Michael Torrie <[EMAIL PROTECTED]> writes:
> Turbos can and *do* increase overall efficiency. Remember that
> efficiency is not measured in fuel per time, but rather fuel per unit of
> work done per unit of time (or something like that :). I only have
> experience in turbos on large diesel engines, but they do increase power
> *and* efficiency in that application. For example, suppose a naturally
> aspirated diesel produces 250 HP. With a turbo, the same engine can
> produce 320 HP, but as you say it does burn more fuel to produce 320 HP.
> But the amount of fuel required to generate 320 HP in the naturally
> aspirated engine would be a lot more than the turbo 320 HP uses.
> Efficiency is about decreasing the rate of increase in fuel consumption
> as you increase in speed, or work done per unit of time.
Well, diesel and gasoline engines are somewhat different beasts. For
example, diesel engines have no spark plugs, and achieve ignition
solely through compression. This means that the more air you shove
in, the better. Diesel's don't have a throttle valve, so they're
cramming in as much air as possible all the time. Higher air input =
higher compression = higher efficiency. More power is generated by
injecting more fuel; if you inject more fuel than you have air for,
you get a huge black cloud behind you. The turbo's pump is always
compressing the intake air, and it's always increasing combustion
efficiency. By adding more air, it also raises the point at which you
stop producing more power and start producing black clouds. :)
A gasoline engine, on the other hand, injects fuel during the intake
stroke. That means that if you compress too much air, it gets too hot
and ignites before the piston is in the right place and the spark plug
goes off, which can bend your rods or blow your pistons apart. So,
there's the throttle valve and the turbo wastegate that prevent too
much air from getting into the intake. During cruise, most of the
blowing the turbo is doing is wasted, and thus the efficiency of the
engine is lower than it would be without that exhaust restriction.
> The ECM cuts fuel flow for a given rpm because the turbo has boosted the
> efficiency of the engine (as well as power, etc), reducing the amount of
> fuel required to maintain that RPM over the amount that would be
> required without it. On Alex's car, for example, At 90 MPH with the
> turbo working at peak, you're definitely burning *more* fuel than you
> did before the turbo kicked in at 70 MPH, but your MPG is way up.
> Efficiency in this case is about how many miles you can go on a gallon.
> Turns out that, on alex's car, driving 90 MPH with the turbo screaming
> is more efficient than driving 70 without the turbo.
Having tuned the ECM in my own gasoline-powered car, I can guarantee
that the non-boosted maps are exactly the same with and without turbo.
Now, the turbo may mean that the engine has to work slightly less hard
to get air in at a given throttle opening, but this is offset by the
exhaust restriction that makes it harder to get it out.
Driving at a constant speed (on the road, anyway) means that the
throttle isn't completely open and there's some degree of vacuum in
the engine, unless you happen to be going up a steep hill or into some
serious wind at high speed. Thus, although the turbo is spinning, the
intake manifold is below atmospheric pressure and most of the air the
turbo is pumping is going straight out the wastegate. The ECM is in
non-boost mode, and is operating exactly the same as if it were
drawing that much air without the turbo.
If you're driving faster than before, it's probably true that you were
able to accelerate to that speed much faster, and perhaps wasted less
fuel getting to that speed. I can guarantee, though, that you're
using more fuel at 90mph than 70mph simply because air resistance
increases dramatically. Power required to overcome aerodynamic drag
increases at the cube of velocity.
I'm not sure how you measured this efficiency gain on Alex's car, or
even what kind of car it is. Gasoline or diesel-powered? What
components changed when the turbo was installed? I'm not doubting
that he's getting better mileage overall now, as I've seen that happen
before after turbo installs on gasoline-powered cars, but that's not
necessarily due to the turbo itself, but on other factors that changed
due to the turbo install.
--Levi
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