Why do heavy duty applications prefer diesel engines over gasoline (petrol) ones despite the latter being more efficient and powerful than a diesel engine of the same size and operating on the same compression ratio?

Ships, railway engines (locomotives), large truck engines, colossal construction equipment, farm machinery, other heavy vehicles, home heating, generators . . . . . a diesel engine is the solution for all heavy duty applications.

Compression Ratio is the volume of the engine cylinder when the piston is at the bottommost position, called bottom dead center (BDC), divided by cylinder volume when the piston is at the topmost position, the top dead center (TDC).

Well, the straightforward answer is this – diesel engines can use a greater compression ratio than gasoline (petrol) engines because petrol starts knocking at elevated compression ratios. And, efficiency and power output of engines rise with increasing compression ratio.

Diesel engines commonly employ a 12 to 20 compression ratio, while petrol (gasoline) engines utilize one between 8 and 10. Heavy duty diesel engines are, therefore, in a better position to deliver.

"Figure

Such capacity is among the significant advantages of diesel engines over gasoline (petrol) engines. This is also why peak pressures in diesel engines are greater than those in gasoline engines. Temperature sensors will also testify of the greater temperatures inside diesel engines.

Petrol (gasoline) engines operate on the Otto Cycle as represented by the pressure-volume (PV) diagram in figure 2.

Figure 2. Pressure-Volume Diagram for Otto Cycle used by Petrol or Gasoline Engines Strokes: 0-1: Suction, 1-2: Compression, 2-3: Power / Ignition, 3-4: Expansion, 4-1: Heat Rejection, 1-0: Exhaust Image Credits: Luc1991 @ https://en.wikipedia.org/wiki/File:P-V_Otto_cycle.svg
Figure 2. Pressure-Volume Diagram for Otto Cycle used by Petrol or Gasoline Engines
Strokes: 0-1: Suction, 1-2: Compression, 2-3: Power / Ignition, 3-4: Expansion, 4-1: Heat Rejection, 1-0: Exhaust
Image Credits: Luc1991 @ https://en.wikipedia.org/wiki/File:P-V_Otto_cycle.svg

Otto Cycle efficiency is given by:

ηotto = 1 – (1 / r γ -1)          (1)

where

r is the compression ratio = V1 / V2

γ is the isentropic (reversible adiabatic) index

Diesel engines operate on the Diesel Cycle as represented by the PV diagram in figure 2.

Figure 3. Pressure-Volume Diagram for Diesel Cycle employed by Diesel Engines Strokes: 1-2: Compression; 2-3: Ignition; 3-4: Expansion; 4-1: Exhaust Image Credits: Tokino & MaxDZ8 @ https://en.wikipedia.org/wiki/File:DieselCycle_PV.svg
Figure 3. Pressure-Volume Diagram for Diesel Cycle employed by Diesel Engines
Strokes: 1-2: Compression; 2-3: Ignition; 3-4: Expansion; 4-1: Exhaust
Image Credits: Tokino & MaxDZ8 @ https://en.wikipedia.org/wiki/File:DieselCycle_PV.svg

Diesel Cycle efficiency is given by:

ηdiesel  = 1 – [(1 / r γ -1) (⍺γ – 1) / γ (⍺ -1)]          (2)

where

r is the compression ratio = V1 / V2

is the isentropic (reversible adiabatic) index

⍺ is the cut-off ratio = V3 / V2

Comparing equations 1 and 2, it is clear that gasoline (petrol) engines are more efficient and, therefore, deliver more power than a diesel engine for the same compression ratio.

However, higher compression ratios escalate internal engine temperature. Now, the chemical composition of petrol (gasoline) is such that it ignites more spontaneously than diesel at higher temperatures.

For optimum efficiency and power, gasoline (petrol) has to ignite slightly before the piston reaches the TDC. Plus, it has to be lighted by the controlled spark from the spark plug.

Uncontrolled combustion means that petrol (gasoline) burns before such required position. And the burning is a result of elevated temperatures, not due to the controlled spark.

Speaking of economics, diesel is cheaper than gasoline (petrol) in most countries across the world because it is easier to refine from crude oil.

Moreover, emission regulations have triggered tremendous improvements in diesel engines. Such upgrades have minimized emissions of carbon dioxide, NOX, and particulate matter from diesel engines.

Indrajeetsinh Yadav has pieced together this short article. For more such interesting and to-the-point content on Engineering and Academic topics, write to us at info@falconwords.com or call us at +91-9822052945.

Leave a Reply

Your email address will not be published. Required fields are marked *