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Turbo Power – Overview
One of the surest ways to get more power out of an engine is to increase the amount of air and fuel that it can burn. One way to do this is to add cylinders or make the current cylinders bigger. Sometimes these changes may not be feasible — a turbo can be a simpler, more compact way to add power, especially for an aftermarket accessory.
When the intake valves open in a naturally aspirated engine, it’s only the atmospheric pressure that fills up the cylinder with an air and fuel mixture. Due to flow losses and the short amount of time, the volumetric efficiency is often below 90%. On a well tuned racing engine, you can come up to just over 110 % in short rpm-ranges. But with a compressor, the volumetric efficiency can be 200% or more if you like. You can of course enlarge the cylinder volume and rpm to achieve more flow through a naturally engine, but this can also be done with a boosted engine. Different types of compressors: Two main types of compressors are available. Supercharger – driven by a belt from the crankshaft of the engine Turbocharger – driven by the exhaust outlet from the engine.
Why do all the car and truck manufacturers almost exclusively use turbochargers instead of superchargers? *A turbocharged engine produces more power and torque at a certain boost. You can expect that at least 30% of the power increase that the supercharger makes, needs to drive for example a roots compressor. For instance: If you have an RX1 with stock 140 hp, you install a turbo and get 260 hp (=120 hp power increase) you could expect about 80 hp increase if it had been a supercharger because it takes 40 hp to drive it. If you design a turbo system in a proper way, the energy that drives the turbine wheel is for free. About 45 % of the energy from the fuel is getting lost thought the exhaust system on a naturally aspirated engine.
Turbos and Fuel Efficiency
Turbochargers allow an engine to burn more fuel and air by packing more into the existing cylinders. The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It’s not perfectly efficient, so you might get a 30- to 40-percent improvement instead.
One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.
Turbos and High Altitudes
The turbocharger also helps at high altitudes, where the air is less dense. Normal engines will experience reduced power at high altitudes because for each stroke of the piston, the engine will get a smaller mass of air. A turbocharged engine may also have reduced power, but the reduction will be less dramatic because the thinner air is easier for the turbocharger to pump.