Technical Minutiae

November 9th, 2009

When it comes to cars, I’m fascinated with technical minutiae.

In the tech Q&A section of one of the car magazines I read, the question was recently posed, “Are there any disadvantages to direct-injected engines?”  Many automakers are switching from port-injection to direct-injection in an effort to increase performance and fuel-efficiency.  (What’s the difference?  In traditional port-injected engines, fuel is squirted into the incoming stream of air before it goes past the intake valve into the combustion chamber where it is ignited.  Direct-injection, on the other hand, involves injecting the fuel directly into the combustion chamber – and air is the only thing that enters via the intake port.)  Direct-injection is used in many high-end and mainstream vehicles now, and will probably make its way further down-market very soon.

MicroscopeTurns out, there are a couple of concerns with direct-injection that aren’t immediately obvious.  One involves the intake valve itself.  Along with the air that flows past it is the small amount of gases that leak past the piston-rings and into the crankcase, returned into the intake stream via the PCV (positive crankcase ventilation) valve. This is essentially the “oily” air that resides in the bottom half of the engine.  Over time, the PCV system could cause oil-residue to build up on the back-side of the intake valve.  In an older port-injected engine, the gasoline washes this residue away, but in a direct-injected engine, this residue never gets cleaned off.  (Ford solved this problem by adding an additional filter to the PCV system.)  Another concern is that directly injecting gasoline into the cylinder could wash the oil-film from the cylinder walls, increasing wear of the rings.  My guess is that this will turn out to be a non-issue, given the amount of testing that goes into proving the fundamental combustion processes of engine technology.  But this is what I find fascinating – there are significant repercussions to even slight design modifications.

It’s sort of the law of unintended consequences.  (Increased ethanol production caused a spike in the price of tortillas, anyone?)  Not long ago, I had a conversation with an automotive engineer that was studying the role that lubricants (i.e., your motor oil) play in auto emissions – a role that is increasing as vehicles become more fuel-efficient.  Motor oil formulations have been (and will continue to be) altered in order to reduce their contribution to tailpipe emissions.  As an example, the amount of zinc dialkyl-dithio-phosphate (ZDDP) was recently reduced in engine oils, due to its detrimental effects on emissions equipment (such as catalytic converters) over time.  Unfortunately, it’s the ZDDP that helped to protect the metal-to-metal impact surfaces in engine valve-trains up until the 1990s.  Owners of cars built before then are now experiencing increased camshaft and lifter wear, upsetting a lot of folks who drive classic (and near-classic) cars. …A slight change somewhere results in unforeseen consequences somewhere else…

At this point, if you’re still reading, your eyes have probably glazed over.  Technical minutiae isn’t for everyone.  But sometimes, it’s the tiniest of details that matter.

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