Racer Brown chapter two

Power Stroke

Now follow the piston motions and the interrelated valve motions through four strokes of the piston for one complete four-stroke cycle. Let's start at exact TC of the power stroke, with both valves closed. Ignition of the air/fuel mixture occurred at an earlier point so that at exact top centre there is seething, flaming, furiously violent activity within the cylinder and combustion chamber, as normal combustion is in a process. This process generates a very rapid but relatively uniform increase in cylinder pressure and temperature. However, at TC the point of maximum cylinder pressure has not been reached. If it had, the piston/connecting rod/crankshaft assembly would simply be shoved out the bottom of the crankcase. The piston, being the only moveable component within the combustion chamber at the moment, is forced toward BC by the still-expanding and pressurising gases in the combustion chamber. At a point represented by about 15 crankshaft degrees of rotation past TC, pressure within the cylinder reaches its maximum value. This places enough of a mechanical leverage advantage from the piston, to the connecting rod, and to the crankpin to force the crankshaft to rotate, producing useful work at the crankshaft, in rotary motion we can capture and measure as torque and/or horsepower. At a point before the piston reaches BC of the power stroke, the exhaust valve starts to open. At this point, most (not all) of the force of the expanding and still-aflame gases in the cylinder has been captured by the piston and transmitted to the crankshaft. Although the gases are still expanding, cylinder pressure is diminishing, and what pressure remains is relatively insignificant to the total process. The exhaust valve opening point before BC gives a period of cylinder "blow-down," during which most of the remaining cylinder pressure escapes from the cylinder past the exhaust valve, through the exhaust port and into the exhaust system. Opening the exhaust valve before BC permits the valve to be fairly well off its seat as the piston reaches BC, although the valve is not yet at full lift, which reduces some of the negative work or pumping losses applied to the piston during the exhaust stroke.
Bottom centre of the power stroke. End of piston stroke 1. Also end of cylinder "blow-down" period. From the piston positions of exact TC to exact BC of the power stroke, the crankshaft rotates exactly ½ revolution, or 180 degrees.

Exhaust Stroke

Next on the sequential piston-stroke agenda is the exhaust stroke. However, evacuation of exhaust gases from the cylinder began with the exhaust valve opening at a point before BC of the power stroke. This represents the first of four periods of the cycle during which valve motion overlaps piston position.
As the piston reaches BC of the power stroke, it changes direction and the exhaust stroke begins, the exhaust valve is still opening, and the piston starts to push the remaining exhaust gases out of the cylinder past the exhaust valve. Whatever pressure is applied to the top of the piston by the exhaust gases represents negative work or pumping loss; the higher the pressure the higher the pumping loss, which reduces the effectiveness of the positive work gained in the power stroke. With this in mind, we can readily see the advantage of the "blow-down" period in reducing residual cylinder pressure, thereby reducing pumping loss during the exhaust stroke. At a point well before TC of the exhaust stroke, the exhaust valve reaches maximum lift and starts to close.
As the piston approaches TC of the exhaust stroke, the intake valve starts to open. This is the second of four periods during the cycle that the valve motion overlaps piston position. This also signifies the start of the valve overlap period, a segment of piston motion and/or crankshaft rotation during which the exhaust valve (still closing) and the intake valve (starting to open) are both open at the same time. The piston is slowing down before it reaches TC, therefore it doesn't have the same amount of force that it did during the first part of the exhaust stroke. As a result, the departing exhaust gases are also slowing and it becomes a practical impossibility to remove all of the exhaust gases from the cylinder before the exhaust valve closes. In fact, some of the exhaust gases are diverted from their intended path toward the exhaust valve and instead point themselves toward the intake valve and the partially exposed intake port. There is no hesitation or indecision. This occurs simply because the pressure around the closing exhaust valve is momentarily higher than it is around the opening intake valve. The lazy exhaust gases take the path of least resistance ahead of the advancing piston as an escape route. This action is inevitable as long as both valves are open at the same time for some amount of piston travel and/or crankshaft rotation. Exhaust gases in the intake port represent a diluting agent to the air/fuel mixture in the immediate vicinity. And, the exhaust gases, having some velocity and therefore inertia, cannot conveniently do an about-face and march toward the exhaust port until the momentary pressure conditions in the combustion chamber and around each valve are favourable for them to do so.
At TC of the exhaust stroke, the piston has completed its second sequential stroke of four, and during the piston travel from BC to TC, the crankshaft has rotated a second ½ revolution for a total of exactly 360 degrees since the cycle began. Exhaust stroke completed. Cycle half-completed. Although the piston motion around top centre is relatively slow, the degree of activity in and around both intake and exhaust ports and valves, and in the combustion chamber, is fast and furious indeed.

