The invention relates to an engine and more particularly to an improved exhaust timing arrangement for an engine and particularly for engines of the 2-cycle type.
As is well known, the performance of an engine can be significantly affected by the exhaust system. The exhaust system can be tuned depending upon the number of cylinders served by a common exhaust pipe, the firing order of those cylinders, the timing of the opening of the exhaust ports of those cylinder served and the length of the exhaust system. This exhaust tuning is particularly advantageous in conjunction with 2-cycle engines.
Because of the scavenging process in a 2-cycle engine, the exhaust system can be more significant in determining the engine performance than with a 4-cycle engine. However, exhaust tuning is utilized with both 2- and 4-cycle engines.
In 2-cycle internal combustion engines, it is a common practice to attempt to utilize the exhaust pulses from one cylinder to effect the exhaust discharge and the scavenging of another cylinder. For example, it is frequently the practice to employ a technique called "exhaust supercharging." With this type of arrangement, the scavenging charge from one cylinder is permitted to pass at least in part through the exhaust port during the scavenging cycle. At the end of the scavenging cycle, an exhaust pulse is transmitted back to the exhaust port so as to drive part of the scavenged gases back into the cylinder and thus provide a charge in the cylinder which is higher than atmospheric pressure and which is substantially free of exhaust gases. Even if exhaust supercharging is not employed, the effective exhaust pulses can be utilized to ensure that scavenging is complete, but also that none of the fresh air charge is permitted to the combustion escape chamber and to also insure that the exhaust gasses do not reenter the cylinder from which they have been discharged.
In order to provide effective exhaust tuning, it is frequently necessary to employ the interaction between two cylinders. However, it is also normally the practice to ensure that the engine cylinders fire in equal intervals so that smooth power transmission is possible. However, with engines having a certain number of cylinders, this then makes the use of exhaust pulses from one cylinder to the other to control the scavenging difficult or impossible.
This condition may be best understood by reference to FIGS. 1 and 2 which are, respectively, a timing diagram for a conventional prior art 2-cylinder in-line engine firing on equal intervals spaced 180.degree. from each other and the pressure pulse curve in the exhaust manifold of the engine. The engine cylinders are indicated at 1 and 2 and their ignition points are indicated on the timing curve in FIG. 1. As will be seen, each spark plug of the respective cylinders 1 and 2 fires at approximately 5.degree. to 10.degree. before top dead center under the engine running condition.
It will also be seen from this figure that there is no overlap in the opening of the exhaust ports of the respective cylinders and, obviously, there is no overlapping of the opening of the exhaust port of one cylinder with the closing of the scavenge port of the other cylinder. As may be seen in FIG. 2, this gives rise to a situation where the exhaust pressure in the exhaust port area of the exhaust manifold is relatively high at the time when the scavenge port is opened. In addition the exhaust port pressure may become less than atmospheric to cause the exhaust gasses to reenter the cylinder.
This has two adverse effects. First, it dilutes the charge and raises the cylinder temperature. Second, it restricts the amount of fresh charge which can enter the cylinder. Thus the power developed is reduced. The exhaust pulses form one cylinder have substantially no effect on the exhaust pulses from another cylinder or the pressure condition at the exhaust port of the other cylinder.
In spite of this situation, it is possible with engines having equal firing intervals and no overlapping of their exhaust port openings to achieve exhaust tuning. In fact, this can also be achieved with a single cylinder by appropriately selecting the length of the exhaust pipe. This is done since it is known that an exhaust pulse is generated in the exhaust pipe which is reflected back from the end of the exhaust pipe to the exhaust port. Thus, by appropriately selecting the length of the exhaust pipe, it is possible to obtain this pulse back effect from a single cylinder or from cylinder-to-cylinder where the cylinders have no actual overlap in their timing.
However, the application of this principle to certain engine applications make such exhaust tuning difficult or impossible. For example, in outboard motor practice, a common environment in which 2-cycle engine are employed, the length of the exhaust pipe cannot be extended beyond a relatively short length. Thus, when the engines operates at relatively low maximum rotational speeds, i.e., 5,000 rpm or lower, it is practically impossible to obtain this type of exhaust control over the scavenging operation.
It is, therefore, a principal object of this invention to provide an improved exhaust tuning arrangement for an engine having at least a pair of cylinders.
It is a further object of this invention to provide an improved engine and exhaust system wherein the engine is configured in such a way so as to achieve the effect of exhaust tuning in assisting the charging of at least one of the cylinders.
It is a further object of this invention to provide an improved outboard motor engine and exhaust system.