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Varying the compression on an engine controls several aspects of input and output from an engine. An engine with high compression is great for racing or torque, but not very easy on gas. Turn it around and use low compression you generally get better gas miliage, but unless your planning on supercharging,
doesn't result in much power.
If a crank could be placed on mounts that can slide up or down on, and then use jack screws to control the sliding of the crank, compression can be controlled. Problem is connecting this thing to a transmission.
SAAB' s variable compression engine
http://autospeed.dr...A_0458/article.html
[Ling, Oct 12 2006]
Description of SAAB engine operation
http://www.saabnet....n/press/000318.html
Buried inside are many references to efficiency and compression ratio. [Ling, Oct 12 2006]
Heat engine thermodynamics, including otto cycle.
http://web.mit.edu/...amics/chapter_5.htm
"the efficiency of an Otto cycle depends only on the temperature ratio of the compression process."
See graph of Otto cycle efficiency with CR. [Ling, Oct 12 2006]
SAAB Variable Compression Engine
http://www.fs.isy.l...e/Lab/SVC/data.html
A pretty clear explanation of what's going on [elhigh, Oct 14 2006]
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//use low compression you generally get better gas miliage//
Is that why diesel engines are so uneconomical? |
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But if you could change the effective volume, that would work. However, several car engines already do this, using hydraulic action on the cylinder head, I think. |
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I'd say changing the head geometry would be far, far easier. I'd also like you to qualify that "lower compression=better economy" coment. |
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In this website we obey the laws of thermodynamics. |
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do some research on your subject before posting. There are an amazing number of incorrect assumptions for such a brief description that is boggles the mind. [-] |
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SAAB has already explored this with a concept engine that has the top half of the block pivot toward and away from the crank center, rather than holding the block still and trying to move the crank's center of rotation. |
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Now, if we really wanted to play with the crank, we could just shift one end and have a sliding scale of compression ratios in one engine. |
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It's just a general idea. Desiel engine's are 15% more effiecint than gasoline anyway, so stop comparing apples an oranges. Desiels also require a high compression ration just to function. The reason this is here is to get information and input, which I appreciate. Lower compression doesn't result in better economy per say, but it's better than a high compression engine of the same specs (e.i. varying compression in the same engine) The SAAB concept sounds hard to do, sounds cool though. |
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coordinating the motion of 6+ jackscrews is easy to do? |
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You are committing the sin of lumping variables together when it comes to engine performance. Compression ratio does not correlate to engine efficiency. Many High performance engines utilize higher compression ratios in order to make more power for a given engine size. In general if you increase compression for a given set of fixed conditions you will increase the available power, however that does not determine whether effeciency will also increase.
The key advantage of a variable compression engine is that it can tune itself to the most efficient parameters for the current operating conditions which vary depending on RPM, Throttle Position, Air Temperature, engine Load etc. |
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Efficiency is also impacted extensively by the rest of the vehicle drivetrain. Many 4 cyl engines are more efficient because they operate most frequently at thier peak efficiency, where as large engines are underutilized. My BMW for example gets better milage at 80 mph than at 60mph. |
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Correct you are, on my first writing (before creating a log in) I mentioned that variable valve timing would be almost mandatory for this to work. using 2 long jack screws attatched via gears to the other 6 (or 8 or 10) moving jack screws was how i initialy planned on accomplishing this task moving all screws simultaniously. |
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[attempting to recall 2nd year thermo] |
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in terms of the otto cycle, efficiency is proportional to compression ratio, in a purely theoretical sense. Operating perameters are the primary factor, though. |
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How is this apples and oranges? It's all a bit fruity, but I thought were talking engine efficiency. 15%. flat figure. nice. wanna qualify that? Them there's some pretty wild statements, fella-me-lad. |
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How is Saab's way harder? I'd hate to try to solve alignment problems with the crankshaft and gearbox. |
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Saabs approach(though they were not the first to try it) is really quite elegant and overall quite simple compared to the alternatives. |
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[custard guts] But not all of us use otto cycle. Prius, (and probably more vehicles in the future) use atkinson cycle. |
