TYPES OF ENGINE: The internal combustion engines for the automotive use may be classified according to the following considerations:
Engine cycle
Number of strokes
Fuel used
Types of ignition
Number and arrangement of cylinders
Valve arrangement
Types of cooling
An automotive I.C. engine may run on Otto cycle or diesel cycle. It may also be classified as a 2-stroke or a 4-stroke engine. The fuel on which the engine runs may be a basis of its class, example petrol engine, diesel engine, etc. THE charge in the engine cylinder after compression required to be ignited. Two types of ignition systems are in uses which classify the engines. These are the spark ignition and the compression ignition. Correspondingly, the engines are named as spark ignition (S.I.) engines or the compression ignition (C.I.) engines. The automotive engines may also be classified, based upon the number and arrangement of cylinders, which has been discussed in detail in the subsequent article. Based upon the arrangement of valves, the engines are classified as side valve type, otherwise valve type etc. These types have been explained in detail in the chapter on engine construction. The engines are broadly divided into two categories depends upon the method of cooling, air cooled engines and the liquid cooled engines. These have been discussed in detail later.
NUMBER AND ARRANGMENT OF ENGINE CYLINDERS
Based upon the number and arrangement of cylinders, the automobile engines may be classified as:
1. Single cylinder engines
2. Two cylinder engines
(a) In-line vertical type (b) V-type (c) opposed type
3. Three cylinder engines
4. Four cylinder engines
(a) In line vertical type (b) V-type (c) opposed type
5. Six and eight cylinder engines
6. Radial engines
Single cylinder engines:
A single cylinder engine has a power stroke every 7200 of crankshaft rotation for a four-stroke engine. These are used normally for scooters and motor cycles. It is seen that the maximum size of the single cylinder is restricted to about 500-600c.c. because of the higher unbalance forces, which become difficult to be balanced. Further the weight of the flywheel required becomes excessive for higher engine sizes.
Two cylinder engines:
This type of engine, apart from providing more power, gives more uniform torque and balancing possible as also better as compared to single cylinder engines. However, in practice two cylinder engines are rarely employed for automotive use.
(a) In line type cylinders placed side by side. This type has two single cylinders placed side by side vertically so that their pistons are in phase. Such an engine will have a power impulse every 360 degree of crankshaft rotation.
(b) In-line type, cylinders 1800 out of phase. In this the two single cylinders are in effect, placed side by side vertically, so that their cranks are 1800 out of phase. This type provides good balancing, but the disadvantage is in unequal firing intervals: the spark takes place at 00, 1800, 7200, 9000 and so on.
(c) V-type. In this two cylinders are placed with their centre lines at some angle to each other. Generally this angle is kept 600, but in some instances angles from 400 to 900 have also been used. Its main advantage is that it can be made more compact, same crankcase are used in a single cylinder engine may be used here also. Further short crankshaft also means more rigidity, due to which the engine runs more smoothly at higher speeds. Besides, overall length and higher of the engine are decreased. The weight/volume ratio is lower compared to an in-line engine. However, there are uneven intervals between power impulses (which take place every 2700 and 4500 of crankshaft rotation), cost of manufacture is high and balancing more difficult. For these reasons it is not used in automobiles.
(d) Horizontally opposed type. The two cylinders are arranged horizontally opposite each other, as shows. In this type power impulses occur ay even intervals of 3600 of crankshaft rotation. As is evident from the figure, the piston and the connecting rod movements are identical, both the piston approach TDC and BDC positions together. This causes the engine to be perfectly balanced, as regards the reciprocating forces. But because the two cylinders are not in line, the forces in the rods produce a rocking couple, which acts alternately in the clockwise and counter-clockwise directions, depending upon the movement of the pistons. Because of the in the clockwise and counter-clockwise directions, depending upon the movement of the pistons. Because of the near perfect balance possible in this type the higher possible speeds at which the engine can run smoothly are much higher. The engine centre of gravity is also low which contributes to vehicle stability. The obvious disadvantage of this type is that the length of the engine increases too much, which has to be placed in the transverse direction in the vehicle. This hinders proper streamlining of the car.
1. Three-cylinder engines:
In three-cylinder in-line engine the power impulse occurs every 2400 of crankshaft rotation. These are dynamically balanced, but there remain some unbalanced rocking forces. However, the three cylinders smooth out the cyclic torque adequately so that this has become a good competitor of the more popular four-cylinder in-line engine for small cars. Its other advantages are reduced weight, length and drag besides improved fuel consumption. Maruti 800 vehicles in India employ this engine.
2. Four cylinder engines:
In this type, the torque obtained, as compared to a single cylinder engine, is much more uniform because two working strokes per revolution are obtained. Further, the balancing is also better. Apart from this, the maintenance is also easier as compared to the engine with larger number of cylinders. The four cylinders are all in a line. The second and the third cylinders are in phase, while cylinders 1 and 4 are also in phase, but in the direction opposite to 2 and 3. Thus the reciprocating forces are also nearly balanced. Firing order is 1-3-4-2 OR 1-2-4-3. Regarding crankshaft torsinal wind-up and uneven breathing spacing between adjacent cylinders, both these firing orders have equal merits and demerits.
V-type. The angle between two cylinder banks is usually from 15 degrees. Thus the width of the cylinder block remains almost the same as in case of in-line type. This gives a more compact engine. The angle of V is kept small in this case, because with large angle the balancing of engines becomes more difficult. Actually a V-4 engine cannot be well balanced like in -line engines whether V-angle may be adopted. Therefore, to keep the vibration level within limits in case of V-4 engines, an extra counter-weighted crankshaft is used.
Horizontally opposed flat type:
This type is similar in construction to the 2-cylinder opposed type. Rather it has advantage over the 2-cylinder type that in this perfect balance is possible. Even the rocking couple is also balanced. Its dynamic balance is even somewhat better than the in-line four cylinder engine, although smoothness of torque is the same in the two cases. Thus it may be operated at much higher speeds. Due to this reason, this type provides very compact engine. Its low flat shape makes it particularly suitable for rear-mounted engines. The only disadvantage, as already mentioned, is its excessive length levels little space for servicing of cylinder head. The famous Volkswagen car has used this type of engine, which is air cooled. To provide the necessary draft of cooling air, a special centrifugal fan is provided.
Six and eight cylinder engines:
For higher H.P. and smoother torque, the six and eight cylinder engines have been used. Like the four -cylinder engines, they also have been designed in the form of in-line, V-type and opposed type configurations. Whereas out of the six-cylinder engine types, the in-line type is most widely used, in case of eight cylinder engines, it is the V-type which is almost universally in use. In six-cylinder in-line engine, there is a power impulse every 1200 of crankshaft rotation. Thus the crankshaft consists of six cranks arranged in pairs in three planes. For heavy diesels, there are seven main journal bearings to support the crankshaft, whereas petrol engines, there are usually have five or even four main bearings. Commonly employed firing order is 1-5-3-6-2-4, whereas equally suitable firing order also used in 1-4-2-6-3-5. This engine has smoothness or torque and excellent dynamic balance. For this reason its use is quite common for engine sizes larger than 2.5 litters, provided length of engine is not the main consideration.