High compression engine
Assume an engine has a compression ratio of 9.5:1. It operates at BDC cylinder pressures up to 14.7 psia. Once compressed the pressure is 9.5 times that or 140 psia. Higher than this running regular gas causes preignition.
Now assume a similar engine is designed which operates with a maximum BDC pressure of 7.35 psia. It has a compression ratio of 19:1. In operation the max chamber pressure is still 140 psia. so there is no knocking at 19:1 compression.
If one is willing to operate an engine with less than atmospheric pressure, then one can use higher compression ratios, yes?
We operate most of our automobile engines at less than 7 psia mfld pressure. wouldn't this work?
opinions?
larry, Iburnh20, mpgmike?
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High compression engine
I believe you just asked an engineering question, so here's an engineering answer.
It all revolves around cylinder "pressure", period.
There are 2 distinct principles at work when discussing "compression ratio". One is the ratio of compression; VBDC/VTDC. The other is "expansion ratio" which is the inverse of compression ratio, or; VTDC/VBDC.
Increasing the compression ratio is desireable for 2 reasons: First it squeezes the fuel to higher pressures, which extracts more energy from the fuel than a lower squeeze. Second, higher compression ratios equate to higher expansion ratios.
There is a limit to how tightly the fuel can be squeezed before it begins to detonate. However, there are a couple fixes for too high of compression. One way is to retard ignition timing. This will begin to kill off the power levels and economy sooner or later. Another way is to increase the octane of the fuel. This will improve power and efficiency on a higher compression engine, but the economy factor (higher priced fuel) takes a hit. The third way is to bleed off some of the pressures by using a custom ground camshaft which keeps the intake valve open long enough that some of the intake charge gets pushed back into the intake manifold, thus reducing net cylinder pressures.
And, as we've recently learned, yet another way of reducing detonation under higher compression ratios is to "better vaporize the fuel" before it enters the combustion chamber. The Powre Lynz achieve this, as does a HyCO and several other devices. And last but not least, "increase activity inside the cylinder" which Somender's Grooves accomplish. Dead stagnant charges inside the cylinder will burn in unpredictable ways (if at all). Often leading to detonation. High swirl activity promotes a cleaner faster burn which reduces the tendency to detonate the fuel under higher pressures.
There is another method used prevelently and that is to add something to the compressed charge that doesn't burn, nor does it oxidize. Exhaust gasses act inertly in the combustion process as they have no oxidizing properties, nor will they burn. EGR reduces combustion "temperatures" which therefore reduce combustion "pressures". By reducing the temperature of the combustion process with EGR, a loss of power and efficiency will be had. The reason for the ceramic thermal barrier coatings on the pistons and combustion chamber is to make 'higher' combustion temperatures, which push harder on the piston than do lower temperatures. EGR is a "Band-Aid" of sorts in that it allows the OEM engineers to reduce NOx emissions without having to increase the efficiency of the engine to do so. When the fuel is properly vaporized before admitting it into the cylinder, and when there is sufficient activity to quickly burn the fuel after the spark plug ignites it, THERE IS VERY LITTLE NOx EMISSIONS FORMED!
It takes time and temperature to form NOx. Although we would have the temperature requirements, we don't have the time requirement when we are getting the fast burn.
Expansion ratio is the amount of time the engine has to turn the high-temp high-pressures into kinetic energy at the crankshaft. Given a long enough cylinder (picture a 2 meter long cylinder here), the spark would ignite the air/fuel charge and begin pushing on the piston. Initially there would be a spike in both pressure and temperature. But as the piston decends this long cylinder, both pressure and temperature drop. Eventually the pressure and temperature would be at atmospheric, though perhaps not at the same time. Allowed to travel far enough, there would be a vacuum in the cylinder and it would be cooler than ambient. At that point, all of the chemical energy from the fuel has been converted into kinetic energy via the piston.
Mike
High compression engine
I think I've been wrong about advancing the cam for efficiency.
A early closing intake improves the compression ratio, but lowers the expn/cmpr ratio. All an early closing intake (advancing the cam) does is increase the volumetric efficiency of the engine at low speeds.
The chrysler 2.2 has a 3.62" stroke, 9.5:1 theoretical compression ratio. As-is cam timing sets the actual compression at 7.8:1 at low speeds. Advance the cam 18 deg and one gets 8.8:1 compression. Retard the cam 18 deg and one gets 6.4:1 - but a great expn/cmpr ratio.
We can retain the high expn/cmpr ratio for efficiency and compensate for the poor compression ratio caused by the late intake timing.
We retard the cam 18 deg, get an intake stroke of 2.31" instead of 3.62". Then we raise the theoretical compression ratio to 13.5:1, so we actually have a low RPM compression ration of 9.5:1, and the advantage of a great expn/cmpr ratio to boot!
But one must rev limit or one will melt one's pistons from inductive charge tuning.
edit: These ideas did not take into account the kinematics of piston movement. The piston, being connected with a rod which has to move left to right during crank pin rotation, actually reaches its bottom of travel at 130 degrees and stays there until 230 degrees. So... the ideas above are bogus. Compression, dynamic compression, does change with cam timing, just not as much as this post thought. *fltcoils*
High compression engine
University of Mich has an answer to getting high compression at low power low mfld press, and avoiding detonation at high mfld press...
...Development ...of a passively controlled, variable compression ratio piston.
