A turbocharger is a centrifugal compressor, which is powered by a high speed turbine, which is driven by the engine's exhaust gases. A turbocharger benefits an engine by providing it with more air which results in a more powerful combustion. The word turbocharger derives itself from turbosupercharger since it is very similar to a supercharger. The only difference between a turbocharger and a supercharger is that turbochargers use the exhaust to power the unit instead of using power from the crankshaft the entire time like a supercharger. Using the exhaust gases makes a turbo more efficient because it is providing pure power instead of using power to create more power.
The turbocharger is initially driven by the engines crankshaft through a gear train and an overriding clutch, which provides aspiration for combustion. Once the engine achieves combustion, and after the exhaust gases reach sufficient temperature, the overriding clutch will disengage the turbo compressor from the gear train. The turbo compressor is now only driven by the turbine, which is driven by the vehicles exhaust gases. Turbo chargers are more efficient and provide maximum power output when the engine is at high output because the engine will be putting out more exhaust gases which will spin the turbo faster.
All naturally aspirated engines rely on the down stroke of the piston to create a vacuum which will suck air into the cylinder. This air is combined with fuel before the piston makes its way back up the cylinder. As the cylinder is pushed back up, the air and fuel will cause combustion which is what powers the engine. The larger the combustion, the more power output will be created. With most engines, the time given to fill the cylinder with air is not enough, which will cause a smaller combustion and less power. The turbochargers job is to force as much air as possible into the cylinder before the piston makes its way back up which will create more power from the larger combustion.
In a turbocharger system, the boost pressure is limited to keep the engine system inside its thermal and mechanical operating range. Over boosting an engine can cause damage to the engine such as pre ignition, overheating, and over stressing the engine's internal components. Boost refers to the intake manifold pressure that exceeds normal atmospheric pressure. To avoid pre ignition, the intake manifold pressure must not get too high which means that it has to be controlled. Controlling this pressure is the waste gate which allows the energy for the turbine to bypass it and pass directly into the exhaust pipes. The turbocharger will then slow because the turbine is starved of exhaust gas which is its source of power. Slowing the turbine will help lower compressor pressure.
All turbochargers experience a condition called turbo lag. This can be problematic when a rapid change in engine performance is required. Turbo lag is the time required for a change in speed and function after there has been a change in throttle position. Turbo lag may be noticeable in all driving conditions but is especially noticeable when accelerating. Turbo lag occurs because the time needed for the exhaust system to spin the turbine may take a bit for the turbine to completely get up to speed. A combination of inertia, friction, and compressor load are the primary contributors to why turbo lag occurs.
Turbo lag is not to be confused with another occurrence called boost threshold. Boost threshold of a turbo system describes the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor may not produce significant boost. This is seen as a limitation of boost at set rpm’s regardless of the pressure from the exhaust. The boost threshold is determined by an engines displacement, engine rpm, throttle opening, and the size of the turbo. Boost threshold has declined steadily with the increase in engine developments and new turbo charger designs. Turbochargers only start creating boost at a certain exhaust mass flow rate.
Since turbochargers compress air to create more power, the air will become warmer. Having warm air is terrible for engine performance and will greatly decrease power production. A higher air temperature can also lead to detonation, which is extremely destructive to engines and the components in them. To counter this, there are methods which can be used to cool the air. The main method used to cool the air in a turbocharger is to use an intercooler. An intercooler works as a heat exchanger that allows the compressed air to give up some of its heat energy to the ambient air. This device is usually placed in the air stream between the compressor outlet of the turbocharger and the engine intake manifold.
Turbochargers also use an anti-surge valve which allows highly pressurized air to escape. This helps protect the turbo from damage and malfunction. The main purpose of this valve is to maintain the spinning of the turbocharger at a high speed. This excess air is usually recycled back into the turbo inlet or vented to the atmosphere.
The reliability of turbochargers has greatly increased over the years. Some common problems that can occur when using a turbocharger happen when the oil system is not working effectively. Dirty or old oil can ruin a turbocharger because it is used to cool the turbo rotating assembly. To help counter this, turbocharged vehicles should get their oil replaced more often than a regular vehicle. Using synthetic oil is recommended because it tends to flow more readily when cold and does not break down as quickly as conventional oils. Since a turbocharger will heat up when running, it is recommended to let the engine run for a few minutes to cool down the turbo before shutting the vehicle off.
A turbochargers light weight and small size make it optimal for smaller vehicles that may not be able to hold a large engine. Turbochargers are most commonly used in gasoline engines which are focused on high performance, but are also seen in a lot of diesel engines which are usually used for transportation in industrial equipment. Improvements to robustness, reliability, and cooling may be required to cope with the extra power put out when using a turbo in a vehicle that may have not been designed to handle it.
Using multiple turbochargers is also common in high performance vehicles. This will increase power and help cope with boost lag due to the fact that there are always two running. When using multiple turbochargers, it is better to use smaller ones which can reach their maximum rpm and performance faster than a single large turbo. This will especially be noticeable in high speed acceleration. When using multiple turbochargers, there has to be separate exhaust streams for each turbo.
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