07 Fuel metering for CI engine

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Fuel metering for Diesel engines Diesel engines use fuel of low volatility, which have to be injected into the combustion chamber. Sufficiently large injection pressures are required to obtain a spray of suitable characteristics (droplet size, spray angle, tip penetration) to be mixed with the air charge and burned in the time available for the combustion process. Mixture requirements In Diesel engines a compression ignition of a heterogeneous charge occurs, because the fuel is atomized in small liquid droplets in the air trapped inside the cylinder and most of the fuel is still in liquid state when combustion begins. The physical states of reactants are different, therefore they cannot be premixed before the combustion start. Most of the fuel burns mixed with the air in a heterogeneous form (from the point of view of reactant physical state). This aspect present some drawbacks (low combustion velocity, pollutant formation,…), but also the advantage to control the load by varying the amount of injected fuel per cycle. Diesel combustion is quite independent on the overall air/fuel ratio, since it mainly relays on local conditions. So the engine can operate (at low loads) with a very lean mixture, because locally (near the single droplets) a mixture of correct (about stoichiometric) composition, suitable for burning, is formed. At partial loads, only the part of trapped air surrounding the fuel-jet is involved in the combustion process. Therefore in a Diesel engine the load can be controlled by varying the mass in injected fuel, changing the global mixture quality. The bmep and the engine power increase by raising the injected fuel mass per cycle. • Diesel engines cannot work for equivalence ratio values greater than 0.8 because: 1) When power and bmep grow, also…