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And if the author was aware of the difference, I would hope that he would have mentioned that he was refering to atkins cycle engines, which are fundamentally different from a thermodynamic standpoint. |
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all other factors being equal, if the engine can handle it, and if preignition can be controlled, higher compression ratios will always result in higher thermal efficiency. |
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-are we really arguing here? moving the crankshaft will cause unsurmountable problems with shaft alignment, vibration, etc. The author has not attempted to adress this issue. |
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varying head geometry is, although new, a very promising idea, in contrast to this one. |
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I thought that high compression ratio was always a good thing in an engine cycle, apart from the following disadvantages: |
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- Higher compression ratio will produce higher temperatures before the spark is introduced, so higher octane fuels are needed to avoid the mixture detonating prematurely |
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- Higher compression and consequent high mixture temperatures produce more NOx, which is bad. |
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If I might take issue with another comment though, co-ordinating any number of mechanically driven jackscrews is easy; you'll get utterly predictable results, plus or minus the backlash in your geartrain. I don't see a significant problem. |
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>>you'll get utterly predictable results, plus or minus the backlash in your geartrain<< |
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Your absolutely correct, and for a one time experimental engine Im sure it could be made to work, however under normal conditions you would require extrordinary precision. Engines have enough wear problems when the Crank is fixed to a very solid chunk of metal. Trying to maintain alignment over 6 to 8 screws in a running engine is just not easy or practical. Exisiting systems use a Hinge and an actuator, very simple, very rugged. I stand on my critisism. |
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I'll agree that a set of driven jackscrews brings unnecessary complication and violates the KISS principle. Controlling them wouldn't be a problem, but I'd suggest that reliability and wear would, however, be a pain, especially as the aforementioned backlash built up. I was splitting hairs, I think. |
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//I stand on my critisism.// |
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The concept of moving a head (something that is generally firmly bolted to the top of a block) was a concept that seemed complicated to me, but I'll take your word that it's a simple design. I'm currently trying to find a qualification for the 15% more efficient desiel comment. As I recall it was on one of those Travel Channel shows. Something that might back that up though, the winner of several of the le mons race events this year was also the first desiel ever to enter the race. However, desiels HAVE TO have a high compression ratio (or an impressive supercharger) just to function, thus the are really no basis for comparision when talking about varying the compression ratio. |
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The Diesel Cycle is a more efficient process than the Otto Cycle from a thermodynamic perspective(more energy can be extracted per a given volume of fuel). |
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RXAaron, you are right about the need for high compression for a diesel engine to function (because it uses compression ignition).
The following is how I understand it, and may need to be confirmed by others:
The reason for high efficiency is *because* of the high compression. Mitsubishi, and probably some other manufacturers, have developed GDI (Gasoline direct injection into the cylinder), which was quite difficult (the fuel must be metered carefully during the injection to stop detonation). The advantage of GDI is that the compression can be increased, thereby improving efficiency. Why can compression be increased in this way? It is because with a normal injector system (into the manifold), too high a compression will cause ignition of the fuel-air mix before the spark happens. With GDI, the performance can be maintained, since it otherwise works the same way as a normal injected engine.
On an alternative track, think about turbo engines. My Scooby had a reduced compression ratio, which allowed for the boost from the turbo. This is great when on full bore, but when cruising, the turbo is not really supplying much boost, and the engine is running on low compression ratio and is therefore not so efficient.
On yet another tack, have you ever heard about an engine being more efficient when the throttle is wide open? It may be an urban myth, but the theory is that when the throttle is mostly closed, less air mass enters the cylinder, and the effective compression ratio is reduced = less efficient. However, if the throttle is wide open, more air mass can be sucked into the cylinder which increases the effective compression ratio and increases efficency. There are obviously other factors to take into account, such as wide open throttle might cause the engine management system to run slightly rich, and so on, but anyway, that's the idea.
