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Heywood 8 (2:26)
Topic(s): Efficiency
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Video Transcript
How efficient are these engines we use? Well, we've got two
types. We've got the spark ignition gasoline engine, and then
we've got the diesel engine. The diesel engine is inherently
more efficient. We know why; it's got a higher compression
ratio, and certain details of how it operates make it 20-25
percent more efficient.
We've got to be careful about efficiency. Engines, if you look
at their best operating point, they're pretty good. They're 30
to 40 percent or so efficient. So 30 or 40 percent of the fuel
energy going into the engine gets delivered by the engine as
work or power; we can use it to drive something. That's the
best efficiency. When you and I are driving around, engines
aren't operating at their operating point. They're operating
more lightly loaded. We don't need much power to drive around.
Under those conditions, friction is much more important in a
relative sense. So the efficiency drops to 20 to 30 percent,
or even lower. After all, if you're standing at a stop light,
the engine's not doing anything. It's just driving the fan for
the heater or the air-conditioning unit. That's all it's
doing, making the lights work. So there, the efficiency's
zero; we're using fuel but we're not driving the vehicle.
So the efficiency varies a lot. And in— If we're honest,
in some average way, as we drive our vehicles around, they're
roughly 10 percent efficient. Sounds bad. But engines are
cold, more friction. Short trips, idling at stop lights, all
of that adds up. Ten percent. And then Amory Lovins, who's one
of the advocates for really working hard to improve vehicle
efficiency, as he said, it all— this big vehicle, me and
my grandson and a few bags, 10 percent of the vehicle weight
at most. So 10 percent of 10 percent is 1 percent. So 1
percent of the energy of the fuel in the tank actually moves
the payload—me and my grandson, and a few bags of
groceries -whatever. So it's much less "efficient," in quotes,
than we think. And we have to ask ourselves: how did we get to
a system where 1 percent of the energy of the fuel in the tank
actually does the task we want done, which is moving me or
you?
