IGNITION TIMING
IGNITION TIMING
For efficient engine operation, the spark in each cylinder must occur at a definite time, with relation to the position of the pistons in the cylinder. This is carried out by timing the ignition.
There are many factors which effect the ignition timing of an engine. Some of them are discussed below.
(i) Engine speed and load :
At low engine speeds and load, the combustion of fuel in the engine cylinder is slower and for complete combustion more ignition advance is required. But at low engine speeds and full load, the combustion is comparatively faster and less ignition advance is needed. And at high engine speed and full load, the combustion is slower and more ignition advance is required.
(ii) Compression Ratio :
If the compression ratio is low, combustion will be slower (due to lesser temperature and pressure obtained during compression stroke ), more ignition advance is required. And if the compression ratio is high, faster combustion will take place due to higher temperature and pressure obtained during the compression stroke which helps in complete combustion of the high pressure gases, less ignition advance is needed.
(iii) Engine Temperature :
The combustion in the engine cylinders depends upon the engine temperature. If the engine is hot, the combustion will take place at faster rate than when the engine is cold. For faster combustion rate less ignition advance is required and vice versa.
(iv) Quality of fuel used :
Depending upon the octane value of fuel used, for low octane value of fuel, the combustion rate is faster, thus less ignition advances is needed whereas for high octane value of fuel, combustion rate is slower, thus needing more ignition advance.
(v) Mixture ratio:
In the case of lean mixture, (air/fuel) he combustion rate is faster and the less ignition advanced is required. And in the case of rich mixture the combustion rate will be slower thus requiring more ignition advance.
(vi) Cylinder bore :
Also depending upon the cylinder diameter. The larger the cylinder bore (diameter) slower the combustion rate and thus more ignition advance will rate and thus less the ignition advance will be required.
Apart from all the above factors the ignition-timing of an engine is also affected by the following:
(a) Combustion rate of fuel
(b) Volatility of fuel
(c) Method of distribution of fuel to the cylinders.
(d) Excessive carbon deposits in the combustion chamber.
(e) Design and shape of the combustion chamber
(f) Location of the spark-plugs in the combustion chamber
(g) Throttle opening.
The high tension current produced in the ignition coil, passes into the centre of the distributor cap and is carried to the centre of the rotor by a carbon brush. The distributor contains brass segments. As the rotor turns, the current jumps from the short gap and passes from the segments to the spark plug. The spark plug produces the spark and ignites the combustible mixture in the cylinder.
It should be ascertained that the spark plug fires in a cylinder when its piston is near the top of its stroke during the compression stroke. the distributor shaft is then meshed with the cam shaft. During compression stroke when the piston is reaching T.D.C. position, the gases are being compressed in the clearance space. If the ignition had been instantaneous then maximum pressure would have developed at this stage. But in actual practice the mixture is ignited towards the end of compression stroke before the piston reaches T.D.C. position.
When the engine is running slow, the piston is moving slowly and the mixture may be ignited towards the end of the compression stroke (T.D.C.) position. There will be sufficient time for complete combustion by the time the stroke is completed. When the engine run at high speed, just as modern automobile engines, the ignition must occur much earlier in the strokes since there will be less time available for complete combustion of the fuel. If the ignition is not advanced in high speed engines, the fuel mixture will continue to burn when the piston has completed the compression stroke and has started moving downward. Under such condition there will be loss of power and maximum pressure will not be developed in the engine cylinder.
When ignition occurs early in the compression stroke, the ignition is said to be advanced. A retarded ignition takes place when the piston is just near the compression stroke.
If the ignition is advanced too much, it will be complete before the end if the compression stroke. Under these conditions, the crank shaft the connecting rod will have to push the piston upward compressing the gases. In such a case the force might not be sufficient to overcome the pressure and the engine would stop or stall.
If the ignition is retarted too much, the combustion of fuel will continue, during the power stroke of the piston (working stroke) and the maximum pressure will not be developed and less work will be obtained from heat energy. Under these conditions comparatively more hot exhaust gases will be going out of the engine cylinder over heating the exhaust valve.
In modern automobiles the ignition is advanced automatically when the engine runs at high speed. The mechanisms used for this advance can be any one of the following.
1. Centrifugal advance mechanism.
2. Vacuum advance mechanism.
3. Combination of centrifugal and vacuum advance mechanism
FIRING ORDER
The order or sequence in which the firing place in different cylinders of a multi cylinder engine is called the firing order. In spark ignition engine, the high tension leads from the distributor are connected to the spark plugs at the different cylinders according to the firing order. A proper firing order reduces engine vibrations, maintains engine balancing and secures and even flow of power. The firing order differs from engine to engine. Probable firing order, for different engines are as follows:
3- cylinder engine. 1-3-2
4- cylinder in line engine-1-3-4-2
4- cylinder horizontal
Cylinder no. 1 is taken from front of the in line engines whereas in v type engines, it is taken from front on right side bank.
References
- Automobile Engineering (Vol. 1 & 2) - K.M.Guptha
- Automotive Mechanics - Joseph Heitner
- Automobile Engineering - Harbans Singh Reyd
- Automotive Mechanics - William H. Course
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