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Rallying and anti-lag systems

The main disadvantage of a turbocharged engine compared to a naturally aspirated one is poor transient throttle response, commonly referred to as 'turbo lag'. This effect is something that roadcar manufacturers continue to improve with all kinds of clever technology such as variable geometry turbochargers.

Turbocharged engines have many advantages. They are compact and, as such, can produce high performance from a physically small and light package. They are typically slightly more efficient than a naturally aspirated engine. In racing they are used widely, in rallying and sportscars. IndyCar has a V6 turbo, and Formula One will follow a couple of years later.

Rallying in particular requires good transient response, as not only the acceleration of the car is affected by poor throttle response. Rally driving techniques use the throttle to control the direction of the car - rally drivers are the originators of the 'drift' driving technique.

For a rally car, immediate and predictable throttle response is more important than in most other forms of motorsport. This requirement gave rise to the development of a number of innovative anti-lag systems in rallying, the aim of which is to provide the level of throttle response that would normally be expected of a naturally aspirated engine from a turbocharged engine.

A number of these systems relied on combustion within the exhaust system, the aim being to keep the turbocharger spinning fast because of high mass flow rates of hot gases. With constant boost available, the engine is therefore ready and able to provide good response as soon as the driver opens the throttle, even from low engine speed. Engines with a good anti-lag system can produce very high torque from low speed, where a turbocharged engine with no anti-lag often compares poorly to a naturally aspirated engine.

One such system involves passing very rich mixtures through the engine when off-throttle, and bypassing air from the inlet straight to the exhaust. When the unburned fuel-rich mixture and the air are recombined in the exhaust, combustion takes place. In a multi-cylinder manifold this can lead to almost continuous combustion. An alternative system is to pass a very lean mixture through the engine and inject fuel into the exhaust. Both systems treat the exhaust as a combustion chamber, much as is the case with a jet engine where fuel is introduced to a flow of compressed air.

The first of these methods requires additional hardware in order to provide 'bypass air' to the exhaust, while the second requires extra injection equipment, a supply of fuel and spare capacity within the control software in order to carry out the strategy successfully.

In addition to the naturally hotter exhaust gas temperatures associated with a turbocharged engine, combustion within the exhaust is a further reliability challenge, giving the exhaust little relief from high temperature. Normally, when a 'conventional' turbocharged engine is on closed throttle, there is some respite from high exhaust temperature. Anti-lag equipped engines are often characterised by loud 'bangs' coming from the exhaust as combustion takes place.

Written by Wayne Ward

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