tailieunhanh - Internal Combustion Engines Fundamentals Episode 1 part 9

Tham khảo tài liệu 'internal combustion engines fundamentals episode 1 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 292 INTERNAL COMBUSTION ENGINE FUNDAMENTALS 3. Because of the low manifold temperature and vacuum fuel evaporation in the carburetor manifold and inlet port is much reduced. Thus during cranking the mixture which reaches the engine cylinder would be too lean to ignite. Until normal manifold conditions are established fuel distribu- H tion is also impaired. To overcome these deficiencies and ensure prompt starts fl and smooth operation during engine warm-up the carburetor must supply a fl fuel-rich mixture. This is obtained with a choke. Once normal manifold condi- fl tions are established the choke must be excluded. The primary element of a fl typical choke system is a plate upstream of the carburetor which can close off fl the intake system. At engine start-up the choke plate is closed to restrict the air fl flow into the carburetor barrel. This causes almost full manifold vacuum within fl the venturi which draws a large fuel flow through the main orifice. When the fl engine starts the choke is partly opened to admit the necessary air flow and fl reduce the vacuum in the venturi to avoid flooding the intake with fuel. As the fl engine warms up the choke is opened either manually or automatically with a fl thermostatic control. For normal engine operation the choke plate is fully open fl and does not influence carburetor performance. A manifold vacuum control is fl often used to close the choke plate partially if the engine is accelerated during fl warm-up. During engine warm-up the idle speed is increased to prevent engine fl stalling. A fast idle cam is rotated into position by the automatic choke lever. fl ALTITUDE COMPENSATION. An inherent characteristic of the conventional fl float type carburetor is that it meters fuel flow in proportion to the air volumt flow rate. Air density changes with ambient pressure and temperature with s changes due to changes in pressure with altitude being most significant. For -ị-example at 1500 m above sea level mean atmospheric