I don't think that the head is moved away from the block in variable geometry engines. elhigh already explained that the head and block is moved away from the crank, by pivoting one side and moving the other. So my earlier comment about moving the cylinder head (by itself) was not quite right. |
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I guess I misunderstood the first explanation of the SAAB engine. Moving the cylinder block makes much more since. I understand your arguement for higher compression, but if that's truely the case, 1, why would SAAB have developed such an engine, and 2, why aren't all engine's high compression? Raising the compression is usually done with higher octane gasoline. |
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RXAaron, to answer your questions, have a read through the links. |
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[ling] Otto cycle engines are most efficient at about 40% of redline. Atkinson/Miller cycle is higher. I think around 65%. At full throttle friction loses and vacuum losses are both quite high. |
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Compression does not equal efficency. Burning as much of the fuel load during the power stroke does. High compression makes it easier to achieve high torque, which gives more horsepower for a given engine size Which is why high compression engines tend to be found in "performance" vechicles. Engines found in "economy" cars are usually lower compression, but highter fuel efficency. |
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The other thing to be considered is that Higher compression engines require tighter tolerances and have reduced longevity compared to a comperable lower compression engine. As engines are not free I suspect these considerations play a much greater roll in determining what engines are used and in what configuration. F1 cars run on 8s and 12s but thier redline is way over 10000rpm and thier service life is measured in hours rather than years for your average automobile. |
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From the SAAB press release: "The compression ratio is one of the most important factors that determine how efficiently the engine can utilize the energy in the fuel. |
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The energy in the fuel will be better utilized if the compression ratio is as high as possible."
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That seems very clear to me. |
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//From the SAAB press release// |
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And putting a eight square meter wing on the back of a Corolla will make it go faster. |
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Corolla = Toyota?
Anyway, see link for non-marketing mit.edu. I'm looking, but I can't see anyone who says that low compression ratio is more efficient. Can you help? |
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Yeah, Corolla is a cheaper model of Toyota favored by young men around here who fit them with gigantic wings on the truck, bore out the muffler, and add a massive sound system. I think their motto is "If you can't go fast, at least go loud." |
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Lower compression does not lead to higher efficency as a rule. It is one factor in engine design. You can't just change one factor and make a significant change in engine design these days. The core elements of ICE design have been pretty much determined for decades, and engine designers work within those parameters to create the engine that meets their needs. |
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Higher compression lets a designer put a larger charge of fuel in a given engine bore. This means more power for a given engine size and weight. It also means more strain on all components and more fuel used for each revolution of the crankshaft. |
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Think of it this way: I have two engines. They are identical except for the connecting rod and offset of the crank. One has a connecting rod ten centimeters long, the other is eleven. Because the bore is the same, the longer rod lifts one centimeter higher , and drops one centimeter lower in the cylinder each stroke. So when the piston is at Bottom Dead Center, the capacity of the combustion chamber is 10% larger. You can see that each cycle (four strokes) will turn the crank shaft of each engine the same distance (two revolutions) but the higher compression engine will have burned 10% more fuel to do this. This gives that engine 10% more power per revolution, but uses 10% more fuel. |
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Of course that is very much an oversimplification, but should be illustrative. |
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I think that's a bit simplified. Have a look at the third link. Do you agree with it? It basically says that the efficiency depends on the ratio of the temperature before compression, and the temperature after compression (T2/T1). Since the temperature after compression depends mostly on the amount of compression, then it's almost the same as looking at the compression ratio, hence the graph shows compression ratio. |
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Honestly I am having a hard time wrapping my head around that. Measuring compression by change in temperature rather than change in volume? Ignoring all intake and exhaust factors? And I can't see that T1 is ever even defined (I assumed that he means baseline temperature) Assuming that Exhaust is at the same temperature as intake? Assuming that combustion is instantaneous? All seems pretty far removed from reality to me. |
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Um, yes, that is a cop out answer. Probably need a real engine designer to answer it properly. |
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I think [Galbinus_Caeli] is a bit mistaken in his illustration. Changing the compression ratio will not affect the volume of gas or the amount of fuel in a cylinder by any measurable amount, and when it is changed, it is almost always done by either a head design change (smaller combustion chamber) or a dome/dish in the piston top. But going with the illustration, a 10 cm rod on say a 8 cm stroke and 8 cm bore would yield exactly the same volume as a 12 cm rod on the same bore x stroke. A longer rod/piston assembly just changes the volume of the combustion chamber at top dead center by a much higher amount than it changes it a bottom dead center. And in reality, a longer rod (or higher compression ratio in general) allows less air and fuel into the cylinder, but only by a small amount. |
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Higher compression allows the charge to burn quicker and more completely (since there is only a certain amount of time it can burn) and gives higher cylinder pressures, which in turn gives more power (torque) from the same displacement engine, but at costs already outlined here by others. |
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I think the principle is similar in concept to the efficiency of any heat engine: it varies according to some function of the ratio of temperatures of the heat source, and heat sink (in Kelvin).