Induction Stroke

As the piston reaches TC, changes direction and starts toward BC, the induction stroke begins. Prior to this, however, the air/fuel mixture within the induction system and intake port has been more-or-less "stacked" around the intake valve, awaiting the time when the intake valve has opened far enough to permit entry of part of the air/fuel mixture into the combustion chamber by weight of its own inertia. Therefore, the penetration of the escaping exhaust gases into the intake port usually doesn't get too far before they are damped out and their direction reversed by the advancing air/fuel charge. However, the exhaust gases do dilute the initial portion of the air/fuel charge.
Opening the intake valve before TC of the exhaust stroke so the valve is pretty well of its seat at TC reduces the pumping loss exerted on the piston in a manner similar to the "blow-down" period, although cylinder pressure conditions and direction of gas flow are both reversed.
The question of overlap breathing is always good for an argument and for good reason: It's never the same between engines of different types, and it can be minimal or substantial, depending upon application, engine speed, load, and a number of other influential factors. It isn't a question of "exchanging breaths" between exhaust gases and air/fuel mixture. It seems to be more a question of how much fuel can be forced out the exhaust port before the exhaust valve closes. There is no doubt at all that this does occur in all engines to some extent. A large amount of fuel forced out the exhaust port in a race engine seems to signify that the remaining cylinder/combustion chamber space not covered by the piston has been forcibly "scavenged" - swept clean of all traces of residual exhaust gases. I think not. Evidence points to air/fuel separation during the valve-overlap period - a divergence of directional paths - whereby the heavier fuel molecules tend to travel in straighter lines - whether or not the straighter lines lead them to the exhaust port - while the air molecules, being very much lighter in molecular weights, are much more easily deflected, turned, bent, whatever, from a straight-line path. The term overlap breathing is perhaps an unfortunate choice of words, because it does refer to a one-directional path from intake port to exhaust port.
One more factor: In spite of the high cylinder pressure, high cylinder temperature, and the intensity of the combustion flame front, some particles of air and fuel within the cylinder escape combustion during the power stroke. These lurk about in relatively isolated and inaccessible pockets of the combustion chamber, around the top piston ting and between the piston and the cylinder bore, etc. This occurs because the turbulence of the advancing flame front is dampened and cooled by the proximity of two or more surfaces to the point where ignition of these particles cannot take place. Being heavier than the exhaust gases and being in a less-active state, these particles are among the last guests to depart from the combustion chamber party and then usually after the piston has passed top centre of the exhaust stroke and has started toward bottom centre of the induction stroke, when they are exposed to the general activity within the cylinder and are more-or-less free to find their way out past the closing exhaust valve. Even then, some of these particles don't make it past the exhaust valve.
In any case, and every case, some residual exhaust gases and unburned air and fuel are trapped within the cylinder after the valve overlap period is one of extreme and intense activity, even though piston motion is relatively slow. Perhaps this will explain part of what happens during this period and why.
After the piston passes the exhaust stroke TC and heads toward the induction stroke BC, the first significant event is the exhaust valve closing. This is the third of four periods that the valve motion overlaps piston position. Meanwhile as the piston descend, the intake valve continues to open until at some point well before BC, it reaches maximum lift and starts to close.
The piston's descent during the induction stroke generates a pressure-reversal within the cylinder to a point less than ambient atmospheric pressure - a negative pressure, partial vacuum or pressure differential. The amount of pressure differential within the cylinder is more-or-less dependent upon how far the intake valve is open as the piston starts its descent toward BC: The higher the valve lift at this point, the less the pressure differential (closer to ambient atmospheric pressure) and vice versa. The pressure differential again represents a pumping loss exerted on the piston but there must be some pressure differential, otherwise the air/fuel mixture would have no incentive for moving into the cylinder to fill the void left by the descending piston. Under favourable conditions, the air/fuel mixture could actually start to enter the combustion chamber before the piston reaches the exhaust stroke's TC.
The force behind the motion of the air/fuel mixture is ambient atmospheric

