re: direct injection

[James Montebello <jamesm@domain.elided>]

All diesels are "direct injection", which is to say, injection directly
into the combustion chamber, not the intake manifold.  VWs acronym is a
bit redundant.  The SPICA injection pump is derived from a diesel injector
pump, which require very high pressures to inject directly into a cylinder
of air near the top of the compression stroke, quite unlike the relatively
low pressures required for manifold injection on gasoline, spark-ignited
engines.

Direct injection for gasoline, spark-ignition engines is also nothing
new, being employed at least as early as the 1930s, when Mercedes used
them in aero engines, and GP car engines.  Some of the advantages are:
no introducing fuel during the overlap phase, so much better fuel economy
and lower emissions, since any fresh charge escaping out the exhaust
valve is only air; better vaporization, since you're injecting into a
high pressure, hot combustion chamber against hot pistons and chamber
walls rather than relatively cool manifold walls; slightly better
accuracy in per-cylinder fueling over port injection (and MUCH better
than throttle-body injection).  Indeed, port injection came AFTER direct
injection, as a cost-cutting measure.

One of the things holding direct injection back in spark-ignition
engines is this significantly higher complexity and cost of a super-high
pressure system where the injectors need to deal with direct exposure
to cylinder heat and pressure.  I suspect that the gains are relatively
small over port injection, probably on the order of 10%, but as we push
ever forward towards extracting the last few micrograms of efficiency
from Otto-cycle engines, that last 10% will become important, and worth
the additional cost.

james montebello

On Sat, 9 Feb 2002, Jay Negrin wrote:

> And, Ryan, for what it's worth, the VW Turbo Diesel is *also* direct
> injecion.  The designation I have seen it TDI, which stands for, I
> believe, Turbo, Direct Injection.
>
> Is there something about Direct Injection, compression or spark
> ignition, that makes things *that* much better/  I guess.....
>
> Jay Negrin
> ARO So Cal
> 76 Alfetta GT - the only direct injection it gets is the irregular
> infusion of parts and/or money to keep it on the road.  All willingly
> done in the name of fun and adventure. (mandatory, though limited, Alfa
> content)
>
> ---- -- - - -      -- - -- -------- -- --- - ----- - - -- ---- -- -
>
> From: Ryan Fellini <rfellini@domain.elided>
> Subject: RE: 156 Direct Injection - 165 bhp, 152lb-ft from 2.0 liters
>
> non-turbo of course. ;>
>
> > Wait a minute, doesn't the new VW 1.8 put out 190hp? I'd say that's
> mass produced engine, or are you just talking about non-turbo engines?
>
> ps. VW 1.8L 20v turbo -> 180 HP, more impressive is their 1.9L Turbo
> Diesel that makes 150 HP and more torque than an Audi S4.. all while
> getting roughly 50mpg.. ah man :>
>
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