The present invention relates to a combustion control apparatus and a combustion control method for an engine that switches the combustion mode between a stratified charge combustion mode and a homogeneous charge combustion mode, and more particularly, to a combustion control apparatus for an engine equipped with a fuel vapor treating mechanism for treating fuel vapor produced in a fuel feed system and a combustion control method.
A fuel vapor treating mechanism for an engine typically performs a procedure for temporarily adsorbing, in a canister, fuel vapor produced in a fuel tank and then purging the adsorbed fuel together with air to an intake system.
In an engine that performs stratified charge combustion, such as a cylinder injection type engine, a large amount of purged gas, if introduced into an intake system during the stratified charge combustion, will deteriorate the combustion state. In stratified charge combustion, a variation in the fuel concentration near the ignition plug significantly affects the combustion state. If the introduction of purged gas causes the fuel concentration to be excessively rich near the ignition plug, normal ignition fails.
To address this problem, conventionally, when it is determined that a large amount of fuel vapor has been produced so that a large amount of purged gas must be introduced into the intake system, stratified charge combustion is prohibited and the combustion mode is switched to homogeneous charge combustion, which is less affected by the introduction of the purged gas. This is exempified by the apparatus described in Japanese Unexamined Patent Publicfation (KOKAI) No. Hei 11-36921.
In general, switching the combustion mode to homogeneous charge combustion improves the capability of the fuel vapor treating mechanism. However, when the amount of fuel vapor is determined to be high, the combustion mode is always switched to homogeneous charge combustion, and stratified charge combustion is prohibited even if it is not necessary to do so. Specifically, although stratified charge combustion is greatly affected by the introduction of purged gas, a certain amount of purged gas in the intake system may be tolerable depending on the particular engine operating conditions. In addition, even if it is determined that a large amount of fuel vapor has been produced, the fuel vapor treatment capability may be sufficient. Thus, if the combustion mode is always switched to homogeneous charge combustion in such cases, the opportunities for performing stratified charge combustion are unnecessarily limited, which reduces the fuel efficiency.
It is an object of the present invention to provide a combustion control apparatus and a combustion control method for an engine that provides as many opportunities to perform stratified charge combustion as possible to improve the fuel efficiency.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a combustion control apparatus of an engine having a fuel vapor treating mechanism is provided. The treating mechanism has a canister, which adsorbs fuel vapor produced in a fuel feed system of the engine. The treating mechanism purges the fuel vapor adsorbed by the canister, along with air, to an intake system of the engine. The flow rate of the purged gas is adjusted in accordance with the running state of the engine. The engine operates in a combustion mode selected from a stratified charge combustion mode and a homogeneous charge combustion mode. The apparatus includes computing means, setting means and control means. The computing means computes a capability value based on the flow rate of the purged gas. The capability value represents the capability of the treating mechanism to treat fuel vapor. The setting means sets a decision value in accordance with the amount of fuel vapor produced in the fuel feed system. The decision value represents a required capability of the treating mechanism. When the capability value is less than the decision value, the control means prohibits stratified charge combustion, which causes the engine to perform homogeneous charge combustion.
The present invention may also be embodied as a method for controlling an engine that operates in a combustion mode selected from a stratified charge combustion mode and a homogeneous charge combustion mode. The method includes adsorbing fuel vapor produced in a fuel feed system of the engine by a canister of a fuel vapor treating mechanism, purging the fuel vapor adsorbed by the canister, along with air, to an intake system of the engine, adjusting the flow rate of the purged gas in accordance with the running state of the engine, computing a capability value based on the flow rate of the purged gas, the capability value representing the capability of the treating mechanism, setting a decision value in accordance with the amount of the fuel vapor produced in the fuel feed system, the decision value representing a required capability of the treating mechanism, and prohibiting stratified charge combustion and causing the engine to perform homogeneous charge combustion when the capability value is less than the decision value.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.