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As I've mentioned in a few of my other ideas, the Miller Cycle is a more efficient take on the supercharged 4-stroke engine. What separates it is that the Miller Cycle design calls for the intake valve to be left open for a small portion of the compression stroke. During this time, air is pushed out
of the intake valve back into the manifold, where it is met with the opposite pressure of the supercharger and compressed back into the engine. This accomplishes two things. The first is that it allows the charge to be cooler, as when it is pushed out of the engne the charge is partially cooled by the intercooler. The second is that, by compresses against the supercharger rather than the cylinder heads, it actually increases efficiency. This is why the Mazda Millenia's conservatively boosted 2.3L V6 was able to make 210HP stock.
So my idea is actually very simple: many newer high performance cars have supercharged engine and variable valve timing (ie: the brand new Audi S4, Jaguar R-models, Chevy Cobalt SS, Shelby GT500, Cadillac CTS-V/ Corvette ZR-1, Mercedes C230, Mercedes E55 AMG, etc.). Why not design a program for the ECUs of these engines to control the variable valve timing and allow the intake valve to stay open for longer? This should be extremely simple given independent tuners' abilities to reprogram ECUs, as well as the flexibility allowed by variable valve timing (which is controlled by the ECU).
The benefits of this would be greatly increased power from the same boost level, and also greater efficiency/ fuel economy. Also, the cost of doing this would only be that of researching and developing the programs necessary to put on the ECU. There would be no other physical parts added to the car, as this would just be a means of using the existing software more efficiently.
Also, I suppose on another note this could be used in tandem with aftermarket supercharger kits as a combo package, but that isn't my main idea.
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//So my idea is actually very simple...// |
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Well, good then, have a bun. [+] |
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Don't you also need to up the compression ratio? |
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Nope; the supercharger is doing that for you. Er well, even more than it normally does that is. The Millenia's engine had a compression ratio of 10:1, which is relatively high for a forced induction engine, but certainly not unusual. |
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The efficiency gain of the cycle you describe is due to an effective reduction in the length of the intake stroke, while retaining the length of the power stroke. |
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Ideally, the cylinder pressure at the end of the power stroke would be equal to ambient pressure. In a typical internal combustion engine, the intake stroke is the same length as the power stroke, resulting in a cylinder pressure well above ambient at the end of the power stroke. The advantage is more power. The disadvantage is wasted energy and fuel. |
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By keeping the intake valve open for a portion of the compression stroke, a smaller quantity of fuel and air is compressed, thereby resulting in a higher expansion ratio during the power stroke. With greater utilization of the energy in the combusted fuel, efficiency goes up. With a greater rate of pressure drop during the power stroke, power output goes down. |
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By adding the supercharger and keeping the geometric compression ratio high, the power output can be brought back up to a more exciting level while retaining much of the efficiency gained by the cycle change. |
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instead of intake valve longer, why not intake valve shorter?, ie: close it halfway through the intake stroke instead of keeping it open halfway through the compression stroke... sry, the idea of pumping the air into the cylinder then pumping it back out, then pumping it back in seems very counterintuitive. |
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[FlyingToaster], I know it seems counterintuitive, but in reality the engine is simply benefiting from the fact that the supercharger can compress the air more efficiently than the piston can, as well as what [Freefall] mentioned. I suppose that with a supercharger you could do what you propose, and maybe that would have its benefits. I know that it would reduce the air that could get into the cylinder, but that is actually an interesting concept. |
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The majority of variable valve timing systems are purely mechanical (or hydraulic or pneumatic - aka -not- electronically controlled). There have been attempts (I believe by Cadillac) to make an engine with electrically-operated valves, but at the time, they proved to be expensive and unreliable. |
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The idea of a programmatically variable Miller-cycle valve timing is not a new one. The logical extreme of this idea is to be able to entirely remove the throttle plate from the intake path, and instead vary the power output by varying the effective length of the compression stroke. At idle, the compression stroke would be very short. At cruise, a bit longer. At "wide-open throttle" (now somewhat of a misnomer due to the absence of a throttle plate), efficiency be damned, the valve closes at the point where the maximum quantity of air has entered the chamber. |
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With the inclusion of a diesel-style direct-injection system, the geometric compression ratio can be cranked way up with less concern for detonation. |
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It's nice to see people coming up with ideas like this independently, but I'm sorry to say it's not a new idea. |
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[Freefall], the idea itself is a simple way of implementing the concept into existing engines; not the Miller Cycle itself. |
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As far as what you stated about removing the throttle plate, that is basically what I came up with in my idea for an Ultimate Atkinson Cycle engine (the Atkinson Cycle is the Miller Cycle sans supercharger). And even with a purely mechanical valvetrain, a simple increase in intake valve lift could be used to translate to a longer intake duration, thus accomplishing this. |
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//supercharger can compress the air more efficiently than the piston can// umwhat? |
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Still wondering why the air is pumped in then back out again, as opposed to the soon-to-be-excess air not being pumped in in the first place. |
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Why not close the intake valve halfway thru the intake stroke (ie: after the required half-stroke's-worth of air is inside the cylinder), instead of halfway thru the compression stroke ? |
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The statement I made is completely true, which is exactly why you see superchargers acting as air compressors for engines rather than pistons. In essence, the compression of the charge against the supercharger vs. against the closed cyclinder head yields a cooler compressed charge as well as more efficient compression. |
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Silly me. I was thinking this might involve the Miller-Urey cycle. |
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If a supercharger is more efficient at compressing air than a cylinder is, then why not ditch the intake and compression strokes in a 4-stroke completely and replace it with a quick shot of compressed air at (near) TDC at the end of the exhaust stroke, powered by a supercharger. |
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Not a traditional 2-stroke where the compression takes place during the upstroke, in this one the upstroke is simply exhaust: at TDC the exhaust valve is closed, the intake valves opened and pre-compressed air forced into the cylinder. |
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(not that I mean to hijack the post more than I have already, and one of those original pastries up there is mine, but it seems very counterintuitive to me since turbines aren't generally as efficient as piston engines) |
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I want a car that can switch between Miller & Otto cycles, too. But I'll probably have to wait until the valves are opened & closed by solenoid, rather than a camshaft with its pesky fixed-duration valve-openings. Without that advance, you're stuck at being able to vary the timing of the valve events relative to the crank, but not to each other. |
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OTOH, you might be able to approach the benefits of Miller/Otto switching with variable valve lift. |
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Now how about variable displacement, where the always-running cylinders are Miller, and the others are Otto-cycle? |
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The Miller Cycle is (most of the time) more efficient than the Otto, because the expansion strike is greater than the effective compression stroke. |
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Extending the inlet valve opening to achieve this is apparently more efficient than shortening it. I don't know why. |
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the way i see it the easy way to describe a miller cycle engine is that the engine itself is the same as normal however, the crankshaft sets into an offset drilled into a wheel that spins at half crank speed effectively changing the swept area so that the engine has a longer expansion stroke. thus allowing more power to be extracted from the same fuel charge. The statement is completely wrong but it helps people to get the right idea in their head, that the whole purpose of the miller cycle is to have a smaller compression stroke than expansion stroke.as far as a chip to run an engine as a miller its feasible. the way that makes it easy would be solenoid driven valves or pneumatic solenoid driven valves. the other way i could think to make it work would be a flash chip, some sort of boost producer, and a new camshaft with both otto and miller profiles (sans boost with an atkinson cycle) and could potentially be easy to do on a 92-95 civic vx or a 96-98 civic hx |
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