Mechanical Basics

# Difference Between Otto Cycle And Diesel Cycle

Mar 20, 2021

## Introduction:

Otto Cycle and Diesel Cycle both are air standard internal combustion engines that are used to convert chemical energy into mechanical energy. There are lots of difference between Otto cycle and Diesel Cycle in construction, working, and applications too. Each cycle is unique in its construction and they are similar in some aspects like

• The heat rejects at a constant volume process.
• The expansion and compression processes take place at the isentropic process (which means the entropy is unchanged throughout the process)

We already covered the topics Otto cycle and Diesel Cycle.

What Can You Learn From This Post?

• Aspects that make Difference Between Otto Cycle and Diesel Cycle
• Brief Explanation of aspects that make Difference Between Otto Cycle and Diesel Cycle
• Summary
• Conclusion
• Frequently Asked Questions in Difference Between Otto Cycle and Diesel Cycle

Let us start reading the difference between Otto cycle and Diesel Cycle precisely point-wise.

## Aspects that make Difference Between Otto Cycle and Diesel Cycle

The main aspects that makes difference between the Otto cycle and Diesel Cycle and make them unique in their invention are

• Working Fuel
• Intake Process
• Firing Process or Combustion Process
• Compression Ratio
• Thermal Efficiency

So, these are the things or aspects that actually show the difference between Otto cycle and Diesel Cycle. Let us read briefly about these four things.

## Brief Explanation of aspects that make Difference Between Otto Cycle and Diesel Cycle:

#### Working Fuel:

The working fuel is the first and foremost topic that we need to speak about in the difference between Otto cycle and Diesel Cycle. The working fuel that used to run the Otto cycle is petrol or gas whereas; it can be called Petrol Cycle or Petrol Engine also and coming to the Diesel Cycle, as the name indicates the Diesel Cycle is run by the fuel Diesel.

#### Intake Process:

As we know that the intake process is the suction process where the fuel is sucked into the cylinder. So the difference between Otto cycle and Diesel Cycle in the intake process is that the air-fuel mixture (air-petrol or air-gas) is sucked in the Otto cycle and only fuel which means diesel is sucked into the cylinder itself.

#### Firing or Combustion Process:

The firing or combustion process is the process of introducing the fire and combusting the fuel. In the Otto Cycle, the petrol or gas is the working fuel; in the suction process the air-fuel mixture is sucked into the cylinder, and spark igniter will release the sparks that result in the combustion process but when it comes to the Diesel Cycle, only air is sucked into the cylinder in suction process and at the end of compression process the fuel injector will release the diesel in terms of small droplets whereas; the cylinder is in a stage that can generate the fire when diesel introduces. Due to this, combustion happens.

Coming to the heat addition process, in Otto Cycle, the heat is added to the system at constant volume process (which means the volume will be remained unchanged) and in Diesel Cycle, the heat is added to the system at constant volume process (which means the pressure will have remained unchanged).

#### Compression Ratio:

Compression Ratio is the ratio of the maximum volume of the cylinder when the piston at the bottom dead center to the minimum volume of the cylinder when the piston is at the top dead center. So, the compression ratio of the Otto or Petrol Engine is around 8:1 and the compression ratio of the Diesel Engine is between 15:1 to 20:1. So by this, we can say that the Diesel Cycle does have a high compression ratio.

#### Thermal Efficiency:

In both cycles (Otto Cycle and Diesel Cycle) thermal efficiency is defined as the ratio of work done to the heat addition. Due to the constant volume heat addition in Otto and constant pressure heat addition in Diesel Cycle, the efficiency will differ.

## Conclusion:

The Otto Cycle and Diesel Cycles are differed in their process, working, and construction. We cannot use diesel in petrol engine and petrol in diesel engines for its working. If we talk about the efficiency, the diesel engine does consist of higher efficiency than the petrol engine. In the two cycles, the compression and expansion processes take place at the isentropic process (which means the entropy remains constant).

The fuel that can be used in Otto Cycle is Petrol or Gas.

The compression ratio of Otto Cycle is taken as Air-Fuel combination and it is around 8:1 ratio which we can say that the 8 parts of Air and 1 part of fuel (petrol or gas) is taken.

The compression ratio of Diesel Cycle is taken as Air-Fuel combination and it is around 15:1 to 20:1 ratio which we can say that the 15 parts of Air and 1 part of fuel (diesel) is taken.

The Diesel Cycle is more efficient than the Otto Cycle.

The heat addition is the process that gives the power to the whole system whereas; the heat to the system is added at constant volume process therefore, the other name of Otto Cycle is Constant Volume Process Cycle.

The heat addition is the process that gives the power to the whole system whereas; the heat to the system is added at constant pressure process therefore, the other name of Diesel Cycle is Constant Pressure Process Cycle.

The four processes in Otto Cycle are

• Isentropic Compression Process
• Constant Volume Heat Addition Process
• Isentropic Expansion Process
• Constant Volume Heat Rejection Process.

The four processes in Diesel Cycle are

• Isentropic Compression Process
• Constant Pressure Heat Addition Process
• Isentropic Expansion Process
• Constant Volume Heat Rejection Process.

The combustion in the Otto Cycle is taken at constant volume whereas; the spark igniter will release the sparks and due to this, the air-fuel mixture will be burned and combustion takes place.

The combustion in the Diesel Cycle is taken at constant pressure whereas; the fuel injector will release the fuel droplets and due to this, the temperature in the cylinder will auto generate the fire and combust the fuel into gas.

The aspects that differentiate Otto Cycle and Diesel Cycle are

• Working Fluid
• Combustion Process
• Intake Process
• Compression Ratio