FUEL COMBUSTION

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Efficient combustion

Is achieved when the fuel has been thoroughly burnt in the time available, achieved by:-

1.    Atomisation of the fuel. High pressure injection through small nozzle holes converts the liquid fuel into a vapour (which has a high surface area to volume ratio) due to high velocity and friction drag with the dense air. This reduces the ignition delay, and the time required to complete combustion. Atomisation is affected by the following factors:-

                                             

where          p =          pressure difference between fuel line and cylinder

                    m =          mass flow rate of fuel

                     m =          fuel viscosity

                     A            =          Nozzle hole area

2.    Distribution/penetration of the fuel. To avoid areas with too low air/fuel ratios, the fuel injector must distribute the fuel sprays evenly without overlapping, and penetrate far enough into the air (60%). Note that the air will centrifuge towards the liner wall under high swirl conditions. Good, even distribution will reduce the time required to complete combustion. Penetration is affected by the following factors:-

                          

where             p          =          Fuel pressure

                        d          =          Nozzle hole diameter

                        t           =          Time of injection BTDC

                        r          =          density of air in the cylinder

We can see that the diameter of the nozzle hole has a large influence on penetration. Note that the density of air will increase as the piston approaches TDC, thus the fuel initially injected at 15^oBTDC will travel further, as there is less air resistance.

Increasing the length of the nozzle holes will also increase penetration, as the jet is more stabilised, but L/d is usually designed at around 3:1, thus included as a constant in penetration calculations.

Increases in fuel viscosity will increase droplet size, but only slightly increase penetration. The more compact fuel jet will increase penetration, but should not cause excessive over penetration.

3.    Sufficiently high air temperatures. In order that the fuel will ignite, the air temperature at the end of compression must be higher than the auto-ignition temperature. Low air temperature will increase ignition delay, and can lead to diesel knock.

4.    Air turbulence. To increase air/fuel mixing, especially after the initial ignition has occurred, some turbulence of the cylinder contents are required. This is provided by the burning fuel/air mixture, swirl from the intake air, and squish from the piston shape. This allows the areas of high fuel/air ratios to be diluted so that combustion can be completed. If low swirl conditions exist, soot or carbonaceous particles in the exhaust stream and the diffusion period of combustion will increase.

5.    Ample excess air. Diesel engines operate under conditions of high excess air, as the time available for a completed combustion cycle is relatively short (11ms for a 600rpm 4/S engine). Hence in order that the majority of fuel particles can find an oxygen molecule to burn with, there must be an oversupply of air, as combustion can rely on cylinder turbulence alone.

Poor combustion

The desirable properties above can deteriorate when each/all of the following occur:-

A.        Incorrect fuel distribution within the cylinder, due to:-

            a)         Incorrect fitting of injector nozzle during overhaul.

            b)         Part blockage of injector nozzle due to carbon trumpets.

            c)         Excess wear of injector holes, which will also increase spray penetration.

B.        Incorrect temperature of the fuel. If too low then,

a)            Fuel droplets become larger, thus much slower burning. The droplet can form a hard layer around it which reduces evaporation has hence burning.

b)            Fuel sprays become more compact, which reduces mixing, and hence increases combustion time and increases exhaust smoke levels.

C.        Incorrect fuel pressure due to fuel pump internal wear. This will reduce the maximum pressure delivered by the pump, as well as producing late injection, thus:-

            a)         Fuel droplet size will increase.

            b)         Penetration will reduce, as fuel supply now injected later, against higher gas pressures in the cylinder.

D.        Incorrect out flow of exhaust due to fouling of the turbocharger or exhaust gas boiler. This will reduce cylinder content purity.

E.        Incorrect charge air pressure. This will:-

a)            Reduce the pressure and hence temperature at the end of compression, increasing ignition delay.

b)            Reduce the quantity of excess air supplied which will increase smoke levels and combustion time.

c)            Reduce air swirl, as velocity through the scavenge ports will be reduced. Smoke levels will also increase.

Causes of poor combustion

Can be traced on all cylinders, requiring measurements of the following:-

1.         Scavenge air pressure and temperature. Turbo-charger, cooler etc performance will affect delivery. The correct air pressure for the engine load could be found from test bed or previous records. Scavenge air temperatures should be above 35^oC

2.         Fuel rail pressure and temperature (viscosity). Checks to be made of the fuel pressure, and viscosity using viscosity/temperature charts, or a calibrated viscometer.

Can be traced on individual cylinders, requiring measurements of the following:-

1.         Indicator cards should be taken to record injection timing, Pmax, Pcompr, and/or afterburning, and check the power balance between cylinders.

2.            Exhaust, liner and cooling water temperatures. Deviation from normal will indicate problems.

If poor combustion is evident whilst the engine is running it should be shut down and the cause confirmed through visual inspection i.e. scavenge port inspection, removal of suspected components for signs of wear, erosion, corrosion etc- injector, pump, rings, exhaust valve etc.

Hence anything which causes the atomisation number to decrease will affect the efficiency of burning. The factors of p, m and A are all monitored and controlled by the engineer on board. Hence he must ensure that close control of the fuel pump wear (for p), the fuel temperature (for m), and the injector nozzle wear (for A) is carried out.

As with the control of the atomisation, the engineer on board can control the size of the nozzle holes (by discarding worn holes and ensuring clean, non-abrasive fuel), and the density of the air (by ensuring that the air supply system is clean, and that the engine is not operated for long periods on loads below 50%)