In the two-cycle motor we find that both the exhaust and transfer ports are open together for certain periods, and it can readily be understood that since we are performing the four operations in two strokes of the piston, the time required for each operation is limited. It was also stated that the exhaust port opened slightly before the transfer port and that a fresh charge was admitted before the spent gases had entirely escaped. From this we can assume that all the spent gases do not escape and that there is a possibility of part of the fresh charge escaping through the exhaust port, and the fresh charge becoming deluged with the spent gases which do not entirely escape. In comparing the exhaust and intake periods of both types of motors, we find that these periods cover but approximately one-half the time in two-cycle motors than in the four-cycle type. The combination of these operations and the short periods naturally permit of less fuel economy in the two-cycle motor. That considerable fuel is wasted has been proven by exhaust gas analysis on numerous occasions.
In the four-cycle type we get a greater charge in the cylinders due to the longer period, as one complete stroke of the piston is required and this is maintained during the compression stroke as both valves remain closed. This is of great importance in high-speed motors, as even with the long periods intake becomes very short when the higher engine speeds are reached and much less gas enters the cylinders. Even in lower engine speeds it presents advantages. The cylinder also maintains its full volume until after combustion occurs and the exhaust gases are forced out of the cylinder by the return stroke of piston, during which time the intake valve remains closed and no dilution of gases occurs. Of course, some spent gases remain in the cylinders of a four-cycle motor; however, the quantity is much less and does not have much effect on the next fresh charge.
There is also considerable power loss in two-cycle motors, due to leaky pistons, crank case joints and cylinders and bearing journals. This is of considerable importance, as the gas is partly compressed in the crank case and can escape through the exhaust port, should the rings leak or through the various joints, should they not remain air tight.
In four-cycle motors power is lost through leaky pistons, and valves; however, it is not as great and is of little importance as the engine will continue to run and develop a small amount of power. This is not the case with the two-cycle, as in the first place the fresh charge is somewhat limited and, second, it can escape before it reaches the cylinder, sometimes causing complete stoppage of the engine.
Leaky valves can readily be reground at a small cost without dismantling the engine, while to eliminate leaks in the two-cycle means complete dismantling, which is an expensive item, as new bearings must be provided in case they permit gas to escape, it being impossible to adjust them with shims and retain an airtight case. Crank case leaks are also of serious nature, as the motor must be dismantled in order to replace the gaskets at the various joints. Replacing rings is probably of equal expense in both types.
The manufacturing cost is lower in the two-cycle type, while it is also somewhat more simple, due to the absence of the valves and their operating mechanism. As there are fewer parts and the limitations of their proportions also means lower weight per horse power for the two-cycle type, and it is also claimed that they develop somewhat more power and have a better mechanical balance. However, these features are obtained at considerable expense in maintenance and it would seem that the logical commercial car motor is one which provides the lowest maintenance cost in proportion to the work done, which refers to the fourcycle type.
The four-cycle motor also has its disadvantages, being more complicated, more expensive to build and perhaps its weakest point is the valves; however, they are readily accessible, and do not mean much expense in regrinding or replacements. Leaky inlet valves alone permit the diluting of the charge with spent gases and indirectly reduce the power of the motor. The valve operating mechanism of course is a source of complication and expense and would be eliminated if this could be done without too great a sacrifice in other directions.
It is also a well-known fact that carburetion is more difficult in the two-cycle motor, which may be due to the fact that the suction of the carburetor is more rapid and uneven in this type than in the four-cycle type.
Two-cycle motors also require considerably more lubricating oil and can not be lubricated by splash, owing to crank-case compression. Forced lubrication from an outside source must be used. While the two-cycle engine has been built for the past twenty or thirty years, it has not been deemed worthy of the serious attention of many engineers, and it has not yet reached anything near the standardization and perfection of the fourcycle engine.