In the link, compression is closely related to temperature since at the high speed of compression, little heat is lost to the cylinder and piston (and there is even less difference when comparing one compression ratio to another). If the starting volume is the same, then remembering that PV/T is constant, the ratio of Pressure to Temperature must be constant. So increasing compression ratio (and final pressure), means a corresponding increase in temperature. That is why compression ratio is used instead of temperature ratio: it's a realistic approximation.
Of course, he is using a simple model, but the principle is there.
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you are forgetting that once you add fuel to the mixture the rules change significantly. Timing becomes a critical component of the mix, if compression is to high the Fuel air mixture will preignite and cause damage to the engine at worst and degraded performance at least. As stated Compression is determined soley by combustion chamber volume at TDC(chamber volume plus piston bore volume at highest point of piston rings less piston volume above that point.) These parameters can be varied through any number of approaches including deck height(the most common way to increase compression is to shave the deck height of the engine block or the Cylinder head) Wrist pin to cyinder face differences, connecting rod length, crank shaft offset, Piston shape(Hemi, dished, Flat). Efficiency is also controlled by valve design and layout, intake runner length, shape, cross section, finish and Volume, Exhaust manifold shape, length, tune, backpressure, finish and temperature. |
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The fundemental challenge in desining car engines is that no one set of parameters is optimal for the range of conditions that are involved in normal driving(ie High Performance cars with high RPM camshaft profiles general idle very poorly as optimal low RPM parameters degrade high RPM operation(timing advancement) |
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I added a link to the list that shows the rationale behind a VC engine and why, though the idea isn't bad, your logic is backwards. |
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A higher compression ratio yields better thermal efficiency, which equals more work from each drop of gas. A lightly loaded engine, say one in a car that's just tooling along the highway at 70 mph, won't detonate even at high compression ratios if it's tuned correctly. But as we've discussed in this venue, one setup doesn't cover all performance regimes, and that high compression engine becomes a pinging hunk of junk when you jam on the gas to pass. Lowering the compression ratio - or changing the valve timing, or injecting water into the cylinder - under heavy load can eliminate the knock and allow the engine to deliver reserve power in performance envelopes that would otherwise cause damage. Changing the CR is just another way to make the engine more adaptable, and another way to make a little engine do more with even less. |
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Though I like it as a concept, I think it's just one more thing that can break. |
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On efficiency's - If you can adjust the combustion chamber based upon the flame propagation of the fuel you can hone in on upper lever efficiencies. The actions and reactions of air and fuel in an engine beholds aspects that most minds can not fathom. This is where CAD kicks butt.
As for designs and concepts on this subject I feel that SAAB has the most fesable one of all mentioned. All others mentioned are interesting however I just can't see any of them brought to production.
I have pondered this a bit and have a plausable idea myself. First keep the head, block, and lower end stationary to avoid complicated mechanical attachments. Now place a movable combustion chamber in between the head and block and hydraulicly move it. As for air/fuel in and exhaust out just sleve the gas passages.
