Factors affecting knock

Factors affecting knock

Knocking is violet gas vibration and audible sound produced by extreme pressure differentials leading to the very rapid rise during the early part of uncontrolled second phase of combustion.

In C.I. engines the injection process takes place over a definite interval of time. Consequently, as the first few droplets injected are passing through the ignition lag period, additional droplets are being injected into the chamber. If the ignition delay is longer, the actual burning of the first few droplets is delayed and a greater quantity of fuel droplets gets accumulated in the chamber. When the actual burning

commences, the additional fuel can cause too rapid a rate of pressure rise, as shown on pressure crank angle diagram above, resulting in Jamming of forces against the piston (as if struck by a hammer) and rough engine operation. If the ignition delay is quite long, so much fuel can accumulate that the rate of pressure rise is almost instantaneous. Such, a situation produces extreme pressure differentials and violent gas vibration known as knocking (diesel knock), and is evidenced by audible knock. The phenomenon is similar to that in the SI engine. However, in SI Engine knocking occurs near the end of combustion whereas in CI engine, knocking thatoccurs near the beginning of combustion.

Delay period is directly related to Knocking in CI engine. An extensive delay periodcan be due to following factors:

·        A low compression ratio permitting only a marginal self -ignition temperature to be reached.

·        A low combustion pressure due to worn out piston, rings and bad valves

·        Low cetane number of fuel

·        Poorly atomized fuel spray preventing early combustion

·        Coarse droplet formation due to malfunctioning of injector parts like spring

·        Low intake temperature and pressure of air

FACTORS AFFECTING KNOCKING

Injection timing

At normal operating conditions min ignition delay (ID) occurs with start of injection at 10 to 15 OCA BTDC.

Cylinder temperature and pressure drops if injection is earlier or later (high at first but decrease as delay proceeds).

Injection quantity (load)

Reducing engine load changes AFR, cools down the engine, reduces wall temperatures, reduces residual gas temperatures and increases ID

Droplet size, injection velocity and rate

Ignition quality within practical limits does not have significant effect on ID including in injection produces only modest decreasing in ID Injector nozzle diameter effects droplet size but has no significant effect on ID

Intake air temperature and pressure

Reducing intake air T and p increases ID.

Strong dependence of ID on charge temperature below 1000 K – above this value effect of intake air conditions is not significant.

Engine speed

Increase in engine speed increases the air motion and turbulence, reduces ID time slightly (in ms), in terms of CA degrees ID increases almost linearly.A change in engine speed, changes

“temp~time” and “pressure~time” relationships.

Combustion chamber design

Spray impingement on the walls effect fuel evaporation and ID

Increase in compression ratio, increases pressure and temperature and reduces ID

Reducing stroke volume, inc surface area to volume ratio, increases engine cooling and increases ID

Swirl rate

Change evaporating rate and air-fuel mixing - under normal operating conditions the effect is small.

At start-up (low engine speed and temperature) more important, high rate of evaporation and mixing is obtained by swirl

Oxygen concentration

Residual gases reduce O₂ concentration and reducing oxygen concentration increases ID.

METHODS OF CONTROLING DIESEL KNOCK

We have discussed the factors which are responsible for the detonation in the previous sections. If these factors are controlled, then the detonation can be avoided.

·        Using a better fuel. Higher CN fuel has lower delay period and reduces knocking tendency.

·        Controlling the Rate of Fuel Supply. By injecting less fuel in the beginning andthen more fuel amount in the combustion chamber detonation can be controlled to a certain extent. Cam shape of suitable profile can be designed for this purpose.

·        Knock reducing fuel injector: This type of injector avoid the sudden increase in pressure inside the combustion chamber because of accumulated fuel. This can be done by arranging the injector so that only small amount of fuel is injected first. This can be achieved by using two or more injectors arranging in out of phase.

·        By using Ignition accelerators : C N number can be increased by addingchemical called dopes. The two chemical dopes are used are ethyl-nitrate andamyle –nitrate in concentration of 8.8 gm/Litre and 7.7 gm/Litre. But these twoincrease the NOx emissions.

·        Increasing Swirl : Knocking can be greatly reduced by increasing swirl ( orreducing turbulence). Swirl helps in knock free combustion.