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Why are all cylinders the same size?
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Why are all cylinders the same size? - May 5, 2008, 11:15 AM

Chevy has a system for shutting some cylinders down when the power isn't needed. We have fancy electronic controls for anything we want (fuel valves... whatever) so why don't we make engines with smaller and larger capacities and use the ones in the formation we need them in. Like couldn't you have a big bore V-twin for off the line and around town that switches itself to a short I4 up high? The same could be done for cars to help acceleration and optimize MPG. So I wonder why this hasn't been explored. Any thoughts?
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May 5, 2008, 11:44 AM

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May 5, 2008, 01:01 PM

I'm at work right now, but I'll think about it and post some ideas later on today.

There are many issues, I'll try to give a good pro/con of comparative advantages


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May 5, 2008, 01:03 PM

Quote:
Originally Posted by birdman
I'm at work right now, but I'll think about it and post some ideas later on today.

There are many issues, I'll try to give a good pro/con of comparative advantages


I was just thinking .... damn it .... I need a Marc post to logically explain what I can only get out in retard!!

Can't wait to read it!!


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May 5, 2008, 01:08 PM

Probably because explosions of gasoline work most efficiently at those exact dimensions?
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May 5, 2008, 01:31 PM

Intricacy, complexity, and cost. Not too many people want to ride around on the two-wheel equivalent of the $pace $huttle. Cheaper to just burn the extra fuel.

You can't just cut fuel and air to a cylinder. You have to alleviate the compression or else the engine works against itself. This means that you will need some sort of valve that remains open all the time when said cylinder is not firing. That valve will have to connect to filtered air. Even then, the piston is still cycling, providing resistance load to the crank.

Not all is lost. Take a little weed eater motor and rig it up to the brake side rear. You could maybe even rig the drive mechanism to work on the rear brake rotor. Then rig an electric start, and you're golden.


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May 5, 2008, 01:48 PM

I was thinking it would be based on balancing the engine, larger bores in the same motor as smaller bores would mean greater mass at one end, and un equal force/pressure on different parts of the engine, the crank for example, it would lead to warping the crank and motor failure at the worst.

But thats just my initial though process.


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May 5, 2008, 01:57 PM

It's cheaper and easier to install two engines and connect them via a gearbox. It's called a hybrid, only the ones in production today use electric motors as a secondary source.

But yes, you'd have issues with balancing, vibration, building an engine that works out geometrically, complexity out the wazoo, etc etc.


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May 5, 2008, 03:30 PM

Why don't they have more gas only hybrids? They could have a gas friendly engine and a big engine or use two gas friendly engines at the same time for more power. I'm sure the single engine version would be more expensive to develop. But if it's as simple as a gear box, it seems like they could cut off an engine when it's not needed.
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May 5, 2008, 03:56 PM

The added weight from the second engine and required related fluids, and also the required space would probably cancel out any benefits.


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May 5, 2008, 04:03 PM

Holding total displacement constant, the most simple and lightest-weight strategy is a single engine with one simple drivetrain. Don't forget that sprung weight is an issue as well. Adding the weight of a small auxiliary engine can quickly cut into any potential fuel consumption savings you gleen from it. Unless, of course, it's a weed eater engine.


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May 5, 2008, 04:04 PM

first off that system chevy has suck in the first place. they praise E85 when it's going to hurt this country more than it could help it.

lets get the gas idea out of the head and move on to hydrogen,

ask "when are we going to get hydrogen bikes?"

Just FYI
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May 5, 2008, 05:41 PM

You will NEVER see widespread hydrogen fuel in our lifetime for the simple reason that you can't trust average joe to pump a substance that's contained at cryogenic temperatures or incredibly high pressures and is explosive in any concentration in air.

On top of this, you're not going to lug around a pressure vessel capable of containing hydrogen at a density such that it will even aproach the energy density of gasoline. All of a sudden, your gigantic SUV either goes ten miles between fill-ups or uses all the cargo volume for fuel. The same goes for just about anything else... Gasoline has an incredibly high energy density and it's dirt cheap. That's why we use it for fuel. It's a bargain at double the price. Unfortunately, we might just test that double the price theory.

Hey, it'll take lots of cars off the road... I might just start riding on the street again.


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May 5, 2008, 06:32 PM

Okay, longer response:

Quote:
Probably because explosions of gasoline work most efficiently at those exact dimensions?
Well, its more the dimensions of the combustion chamber in terms of relative bore and height that matter for efficiency of combustion, so scaling to a smaller cylinder is totally reasonable with only minor efficiency loss.

