Blessed by the Potato

Hybrids (like my Prius) contain many awesome innovations that give it that great efficiency, leading to lower fuel use (and lower expenses!). With plug-in hybrids starting to hit the market, the ways that technology provides efficiencies increase. Yet even though hybrids have been around for well over a decade, they are still very poorly understood (particularly by auto journalists).

Quick, which one innovation contributes the largest portion of the fuel savings in a hybrid like the Prius, is it:

a) regenerative braking
b) exhaust gas recirculation
c) atkinson-like cycle engine
d) electric motors and a battery to power acceleration
e) aerodynamics, including a tear-drop shape and a plastic cover for the underside to smooth airflow
f) lightweight aluminum construction
g) low rolling resistance tires
h) continuously variable transmission
i) an engine that turns off when not in use (coasting, at stops)

All of those innovations (and I’m sure a few I’ve forgotten) add to the efficiencies. Odds are good that you answered a, or d, or i, since those are the items that are the most distinctive about a hybrid, and which get the most press. And of course, none of those three are of much good when smoothly cruising on the highway, which is why auto writers keep saying stupid, easily disprovable things like “all that hybrid gear is useless on the highway.”

Yet a quick check of fuel consumption ratings shows that indeed, hybrids get fantastic highway mileage. The gap between them and a regular car isn’t as wide in the highway cycle as in city driving, but it’s still a really good improvement. So why is that?

It’s because the actual answer is (c): the atkinson-like cycle engine. Estimates vary, but something like half of the total improvement in overall mileage (and nearly all of the highway-rating boost) is because the engine is simply more efficient at turning hydrocarbons into motion (the “thermal efficiency”). All that business with the electric motors helps, and there certainly is some benefit from pairing the properties of an electric motor (which generates maximum torque at low RPM, and is great for helping to meet peak demand) with a gas engine (which needs to rev up to produce acceleration, and which has a high-density fuel source for steady-state power demands), but it’s there largely because with an Atkinson engine, you’re not going to get anything approaching an acceptable 0-60 time. That efficient engine needs help getting going.

If you wanted (e.g.: if you’re an automotive engineering student looking for a summer project), you could strip out all the electronic stuff from a hybrid and just drive around with the Atkinson engine, and you’d get a big part of the benefit. Of course, you couldn’t actually use that in traffic since it would take you forever to accelerate, but on a closed course to prove a point…

Anyway, once you’ve got the electric motor in there for the acceleration boost, you can do all the extra tricks with it to squeeze out even more efficiency, like shutting off the engine when coasting, regenerative braking, etc.

So now that you know about the importance of the Atkinson-cycle engine, you see why hybrids still rock on the highway. In fact, improved thermal efficiency is also the reason why diesels get decent fuel consumption numbers on the highway: the higher compression of a turbo diesel is also more efficient at turning hydrocarbons into motion. In terms of litres burned per hundred kilometers, diesel (like a Jetta or Golf TDI) is on par with a hybrid like the Prius; though once you factor in the higher energy density of diesel, well, you can see who the winner is (and I won’t get into the issue of city driving, too).

And of course, knowing that this is the single biggest source of efficiency is why I was so upset that GM didn’t include an Atkinson-like engine in the Volt, instead just slapping in an off-the-shelf engine, which is why its efficiency isn’t so great once the charge from the plug-in is used up.