DIESEL CYCLE: VOLUMETRIC COMPRESSION RATIO, EXPANSION RATIO, CUT-OFF RATIO AND EFFICIENCY

We were discussing Otto cycle, an ideal cycle for internal combustion spark ignition reciprocating engines or simply petrol engines and also Diesel cycle, the ideal cycle for the operation of internal combustion compression ignition reciprocating engines in our previous posts. We have also discussed the derivation of efficiency of Otto cycle and efficiency of Diesel cycle.

Today we will see here few more very important terms i.e. volumetric compression ratio, Expansion Ratio, Cut-off ratio and simultaneously we will also see here the efficiency of Diesel cycle in terms of these ratios with the help of this post.

First we will have to see brief introduction and background to remind the concept of Diesel cycle. Diesel cycle is one type of air standard cycle which is designated as the ideal cycle for the operation of internal compression ignition reciprocating engines.

Let us see an overview of Diesel cycle with the help of PV diagram and TS diagram as displayed here in following figure. As we can see in below figure, there will be two isentropic or adiabatic processes, one constant pressure and one constant volume process.

Let us understand now the term volumetric compression ratio, Expansion Ratio and Cut-off ratio

Volumetric compression ratio will be defined as the maximum volume to minimum volume and it will be indicated by r_v.

r_v = V₁/V₂

Now we must have to understand here that why we are focusing here to understand the volumetric compression ratio and why we are giving stress to determine the result or formula to calculate the efficiency of the Diesel cycle in term of volumetric compression ratio.

As we know very well that Diesel cycle is one theoretical cycle or ideal cycle for internal compression ignition reciprocating engines or simply Diesel engines. Space is one very important design criteria for any automobile and automobile designer. Designer will have to surely consider this design criteria i.e. space restriction.

We will have to design our engine with considering the space permitted for respective automobile as we may not design one engine which accommodate large area as it will not be in favour of the user of automobile and therefore there will be one maximum volume and one minimum volume and we will have to design our engine with considering the maximum and minimum volume.

Now we will write here the expression of efficiency of the Diesel cycle in terms of above mentioned ratios. As we have discussed in our recent post, efficiency of diesel cycle will be determined with the help of following formula.