# Otto and Diesel Cycle: Heat Addition & Cutoff Ratios

• venkata
In summary, there are key differences between the Otto cycle and the diesel cycle, particularly in the way heat is added to the air-fuel mixture. In the Otto cycle, heat is added at constant volume while in the diesel cycle, it is added at constant pressure. This is because the diesel engine was initially developed with coal powder as fuel, which caused explosions due to the high pressure. To prevent this, the fuel was later changed to petroleum oil and injected at lower rates to maintain constant pressure. This has also led to discussions about the use of cutoff ratios in the diesel cycle. Further information can be found by searching online or reading articles on the topic.

#### venkata

In case of Otto cycle we consider that heat is immediately added to air fuel mixture(i.e constant volume heat addition) but in case of diesel cycle we consider heat addition at constant pressure and we also talk of cutoff ratios.why so??

venkata said:
In case of Otto cycle we consider that heat is immediately added to air fuel mixture(i.e constant volume heat addition) but in case of diesel cycle we consider heat addition at constant pressure and we also talk of cutoff ratios.why so??
There are pros and cons to each. have you done a google search for this question?

venkata said:
In case of Otto cycle we consider that heat is immediately added to air fuel mixture(i.e constant volume heat addition) but in case of diesel cycle we consider heat addition at constant pressure and we also talk of cutoff ratios.why so??
http://charming.awardspace.com/otto_diesel/otto_diesel.html
In this website you can find explanations about the diesel engine, how and why it was developed,etc..
I read in an article that Rudolf diesel initialy used coal powder as fuel(sprayed over hot compressed air)but the combustion increased the pressure higher than the permissible limits so that the cylinder exploded during a trial.Later petroleum oil was used and the fuel was injected at lower rates so that combustion occurs approximately at constant pressure rather than constant volume.

## 1. What is the Otto Cycle?

The Otto Cycle is a theoretical thermodynamic cycle that describes the operation of a spark-ignition engine, such as a gasoline engine. It consists of four processes: intake, compression, power, and exhaust.

## 2. How does the Otto Cycle differ from the Diesel Cycle?

The main difference between the Otto and Diesel Cycles is the type of fuel used. The Otto Cycle uses a spark plug to ignite a fuel-air mixture, while the Diesel Cycle uses compression to ignite a fuel-air mixture without a spark plug. This results in different heat addition and cutoff ratios for the two cycles.

## 3. What is heat addition in relation to the Otto and Diesel Cycles?

Heat addition refers to the process of adding heat to the fuel-air mixture in the power stroke of an engine. In the Otto Cycle, this is done by igniting the fuel-air mixture with a spark plug. In the Diesel Cycle, heat addition occurs as a result of compression, which increases the temperature of the fuel-air mixture until it self-ignites.

## 4. What are cutoff ratios and why are they important?

Cutoff ratios are the ratios of the volumes of the air-fuel mixture before and after compression in an engine. In the Otto Cycle, a higher cutoff ratio results in a more efficient engine, as it allows for more air and fuel to be compressed and burned. In the Diesel Cycle, a lower cutoff ratio is preferred, as it allows for more of the fuel to be compressed and achieve self-ignition.

## 5. How do heat addition and cutoff ratios affect the performance of an engine?

The heat addition and cutoff ratios are directly related to the thermal efficiency of an engine, which is a measure of how much of the energy in the fuel is converted into useful work. In general, higher heat addition and cutoff ratios result in a more efficient engine and increased power output. However, these ratios must be carefully optimized to balance efficiency and power for the specific type of engine and fuel being used.