Could someone explain turbo-lag?

Question

i know what its like, delayed acceleration in cars that have turbo/turbo charged engines, but i would like to know more about it, and why it happens.

Answer 1

This means the engine must reach a certain RPM (revolutions per minute) before the Turbocharger even kicks in. If you are driving slow around a heavily trafficated area, meaning 30 miles per hour or less, you will never use your turbos. You need open road or taking of at the light while pushing the pedal down and lettin her rip. When turbo kicks in, I assure you , your head will slam into the headrest~!!!

Answer 2

Picture a pinwheel that is attatched to your exhaust system. When your engine is running and kicking out exhaust the pinwheel turns, and of course it turns faster as more exhaust comes out. Well picture that pinwheel being attatched to another wheel (connected by a shaft). The other wheel spins at the same time as the pinwheel, and as it turns, it forces air into your engine. This creates your turbo boost. Well turbo lag occurs when you first accelerate and you need to build up enough exhaust gas to make the pinwheel move fast enough to make boost on the intake side. At which point, the boost supplies enough power to the engine to keep the exhaust pinwheel moving fast enough to continue boosting. Some people avoid turbo lag by keeping thier engine rpms up and slamming the clutch into gear. Which could be damaging, but works. I hope I explained it good enough.

Answer 3

Turbo lag is the time it takes for a turbo to spool up. When you slam the gas to the floor, more exhaust gasses exit into the turbo, which makes the compressor spin faster, which forces more air into the engine. At idle, most turbos are barely spinning and the engine is sucking in air. Once the compressor in the turbo starts spinning really fast, it jams more air and fuel into the engine. The time between sucking and being force fed is turbo lag.

Answer 4

Turbo lag is sometimes felt by the driver of a turbocharged vehicle as a delay between pushing on the accelerator pedal and feeling the turbo kick-in. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly-driven compressor in a positive-displacement supercharger does not suffer this problem. (Centrifugal superchargers do not build boost at low RPMs like a positive displacement supercharger will). Conversely on light loads or at low RPM a turbocharger supplies less boost and the engine is more efficient than a supercharged engine.

Lag can be reduced by lowering the rotational inertia of the turbine, for example by using lighter parts to allow the spool-up to happen more quickly. Ceramic turbines are a big help in this direction. Unfortunately, their relative fragility limits the maximum boost they can supply. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response helps but there are cost increases and reliability disadvantages that car manufacturers are not happy about. Lag is also reduced by using a foil bearing rather than a conventional oil bearing. This reduces friction and contributes to faster acceleration of the turbo's rotating assembly.

Another common method of equalizing turbo lag is to have the turbine wheel "clipped", or to reduce the surface area of the turbine wheel's rotating blades. By clipping a minute portion off the tip of each blade of the turbine wheel, less restriction is imposed upon the escaping exhaust gases. This imparts less impedance onto the flow of exhaust gases at low RPM, allowing the vehicle to retain more of its low-end torque, but also pushes the effective boost RPM to a slightly higher level. The amount a turbine wheel is and can be clipped is highly application-specific. Turbine clipping is measured and specified in degrees.

Other setups, most notably in V-type engines, utilize two identically-sized but smaller turbos, each fed by a separate set of exhaust streams from the engine. The two smaller turbos produce the same (or more) aggregate amount of boost as a larger single turbo, but since they are smaller they reach their optimal RPM, and thus optimal boost delivery, faster. Such an arrangement of turbos is typically referred to as a parallel twin-turbo system.

Some car makers combat lag by using two small turbos (such as Kia, Toyota, Subaru, Maserati, Mazda, and Audi). A typical arrangement for this is to have one turbo active across the entire rev range of the engine and one coming on-line at higher RPM. Early designs would have one turbocharger active up to a certain RPM, after which both turbochargers are active. Below this RPM, both exhaust and air inlet of the secondary turbo are closed. Being individually smaller they do not suffer from excessive lag and having the second turbo operating at a higher RPM range allows it to get to full rotational speed before it is required. Such combinations are referred to as a sequential twin-turbo. Sequential twin-turbos are usually much more complicated than a single or parallel twin-turbo systems because they require what amounts to three sets of pipes-intake and wastegate pipes for the two turbochargers as well as valves to control the direction of the exhaust gases. An example of this is the current BMW E60 5-Series 535d. Many new diesel engines use this technology to not only eliminate lag but also to reduce fuel consumption and produce cleaner emissions.

Lag is not to be confused with the boost threshold; however, many publications still make this basic mistake. The boost threshold of a turbo system describes the minimum turbo RPM at which the turbo is physically able to supply the requested boost level [citation needed]. Newer turbocharger and engine developments have caused boost thresholds to steadily decline to where day-to-day use feels perfectly natural. Putting your foot down at 1200 engine RPM and having no boost until 2000 engine RPM is an example of boost threshold and not lag.

Race cars often utilise anti-lag to completely eliminate lag at the cost of reduced turbocharger life.

On modern diesel engines, this problem is virtually eliminated by utilising a variable geometry turbocharger.