Diesel engines once had a reputation for being efficient but sluggish. Sure, they sipped fuel, but they lacked urgency. This perception has changed dramatically. What is the main reason? The popularity of turbochargers.
Modern diesel engines are characterized by explosive torque and quick response. This isn’t just about one part. High pressure direct injection plays an important role. However, the turbocharger is the engine that powers the engine. Interestingly, turbocharging is not unique to diesel engines. F1 teams have used these years to squeeze more horsepower out of their units. The technology is universal. However, its application has developed greatly in the automotive industry.
Mechanism of forced induction
To understand the importance of a turbocharger, you need to understand how an engine produces power. It comes from burning. More precisely, a mixture of fuel and air is burned. More mixture equals more power.
Historically, engineers have achieved this in two ways. They made the cylinder bigger. They added cylinders. It is equipped with a V8 engine instead of an inline 4-cylinder engine. Large displacement engine. These methods work. But they add weight and complexity.
There is a third way. You can force more air into the existing cylinders. That’s the turbocharger’s job.
Naturally aspirated engines relies on atmospheric pressure to pull air in, while turbo engines use exhaust gases to draw in air. This is a closed efficiency loop. The exhaust energy that would otherwise dissipate into the atmosphere is captured. It turns the turbine. The turbine drives the compressor. The compressor forces dense air into the combustion chamber.
A step-by-step process
When you visualize the process, it’s easy to see how it works. This doesn’t happen by magic. It happens through a certain series of physical events.
- Exhaust gas flow: Hot exhaust gases leave the engine and hit the turbine blades. The pressure makes the turbine spin at incredibly high speeds.
- Compression: The turbine is connected to the compressor wheel via a shaft. As the turbine rotates, the compressor sucks in the surrounding air and compresses it.
- Cooling: Compressing air heats it up. Hot air is less dense. Solve this problem by routing the air through the intercooler (or intercooler). This lowers the temperature. Cooler air is denser. Higher air density means that there are more oxygen molecules per cubic centimeter.
- Fire: Dense cold air enters the cylinder. A fuel injector injects diesel fuel into this pressurized environment. The result is a more powerful explosion.
“The main advantage is density. Cooling the compressed air maximizes the oxygen available for combustion without increasing the size of the engine.”
Overcoming hysteresis with variable geometry
Early turbochargers had flaws. Lag. This is not a software bug. This is physics. At low engine speeds, the exhaust gas flow is weak. It isn’t enough to spin the turbine fast. The driver presses the gas pedal. The engine hesitates. Then, suddenly, the turbo spools up. Power arrives late.
Engineers solved this problem by using variable geometry turbines (VGT). The blades inside the turbine housing can change their angle. At low speeds, the blades narrow the exhaust pipe. This increases the speed of the gas. The turbine spins faster despite the low volume. At a high speed, the blades open. This prevents the engine from clogging due to back pressure.
This technology eliminates the “dead zone” between idle power and peak power. It made diesel engines responsive in a way they never were before.
BMW’s two-step approach
Some manufacturers want to go further. BMW introduced a variable twin turbocharger in the 535d. This is a two-stage setup. It uses two turbochargers of different sizes.
The smaller turbo spools up immediately at low speeds. It offers instant torque and quick acceleration for city driving. It works efficiently in a lower speed range, which is difficult with larger turbines.
As engine speed and displacement increase, a larger turbocharger becomes more efficient. It handles the high-flow demands of high-speed driving. The system switches seamlessly between the two units. The result is strong performance throughout the rev range. There is no lag at the bottom.
