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Old 04 November 2014, 08:00 PM
Alchemy Alchemy is offline
Join Date: 05 March 2001
Location: Plymouth, MI
Posts: 4,407

Originally Posted by RichardM View Post
EGR is primarily to reduce pollution. It doesn't improve power output at all.
"Reduced power loss" as in "Less of the engine's power output is spent pulling air across a high-drag-coefficient obstruction, i.e., a mostly-closed throttle plate."

I should have just said "improved fuel efficiency at low power demand."

EGR always reduces engine power output because it's cutting down how much fuel and air you can get into the cylinder. (Which is why EGR is not active during high power demand.)

But when the engine is at low load and low throttle angle, a lot of the power being produced by the engine is being spent sucking air across the throttle body. Recirculating exhaust back into the cylinder reduces the amount of fresh air you need to draw through the throttle body, and increases manifold pressure so that the throttle can be opened further for the same flowrate. Both of these things reduce the amount of energy the engine spends on sucking air from outside.

While EGR will in some cases reduce the efficiency of combustion (the charge still burns completely but the energy isn't as easily extracted), there are a lot of low-load conditions where the reduced throttle loss more than makes up for it, giving you improved fuel economy overall.

Since diesels don't control power by throttling, and can handle a wide range of air-fuel ratios, there's no real benefit, fuel-efficiency-wise, in displacing the charge with exhaust. So while EGR also reduces NOX in diesels, it does so at a significant fuel economy cost.
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