Compression Stroke

Now the piston changes direction and heads for TC again on the compression stroke.
Meanwhile, the intake valve is closing but is not yet seated. Piston motion before and after BC is at its "laziest" during the cycle, which means the crankshaft swings through a fair-sized arc before the piston moves significantly toward TC. The cylinder-filling action of the air/fuel mixture is continuing because the inertia of the mixture charge outweighs the effect of the rising piston. At least for the moment.
You know what inertia is. This is what hurts when you whack you thumb with a hammer. Use a lighter hammer, or swing it at a lower speed, or both, and it won't hurt as much nor for as long. Swing the hammer at a faster speed, or use a heavier hammer, or both, and it will hurt more and for longer. More scientifically and in this context, inertia is a property of the air/fuel mixture which causes the mixture to resist any change in its motion. Your thumb represents resistance to the motion of the hammer. Similarly, the air/fuel mixture continues to fill the cylinder until the rising piston becomes enough of a resistance to slow the mixture, stop it, or reverse it.
The time to close the intake valve is before the flow of the air/fuel mixture into the cylinder is stopped, at some point well before the compression stroke TC. Closing the intake valve represents the fourth and last period during the cycle that valve motion overlaps piston position. From this point until the exhaust valve opens during the latter part of the power stroke. Both valves remain closed.
Whatever volume of air/fuel mixture in the cylinder as the intake valve closed is trapped and is the source of energy for the power stroke and to carry the piston and crankshaft through subsequent strokes, with enough left over to measure as usable power. Being a compressible gas, the air/fuel mixture is compressed by the rising piston and as a function of compression, the mixture is also heated, which further increases the cylinder pressure. At this stage, the mixture is a highly-agitated mass caused by the motion of the piston and its compressive action, during which time the air particles are heated further and attempting to expand in a volume that is progressively decreasing as the piston approaches top centre. Meanwhile, the fuel particles are being forced into a state of separation and vaporisation and into more intimate contact with the air particles by the same heat of compression. And there is motion. Oh, boy! Is there motion! As the piston approaches top centre, this violently turbulent mass is just ripe for…
No. I won't say explosion BANG! But it is time to light the fire. At a point arbitrarily chosen to be 30 crankshaft degrees before top centre of the compression stroke, several thousand volts of jolt (electrical energy) are delivered to the centre spark plug electrode, causing a spark to jump the gap between the centre and ground electrodes in the spark plug. The spark intensity and duration ignites the air/fuel mixture closest to the plug electrodes, setting of a chain reaction that will envelope the entire combustion chamber area, except those few relatively tiny isolated inaccessible pockets that resist flame penetration. The flame front moves away from the point of ignition and expands more-or-less uniformly while the unburned portion of the mixture is forced into even higher degrees of turbulence and compression by the advancing flame front, all of which causes a relatively abrupt but uniform increase in cylinder temperature and pressure.
All of this is going on as the piston reaches the compression stroke TC. The piston has completed the last stroke of the four-act-stroke cycle comedy. This is accompanied by an additional 1/2 revolution (180 degrees) of the crankshaft, which brings the total number of crankshaft degrees of rotation to 720 for the complete cycle. Now any dumbhead knows there are just 360 degrees in exactly one revolution of anything that turns, so it should be obvious by now that it takes exactly two revolutions (720 degrees) of the crankshaft to complete the four piston strokes that form one cycle of the four-stroke cycle engine. Compression stroke complete. Cycle complete.
But is it? Earlier, it was stated that the cycles overlap each other, so we just can't walk off and leave it. Besides, hell hath no fury like a chamberful of ignited violent vapors and an immovable piston. From the arbitrary 30-degree before top centre ignition point until maximum cylinder pressure is reached at about 15 degrees after TC, the crankshaft has rotated about another 15 degrees after the theoretical end of cycle. But now we can see how the finishing phase of one cycle is the beginning phase of the following cycle, and also how on cycle overlaps another.