![]() As it grows in size, its heat output increases, which allows it to grow at an accelerating rate, expanding rapidly through the combustion chamber. The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. ![]() This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases. The combustion is started by the spark plug some 10 to 40 crankshaft degrees prior to top dead center (TDC), depending on many factors including engine speed and load. Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. It was further investigated and described by Harry Ricardo during experiments carried out between 19 to discover the reason for failures in aircraft engines. In the letter they stated that an early ignition can give rise to the gas detonating instead of the usual expansion, and the sound that is produced by the detonation is the same as if the metal parts had been tapped with a hammer. The phenomenon of detonation was described in November 1914 in a letter from Lodge Brothers (spark plug manufacturers, and sons of Sir Oliver Lodge) settling a discussion regarding the cause of "knocking" or "pinging" in motorcycles. However, pre-ignition can be followed by knocking. Knocking should not be confused with pre-ignition-they are two separate events. Effects of engine knocking range from inconsequential to completely destructive. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The fuel–air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. In spark-ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. Please help out by adding images to it so that it can be better illustrated. As a result, the timing moves to 32 degrees before top dead center which allows the fuel to be ignited early enough so that it is burning while the piston is rising and completely burnt when the piston reaches top dead center.This article needs additional or more specific images. In order to have the same result, the fuel must be ignited much sooner in the compression stroke. This would equate to 10 degrees of crankshaft rotation before reaching top dead center to give the fuel sufficient time for a complete burn.Īs the engine rpm increases to 3,000 rpm, the fuel, - still requiring the same time to burn - would never have sufficient time to burn if ignited at the same 10-degree timing. Using an engine idling at 900 rpm further illustration, the piston is moving upward at a speed that the fuel, given the time it takes to burn completely, is ignited 1/16-inch from the top. This would mean that the fuel is still igniting while the piston is descending in the power stroke and would result in a massive loss of power. It would not be efficient having a small percentage of the fuel consumed before the piston hits top dead center. As the piston rises, the ignition spark plug ignites the fuel and the process begins again.Ĭonsider that the fuel must be burnt as completely as possible before the piston reaches the top of the compression stroke in order to force the piston downward in the power stroke. This is the fourth or compression stroke. The crankshaft turns again and the piston begins to move upward, compressing the raw fuel and air in the process. Just prior to reaching the bottom of this stroke the intake valve closes. As the piston reaches top dead center and continues downward once again it creates a vacuum sucking more fuel into the cylinder. Just before the piston rises completely in the exhaust stroke the intake valve opens, using the vacuum produced by the rapidly exiting exhaust gases to help draw in more fuel from the intake valve. The upward moving piston forces the burnt gases out of the cylinder. As the crankshaft turns, the piston begins to go back up and the camshaft opens the exhaust valve. The burning fuel expanding forces the piston downward. The piston begins all the way up at top dead center. The crankshaft turns two revolutions, which moves the pistons up and down to one turn of the camshaft that opens and closes the valves. Let's use a single cylinder in an engine as an illustration to demonstrate how all four strokes work. All automotive engines today have four strokes.
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