A car engine adjusts its output by mixing air and fuel in varying ratios. In diesel engines (as well as in gasoline and LPG engines), there are several methods of injecting fuel and several ways of drawing in air. These days, many buses are abandoning V8 engines in favor of turbo intercooler engines, so let’s take a closer look at what they are.
About the Intercooler
From Korean, AI-translated
A car engine adjusts its output by mixing air and fuel in varying ratios. In diesel engines (as well as in gasoline and LPG engines), there are several methods of injecting fuel and several ways of drawing in air. These days, many buses are abandoning V8 engines in favor of turbo intercooler engines, so let’s take a closer look at what they are.
Originally, car engines used a naturally aspirated system. However, to improve power output, fuel efficiency, and reduce exhaust emissions, the turbo engine (turbocharger) was developed.
In a naturally aspirated system, as the name suggests, there are no additional devices. There is an air intake opening, and between this opening and the engine is an air cleaner. Air is drawn in without resistance as it flows in naturally. The engine should ideally draw in a mixture equivalent to its displacement, but in reality, it only draws in about 80% of that volume. For example, a 2000cc engine can only intake the equivalent mixture of about 1600cc. To increase intake volume, manufacturers increase the number of valves or enlarge the valve diameter. Another method is to use a turbocharger or supercharger to force air in by spinning a turbine or pump, increasing the air pressure by about 1.5 times, allowing more air to enter the cylinder. As more air enters, more fuel can be burned, resulting in increased power. Using the cylinder’s vacuum to draw in air is called a naturally aspirated system, while forcing air in with a turbocharger is called a forced induction system.
The turbo intercooler engine was originally developed for aircraft to maintain a constant air pressure inside the engine even at high altitudes where the air becomes thin. This concept was later applied to car engines. A turbo intercooler cools the high-temperature compressed air in the intercooler, significantly increasing air density, which increases the absolute volume of intake air supplied to the cylinder, thereby improving engine output. Compared to naturally aspirated engines, output can increase by nearly 30%, providing exceptional performance. It can produce the same output at lower speeds, extending engine life. Additionally, vibration, noise, and exhaust gases are reduced, and fuel efficiency is said to improve as well.
The turbo intercooler engine is essentially a naturally aspirated engine with two additional major components (though there are many other differences in reality): the turbocharger and the intercooler. These two devices greatly enhance engine performance.
Turbocharger
The turbocharger, often simply called a “turbo,” is a device shaped somewhat like a snail. Inside its housing, a single shaft has two turbines mounted at each end, each with blades angled differently. One side is connected to the intake manifold, and the other side is connected to the exhaust manifold. When exhaust gas pressure spins the turbine on the exhaust side, the impeller on the intake side also spins. The air near the center of the impeller is accelerated outward by centrifugal force into the diffuser. Because the diffuser has a larger cross-sectional area, the air’s kinetic energy is converted into pressure energy before being supplied to the cylinder, thereby improving volumetric efficiency. The rotation of the exhaust turbine also improves exhaust efficiency. For this reason, the turbocharger is sometimes called an “exhaust turbine supercharger.”
Not all the combustion heat in an engine cylinder is converted into power. Only about 30% of it is used to push the pistons and turn the crankshaft. Around 5% is lost to piston and cylinder friction, another 30% or so is lost to cooling, and the remaining 35% is expelled in the exhaust gases. Exhaust temperatures can reach as high as 900℃, and the expansion pressure in the cylinder pushes exhaust gases into the exhaust manifold at speeds approaching the speed of sound (Mach 1). A turbine installed at the exhaust manifold outlet harnesses this high-speed exhaust flow to generate powerful rotation, functioning like a pump to compress air to about 1.5 times atmospheric pressure and feed it into the engine’s cylinders.
(1) Structure of a Turbocharger
The turbocharger consists of: - An impeller that compresses intake air through high-speed rotation powered by exhaust gas pressure, - A turbine that converts the thermal energy of exhaust gases into rotational energy, - Floating bearings that support the turbine shaft, - A supercharging pressure regulator that prevents boost pressure from exceeding the specified limit, - An intercooler that cools the compressed air, - And a knock prevention system that adjusts injection timing to prevent knocking.
① Impeller
The impeller is a set of blades installed on the intake side that compresses air into the cylinders. Diesel engines typically use a radial type impeller (straight blades arranged radially) because it is suitable for high-speed rotation. Gasoline engines often use a backward type impeller (spiral-shaped blades) that is more efficient at lower rotational speeds.
② Turbine
The turbine is installed on the exhaust side. It spins the compressor using the pressure of exhaust gases, converting the thermal energy of exhaust gases into rotational energy. Turbochargers typically use a radial type turbine. Because the turbine operates at high speed while exposed to exhaust heat, it must have excellent strength and heat resistance to withstand centrifugal forces. Exhaust gases from each cylinder pass through the turbine housing’s outer circumference, contact the turbine blades to spin them, and then exit through the exhaust pipe. Since the impeller is mounted on the same shaft, it spins simultaneously.
