We were discussing Rankine cycle and its various terminologies in our recent post, where we have discussed the basic concept of ideal cycle for steam power plants i.e. Rankine cycle and also we have seen there the basic operation and arrangements of various components of steam power cycle or Rankine cycle.

Today we will be focused here to understand the basic concept of Otto cycle.

### Otto cycle: The ideal cycle for internal combustion spark ignition reciprocating engines

Otto cycle is one type of air standard cycle which is designated as the ideal cycle for the operation of internal combustion spark ignition reciprocating engines. Before understanding the Otto cycle, we must be aware about the operations performed by a reciprocating spark ignition internal combustion engine or simply petrol engine.

Therefore first let us see an overview of internal combustion spark ignition engine. Basic components of internal combustion spark ignition reciprocating engines are displayed here with the help of following figure.

We can see here the arrangement of cylinder and piston with two valves i.e. inlet valve and exhaust valve. Piston will reciprocate within the cylinder between two fixed positions i.e. IDC and ODC

The inner most position will be termed as inner dead centre (IDC) and at this state piston will make lowest volume in the cylinder. IDC i.e. inner dead centre will also be termed as top dead centre (TDC) if engine is one vertical engine.
Second fixed position will be termed as ODC i.e. outer dead centre and at this state piston will make largest volume in the cylinder. Outer dead centre will also be termed as bottom dead centre (BDC) if engine is one vertical engine.

Piston will also be connected with crank-connecting rod mechanism and therefore reciprocating motion of piston will be converted in to rotary motion and vice versa. Piston will move within the cylinder and when piston will move from one dead centre to another dead centre such as IDC to ODC then that movement of piston will be termed as stroke.

Now we will start to understand the Otto cycle with step by step and simultaneously we will draw here the PV diagram also for the Otto cycle.

So let us start our cycle with IDC i.e. from inner dead centre. When piston will be at IDC, intake valve will be in open condition and exhaust valve will be closed. In this situation, pressure inside the cylinder above the piston will be approximately equivalent to atmospheric pressure. When piston will start to move towards ODC or BDC, gas above the piston will start to expand as piston will be moving downward towards ODC.

Therefore pressure will be reduced below the atmospheric pressure and hence suction pressure will be created and hence fresh mixture of air and fuel will be admitted in to the cylinder through the intake manifold.

Now let us consider this situation clearly, piston is moving downward and suction pressure created and therefore fresh mixture of air and fuel is coming in to the cylinder and therefore pressure will be equalise here due to admission of fresh mixture of air and fuel. In simple we can say that admission of mixture of air and fuel in to the cylinder will be done at approximately constant pressure. This process is displayed in PV diagram by process 0 to1.

When piston will reach to ODC, intake valve will be closed and hence we can say that in this situation both valve will be closed. Intake valve and exhaust valve both will be in closed position when piston will reach to ODC or BDC.

Now when piston will start to move towards IDC, mixture of air and fuel will be compressed within the cylinder due to movement of piston towards IDC. When piston will reach IDC, piston will made lowest volume in the cylinder and hence mixture of air and fuel will be compressed. Hence pressure and temperature of working fluid i.e. mixture of air and fuel will be high and we must note it here that cylinder will be insulated and hence there will not be any transaction of heat energy during this process of compression of mixture of air and fuel. This process is displayed in PV diagram by a process 1to 2 and this process will be isentropic process.

As we can see above figure, there will be one igniter and this igniter will start the burning of mixture of air and fuel. Hence pressure and temperature of the product of combustion will be increased due to the exothermic reaction. We must note it here that this combustion process of charge (mixture of air and fuel) will be so instantly that we can assume that this process will be constant volume process. This process is displayed in PV diagram by a process 2to 3 and this process will be constant volume process.

Now due to high pressure and temperature of the combustion product, piston will start to move towards ODC. We can say that piston will move from IDC to ODC and we must note it here that for this process also entropy will be constant because there will not be any heat energy transfer during this process 3-4.

As piston will move from IDC to ODC due to high pressure and temperature of the combustion product, crankshaft will start to rotate and will develop the useful work.

Now piston will be at ODC and in this case exhaust valve will be opened and therefore combustion product will leave the cylinder through the exhaust valve. This process will be so instantly that we can assume that this process will be constant volume process. This process is displayed in PV diagram by a process 4 to 1 and this process will be constant volume process.

So let us see the PV diagram, Heat energy will be added to the system during the process 2-3 and heat energy will be rejected during the process 4 to 1.
Work energy will be done over the system during the compression stroke or during the process 1 to 2 and work energy will be produced as useful work during the expansion process 3 to 4.

Now piston will start again to move towards IDC in order to remove the rest combustion product from the cylinder, piston will be moving towards IDC and compressing the combustion product but same time combustion product leaving the cylinder through the exhaust valve and hence we can say that pressure will be constant. In simple way, we can say that exhaust stroke will be done at constant pressure process and this process is displayed by the process 1 to 5.

Cycle 1-2-3-4-1 will be termed as Otto cycle. Petrol engine or internal combustion spark ignition reciprocating engines are working on the principle of Otto cycle. Piston executed here four complete strokes and crankshaft will rotate by two revolutions for each thermodynamic cycle.

Do you have any suggestions? Please write in comment box.
We will see another topic in our next post in the category of thermal engineering.

### Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som