When you open up a bottle of soda, the pressure outside the bottle is less than the pressure outside, drawing the carbon dioxide out of the soda, creating fizz. A normally-aspirated engine uses this same principle: during the intake stroke, air or a mixture of air and fuel are drawn into the cylinder by this pressure difference. There is one problem with this: the amount of air that goes into the cylinder is limited by the amount of pressure difference, so there is much less air going in at high altitudes, and even at sea level this could be increased. This is where supercharging comes in.
A supercharger is a massive air pump. More air can get into the cylinder, so more power can be made. A regular supercharger is belt-driven by the engine, which takes power. A turbocharger is a supercharger driven a second pump spun by exhaust gasses. Since the exhaust pressure is normally wasted, this is essentially “free” energy, producing more power than a similarly-sized shaft-driven supercharger and improving gas mileage. While there are a variety of supercharger pump designs, turbos almost exclusively use turbine-style pumps.
How does turbocharing work? How can I improve my turbocharger’s performance?
A turbocharging system is made up of several parts, not just the turbo. Each piece can be tuned or upgraded to improve performance.
Turbo: Once air has passed through the air filter, it’s piped into the turbo. The speed the intake turbine spins at is determined by the pressure of the exhaust gases entering the exhaust turbine. Higher engine speeds increase pressure, as does heat: stock systems put the turbo very close to the exhaust manifold, or place it after the first catalytic converter to take advantage of the heat produced burning off leftover fuel in the exhaust.
A larger turbo is the surest way to large increases power, but also the most expensive. It also may require adding larger fuel injectors, and stronger engine components to handle the stress.
Wastegate: A wastegate lets gases pass around the exhaust turbine, controlling boost pressure. It’s commonly believed that the chattering noise a turbocharged car makes when decelerating comes from the wastegate, but its actually the air backing up as the intake pump slows. Older cars have a mechanical wastegate which can be replaced with a unit that opens later, increasing maximum pressure.
Boost controller: Most turbocharged engines from the late 1980s on come with electronically-controlled wastegates, which can be adjusted inside the car with one of these devices. This can be part of a modified engine control unit (ECU), or a separate control where maximum boost pressure can be altered. This can affect drivability, since most stock systems are designed to add pressure in stages, giving a more linear pedal feel. It’s also very easy to increase boost past a point the engine can handle.
Intercooler: Hot air is less dense, reducing power. It may seem that air in a turbo would be heated by the nearby exhaust turbine, but it’s actually heated by being compressed. An intercooler is a radiator that cools the air after its been through the turbocharger, increasing its density and reducing predetonation. This can be either a direct air-to-air intercooler, or a water-to-air system which uses a separate radiator like the engine’s cooling system.
Knock sensor: Turbocharged motors are susceptible to predetonation. Also called knocking or pinging, the air-fuel mixture inside the cylinder ignites before the piston is in the optimum, which reduces power and can cause engine damage. Stock knock sensors are able to adjust turbo boost and ignition timing to prevent predetonation when lower octane gas is used to protecting the engine, but this also reduces power. Large increases in boost will require aftermarket equipment to handle adjustments past stock knock sensor’s range.
What’s turbo lag?
It takes time for the pump to build up speed, which results in a delay between pushing on the gas pedal and accelerating. Lag was a major problem for early cars, but it has been reduced by a variety of technologies, including lighter turbines, electronic boost control, low-pressure turbos, and sequential turbos, which use a small turbo for low engine speeds and a large one for high speeds.
Do I need special fuel or oil to keep my engine running?
The bearings that hold the turbine shaft in place get incredibly hot, requiring high-grade motor oil.
Most manufacturers also recommend letting the engine idle for a few seconds before shutting it off to let some cooler oil run through the turbo.
Most manufacturers recommend high-octane gasoline in their engines to prevent predetonation. Octane is a measurement of how fast the gas ignites, so a high-octane fuel mix can burn closer to the optimum time for detonation. The risk of predetonation increases as boost is increased, leading most tuners to switch to higher octane fuels.
Why are turbochargers becoming popular on cars again?
With tightening gas mileage regulations, turbos are a good way to get more power out of a quantity of fuel. However, this extra power has always come at the cost of reliability. This is changing with the introduction of direct injection.
In a normal engine, fuel is mixed into the incoming air before it reaches the cylinder. This can be an unpredictable process, and offers little in the way of cooling to the combustion chamber. By relocating the fuel injectors to the cylinder, fuel can be sprayed multiple times during the intake and compression cycle, improving mixture with the air while also cooling it. In this way, knocking is all but eliminated.
Tags: exhaust manifold, improving gas mileage, supercharger, turbocharger
Bookmark This Article