In this blog post, we’ll compare a car engine to the human digestive system to take a detailed look at the principles of the four-stroke cycle, how gasoline and diesel engines operate, and technologies that improve engine efficiency.
Engines and the Human Digestive System
You’ve probably driven a car and noticed on the dashboard that you’re running low on fuel. When that happens, you naturally look for a gas station to refuel. The fuel you put in ultimately becomes the driving force behind the car, and the central component that converts this fuel into actual power is the engine.
A car is a complex system where countless parts work together organically, but at its heart lies the engine, which generates power to turn the wheels. Humans consume food to move and generate strength, digesting it to convert it into energy. Engines are very similar to the human digestive system in that they burn fuel to produce mechanical energy. Just as digestive juices are necessary for the human digestive process, engines absolutely require air to burn fuel, and the process of air and fuel mixing properly and combusting drives the engine’s operation.
Inside the engine, air and fuel enter the cylinder, where they are compressed by the piston. The high pressure generated by the combustion of this compressed mixture pushes the piston downward; this motion rotates the crankshaft, generating the rotational force that propels the car. After combustion is complete, the piston moves upward again, expelling the exhaust gases, and the process repeats. This sequence of four stages—intake, compression, combustion, and exhaust—forming a single cycle is called the four-stroke cycle, and most gasoline and diesel passenger car engines today operate on this basic principle.
Detailed Process of the Four-Stroke Cycle
During the first stage, the intake stroke, the piston moves from the top dead center (TDC)—the highest point in the cylinder—to the bottom dead center (BDC)—the lowest point—causing the pressure inside the cylinder to drop. At this point, the intake valve opens, allowing air to flow into the cylinder due to the pressure difference. In gasoline engines, air and fuel are typically drawn in together during this stroke, and most modern direct-injection gasoline engines also inject fuel during either the intake or compression stroke. In contrast, diesel engines draw in only air during the intake stroke and inject fuel near the end of the compression stroke.
A key concept in this process is displacement. Displacement refers to the maximum volume that each cylinder can hold during a single intake stroke. For a car to produce higher power, it must burn more fuel, which requires taking in more air. Therefore, an engine with a larger displacement can intake more air and fuel at once, enabling it to produce higher power. This principle is similar to a person consuming more food to generate more energy.
In the second stage—the compression stroke—after the intake valve closes, the piston moves upward toward top dead center (TDC), strongly compressing the air or the air-fuel mixture inside the cylinder. During this process, the pressure and temperature inside the cylinder rise significantly, creating an environment conducive to efficient combustion.
Gasoline engines typically compress the air-fuel mixture together and then initiate combustion with a spark from the spark plug. In contrast, diesel engines compress only air to very high pressures to create high temperatures, and then inject fuel at the end of the compression stroke. At this point, the fuel comes into contact with the high-temperature air and ignites spontaneously; this is called compression ignition.
To improve engine efficiency, it is crucial to retain as much of the intake air as possible inside the cylinder. To achieve this, modern engines utilize technologies such as variable valve timing to precisely control the opening and closing of the intake valves, thereby increasing charge efficiency. This is because the more air that is effectively trapped inside the cylinder, the more fuel can be completely burned, resulting in higher power output and greater efficiency.
In the third stage—the combustion (power) cycle—which follows the compression stage, gasoline and diesel engines generate power in different ways. In a gasoline engine, when the piston is nearly at top dead center (TDC), the spark plug fires, causing the compressed air-fuel mixture to burn rapidly. At this moment, the pressure and temperature inside the cylinder rise sharply, forcefully pushing the piston downward; this force rotates the crankshaft, generating the power that propels the vehicle.
In contrast, in a diesel engine, fuel is injected into air that has been compressed to high temperature and pressure, causing the fuel to ignite spontaneously. In a diesel engine, combustion continues throughout the fuel injection process, and power is generated as the piston moves downward. In other words, diesel engines differ fundamentally from gasoline engines in that they are designed to burn fuel using only compression heat, without spark plugs.
During the fourth stage—the exhaust stroke—the piston reaches bottom dead center (BDC) and then moves back up to top dead center (TDC), forcing the exhaust gases—from the completed combustion—out of the cylinder. At this point, the exhaust valve opens to discharge the exhaust gases into the exhaust system, after which the intake stroke begins again, repeating the four-stroke cycle.
The exhaust stroke also has a significant impact on engine efficiency. If the exhaust valve opens too early, high-temperature, high-pressure combustion gases that have not yet done enough work may escape, resulting in energy loss. Conversely, if it opens too late, the exhaust gases cannot be expelled smoothly, which may reduce the efficiency of the subsequent intake stroke. Therefore, modern automotive engines utilize technologies such as Variable Valve Timing (VVT) to precisely control the timing of valve opening and closing based on driving conditions, thereby simultaneously improving both power output and fuel economy.
Improving Engine Efficiency and the Future
The four-stroke cycle is the most fundamental principle that enables a car to run on the road, but it also has clear limitations. For example, simply increasing the displacement to achieve higher power output leads to an increase in the engine’s size and weight. Just as it is difficult for a person to walk for a long time while carrying a heavy load, a car’s fuel economy and energy efficiency can also decline as the weight of the body and engine increases.
To address these issues, automotive engineers have developed various technologies to supply more air into the cylinders or increase the pressure of the intake air—even with the same displacement. A prime example of this is “downsizing” using a turbocharger. A turbocharger uses the energy from exhaust gases to drive a compressor, forcing more air into the engine. This enables even small-displacement engines to produce power and performance comparable to those of conventional large-displacement engines. The “turbo engines” used in many passenger cars today are prime examples of this technology in action.
In addition, various other technologies to improve engine efficiency are continuously evolving. For example, “downspeeding” is a technology that reduces friction losses by lowering the engine’s rotational speed, thereby decreasing the reciprocating speed of the pistons. Since internal engine friction increases as rotational speed rises, securing sufficient torque in the appropriate low-rpm range can improve both fuel economy and efficiency.
Another technology, “Fast Burn,” accelerates the combustion rate so that fuel burns efficiently in a shorter amount of time. Since combustion in a real engine does not occur instantaneously but requires a certain amount of time, thermal efficiency can be improved by shortening the combustion time through the optimization of the combustion chamber shape, fuel injection method, and airflow. While these technologies are sometimes applied independently, they have recently been evolving toward being combined to achieve even higher performance and fuel economy.
Since the advent of primitive internal combustion engines around 1860, automotive engines have undergone continuous development for over 160 years. Today, not only gasoline and diesel engines but also hybrid systems, plug-in hybrids, hydrogen fuel cells, and electric vehicles are advancing in tandem, while internal combustion engines continue to evolve through high-efficiency combustion technologies and emissions reduction technologies.
While the automotive industry is expected to see further expansion of electrification in the future, internal combustion engines are also likely to continue evolving—either by integrating with hybrid technology or by utilizing eco-friendly fuels. Ultimately, automotive propulsion technology is constantly changing to meet the demands of the times, and we can expect to see the emergence of various types of propulsion systems with even higher efficiency and environmental friendliness in the future.
