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There are 3 types of EGR's: vacuum, digital, and PWM. Vacuum is vacuum so I'm not talking about that one. The purpose of the EGR is to reduce oxides of nitrogen (NOx) emissions.
From AllData:
"The atmosphere is made up of mostly Nitrogen, with a smaller percentage of oxygen, and a mixture of other gases. Oxygen and Nitrogen do not normally combine except at very high temperatures and pressures, conditions which are present in the combustion chamber especially during hard acceleration. When the engine is under load, the EGR valve admits a small amount of exhaust gas into the intake manifold to mix with the air/fuel charge. The exhaust gas is essentially inert (contains no fuel or oxidizer) and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion without participating in the actual burn. Greater amounts of exhaust gas are metered in as engine speed and load are increased."
The digital EGR uses 3 different sized solonoids, think of it as a low/meduim/high setting and the combonation of the 3 solonoids activating can vary how much exhaust is let into the intake.
The new design is the Pulse Width Modulation (PWM) solonoid which is basically infinate in its adustability while the digital is somewhat stair-stepped.
From AllData:
"The atmosphere is made up of mostly Nitrogen, with a smaller percentage of oxygen, and a mixture of other gases. Oxygen and Nitrogen do not normally combine except at very high temperatures and pressures, conditions which are present in the combustion chamber especially during hard acceleration. When the engine is under load, the EGR valve admits a small amount of exhaust gas into the intake manifold to mix with the air/fuel charge. The exhaust gas is essentially inert (contains no fuel or oxidizer) and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion without participating in the actual burn. Greater amounts of exhaust gas are metered in as engine speed and load are increased."
The digital EGR uses 3 different sized solonoids, think of it as a low/meduim/high setting and the combonation of the 3 solonoids activating can vary how much exhaust is let into the intake.
The new design is the Pulse Width Modulation (PWM) solonoid which is basically infinate in its adustability while the digital is somewhat stair-stepped.