Cutoff Ratio and Compression Ratio of Engines

Cutoff ratio

The term 'cut-off ratio' applies specifically to diesel engines and does not pertain to petrol engines. In petrol engines, there is no cut-off. Cut-off refers to the cessation of fuel spray, which occurs in diesel engines but not in petrol engines. Therefore, this term is exclusive to diesel engines. Please note the PV diagram of the diesel cycle below.

Air is isentropically compressed in process 1-2. Once the compression process is complete, the fuel spray initiates at point 2. With the onset of the fuel spray, combustion begins, leading to gas expansion. This spray and combustion phase continues until point 3, representing the gas expansion from point 2 to point 3. At this stage, V3/V2 equals the cutoff ratio, where V3 represents the volume at point 3, and V2 represents the volume at point 2.

A potential question may arise: why does the fuel spray organize combustion? The answer lies in the fact that the compression of air during process 1-2 elevates its temperature significantly, leading to the ignition of the fuel spray and the commencement of combustion.

Basic usages of cutoff ratio

Cutoff ratio along with compression ratio is used to determine efficiency of diesel engines. The formula for efficiency of a diesel engines is given below:

Effects of cut-off ratio on diesel cycle efficiency

In the formula for the diesel cycle efficiency given above the value inside the red parenthesis increases with an increase in cutoff ratio resulting decrease in efficiency. Therefore, it can be generally stated that an increase in the cutoff ratio decreases the efficiency of a diesel engine.

Compression ratio 

The term compression ratio applies to both diesel and petrol engines. See the petrol cycle figure below.

Here, the air is isentropicaly compressed from 1 to 2, similar to the process in the diesel cycle illustrated in the previous diagram. Here V2 represents the volume after compression, while V1 represents the volume before compression. The compression ratio is defined as the ratio of V1 to V2, and this principle is applicable to both diesel and petrol engines. Upon closer examination, it becomes evident that V2 is equivalent to the clearance volume (Vc), which approaches near top dead center (TDC). On the other hand, V1 corresponds to the total volume, and when the piston is at bottom dead center (BDC), V1 can be expressed as the sum of Vc and the swept volume (Vs). The swept volume (Vs) is determined by subtracting the volume at BDC from the volume at TDC. Thus, the compression ratio (Rc) can be calculated as Rc = (Vc + Vs) / Vc, where Vc represents the clearance volume and Vs stands for the swept volume. It's important to note that the compression ratio formula is applicable to both diesel and petrol engines.

Basic usages of compression ratio

Compression ratio is used to determine the efficiency of both diesel and petrol engines. The formula for petrol engine efficiency is:

Effects of compression ratio on otto/petrol or diesel cycle

The compression ratio has a significant impact on both the Otto and Diesel cycles. A higher compression ratio generally improves the thermal efficiency and fuel economy of both Otto and Diesel cycle engines. It can be simply verified by inspecting efficiency formulae. For the same compression ratio, Otto cycle or petrol engines have higher efficiency than the diesel cycle. However, generally diesel engines use a higher compression ratio than petrol engines because diesel is tougher to ignite than petrol. This is the reason why diesel engines practically have higher efficiency than petrol engines. The compression ratio of a petrol engine typically varies between 6:1 to 12:1 whereas a diesel engine typically uses a compression ratio between 14:1 to 25:1.