Quote:
Intricacy, complexity, and cost.
TRUE! and the dominant reason...easier and cheaper to make 4,6,8 of the exact same piston, rod, etc.

Quote:
You can't just cut fuel and air to a cylinder. You have to alleviate the compression or else the engine works against itself. This means that you will need some sort of valve that remains open all the time when said cylinder is not firing. That valve will have to connect to filtered air. Even then, the piston is still cycling, providing resistance load to the crank.
Actually its the opposite. In that version, the air is being forced into and out of the non-firing cylinder...that flow through the valves is pumping loss and can be a significant loss. In ALL displacement on demand systems, the shut-down cylinders operate with the valves totally closed, providing an air-spring to return the energy of compression. While there are heat of compression losses with this method, it is more efficient than leaving the valves open.

Quote:
I was thinking it would be based on balancing the engine, larger bores in the same motor as smaller bores would mean greater mass at one end, and un equal force/pressure on different parts of the engine, the crank for example, it would lead to warping the crank and motor failure at the worst.
True, but you could have operating pairs of piston/rods that would be balanced. Think of a V-12 as two inline sixes each with differently sized bore/stroke. Since an I-6 is the only totally ballanced configuration, each set would be balanced on its own.

You could do the same thing with a 90-deg V-8, operating as 4 V-twins...the only uncompensated motion would be the tertiary rocking motion (like all 90deg V-twins) which can be taken out with balance shafts

Quote:
Why don't they have more gas only hybrids? They could have a gas friendly engine and a big engine or use two gas friendly engines at the same time for more power. I'm sure the single engine version would be more expensive to develop. But if it's as simple as a gear box, it seems like they could cut off an engine when it's not needed.
The reason electric/gas hybrids are an advantage is they allow:
A smaller steady-state operating engine to operate at its most efficient point, all the time
additional power provided on-demand by a motor with a broad efficiency range (electric) whose energy is provided by regenerative braking (the big factor why city mileage is more on a hybrid than highway

a gas/gas hybrid would offer none of those advantages (additional power provided by a larger gas engine is just like displacement on demand...but, as mentioned above, all the ancillary complexity, parts, fluids, add weight and cost.

Quote:
You will NEVER see widespread hydrogen fuel in our lifetime for the simple reason that you can't trust average joe to pump a substance that's contained at cryogenic temperatures or incredibly high pressures and is explosive in any concentration in air.

On top of this, you're not going to lug around a pressure vessel capable of containing hydrogen at a density such that it will even aproach the energy density of gasoline. All of a sudden, your gigantic SUV either goes ten miles between fill-ups or uses all the cargo volume for fuel. The same goes for just about anything else... Gasoline has an incredibly high energy density and it's dirt cheap. That's why we use it for fuel. It's a bargain at double the price. Unfortunately, we might just test that double the price theory.
I'll partially agree with my protege here on the first part...hydrogen is a bitch to transfer under even ideal conditions...but then again, who would thought something as dangerous as gasoline could be moved around, pumped into a wide vareity of vehicles by your average joe (read, average retard)

As for the density part, true, its volumetric density, even at high pressures (most hydrogen storage systems operate around 7-15,000psi for good efficiency, they still only store about 30-50g/liter of hydrogen (compared to about 800g/l for gasoline, and that is with a VERY expensive tank). BUT, what matt neglects is the efficiency factor...your average fuel-cell + electric motor is ~60-70% efficient, or 2-3x a gasoline engine, so the storage volume is in effect only about 5-6x worse (so its not the WHOLE SUV, but think a large size BMW 7 series trunk for similar ranges).

That all being said, hydrogen is a crappy idea for motor vehicles, so I'll agree with matt on that.

If it wasn't for cost and complexity, an engine with two different sizes of cylinders and displacement on demand, would likely be more efficient. There are easier ways of achieving that last little bit though. The big question is...with proper exhaust treatment and refining, diesel is no more polluting than gasoline AND is about 20-30% more efficient.

IMHO, the future fuel for motor vehicles is likely to be some sort of alcohol, but its going to be WAY in the future.

For a random tidbit of interesting ideas, look up Saab's concept for a variable displacement engine.


Marc
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May 5, 2008, 10:43 PM

Larger/more batteries with plug-in recharging for home and office is probably alot more around the corner than any of the other technologies mentionned.

Development of Nuclear power with more battery supplemented vehicles will take care of a majority of the transportation needs of the populus.

Heavy commerical will still rely on diesel and other fuel sources.

Ethanol is a joke and has a larger carbon footprint that gasoline right now.
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