- 21-Jul-2022

**Otto cycle has the highest work output and efficiency.** **Diesel cycle has the least efficiency.** **The dual cycle has the efficiency between the Otto and diesel cycle**. Same as the same compression ratio and hear addition.

**The Otto cycle has less compression ratio (7:1 to 10:1) but in diesel cycle the compression ratio is high (11:1 to 22:1)**. In Otto cycle as well as in diesel cycle heat rejection takes place at constant volume. The efficiency of Otto cycle is less as compared with the Diesel cycle.

The thermal efficiency of a Carnot cycle operating between these two reservoirs is **η = 1−T _{C}/T_{H}**. This value is significantly higher than that of the Otto cycle operating between the same reservoirs.

The Brayton cycle thermal efficiency **contains the ratio of the compressor exit temperature to atmospheric temperature**, so that the ratio is not based on the highest temperature in the cycle, as the Carnot efficiency is.

**Otto cycle has the highest work output and efficiency.** **Diesel cycle has the least efficiency.** **The dual cycle has the efficiency between the Otto and diesel cycle**. Same as the same compression ratio and hear addition.

**Otto cycle** has the lowest thermal efficiency for a fixed maximum and minimum temperature.

**The Carnot cycle**, due to its fully reversible nature, represents the highest cycle efficiency possible for the two temperature limits of T_{H} and T_{L}; where T_{H} represents both the heat source temperature and the temperature of the working fluid, and T_{L} is both the working fluid temperature and the temperature of the heat

Otto cycle is used for petrol or spark ignition engine while diesel cycle is used for diesel or compression ignition engine. The main difference between Otto cycle and Diesel cycle is that **in Otto cycle heat addition takes place at constant volume and in diesel cycle heat addition takes places at constant pressure**.

For different compression ratio:

The **diesel cycle** is more efficient than dual and otto cycle.

The Efficiency of Carnot's Cycle

The Carnot cycle achieves maximum efficiency because **all the heat is added to the working fluid at the maximum temperature**.

In modern nuclear power plants, which operate the Rankine cycle, the overall thermal efficiency is about **one-third (33%)**, so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power.

A typical diesel automotive engine operates at around **30% to 35%** of thermal efficiency.

The simple answer is ,**In the otto the energy available is fully used to raise the pressure in the cylinder**. Higher the pressure implies larger work done. But in diesel cycle, Part of the energy is added only during the expansion, So it does not contribute to the work. Hence Otto cycle is more efficient.

In practice, **Diesels are more efficient than Otto engines**, because the latter must operate at lower compression ratios to avoid knocking (Section 3.6).

Although for a given compression ratio the Otto cycle has higher efficiency, **because the Diesel engine can be operated to higher compression ratio**, the engine can actually have higher efficiency than an Otto cycle when both are operated at compression ratios that might be achieved in practice.

**A Carnot engine operating between two given temperatures** has the greatest possible efficiency of any heat engine operating between these two temperatures. Furthermore, all engines employing only reversible processes have this same maximum efficiency when operating between the same given temperatures.

The Carnot cycle

In this equation, the ratio V_{1}/V_{2} is known as the compression ratio, CR. When we rewrite the expression for thermal efficiency using the compression ratio, we conclude the air-standard Otto cycle thermal efficiency is a function of compression ratio and **κ = c _{p}/c_{v}**.

In general, the thermal efficiency, η_{th}, of any heat engine is defined as **the ratio of the work it does, W, to the heat input at the high temperature, Q _{H}**. In the middle of the twentieth century, a typical steam locomotive had a thermal efficiency of about 6%.

+

## Your comment