Otto Cycle and Diesel Cycle (With Diagram) | IC Engine | Thermal Engineering

In this article we will discuss about the Otto Cycle and Diesel Cycle of IC engine.

The question arises of these engines, which are most efficient and what are the factors that impose limitation on the maximum efficiency.

Sadi Carnot was the first scientist to study this problem and he found that the maximum possible efficiency of an engine, working on Carnot cycle, is given by the expression-

Where, T₁ and T₂ = upper and lower temperature limits of the Carnot cycle.

This efficiency depends only on the limits of temperatures between which the engine works and it is independent of the working substance used. It can be proved that no engine can be more efficient than Carnot’s reversible engine. Although the Carnot’s cycle is impracticable both for steam engines and internal combustion engines, the importance of wide temperature difference is fully confirmed in actual working.

As regards the upper temperature limit, the internal combustion engine has a higher temperature than steam engines, while the lower temperature limit in steam engines is lower than that in internal combustion engines.

The upper temperature in internal combustion engines may be as high as 2200°C, while the lower temperature in internal combustion engines may be of the order 450°C. Fig. 13-16 shows P-V and T-ɸ diagram of carnot cycle.

As the Carnot cycle is impossible to attain in actual practice because of two main reasons:

(1) Head addition at constant temperature is impossible

(2) Heat rejection at constant temperature is impossible.

The following cycle have been suggested for reciprocating internal combustion engines:

(1) Otto cycle or constant volume cycle

(2) Diesel cycle or constant pressure cycle

(3) Dual cycle.

Of all the cycle mentioned above, the Otto cycle, the Diesel cycle and the Dual cycle are employed in present day reciprocating internal combustion engines. The Otto cycle is employed for petrol and gas engines, Diesel cycle is used for slow speed oil engines and the dual cycle is employed for high speed Diesel engines.

We shall consider now some of the points associated with the thermodynamic cycles employed in internal combustion engines.

(1) Otto Cycle:

The otto cycle or the constant volume cycle is employed for petrol and gas engines, in which combustible mixture is drawn in the engine cylinder during suction stroke or induction stroke. From the expression for efficiency of Otto cycle we observe that as the compression ratio is increased, the thermal efficiency increases.

However the return for the given theoretical increase of compression ratio diminishes as the ratio of compression increases. Thus from the efficiency expression we may conclude that in order to increase the efficiency of the engine, we may employ higher compression ratios. However, there are practical difficulties in employing higher compression ratios.

In case of petrol engines, the ratio of compression is limited by the ignition temperature of the mixture. If the ratio of compression were too great, the temperature due to compression may be higher before the end of compression than the ignition temperature of the mixture and pre-ignition would occur.

This must be avoided and therefore the compression ratio is limited in these types of engines. Thus, the practical limits are set by the onset of pre-ignition and/or Detonation so that most spark ignition engines have compression ratios of about 7:1. Fig. 13-17 shows P-V diagram of otto cycle.

(2) Diesel Cycle:

The octane number of a fuel is measure of its ability to resist spontaneous combustion. This resistance can be improved with additives-tetra-ethyl lead being the most common additive for petrol. Now-a-days due to air pollution caused by tetra-ethyl lead, its use is discouraged.

In the case of a compression ignition engine, since air alone is contained in the cylinder during the compression stroke, the problem of detonation does not arise. Consequently high compression ratios of upto about 23 to 1 can be used. Higher values, however, will cause pressures above practical possibilities. Fig. 13-18 shows the P-V and T-ɸ diagram of diesel cycle.

Fig. 13-19 shows the effect of compression ratio on work of the cycle. The work increases with increase in compression ratio.

Fig. 13-20 shows the pumping loop in otto cycle.

During the suction stroke the pressure in the engine cylinder will be slightly less than the atmospheric and during the exhaust stroke the pressure will be in actual engine the suction and exhaust process do not occur at constant pressure greater than the atmospheric pressure.

The loop is known as the pumping loop and the power expended on the gases in drawing the fresh mixture into the engine cylinder and in exhausting burnt gases is known as the pumping power, which can be obtained with the usual formula. In case of gas engines working on hit and miss method of governing the area of the pumping loop for the working cycle will be less than that for the idle cycle or missed cycle.

The net indicated power of the engine will be the gross indicated power minus the pumping power.

The efficiency of I.C. engines derived by assuming working substance as air is known as the air standard efficiency. This is the ideal efficiency of the engine.

The actual thermal efficiency will be about 60% of the air standard efficiency for the following reasons:

(i) Properties of the actual working fluid reduce the pressures and temperatures round the cycle due to variation of specific heats, dissociation and changes in the number of moles present during combustion.

(ii) Compression and expansion are not adiabatic or isotropic and heat being lost to the jacket water.

(iii) Combustion and heat rejection are not at constant volume since they require a definite time.

The actual work delivered by the engine will be less than the work developed during the power loop due to pump loop, friction and other mechanical losses. The brake thermal efficiency is, therefore, defined as the actual work delivered to the output shaft, the so called brake work divided by the energy released during combustion of the fuel used. The brake thermal efficiency for automobile engines averages around 25% and has increased very little in the last sixty years.

In the engines working on Diesel cycle or dual cycle, only air is taken in during the suction stroke, therefore, the compression ratio is made higher than that in engine working on Otto cycle because there is no danger of pre-ignition and so by increasing the compression ratio say 11 to 22, a higher efficiency is obtainable in Diesel cycle than is possible with Otto cycle. It should be noted that for the same compression ratio, the efficiency of the Otto cycle is greater than that of the Diesel engines.

(iv) Due to higher compression ratio, the temperature at the end of compression is sufficient to ignite the fuel oil which is injected into the cylinder at the end of compression stroke. Hence such engines are known as Compression.