Experimental testing of protoype pistons was conducted.... Dynamometer engine testing showed that Brake Specific Fuel Consumption (BSFC) improvement ...ranged from 8 to 18 % up to 70% load. Knock free full load operation was also achieved.
http://me.engin.umich.edu/autolab/Projects/Completed/PRP.html http://me.engin.umich.edu/autolab/Publications/P2005_06.htm http://me.engin.umich.edu/autolab/Publications/Adobe/P2005_06.PDF
http://me.engin.umich.edu/autolab/index.html
pics deleted due to upload limit. see pdfs above
High compression engine
Hehe, a lot of this is still over my head, but I think that dynamic compression thing would not last long. I think metal fatiuge would either affect the spring rate or break the mechanism, that or be insanely expensive : )
To make an engine like you were talking about in your first post would just require a custom ground camshaft. The benefit I see to that is that the fuel would be under a high amount of vaccum and therefore allowed to atomized more freely.
Some of the problems are, like mpgmike stated, you aren't getting the benefits of the higher pressures, and if the vaporization is not helpfull then you are doing both unproductive work on the gas and unproductive work from the cylinder walls during the decompression and recompression at the bottom of the crank swing. I need my own shop with a little 1 cylinder engine so I can play around with all this stuff. : D
High compression engine
We never have enough "tools" (my father-in-law references my "toys").
Mike
High compression engine
Let's pretend we're running on propane. Take vaporization out of the picture, not the issue.
Consider....I'm from a flat area, near chicago. So around here a basic mother-in-law putt putt automatic transmission car,driving on surface streets, doesn't get run above 2800 rpm, and certainly does get throttled beyond 50% most times. Looking at the graph for the 2.2 NA, http://www.thedodgegarage.com/turbo/sae_22_dyno.gif (http://www.thedodgegarage.com/turbo_sae_22.html)
That means 12 to 25 HP is being used (because of the throttling). Not 100 or 120 HP, max output rating for this engine. One doesn't need a better HP/engine mass ratio, one needs efficiency at this operation point.
See the cam diagram for the 2.2 http://www.thedodgegarage.com/turbo/sae_22_cam.gif
Retarding the cam 2 belt notches will move the intake closing to near 90 deg ABDC, and move the exhaust opening to 164 deg ATDC. Both of these conditions enhance the Atkinson cycle effect. The "late" intake and exhause timing carries the valve overlap into the ATDC section as the pistion is moving 10% or so, allowing for some EGR to help avoid detonation.
So one does this, and corrects the computer to give correct mixture. One also then mills or decks to raise the chamber "real" compression back to 9.2:1 (or better depending on Grooves etc.)
Result, Atkinson cycle pumping losses beat, a narrowing of the 90% use max expansion pressure range, and some exhause gas getting recirculated to help prevent detonation.
Sounds promising to me, on a "normal" 8 valve engine. Just need to increase CR to 13-14:1, mill head, and lean out computer calibration.
pics deleted due to kb upload limit, see links above
High compression engine
Hrmm, I can see the EGR being somewhat helpfull, but I think too much EGR causes bad things, and it would be difficult to tune for that. Isn't tetarding the cam going to open the exhaust during the power stroke, letting usefull heat and pressure out of the engine? I'm not sure what the atkins cycle is, I'll have to read about it when I get home from work. I don't mean to be negative on your plan, I'm just kinda confused at where the efficiancy gains happen.
I was thinking about the dynamic compression ratio engine today, 1st, I don't think that design is good because as the piston crown moves up and down it will affect the balance of the engine (I think it will at least, or does it not since the piston moves in a track?). 2nd I'm pretty sure saab already did it, they had a cylinder head that moved up and down. Probably would be more reliable if you could pull your spark plugs in and out to change the volume. Theoretical engines always produce theoretical results : )
High compression engine
Retarding the cam will cause the exhaust valve to be less open at the end of the exhaust stroke, leaving a greater opportunity for use of the power.
The engine in the Toyota prius is an Atkinson cycle engine. It doesn't use a throttle to control engine power/charge, I don't think, but rather varies the late timing of the intake valve. It literally spits out, during the compression stroke, the air/fuel charge it just sucked in, except for the portion it needs for the power demand. neat.
The Mazda Millinium uses a Miller cycle, late closing intake valve and a supercharger to provide the psi charge one is loosing on the compression stroke. MIller cycle engines were the top high tech at Indy in the 20s.
The dynamic compression piston crown only moves up/dn during the last bit of the compression stroke, otherwise it is always up. I don't know enough of engine dynamics to know if that affects balance. I thought one only balanced for rotating mass, so perhaps the piston crown can be assumed to be reciprocating mass and not affect the crank weights for balance. (Pre 1913(?) engines didn't have balancing weights on the crank)
Saab variable compression http://www.saab.com/main/GLOBAL/en/vepsilon/index.html http://www.edmunds.com/edweb/editorial/innovations/saabengine.html "Due to its variable compression ratio, the SVC engine can run at an optimum compression ratio of 14:1 at low engine loads to maximize fuel efficiency..."
http://www.saab.com/main/GLOBAL/en/vepsilon/index.html
Eh?
High compression engine
Reciprocating mass, pistons, rings, wrist pins (and locks, if used), and the small end of the rods. Rotating mass, crankshaft, harmonic dampner, flywheel or flex-plate, big end of rods, rod bearings.
High compression engine
http://me.engin.umich.edu/autolab/Publications/Adobe/P1998_02.PDF
I found this analysis contrasting a Miller Cycle to an Otto cycle, apparently it is good for only 6% gain in a normally aspirated application. The "spat out" intake charge messes up the heat transfer and reduces the gains.
So the miller cycle doesn't really look worth it.
High compression engine
The University variable comp piston will work because it's limiting comp at max throttle/power but allowing higher comp at reduced throttle openings. I fear like tkelly27 that reliability AND EXSPENCE would be a problem. See my post in "Max Compression Ratio".
re:High compression engine
I agree mate I also read that Saab already done this on their cars. Awesome innovation. This make their engine pretty awesome. That is also the reason why Saab engines has a lot of new additional Saab accessories in them. They make the engine more powerful and with high MPG.