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When tractors were first developed, they used enormous steam engines that were notoriously unreliable and hard to maintain. These were phased out around the turn with the twentieth century and substituted for internal combustion engines which were smaller though powerful and ran on a number of fuels including kerosene, ethanol and gasoline. By the 1960s several of these engines were phased out and only more effective internal combustion engines that ran on diesel and after this, biodiesel. The EGR system routes a number of the exhaust gases (6 ? 10%) through the EGR valve, back in for the Intake System, and finally inside combustion chamber. The exhaust gases are inert i.e. they do not burn inside the combustion chamber; also, the exhaust gases occupy a few of the space which may otherwise be taken from the intake air. This cuts down on level of intake air readily available for the combustion process during each Power Stroke; which often decreases the peak combustion temperatures. This ends in lower NOx emissions.