③ Floating Bearing
The floating bearing supports the turbine shaft, which rotates at around 10,000–15,000 rpm. It is lubricated with oil supplied from the engine, allowing it to spin freely between the housing and the shaft. One important precaution: if the engine is shut off immediately after high-speed driving, oil supply to the floating bearing stops, which can cause sticking (seizure). Therefore, the engine should be idled for a while to cool the turbocharger before turning it off.
④ Supercharging Pressure Regulator – Wastegate
The supercharging pressure regulator (commonly called a wastegate) prevents boost pressure from rising above the specified limit. If boost is not regulated, the pressure can exceed the engine’s tolerance, leading to damage. There are two main control methods: diverting part of the exhaust gases or controlling intake air flow.
a. Exhaust Gas By-Pass Type: This method diverts a portion of exhaust gases away from the turbine to prevent boost pressure from exceeding the specified value. In the remote by-pass type, a bypass valve is installed away from the turbocharger (see diagram) to regulate boost. In the swing-valve type, integrated into the turbocharger housing, the actuator opens a swing valve to bypass exhaust gases. Most turbochargers used domestically adopt the swing-valve type.
Wastegate function – limiting boost pressure
b. Suction Relief Type: In this electronically controlled method, a relief valve is installed on the intake side. When boost pressure exceeds the specified limit, the relief valve opens, releasing compressed air into the atmosphere and controlling boost before it enters the cylinders.
⑤ Knock Prevention System
The knock prevention system installs a knock sensor on the cylinder block to detect knocking vibrations and delays injection timing when knocking occurs. Knocking is more likely at low boost or low intake air temperatures. The system consists of a knock sensor that detects vibrations at 6–8 kHz, a knock control unit, and an ignition timing control section.
Knock prevention system overview
(2) Turbocharger Lag
When partially loaded or under steady cruising, the turbocharger is essentially idling. At this time, the intake manifold is under vacuum, and little exhaust gas passes through the turbine, so the compressor spins too slowly to compress intake air. When the driver presses the accelerator pedal to demand more power, the throttle valve opens, more air-fuel mixture enters the engine, and manifold vacuum decreases. The combustion of the added mixture increases exhaust gas flow, which in turn accelerates the turbine and compressor until boost is supplied.
Many drivers of turbocharged vehicles complain about “turbo lag” — the delay between opening the throttle and the turbo providing extra power. Turbo lag is the time required for an idling turbine to reach boost pressure. Additional time is also needed for the intercooler and intake piping to fill from vacuum to pressurized state. Overall lag time can be 0.5 seconds or more, which is noticeable to many drivers.
Partial solutions include making the rotating assembly (such as the compressor wheel and turbine wheel) as lightweight as possible, or using two smaller turbochargers instead of a single large one. Smaller, lighter rotors reduce lag. Engines with two turbochargers are called “bi-turbo” or “twin-turbo” engines.
Engine equipped with twin turbos
Intercooler
An intercooler is installed between the impeller and the intake manifold to cool the compressed air. When air is compressed by the impeller, its temperature rises, which reduces the rate of air density increase. This can lead to knocking or a drop in charging efficiency. To prevent this, the intercooler is designed similarly to a radiator and cools the air either by ambient airflow during driving (air-cooled type) or by circulating coolant (water-cooled type).
(1) Air-Cooled Intercooler
This type cools the charged air using airflow from vehicle motion. The structure is simple compared to water-cooled types, but cooling efficiency is lower. Since efficiency increases with vehicle speed, air-cooled intercoolers are often used in racing cars equipped with turbo engines.
(2) Water-Cooled Intercooler
This type circulates coolant from the engine’s radiator or a dedicated radiator to cool the charged air. When intake air temperatures exceed 200°C, coolant at 80–90°C is used, often combined with airflow cooling during driving. While more complex in structure compared to the air-cooled type, it offers better cooling efficiency even at low speeds.
Summary of Turbocharger and Intercooler Benefits
A supercharger (forced induction device) is essentially an air pump installed in the intake path to increase engine power output, torque, and fuel efficiency. Naturally aspirated engines draw in air by vacuum created during the piston’s intake stroke, limiting performance. By forcing more air into the cylinders, volumetric efficiency is increased, leading to improved power output and torque, as well as better fuel economy. The main benefits are:
① Power output increases by 35–45%.
② Higher volumetric efficiency increases mean effective pressure.
③ Increased volumetric efficiency boosts torque.
④ Less power loss at high altitudes.
⑤ Higher compression temperatures shorten ignition delay.
⑥ Reduced cooling loss and a 3–5% improvement in fuel efficiency.
The reasons turbo intercoolers are replacing the once-dominant V8 engines lie here. Since many V8s consumed excessive fuel, they have been pushed aside. Although the deep rumble of V8s is becoming rare, perhaps one day, with further technological advancement, turbo intercoolers will be fitted to V8 engines — creating twin-turbo V8s.
