New Bosch lambda sensors are smaller, more accurate

August 18, 2011 // By Christoph Hammerschmidt
With the emission limits in exhaust legislation becoming ever stricter, exhaust sensors are becoming increasingly important. At the same time, automakers are becoming increasingly demanding with respect to sensors' installation space, measurement accuracy, and robustness. For this reason, Bosch is now launching the LSF Xfour, the successor generation of the tried and true LSF4.2 switching-type lambda sensor. The new sensor is smaller and more robust, and its measurement accuracy has been improved. In addition, it responds faster following a cold start.

Compared with the LSF4.2, it takes half as much time to reach control mode following engine start. This significantly reduces the amount of time in which the engine's emissions are uncontrolled. Combined with greater measurement accuracy, this will make it easier to comply with future exhaust standards. In addition, environmental impact is reduced. As overall length is shorter, the amount of installation space needed for the entire sensor assembly has been significantly reduced, while increasing temperature resistance at the same time. As a result, there is greater choice when deciding where to install it in a passenger car's exhaust tract. All variants of the sensor element can be equipped with a thermal shock protective coating as an option. This improves the sensor's resistance to condensation in the exhaust tract following a cold start.

Upstream of the catalytic converter, and occasionally downstream of it, the lambda sensor measures the oxygen content in the exhaust pipe. This is an indicator of combustion quality. For one kilogram of gasoline to be burned completely, roughly 14.7 kilograms of air are needed. The ratio of the mixture in the combustion chamber to the optimum mixture is known as lambda. Conventional gasoline combustion processes ideally operate at a setting of “lambda equals one.” If lambda is smaller than one, the mixture is rich, with too little oxygen. If the ratio is greater than one, combustion is lean, with an excess of oxygen. This is the case with diesel engines, for example, or lean-burn gasoline engines. The switching-type sensor, which delivers an abrupt sensor signal when it detects a transition from a lean to a rich mixture, allows the optimum stoichiometric air-fuel mixture to be precisely adjusted.