Turbo lag is a unique phenomenon encountered in turbocharged internal combustion engines, whereby an operator experiences a short delay in full engine response after pressing the accelerator pedal. This occurs because a turbocharger relies on pressure from exhaust gasses, and needs a short amount of time to generate the pressure needed — known as spooling up. Turbo lag is considered a negative characteristic in automobiles, and one that engineers strive to mitigate in a number of different ways.
To understand turbo lag, a working knowledge of how turbochargers work and why they are used is helpful. The idea behind adding a turbo system to an engine is to augment the power generated by the engine alone through simple combustion. This basic concept is known as supercharging, of which turbocharging is but one variant.
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A turbo works by using exhaust air to spin a turbine, which is attached to the same shaft as a compressor. Compressed air created as the turbine spins the compressor is, in turn, fed into the engine. This allows more horsepower to be generated by improving the engine's volumetric efficiency, a trait based in part on the fundamental precept that the more oxygen in a given volume of air, the more potential energy that volume has.
Compared to alternatives like belt-drive superchargers or simply increasing the displacement of an engine, turbocharging is an attractive option. This is because the proportion of horsepower a turbo creates, as compared to the weight of its parts — a characteristic known as power to weight ratio — is favorable compared to these other options. Turbos are thus relatively common in gasoline engines, and almost standard in mass-produced diesel engines, which are known as turbodiesels. Turbo engines have been particularly embraced by several automobile manufacturers, including Saab®, Mercedes Benz®, and Volkswagen®.
The basic design of a turbocharger consists of a metal — usually aluminum — center housing and hub rotating assembly (CHRA), a turbine, a compressor, and a central shaft. The size of the CHRA, the turbine, and the compressor dictate how much extra horsepower they can generate, and generally also how much turbo lag is going to be created. The larger the parts, the longer the turbo typically takes to spool, and the more turbo lag there will be.
The most common way engineers get around turbo lag is simply to use the lightest components possible, as less inertia means less lag. A more complex way is to pair a large turbo with a smaller one, or with a supercharger. The instant or near-instant spooling of these secondary units helps compensate for the lag, while the larger one builds pressure, minimizing or eliminating it completely.