Air-fuel ratio requirements

Air-fuel ratio requirements

The task of the engine induction and fuel systems is to prepare from ambient air and fuel in the tank an air-fuel mixture that satisfies the requirements of the engine over its entire operating regime. In principle, the optimum air/fuel ratio for a spark-ignition engine is that which gives the required power output with thelowest fuel consumption, consistent with smooth and reliable operation. In practice, the constraints of emissions control may dictate a different air/fuel ratio, and may also require the recycling of a fraction of the exhaust gases (EGR) into the intake system. The relative proportions of fuel and air that provide the lowest fuel consumption, smooth reliable operation, and satisfy the emissions requirements, at the required power level, depend on engine speed and load. Mixture requirements and preparation are usually discussed in terms of the air/fuel ratio 0' fuel/air ratio and percent EGR. While the fuelmetering system is designed to provide the appropriate fuel flow for the actual air flow at each speed and load, the relative proportions of fuel and air can be stated more generally in terms of the fuel/air equivalence ratio, which is the actual fuel/air ratio normalized by dividing by the stoichiometric fuel/air ratio. The combustion characteristics of fuel-air mixtures and the properties of combustion products, which govern engine performance, efficiency, and emissions, correlate best for a wide range of fuels relative to the stoichiometric mixture proportions. Where appropriate, therefore, the equivalence ratio will be used as the defining parameter. A typical value for the stoichiometric air/fuel ratio of gasoline is 14.6.t Thus, for gasoline,

A brief summary is sufficient here. Mixture requirements are different for full-load (wide open throttle) and for part-load operation. At the former operating condition, complete utilization of the inducted air to obtain maximum power for a given displaced volume is the critical issue. Where less than the maximum power at a given speed is required, efficient utilization of the fuel is the critical issue. At wide-open throttle, maximum power for a given volumetric efficiency is obtained with rich-of-stoichiometric mixtures, 4 X1.1. Mixtures that are richer still are sometimes used to increase volumetric efficiency by increasing the amount of charge cooling that accompanies fuel vaporization, thereby increasing the inducted airdensity.At part-load (or part-throttle) operating conditions, it is advantageous todilute the fuel-air mixture, either with excess air or with recycled exhaust gas.This dilution improves the fuel conversion efficiency for three reasons:' (1) theexpansion stroke work for a given expansion ratio is increased as a result of thechange in thermodynamic properties of the burned gases-see Sees. 5.5.3 and5.7.4; (2) for a given mean effective pressure, the intake pressure increases withincreasing dilution, so pumping work decreases; (3) the heat lossesto the walls are reduced because the burned gas temperatures are lower. In theabsence of strict engine NO, emission requirements, excess air is the obviousdiluent, and at part throttle engines have traditionally operated lean. When tightcontrol of NO,, HC, and CO emissions is required, operation of the engine witha stoichiometric mixture is advantageous so that a three-way catalyst can beused to clean up the exhaust. The appropriate diluent is then recycled exhaustgases which significantly reduces NO, emissions from the engine itself. Theamount of diluent that the engine will tolerate at any given speed and loaddepends on the details of the engine's combustion process. Increasing excess air or the amount of recycled exhaust slows down the combustion process and increases its variability from cycle to cycle. A certain minimum combustionrepeatability or stability level is required to maintain smooth engine operation.

Deterioration in combustion stability therefore limits the amount of dilution ancan tolerate. As load decreases, less dilution of thefresh mixture can betolerated because the internal dilution of the mixture with residual gas increases. At idle conditions, the fresh mixture will not usually tolerate anyEGR and may need to be stoichiometric or fuel-rich to obtain adequate combustion stability.

If stoichiometric operation and EGR are not required for emissions control, as load increases the mixture is leaned out from a fuel-rich or close-to-stoichiometric composition at very light load. As wide-open throttle operation is approached a teach engine speed, the mixture is steadily enriched to rich-of-stoichiometric the maximum point. With the stoichiometric operating conditions required for three-way-catalyst-equipped engines, when EGR is used, the percentage of recycledexhaust increases from zero at light load to a maximum at mid-load, and then decreases to zero as wide-open throttle conditions are approached so maximum break can be obtained. Combinations of these strategies are possible.

For example, lean operation at light load can be used for best efficiency, and Stoichiometric mixtures (with a three-way catalyst) and/or EGR can be used atmid loads to control NO, emissions.

Typical mixture requirements for two common operating strategies: Top diagram shows equinlence ratio variation with intake mass flow rate (percent of maximum flow at rated speed) at constant low and high engine speeds. Bottom diagram shows recycled exhaust (EGR) schedule as a function of intake flow rate, for low, mid, and high speeds for stoichiometric operation.

In practical spark-ignition engine induction systems, the fuel and air distribution between engine cylinders is not uniform (and also varies in each individual cylinder on a cycle-by-cycle basis). A spread of one or more air/fuel ratio between the leanest and richest cylinders over the engine's load and speed range is not uncommon in engines with conventional carburettors. The average mixturecomposition must be chosen to avoid excessive combustion variability in theleanest operating cylinder. Thus, as the spread in mixture nonuniformityincreases.The mean equivalence ratio must be moved toward stoichiometric and away from the equivalence ratio which gives minimum fuel consumption.