Most people confuse fuel economy with tailpipe emissions.... A common
mistake so no fault, no foul, I guess...
Several factors combine to make exhaust cleaner even though we wind up
burning more fuel to achieve it...
To reduce hydrocarbon emissions and carbon monoxide emissions, the engineers
have resorted to running mixtures on the lean side. This reduces the power
output of the power stroke and that means we need to open the throttle wider
to achieve the same work that a "dirty" engine would do... Even though we
have opened the throttle wider, the mixture is still lean and hydrocarbon
and carbon monoxide levels are low enough that the catalytic converter can
deal with them without going into meltdown....
Another gas mileage killer is the EGR (exhaust gas recirculation) system..
As combustion temperatures pass (caution, memory thing happening) about 1200
degrees F or so, the combustion event starts to do some different things...
If we keep combustion temperatures lower, we don't form oxides of nitrogen.
A we allow combustion temps to rise, NOx becomes a very real problem...
So... when we operate the engine at higher power output levels, we need to
do something to reduce the temperature of the combustion events... Enter the
EGR valve. Exhaust gases are considered "inert" .. we can dilute the working
fluid in the combustion chamber with spent exhaust gases without making the
mixture richer or leaner. Since we will react a smaller amount of working
fluid (the exhaust gases taking the place of reactable constituents),
combustion temps will remain below the level where NOx can for or at least
keep it in a range where it can be either acceptabloe or easily dealt with
in a catalytic converter. Once again, diluting the working fluid makes us
open the throttel more to schieve the same work as the dirty engine.
But, the net result is that tailpipe emissions of the really harmful
components are reduced or eliminated.
The technology is imperfect but then these engines are imperfect as well...
For some interesting insights, there are many presentations on the web
regarding the EcoBoost engine (featuring direct gasoline injection - the
fuel injectors inject fuel directly into the cylinder -and twin
turbochargers). You might also check out the new 6.2 liter gasoline engine
which has two spark plugs per cylinder - unlike the older dual plug 2.3,
these plugs BOTH fire ALL THE TIME.
If you're a real glutton for punishment the nes 6.7 liter diesel is more
complex than anything Ford has had before.. With three new engines coming on
line within a year of each other, this is going to be a hell of a ride for
the dealer techs...