COMBUSTION IN C.I. ENGINE

Review of Combustion In S.I. Engine In the S.I. Engine a Homogeneous Carburetted mixture of petrol vapour and air, in nearly chemically correct ratio, is compressed in the compression stroke through a small compression ratio (6to 10) and the mixture is ignited at one place before the end of compression stroke by means of an electric spark. After ignition a single definite flame front progresses through the air fuel mixture, and entire mixture being in the combustible range.

Combustion In C.I. Engine In C.I. Engine, air alone is compressed through a large compression ratio (12 to 22) during compression stroke, so that the temp. & press. of the air becomes very high ( well enough the ignition point of the fuel). In this highly compressed & highly heated air in the combustion chamber ,one or more jets of fuel are injected in the liquid state,(which is compressed through a high pressure of about 110 to 200 bar , by means of a fuel pump.)

Combustion In C.I. Engine Each minute droplet as it enters the hot air is quickly surrounded by an envelop of its own vapour.& this in turn, and after an appreciable interval, is inflamed at the surface of the envelop. To evaporate the liquid ,latent heat is abstracted from the surrounding air which reduces the temperature of a thin layer of air surrounding the droplet & sometime must elapse before this temp. can be raised again by abstracting heat from the main bulk of air in its vicinity (near atmosphere of air). As soon as, this vapour & air in contact with it ,reaches a certain temp. & local A-F ratio is within the combustible range ,ignition takes place.

Combustion In C.I. Engine Once the ignition has been taken place & a flame established, the heat required for further evaporation will be supplied from that released by combustion. The vapour would be burning as fast as it can find fresh O2. Thus we see that there is a delay period before the ignition takes place. The duration of delay period depends, among other factors ,temp. & pressure of the air & self ignition temp. of the fuel. Higher the air temp. or lower the self ignition temp. ,the shorter the delay. Higher press. also results in shorter ignition delay because of increase in the rate of heat transfer& more intimate contact between the hot air & cold fuel.

Combustion In C.I. Engine * In the C. I. engine the fuel is not ignited at once ,but it is spread over a definite period of time corresponding to 20-40 degree crank rotation. (This period in most cases greater than ignition delay.) The initial fuel droplets meet air, whose temp. is only little above their self ignition temp. & they ignite after ignition delay. The subsequent fuel droplets finds air already heated to much higher temp. by burning of initial droplets & therefore light up much more quickly ,almost as they injected from the injector/ nozzle. But their movement is stopped due to less quantity of air availability.

Combustion In C.I. Engine If the air within cylinder is motionless, only a small portion of the fuel would find sufficient O2 & even burning of this fuel would be slow or even choked ,as it surrounded by its own products of combustion. It is hence required to given an orderly & controlled movement of air & fuel so that a continuous supply of fresh air is brought to each burning of droplets & the products of combustion will be swept away. The effect of this air motion is called is “air swirl”. Turbulence which is required in S.I. engine implies disordered motion, with no general direction of flow, to breakup the surface of flame front & to distribute the flame throughout in externally prepared ,homogeneous combustible mixture.

Combustion In C.I. Engine Swirl : Which is required in C.I. engine implies an orderly movement of the whole body of air with a particular direction of flow, to bring a continuous supply of fresh air to burning of each droplet& swept away the products of combustion.

Stages of Combustion in C. I. Engine

Stages of Combustion in C. I. Engine Stage 1 Ignition Delay Period: Physical Delay: Beginning of fuel Injection Atomization & Vaporisation of fuel Mixing of fuel & Air Attainment of Self ignition temp. of fuel Chemical Delay: Attainment of the condition of chemical reaction Start of reaction Accelerated reaction Fuel Ignition

Stages of Combustion in C. I. Engine Stage 2 (Rapid or Uncontrolled Combustion): In this period the pressure rise is rapid because during the delay period ,the fuel droplet have had sufficient time to spread themselves over a wide area & they have fresh air all around them. About 1/3rd heat is evolved during this period. Stage 3 ( Controlled Combustion): At the end of second stage the temp.& press. are so high that the fuel droplets injected during the last stage burn almost as they enter & any further pressure rise can be controlled by purely mechanical means i.e. by the injection rate. About 70-80% heat is evolved during this period.

Stages of Combustion in C. I. Engine Stage 4 ( After Burning): Theoretically, it is expected that combustion process shall end after the third stage. However ,because of poor distribution of fuel particles, combustion continues during part of remainder of the expansion stroke. This after burning can be called the fourth stage of combustion.

Variables Affecting Delay Period Fuel: A higher the self ignition temp. of the fuel , higher will be the delay period. Injection pressure :To achieve the complete combustion in the very short time available, the liquid fuel should be injected in the droplets of smallest size to obtain largest surface-volume ratio. But at the same time the rate of combustion also depends upon which the products of combustion can be removed from the from the surface & replaced by the fresh air. A smaller droplets have lesser momentum & hence have lesser velocity resulting in partial suffocation by its own products of combustion.

Variables Affecting Delay Period Secondly ,as the pressure rise after ignition depends on the area of inflammation ,the smaller the size greater the number of droplets & therefore larger the area of uncontrolled pressure rise. The disadvantage of larger droplets is of course the subsequent rate of burning is too slow & hence a compromise has to struck. As the size of droplets depends upon the injection pressure, it can be said that lower the injection pressure lower the rate of pressure rise during uncontrolled combustion phase & smoother the running. 3. Compression Ratio: Increase in compression ratio reduce the ignition delay , as it raises both temp.& pressure.

Variables Affecting Delay Period 4.Intake Temp.: Increasing the intake temp. would result in increase in compressed air temp., which would reduce the delay period. 5.Jacket Water Temperature: increased in jacket Water temp. also increases compressed air temp. & hence reduce delay period. 6. Fuel temp.: Increase in the fuel temp. would reduce the delay period. 7.Intake pressure or supercharging: Increase in intake pressure or supercharging ,reduces the auto ignition temp.& hence reduces delay period. 8. Load on engine: Increase load on engine reduces delay period, since A-F ratio decreases with the increase in operating temp.

Knock in C.I. Engine If the delay period is long ,a large amount of fuel will be injected and accumulated in the chamber. The auto ignition of this large amount of fuel may cause high rate of pressure rise high maximum pressure which may causes knocking in diesel engine. The longer the delay period ,the more rapid and higher will be the pressure rise since more fuel will be present in the cylinder before the rate of burning comes under control. This cause rough running & may cause diesel knock. Therefore objective is to keep delay period as short as possible .but at the Same time some delay period would be necessary otherwise the droplets would not be dispersed in the air for complete combustion & this would cause the high smoke & high fuel consumption. Short ignition delay & low self ignition temp. of the fuel is desirable ,to eliminate the knock in case of C.I. engine.

Compression of Knocking Factors Tending to reduce Knocking in S.I. & C.I. Engine Sn. Factors S.I. Engine C. I. Engine 1 Self Ignition temp.of fuel High Low 2 Time lag or delay period for fuel Long short 3 Compression Ratio 4 Inlet Temperature 5 Inlet Pressure 6 C.C. wall temp. 7 Speed 8 Cylinder Size Small