I know that in SI engines the air-fuel ratio is controlled by the carburetor (using the throttle valves). But in case of direct injection system how the air-fuel ratio is controlled by the petrol engine? How the air-fuel ratio is controlled in CI engine?
Parag
In direct injection gasoline engines there is no throttle valve. It is just like CI engines. In these types of engines A/F ratio is not required to be constant where fuel supply is determined by the load and speed demand. Inside the combustion chamber the A/F ratio is widely variable.
Control system has been developed for having better control the air fuel ratio. There will be many sensors connected to a central processor which get the feed back from many sensors dealing with various output parameters of the engine. depending the on the engine requirements the sensors actuate the devices like fuel pump, throttle, inlet temp, pressure and etc. to meet the best operating conditions. Using the same one can have better control over the air fuel ratio in CI and SI engines.
A/F ratio for conventional diesel.engines is maintained by adjusting the fuel delivery supplied to the engine based on the.measured air flow or if. You plan yo alter your engine components you improve the ratio by adding turbo chargers or increasing the valve lift of your cam shaft but the emission ( Nox ) to be.maintained in appropriate level if exceeds a limit you can also use EGR
In SI engines the air/fuel ratio vary a little bit round the stoichiometric value of about 14.5:1. In cases when the vehicle is started from cold or accelerating or required to be loaded a rich mixture is required. When the engine is cruising at a constant speed a lean mixture is required.
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