dual fuel engine

The dual fuel engine works on diesel cycle. As the name suggests two fuels come into use.The gaseous fuel, which is the primary fuel (usually air/natural gas) is first compressed and then the pilot fuel(small quantity of liquid fuel) is injected for the rapid combustion. The total working of the engine is described under the working heading. Thus as two fuels are being used for the engine power this is called dual fuel engine.

The dual fuel engine is the combination of both Otto and diesel engine. Firstly going to the concept, the otto cycle is constant volume cycle and diesel cycle is constant pressure cycle. Now, these cycles are theoretically enough correct but in practice they become unreal.

For the chemical reaction to take place during the combustion process, some time interval is required. Hence the combustion process doesn’t occur at constant volume.

Also Read: What will happen If we use Petrol in Diesel Engine or Vise Versa?

Likewise in diesel cycle, due to uncontrolled combustion, the process doesn’t take place at constant pressure. Thus dual cycle which is also called as limited pressure cycle is a combined Otto and diesel cycle.


In the dual cycle, some amount of heat is added at constant volume and some at constant pressure. (Qs) as shown in the diagram is heat addition.

                          dual fuel engine

1-2: reversible adiabatic compression

2-3: constant volume heat addition

3-4: constant pressure heat addition

4-5: reversible adiabatic expansion

5-6: constant volume heat rejection

Working Of The Dual Fuel Engine-

The dual fuel engine works on compression ignition principle. At first the air-fuel is drawn into the engine cylinder which is highly compressed in the cylinder. Next step is the ignition process. But this fuel (also called gaseous fuel) is not a good compression ignition fuel. Hence a small amount of diesel (called as pilot fuel) is injected, which acts as a catalyser to the ignition process. As the fuel is ignited due to the temperature rise, the whole air-fuel mixture ignites and rapid combustion takes place.  Due to this rapid combustion the pressure inside the cylinder is increased causing the piston to move and produce engine power.

Also Read: What will happen If we use Petrol in Diesel Engine or Vise Versa?

The sudden increase in the dual fuel engine causes knocking problem, which can be controlled by injecting a definite quantity of diesel fuel.

One intresting fact about the dual fuel engine is that the combustion takes place as  a CI engine fashion, but the propagation of the flame front is just like the SI engine.The air standard efficiency for any engine is given by, the ratio of Heat added minus heat rejected to total heat added.

The mathematical formula for the efficiency is given as,

The efficiency of the dual fuel engine lies between the Otto and Diesel cycle.

When the rc (cut-off ratio) is equal to 1, it becomes an otto engine  and when the rp (pressure ratio) is equal to 1, it becomes a diesel engine.


  • Less fuel usage i.e., as the cost of diesel fuel is more converting an engine to work on air and a small amount of diesel is economical.
  • Spark plug and ignition system is not required.

Do leave comments. 🙂


  1. yes they are in use. Infact you can convert a diesel engine to dual fuel engine. Refer to this article for better idea files.aiscience.org/journal/article/pdf/70180029.pdf . 🙂

  2. -Spark plug and ignition system is not required.

    2 stroke marine diesel engines dont have spark plug/ignition system too so i think this is not an advantage..

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