Classification of Internal Combustion Engines

Internal combustion engines convert the chemical energy in the fuel (gasoline, diesel fuel, LPG, natural gas, etc.) into mechanical energy. The fuel produces heat energy by engaging in a chemical reaction with the air in the combustion chamber in the engine. The heat produced increases the pressure of the gas in the combustion chamber, which causes the piston to move.
Engines can be classified based on the following criteria:
•          Fuel Type
•          Cylinder Arrangement
•          Operation Time
•          Mixture Formation
•          Type of Ignition (Spark-Ignition - Compression-Ignition)
•          Cooling Technique (Air-Cooled - Water-Cooled)
•          Cylinder Filling Method (Naturally Aspirated - Turbo-Charged - Supercharged)
•          Valve Arrangement
Lubricants used in vehicle engines are evaluated based on the type of fuel and relevant oil standards and specifications are established by certain authorities.

We can classify the engines based on their fuel types as gasoline, diesel, LPG, and CNG, and compare and contrast some of their basic characteristics as follows.
Diesel Engines and Gasoline Engines
  • Diesel engines do not require spark plugs.
  • They have a higher compression rate and a higher thermal efficiency.
  • There is no risk of knocking as it is only air that is compressed.
  • Since combustion is less controlled, higher levels of vibration and noise are produced.
  • They have higher torque rating but operate at lower speeds. They reach the maximum torque at lower revolutions.
  • As they are exposed to a higher level of pressure, they have to be made of more durable parts and are therefore heavier.
  • Their maintenance intervals are usually longer; however, their maintenance costs are higher.
  • Overheating occurs less frequently as they work more efficiently.
  • Cold start problem under low temperatures is more common.
  • While diesel engines are more prone to the formation of soot and NOx due to the high sulfur and nitrogen content in the fuel and higher in-cylinder temperature, gasoline engines are inclined to higher CO formation due to their higher operating revolutions.
  • Although diesel fuel is more prone to CO2 formation due to the excess number of carbons in its molecule, gasoline engines usually have more CO2 emissions due to less fuel consumption per km.
  • Since gasoline engines use a more refined, light fuel, particles are usually a bigger problem for diesel engines. NOx is more toxic than CO2 emissions, which is why diesel engines are typically considered less environmentally friendly.
LPG Engines & CNG Engines
  • CNG (Compressed Natural Gas) is the methane gas compressed at 200 - 250 bars pressure (CH4). LPG (Liquefied Petroleum Gas) is the liquefied form of Propane (C3H8), Propylene (C3H6), Butane (C4H10) and Butylene (C4H8) gases at ratios depending on the region at 15 °C temperature and 1.7 - 7.5 bars pressure.
  • LPG is obtained from crude oil through distillation, and although it emits CO2 when used in a vehicle, it is a cleaner fuel compared to gasoline (25% less CO2). CNG is a cleaner fuel compared to LPG (80% less greenhouse gas emission than gasoline vehicles).
  • Since CNG is lighter than air, it disperses into the air in case of any leakage, and it is safer than gasoline. LPG, on the other hand, collapses to the ground since it is heavier than the air. It is a difficult gas to ignite; however, it can be dangerous in the event of an accident.
  • Since LPG and CNG have less hydrocarbon bonds than gasoline and diesel fuel, they contain less energy. LPG (propane) has approximately 2.5 times more calorific value than CNG.
  • All gasoline engines can be converted to LPG and CGN. Since LPG and CNG contain less energy than gasoline, it may lead to a loss of power in gasoline vehicle conversion (about 10% for LPG).
  • Since CNG engines have less combustion residue (soot) (does not contain lead, benzene, etc.), the engine oil remains cleaner and the spark plugs do not clog.
  • LPG and CNG are less lubricating than gasoline and diesel fuel, which causes an increase in valve wear, but positively affects the lubrication in the piston rings.
  • Since LPG takes up less space, it is more convenient for use in passenger cars.
  • The fuel used to achieve the same level of power in CNG increases the in-cylinder temperatures by approximately 200oC, which shortens the life and reduces the strength of these metal parts, and accelerates the oxidation of engine oil.