so that when you press on the accelerator up to say 20%-30% or use cruise control, the throttle opens all the way, but leans the mixture enough so that it can maintain speed. So instead of using the throttle to control speed by regulating how much fuel comes in, you regulate how much fuel comes in instead, kind of like a diesel engine. Then past 30%, the mixture is richened so that you can accelerate accordingly.
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Old 06-07-2008, 04:51 AM   #2
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Lean mixtures want to explode and explosions are a no no.. Quick but controlled burning is what you want, not explosions.

That's why it takes the resources of someone like a Honda to make a lean burn motor work, lean burn is tricky and hard.

If you are going to do a hybrid it could be that lean burn is a dead end.
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Old 06-07-2008, 05:08 AM   #3
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Just making the mixture more lean is not practical, but that could have changed any day now.

Google "Homogenous Charge Compression Ignition". I have a 42 page document, dated 8-5-03 that illustrates a design for an engine that has variable compression. One of the key components of HCCI is the ability to make changes in the compression ratio to allow HCCI under different loads.
The reason too lean won't work is because the mixture contains areas of rich mixture and areas of lean mixture, which causes preignition. This engine design also has the ability to eliminate it's stroke altogether and become a flywheel, capable of storing it's own energy in the engine's rotating mass. It also has no reciprocating components, and in its current configuration, no valve train. It is capable of compression ratios of 50 to 1 with piston to "cylinder head" clearances of less than 1 millimeter.

Think of the movie "Transformer". This engine can transform itself from active power generation to storage in tenths of a second. It would never idle, or operate at anything other than its highest BFSC. All combustion pulses would be WOT with the same volume of fuel delivered per pulse.

I hope some day I have the means to build it. I believe it could reach 60% efficiency maybe even more. My handle R.I.D.E stands for Rotational Inertial Dampening Engine. The engine itself hypermiles cycling between fuel consuming power generation and storage of hundreds of horsepower seconds of energy.

In this concept vehicle when you accelerate from a stop, the engine isn't running. Like one of the old flywheel cars some of us old guys remember your power comes from storage, with an Infinitely Variable Transmission converting your rotational storage into power at the wheels. Acceleration would be fantastic, similar to being launched of an aircraft carrier, controlled only by the "gear" ratios in the transmission of which you have an infinite number.

Another way to achieve lean burn is to use a stratified charge. Honda did this in the 70's but they certainly weren't the first. Basically a stratified charge is where you modify the engine to control the lean and rich areas in the combustion chamber, which allows you to control preignition. Honda's CVCC engines in the 70's did this with a prechamber design that used a tiny valve with its own circuit in the carburetor, combined with another much larger valve. The smaller valve fed a 12 to 1 AF ratio into a prechamber, while the larger valve fed a 18 to 1 very lean misture into the main combustion chamber. This allowed the engine to run at lower overall AF ratios than normal.

My VX uses controlled valve lift to accomplish the same basic thing. It was the first step towards practical HCCI, by using turbulence to better atomize the mixture for lean burn operation. I'ts a shame the NOX emission regulations basically made it impossible for them to continue development of lean burn.

A case of legislation stifling innovation, that was actually caused by the fuel formulation of the time that has since been changed to the point where its now possible to go back to the lean burn principle, with higher pressure injection and increase mileage significantly.

Just my opinions here.

regards
gary
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Old 06-07-2008, 05:25 AM   #4
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What fumesucker sates is essentially correct.

Committing to "dead ends" in expensive engine developments is corporate suicide for any but the most wealthy auto manufacturers.

My disagreement with fumesucker is my core belief that hybrids are a "dead end". This belief is shared by many.

Hypermiling a hybrid is absolute proof that hybrids are not the solution. They represent a transitional phase. The real car of the future will hypermile itself, moving the hypermiling responsibility to the ECU of the vehicle. The driver only needs to control the desired acceleration or deceleration. Hypermilers are the pioneers of a principle that desperately needs to be incorporated into the vehicle instead of being the drivers responsibility.

By definition a hybrid uses two sources of power, which is not necessary. Instead the single source of power (by definition not a hybrid) should be capacitive storage with the fuel consuming powerplant restoring capacitive energy.

Since the capacitive storage would be the sole source of motive energy (including regenerative braking) in the strictest technical definition the vehicle would not be a hybrid.

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Old 06-07-2008, 06:21 AM   #5
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I'm just spitballing here but......

If the air fuel mixture is too lean, it won't ignite (misfire) and you will have wasted that particular charge of air/fuel.

If the AF mixture is too rich, it will ignite but you still will waste some of the fuel, albeit not as much when the mixture was too lean.

So constantly being lean would cause the engine to misfire too much (if the engine was not meant to run that lean of a AF mixture), hence crappy fuel economy.
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Old 06-07-2008, 06:45 AM   #6
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I'd say you're right on it. I once had a 283 I built and put the smallest Carter 2bbl carb you can find on it...you couldn't put a quarter down the intakes...and it had adjustable metering rods, which I played with to lean the burn. What I got was a choked engine that had great power in low rpm, no high rev capability, and got 13mpg in an old 1960 chevy pickup (low gearing, of course). Later, an enginebuilder I knew told me to put a 4bbl back on it, with a cam for my driving style (lowrev rv cam) and I'd have power and mpg, but I sold the truck.
They do have to breathe.
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Old 06-07-2008, 08:15 AM   #7
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With an engine completely optimized for a certain rpm and wide open throttle you can get a very good efficiency figure when its running.

By running the engine only at peak efficiency and otherwise keeping it turned off you can optimize the fuel usage for every unit of energy you extract.

Driving style is really another matter than optimizing energy usage/storage in the drivetrain itself.

Let the drivetrain have enough computing power and sensors to most efficiently match engine to load but the driver will ultimately be responsible for the load.
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89 Yamaha FZR400 Crotch rocket, semi naked with only the bikini fairing, no lowers, 60 plus mpg

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Old 06-07-2008, 05:07 PM   #8
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running lean itself doesn't have any benefit. the ideal of running lean that everyone talks about here is as a result of removing some of the fuel from the process.
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