Classification of Internal Combustion Engines

Internal combustion engines convert the chemical energy found in fuels such as gasoline, diesel, LPG, or natural gas into mechanical energy. Inside the engine’s combustion chamber, fuel reacts chemically with air to produce heat energy. This heat increases the pressure of the gases in the chamber, forcing the piston to move.

Engines can be classified according to the following criteria:

• Type of fuel
• Cylinder configuration
• Operating cycle
• Method of mixture formation
• Type of ignition (spark ignition – compression ignition)
• Cooling method (air-cooled – water-cooled)
• Method of cylinder filling (naturally aspirated – turbocharged – supercharged)
• Valve configuration

The lubricants used in vehicle engines are defined according to the fuel type, and oil standards and specifications are set by authorized global institutions.

Broadly speaking, engines can be categorized by fuel type—gasoline, diesel, LPG, or CNG—and their core features can be compared as follows.

Diesel engines do not require spark plugs. They operate with a higher compression ratio and greater thermal efficiency. Because only air is compressed, there is no risk of engine knock. However, combustion is less controlled, resulting in higher vibration and noise. They produce higher torque but operate at lower speeds. Peak torque is achieved at lower engine revolutions. As they are exposed to higher pressure, their components must be more durable, which makes them heavier. Maintenance intervals are generally longer, but maintenance costs tend to be higher. Since they run more efficiently, overheating issues occur less frequently. However, cold-start difficulties are more common in low temperatures. Diesel engines are more prone to soot and NOx formation due to the higher sulfur and nitrogen content in fuel and higher in-cylinder temperatures, while gasoline engines, operating at higher speeds, are more likely to produce CO emissions. Although diesel fuel tends to produce more CO₂ because of its higher carbon content, gasoline engines typically emit more CO₂ overall since they consume more fuel per kilometer. Gasoline, being a more refined and lighter fuel, produces fewer particulates—hence particulate emissions are a more significant issue in diesel engines. Since NOx emissions are more toxic than CO₂, diesel engines are often considered less environmentally friendly.

CNG (Compressed Natural Gas) is methane gas (CH₄) compressed to a pressure of 200–250 bar. LPG (Liquefied Petroleum Gas) is a blend of propane (C₃H₈), propylene (C₃H₆), butane (C₄H₁₀), and butylene (C₄H₈) gases, liquefied at 15°C and between 1.7–7.5 bar, with ratios varying by region. LPG is derived from crude oil through a distillation process and, although it emits CO₂ when used, it is a cleaner fuel compared to gasoline—producing about 25% less CO₂. CNG is even cleaner, emitting up to 80% less greenhouse gas than a gasoline engine. Because CNG is lighter than air, it disperses easily in case of leakage, making it safer than gasoline. LPG, on the other hand, is heavier than air and tends to sink; while it is less flammable, it can pose safety risks in the event of an accident. Compared to gasoline and diesel, LPG and CNG contain fewer hydrocarbon bonds, which means they have lower energy content. LPG (propane) has roughly 2.5 times higher calorific value than CNG. All gasoline engines can be converted to run on LPG or CNG. Because LPG and CNG contain less energy than gasoline, converted engines experience a slight power loss (around 10% for LPG). CNG engines produce less combustion residue (such as soot) since they do not contain lead or benzene, keeping engine oil cleaner and preventing spark plug fouling. However, because LPG and CNG have lower lubricating properties than gasoline or diesel, they can cause increased valve wear, although they positively affect piston ring lubrication. As LPG requires less storage space, it is generally more suitable for passenger vehicles. In CNG applications, achieving equivalent power output raises combustion chamber temperatures by approximately 200°C, which shortens the lifespan and durability of metal components and accelerates oil oxidation.