The reason for hydralics to move the cylinder bank is simple it gives under force but at negligable amounts, the hydralics can be fine tuned, and will transmit leverage equally efficiently and precise without mechanical failure for the life of the engine. (AKA jack screws) diagrams to come soon and perhaps patients as well. |
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[dr], the wonderful thing about jackscrews, aside from the steampunk brute-strengthiness, is that they continue to hold the engine at their setting even if power or hydraulic pressure quits. |
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It doesn't take a very large change of head-to-crank distance to radically change the CR. The jackscrews wouldn't be very long at all - a centimeter of travel would be all you would ever need. |
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I haven't even run the numbers and I'm already sure of this. |
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Example: moving the crank in my Toyota 22R 5mm closer to the head raises my compression ratio from a healthy 9.0:1 to a striking 18:1. The engine merely wants to blow itself apart at the lower number; at the higher compression ratio it will be quite determined to separate both the head and the variable CR mechanism. Hydraulics will do a fine job of raising the block and lowering it back down, but the jackscrews raise it while continuing to hold it together. Hydro doesn't do that. |
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I can agree there could be issues with hydralics however the hydralic system I am talking about is a closed system like a floor jack. My thinking is with my design idea that you would need several pistons or jack screws. with a closed hydro system controling the movement of all the points is easier than jack screw / drive shaft to me. I think this will fix both of us:
use jack screws that are controled individually by micro servos like in some EGR valves. this would eliminate variances in a jack screw drive train and It would also be very preciese. |
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Varying the compression ratio is probably good, but I'd think the most practical way would be to delay the closure of the intake valve. This would leave the expansion ratio of the engine maximized, but allow the compression ratio to be varied without throttling. |
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[supercat] That's called the Atkinson Cycle and it is used in the Toyota Prius. About 10 minutes after reading about the Atkinson Cycle and its power problems, I wondered why this wasn't variable. The problem I believe is that you need an extra camshaft (there would be 2 cams operating the inlet valve) which would need to be moveable so that it could be disengaged or engaged at different amounts. |
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I thought it was the Miller cycle. In any case, one wouldn't need to cam shafts if one had a solenoid to keep the valve open. Since the valve is held open against pressure, releasing the solenoid would let the valve close. |
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To be sure, the mechanism has to be built to withstand millions of actuations, but I think that should be doable. |
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The Miller cycle is the Atkinson cycle with a supercharger. Instead of compressing the charge out the intake valve (like in an Atkinson), it compresses it into a supercharger which compresses it back into the cylinder. |
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Also, if BMW has their Valvetronic technology (which uses variable valve timing/ variable lift to control how much air gets into the cylinder and means that the car doesn't need a throttle), I'm sure something like that could be used to near-infinitely adjust how much air is compressed back out to control the compression ratio. |
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// Also, if BMW has their Valvetronic technology... I'm sure something like that could be used to near-infinitely adjust how much air is compressed back out to control the compression ratio.// |
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There are limits as to how much late intake closure may be used to throttle an engine, so some sort of throttling will still be necessary. On the other hand, even if one can't eliminate all of the energy waste associated with a throttle, eliminating even half of it would be a big win. |
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I don't understand what you mean when you say "closed system like a floor jack." My hydraulic bottle jacks are indeed closed, but if you pull upward on the head hard enough - which isn't really that hard - the oil starts to vaporize into the vacuum. Suspend a hundred pounds or so from the bottom of a bottle jack and you can boil all the oil in the cylinder, at room temperature. Jackscrews will restrain the tension up to the tensile limit of the metal they're made of. You can hang a ton or more from a jackscrew that's only a half-inch in diameter. |
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What you need to realize is that the main stress in the engine is tension. The forces within the engine are trying to blow it apart: the cylinder walls outward, the head upward, the crankshaft downward. All those things have to be held IN. |
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Once you go to Direct Injection, the need for a throttle is obviated. All the engine control goes via the fuel injection at that point. |
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For that matter, DI does away with a lot of the problems of engine knock under high compression, to the extent that some manufacturers are now exploring a hybrid engine that is a quasi-diesel under light loads, and spark ignited under heavier demands. VW and MB are both looking into this, there may be others. Audi is selling a DI engine that runs 15:1 compression. It's pretty small, but the horsepower numbers are pretty good, as is the fuel efficiency. No, I don't have exact numbers on hand. |
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