EGR Control
Why have an EGR valve? In practice, combustion using the exact ratio of fuel to air has some significant problems. The formation of soot, nitrous oxides and diesel knock (explosive detonation). To counter these problems the air is mixed with cooled, inert, exhaust gas to dilute the oxygen content.When does the EGR operate? The EGR in most modern engines is controlled by the ECU. In order to decide when to operate the valve the ECU must have to know two things. The mass flow of gas through the engine and the flow of air entering from the atmosphere. Since the exhaust gas is being recirculated the two are not equal. However, the volume of gas being 'pumped' by the engine can be calculated from the engine revolutions, engine size, air pressure and temperature. The flow entering from the atmosphere can be measured by the MAF. So the ECU should be able to calculate and control how much exhaust gas is being recirculated by the EGR valve.
So when does the EGR operate. Unlike a petrol engine a diesel does not have a throttle. A diesel simply alters the amount of fuel to control the power.Less fuel less power, more fuel, more power, up to the point where the quantity of oxygen in the air will burn no more.
As this point is reached the combustion process begins to get problematic as detailed above. The combustion process is at its maximum temperature and air fuel mixing becomes difficult. Therefore a solution is to recirculate the now oxygen depleted exhaust gas back into the engine as this point is reached.
So when the accelerator is depressed it effectively is requesting the ECU to increase the fuel flow. From the engine revs and MAF the ECU knows when the critical maximum fuel to air ratio is being reached and opens the EGR valve.
Turbo Control
Why a turbo? A turbo, like a supercharger, is a way of forcing more air and therefore oxygen through an engine. This means more fuel can be burnt and more power produced. This also results in higher efficiency by utilising some of the wasted energy contained in the exhaust gas.How does it operate? Most modern turbos use variable vanes on the exhaust side. When closed they create the maximum pressure differential across the turbo and generate the maximum power.
However, under normal driving conditions the amount of oxygen entering the engine will exceed the power and fuel required. Therefore, the ECU will open the vanes and maintain the flow at atmospheric pressure.
However, as more power is requested and the maximum fuel:air ratio that can be achieved at normal pressure is reached, the vanes are closed and the turbo begins to raise the pressure and consequently air flow. Therefore, more fuel can be burnt and more power produced.
