In this blog post, we’ll take an easy-to-understand look at the operating principles of diesel and gasoline engines, the differences in their combustion methods, and how their performance characteristics—such as fuel economy, torque, and noise levels—differ.
The 4-Stroke Cycle: The Basic Principle of Engines
Most cars use 4-stroke engines. A single 4-stroke cycle consists of four stages: Intake, Compression, Power (or Expansion), and Exhaust. While the theoretical 4-stroke cycle is a simplified model of actual engine operation, it is still widely used today as a standard for understanding the basic principles of internal combustion engines.
The first stage is the intake stroke. With the intake valve open and the exhaust valve closed, the piston moves downward, drawing air into the cylinder. In gasoline engines, a mixture of air and fuel typically enters the cylinder; however, in direct injection (GDI) systems, only air is drawn in initially, and fuel is injected during the compression stroke. In contrast, in a diesel engine, only air enters the cylinder during the intake stroke.
The second is the compression stroke. With both the intake and exhaust valves closed, the piston moves upward, compressing the gas inside the cylinder. In a gasoline engine, the air-fuel mixture is compressed, while in a diesel engine, only air is compressed. Diesel engines use a much higher compression ratio to facilitate spontaneous ignition.
The third is the power stroke. The high pressure generated by combustion pushes the piston downward, producing power. In a gasoline engine, the spark plug generates a spark toward the end of the compression stroke to ignite the mixture. In contrast, in a diesel engine, fuel is directly injected into the high-temperature, high-pressure compressed air, causing the fuel to self-ignite and initiate combustion.
The final stroke is the exhaust stroke. The exhaust valve opens, and as the piston moves upward again, it forces the exhaust gases remaining after combustion out of the cylinder. Once this process is complete, the intake stroke begins again, and the four-stroke cycle repeats.
Differences Between Diesel and Gasoline Engines
As interest in fuel economy and driving performance has grown in the automotive market, comparisons between the characteristics of diesel and gasoline engines have become more common. Although the share of electric and hybrid vehicles has increased significantly in recent years, internal combustion engine vehicles still play a vital role in both the passenger car and commercial vehicle markets. Diesel engines offer excellent fuel economy and high torque, while gasoline engines are known for their quiet operation and smooth power delivery. Therefore, when comparing the two engines, fuel economy, torque, vibration, and noise are the primary factors considered.
The biggest difference lies in the combustion method. Diesel fuel has a higher tendency for spontaneous ignition than gasoline. Consequently, while gasoline engines ignite the fuel-air mixture using a spark from a spark plug, diesel engines cause combustion by spontaneously igniting the fuel solely through the high pressure and temperature inside the cylinder.
Diesel engines use a higher compression ratio to facilitate spontaneous ignition. Since, thermodynamically, thermal efficiency tends to improve as the compression ratio increases, diesel engines can achieve higher efficiency with the same amount of fuel. Thanks to this characteristic, they generally offer better fuel economy than gasoline engines.
High thermal efficiency is a key advantage of diesel engines. Furthermore, with the adoption of the latest technologies—such as turbochargers and high-pressure common-rail fuel injection systems—power output and responsiveness have also improved significantly. On the other hand, because they must withstand high pressure and temperature, the engine block and major components must be designed to be more robust; these structural characteristics contribute to higher manufacturing costs and technical complexity.
Vibration and noise are also key differences between the two engines. While older diesel engines were widely perceived as being noisy and vibrating heavily, recent advancements—including high-pressure fuel injection technology, multi-stage injection, improvements to the intake and exhaust systems, soundproofing technology, and precise engine control systems—have significantly improved their quietness. Nevertheless, gasoline engines generally produce less vibration and noise than diesel engines and tend to provide a smoother driving experience. Conversely, diesel engines deliver high torque even at low RPMs, offering outstanding performance in situations that require significant power, such as highway driving or freight transport.
Furthermore, today’s internal combustion engines are not simply categorized as gasoline or diesel engines. Various combustion technologies—such as GDI (Gasoline Direct Injection), HCCI (Homogeneous Charge Compression Ignition), and SPCCI (Spark-Controlled Compression Ignition)—have been researched and developed to combine the advantages of both types, and some have already been applied to mass-produced vehicles. These technologies are the result of efforts to simultaneously improve fuel efficiency and reduce emissions.
Ultimately, the most fundamental difference between gasoline and diesel engines lies in their fuel ignition methods. This difference leads to variations in various performance factors, including compression ratio, thermal efficiency, fuel economy, torque, vibration, noise, emissions characteristics, and manufacturing costs. Recently, due to stricter environmental regulations and advancements in electrification technology, internal combustion engines have also been continuously evolving; understanding their basic principles allows for a more accurate grasp of the characteristics and pros and cons of various automotive technologies.
