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Old 03-18-2013, 03:30 PM   #43
fielderLS3


 
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Drives: 2016 Mazda6, 2011 Mustang 5.0
Join Date: Mar 2010
Location: Portage, Wisconsin
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Quote:
Originally Posted by CAP'N B View Post
Maybe I got the concept wrong, as far as I know direct injection is fuel that is injected directly into the cylinder, not the intake port, therefore there is no fuel in the incoming air charge until the injector sprays it into the cylinder. The same way a cam sensor tells the coils when to fire so it could tell the injector when to fire causing the heated by compression air charge to explode. We already use a throttle sensor and electronics to control the air/fuel ratio, the only thing a diesel has different than a gas engine is a governor. Diesels do have throttles. LOL
Direct injection does directly spray the fuel into the cylinder, but if I understand it correctly, it injects it during the intake stroke, so an air/fuel mixture is still there during the compression stroke. The evaporation of some of the fuel right in the cylinder cools it, allowing for a somewhat higher compression ratio without detonation. The timing is still determined by a spark. With a diesel, only air is compressed for the compression stroke (at least most of it) with the timing of ignition determined by injecting the fuel at exactly the right moment.

And yes, some newer diesels may have "throttles" but the purpose and workings is different (mainly for emissions...also having a way to close off the intake in the rare event of a runaway, too). They are still not "throttled" in the same sense that a gas engine is, and it is not strictly necessary to have one for a diesel to work. A diesel's air/fuel ratio range is large, with power determined more by how much fuel is put in alone (up to an upper limit depending on the available quantity of air), where with gas, the fuel has to be carefully metered to stay fairly close to its 14.7 ratio, so power is more strictly controlled by the amount of air allowed in, as that more closely determines how much fuel can be mixed in.
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