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Théorie De La Compression Sur Un Moteur
Compréhension De Son Focntionnement

*** Décembre 2009 ***


(Traduction en cour de réalisation... dés que j'ai un peu de temps...merci pour votre patience ...Sauf si vous lisez l'anglais techniques sans soucis...tant mieux pour vous !)

(By David Vizard)
Compression Comprehension :
It takes the barest of mental agility to appreciate that increasing the CR will raise cylinder pressure, thus causing torque output throughout the rpm range to simply follow suit. What is less obvious is that the increase in output from the higher CR comes about largely due to an increase in thermal efficiency. The thermal efficiency is a measure of how effectively the engine converts the heat-generating potential of the fuel, when burned with an appropriate amount of air, into mechanical power. To explain all this (starting from the raw fuel and air to the output at the flywheel) is rather more complex than we have the space (or inclination) to deal with, but no matter, as the most pertinent and relatively simple part applying here is not.
If a 2:1 and a 15:1 cylinder start at the same pressure, the 15:1 cylinder's pressure decays at a far higher rate and in so doing delivers the power to the crank mostly before 90 degrees after TDC is reached. Unless supercharged, a 2:1 cylinder would only achieve about 200 psi and as such, the difference in power be-tween the two ratios would be represented by the shaded green area.
Simple Theoretical Power Gains :

At low rpm, port velocity and pressure waves are too weak to produce any cylinder ramming. Couple this to the fact that even a short cam of some 250 degrees of off-the-seat timing will not close the valve till about 50 degrees after BDC. Fig. 3 shows the typical extent of piston motion back up the bore before the intake closes for three cams. Because of the delayed intake closure we find that during the period the piston moves up the bore from BDC until the valve closes, a significant amount of the induced air is, at low rpm, pushed back into the intake manifold. This means the volumetric efficiency (breathing efficiency) and thus the effective displacement of the cylinder is well below 100 percent. In other words, a 100cc cylinder with a static CR of 10:1 may only trap 75cc of air. This means the dynamic CR, at about 8.5:1, has dropped well below the static CR of 10:1. The bigger the cam, the more this effect comes into play.

The controlling factors influencing the best intake-to-exhaust ratio for maximum output (and this does assume all the available space for valves is used) has been a much-debated subject that, for the most part, has left the reader little or no wiser. The often-touted 75-percent rule is usually accepted without further question. In reality, the value is far from fixed. The optimum intake-to-exhaust ratio could range from as little as 0.75:1 (for a low CR supercharged engine) to as much as 1:0.6 (for a very high-compression naturally-aspirated engine). What is usually not appreciated here is that the CR is, for the most part, the controlling factor. Because the high-compression cylinder delivers energy to the crank much earlier in the power stroke, there are implications we can take advantage of. The most obvious is that the exhaust valve opening can be made earlier and held open longer. This can be done for improved high-rpm output without significantly impacting the engine's low-speed output. The rule here then is that the higher the compression ratio goes, the smaller an exhaust valve is needed to get the job done. This in turn leaves more room for a larger intake.

Note the minimal raised crown on this Calico-coated Lunati piston. It was used in a 441-inch small-block Chevy to achieve a 13:1 CR in conjunction with this chamber form (also coated) in a conventional 23-degree heads.

Top-notch ring seal starts with an equally top-grade bore and hone job. Always have the bores honed with a deck plate and with a finish as recommended by the ring manufacturer.

At this point it is pretty clear that making the most of the potential that can be had from high compression is a goal worth pursuing. But as the ratios sought get higher, counter-productive problems can begin to arise. Probably the most commonly seen of these is due to the final combustion chamber shape achieved when all the stops have been pulled out. The problem here is that as ratios much above about 10:1 are required, the only way to further minimize the volume after maximizing head milling is to have a raised crown piston. Up to a point, this is okay, but if the crown intrudes into the chamber too far, it can severely compromise the flame travel, resulting in a very ineffective combustion process. As to how much can be lost, suffice it to say I have seen a hundred horsepower disappear because of a piston crown intruding an eighth inch too much. The rule here is that unless you know what combination of chamber and crown form works or are prepared to do the necessary R&D, don't go overboard on crown intrusion into the chamber. For typical small-block V-8 from Chevy, Chrysler or Ford, a good rule of thumb is to use no more than 100 to maybe 125 thousandths crown height in your quest for a high CR.

My extensive testing has shown Total Seal rings can produce zero leakdown and can continue to do so for as much as 100,000 miles. Here is how they work: Firstly, the gap for the lower ring is on the opposite side of the bore where it is sealed off by the upper ring. Gas pressure, communicated from the top side of the piston, passes down and through the gap as shown or through the radial gas ports if the piston used has them. This pressurizes the backside of the ring, ensuring firm contact with the cylinder wall. Because both upper and lower rings are virtually in contact with the bore, no leakage route exists.

Gas porting is a technique whereby compression and combustion pressure is communicated to the backside of the ring, thus pressing it more firmly against the cylinder bore wall. Piston deck-located ports such as seen here work well for a drag race engine but plug up too easily for an endurance engine.

Compression Ratio Gas Porting
Gas porting is a technique whereby compression and combustion pressure is communicated to the backside of the ring, thus pressing it more firmly against the cylinder bore wall. Piston deck-located ports such as seen here work well for a drag race engine but plug up too easily for an endurance engine.
Having a high compression ratio brings about greater demands on cylinder sealing. The higher the pressures involved, the more attention needs to be paid to details. The first part of the equation toward sealing up the cylinder is to make sure your machine shop hones the block right. This should involve the use of a deck plate to simulate the distortion brought about by the stresses of head bolt tightening. Next, make sure your machine shop is aware of the type of piston ring material being used so they can apply an appropriate finish. Then give the bores a good rub down with a new Scotch Brite pad and plenty of Gunk engine cleaner. After that, scrub (with a stiff brush) the bores with a strong liquid detergent and hose off with hot water. After you are sure they are clean and grit-free, hose the block down and spray the machined surfaces with WD-40 to prevent rust.

Seen here is the mode of operation for both vertical and radial gas ports. The vertical gas ports have been favored by drag racers for many years but the trend is toward the radial type, which is used for endurance engine.

Compression Ratio Gas Porting
Seen here is the mode of operation for both vertical and radial gas ports. The vertical gas ports have been favored by drag racers for many years but the trend is toward the radial type, which is used for endurance engines.

Date de création : 29/12/2009 @ 20:00
Dernière modification : 30/12/2009 @ 01:35
Catégorie : Technique automobile
Page lue 26919 fois

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