On July 12, 2012 I posted a critique on the “flathead” or side-valve engine design. I made some comments about air flow in that posting and I now realize a discussion on some of the technical issues about engine and automotive design should start from a discussion of some of the basics of engine operation in order to provide the necessary background for future postings. To that end, the topic of this posting is the sequence of what happens inside an engine during the normal four-stroke engine cycle.
I prepared the following schematic which shows, in a linear manner, the typical operation of a four-cylinder engine. I think that it is useful to have this concept clearly in mind when considering the design features of various engine designs and in order to assess the effects of high performance modifications to all engines.
Four-Cylinder Four-Stroke Engine Cycle
There is a lot of information on the above sketch, therefore I will point out some of the more important points to note.
Note that the four-stroke engine cycle requires two revolutions of the engine. I have indicated this by the degree markers along the top of the sketch. Each revolution is 360 degrees, so two revolutions total 720 degrees. Also note that the pistons move between Top Dead Center and Bottom Dean Center with every 180 degrees of engine rotation.
The four cycles that take place in an engine are as follows:
These cycles are repeated, in that order, in each cylinder time after time.
Looking at Cylinder No. 1, it can be seen that the ignition point is at 360 degrees. Starting from Cylinder No. 1, it can be seen that the next cylinder to fire is Cylinder No. 3 at 540 degrees, then Cylinder No. 4 at 720 degrees, and finally Cylinder No. 2 fires at 180 degrees. Note that 720 degrees of one cycle is equal to 0 degrees of the next cycle. As the firing order is almost always referenced from Cylinder No. 1, the firing order for this engine is 1,3,4,2.
Note that the firing point occurs when the piston reaches top dead center at the end of the compression stroke. During the most of the compression stroke both the intake and exhaust valves are closed and the upward movement of the piston compresses the air/fuel mixture. Top dead center is defined as the point when the piston reaches the highest position in the cylinder bore.
Note that the firing or ignition is shown as occurring right at the Top Dead Center (TDC). It is necessary to set the timing of the ignition so that it does occur at the desired moment. The timing point is always a few degrees in advance of TDC. When an engine is running at 4,000 rpm that means that it is running at 1,440,000 degrees per minute, as each revolution is 360 degrees. Therefore if the timing is set an advance of 6 degrees, then this is an advance of 6/1,440,000 minute or 0.00025 seconds. This represents the time from starting the ignition to the time that the spark goes off in the cylinder.
Also note that there is an overlap between both ends of the intake and exhaust cycles. This is a point that I mentioned in the critique of the “flathead” or side-valve engine design, as air can be both entering and leaving the cylinder at the same time. This means that the relative positions of the intake and exhaust valves in the combustion chamber is important.
In upcoming posts I will be discussing the specifics of the four-stroke engine cycle, when looking at engine designs or component designs. I hope that this sketch will help in the understanding of those upcoming posts.