If we look at the composition of the air, we see that a considerable portion, 78% in fact, is nitrogen. Despite this high proportion, if possible, every oxygen atom (O2) must find one carbon atom (C), or two hydrogen atoms (H2). Unfortunately however, with Lambda 1, the oxygen atoms are counted off exactly. Although, in the Diesel engine (Lambda > 2) enough oxygen exists, the time-span for mixing and combustion is lower compared with the petrol suction-injection. Even more important here, is the higher combustion temperature, which encourages the build-up of nitrogen oxides. In utility vehicles, the addition of Urea solution, e.g., from Euro 5 onwards, is standard practice. The downstream-installed catalytic converter now allows even higher combustion temperatures, thus, a higher fuel efficiency.

In the meantime, the exhaust gas recirculation reduces the amount of harmful nitrogen oxide in the exhaust gas. Only a portion of the fresh gas to be taken in, is supplied to the engine. In this case, one speaks of fuel saving through not performed intake work. Nonetheless, one must pay careful attention to the controlling, to ensure that the efficiency does not deteriorate, and that the carbon monoxide (CO) and hydro carbon (HC) amounts do not increase. Torque and are not really improved anyway. For this reason, it is accordingly regulated for each respective operational range, and in the meantime, is cooled by air or cooling liquid (see above figure). In an engine which has not yet reached the operating temperature, the coolant is heated more quickly, but only if the system is not divided into various circulations.

Internal exhaust gas recirculation is possible through a greater valve-overlap and can be altered by the camshaft adjustment or by electric/hydraulic valve control. In the external exhaust gas recirculation, the exhaust- and inlet tracts are linked by a pipe. This can take place either inside the cylinder head over a very short distance, or externally as a connection between the exhaust- and the inlet manifolds. An exhaust gas recirculation valve in the piping, regulates the flow-through rate. An electro-magnet (see figure 4) in the valve is given electric signals from the control device, with which it controls the pressure chamber (see figure 4, below the electric connection). This opens or closes the exhaust gas recirculation pipe. At full strain and similar low-air operation ranges, the exhaust gas recirculation must be de-activated, otherwise, (e.g., in the Diesel engine) there would be not enough surplus oxygen. The engine would begin to produce soot.

To reduce the heat-stress to the combi-valve, it should actually, be placed a certain distance away from the exhaust pipe. On the other hand, the distance between the exhaust gas diversion and the combi-valve should not be too great, because, during the de-activation of the recirculation, this dormant piece of piping would have negative effects on the exhaust gas composition. Figure 3 shows a vacuum operated flap for the regulation of the cooling. Apart from the exhaust gas recirculation itself, the exhaust gas recirculation valve is also cooled. Here, recently - different than in figure 5 - a vacuum is also no longer necessary, because the valve is not only electrically controlled but also electrically actuated. The advantage: An (almost) complete diagnosis becomes possible through the control device.

EGR implicates pressure losses with turbocharging. In order to keep this noticeably low, it is e.g. the possibility to connect only three cylinders to the exhaust gas recirculation in six-cylinder trucks. The other three then supply the turbine the full pressure at any time and thus reduce the turbo lag (asymmetrical exhaust gas turbocharger).

You suspect an error in the exhaust gas recirculation or the EGR-valve, although the fault-memory shows no such indication? In this case, you can shut down this equipment for the purpose of testing. However, don't pull any plugs out, because the control device will notice that. More success is promised - particularly in older - by mechanically blocking the pipe. If you don't want to sacrifice the time and unnecessarily buy a new EGR-valve, block the exhaust gas recirculation for a test drive with a specially manufactured sealing. In new vehicles (e.g., OBD II) the engine management can go into emergency running status, which nevertheless, allows conclusions to be made concerning the error.

A sticky exhaust gas regulation valve can confuse the engine-management-control. Should the engine not start when the EGR-valve is connected but does when it's disconnected, then it should be replaced.