OT: Hybrid cars

[Oleg Goldshmidt]

guy keren <guy.choo.keren at gmail.com> writes:

> actually, driving at 70-80kmh is usually MUCH MORE fuel-efficient then
> driving at 110kmh, in most cars and under most road conditions...

This may *still* be true for many cars on Israeli roads, but it should
not be regarded as some law of nature or engineering, and it is not a
universal constant. Chances are, it no longer holds for many newer
models (or for any: I'd say the sweet spot was around 90km/h 10-15 years
ago for family-sized cars). The exact number is less important than the
dynamics, in my mind. So let's consider how it evolves.

I'll skip the basic physics (unless someone requests), but you can
design a car for particular requirements, including for most efficient
speed.  Different engines are designed, and can be tuned differently by
default, depending on the market. For instance, cars sold here are not
identical to those sold in Europe. They drive *much* faster on European
highways than we drive in Israel. Typical highway speed limits are
120-130 km/h, 110 km/h on smaller roads. Their requirements are
different. Actually, our highway speed limits are increasing, albeit
slowly: 100-110 km/h is quite common today, as opposed to the universal
90 km/h just a few years ago. This may already be reflected in how
engines are tuned here (and car manufacturers cannot keep developments
in check because of a small and "retarded" market such as Israel, so we
have to accommodate newer models), but you certainly need to talk to a
qualified mechanic and not to me.

I do not keep records or statistics, but my unscientific observations
lead me to believe that my 6 year old Passat is more efficient at
110km/h on Highway 1 or 2 than at 80-90. But it may be very different
from most contemporary Mazda 3's and Corollas and what not, e.g., the
6th gear must be somewhat helpful at higher speeds, it may use a leaner
air-fuel ratio than many other engines, etc. Most importantly, its
engine power vs. aerodynamics may be very different (I promised to skip
the physics, but aerodynamics are crucial).

Generally, smaller cars are often intended for urban driving - shorter,
slower rides, easy parking, etc. It makes sense to optimize for lower
speeds, frequent gear shifts (at low gear), shorter gear ratios,
etc. Note that you cannot optimize much for too low speeds (basic
physics again).

An important relevant consideration when discussing the dynamics of this
is that it makes more sense to improve highway consumption than urban
one to improve the mixed number, since you typically drive longer
distances on highways and you quote average mileage that is skewed
towards highways. E.g., if your daily mix when driving to the city to
work and back is 100km on the highway (at 20km/l) and 10km in the city
(at 10km/l) then you use 6l over your trip (5 on the road, 1 in the
city). Improving highway consumption by 10% will reduce your daily total
to ~5.5l. Improving your city consumption by 10% will reduce your total
to ~5.9l. If you improve highway by 10% but screw up the city part by
10% you will still be better off at less than 5.6l. If you improve the
city part by 10% but your highway becomes 10% worse you'll be at almost
6.5l.

>From this it follows that as your roads and cars get better and faster
(and safer) it makes a lot of sense to optimize for higher road
speeds. If you can drive on Road 2 at 110km/h rather than 90km/h (my
example above roughly corresponds to commuting from Hadera to TA or to
Haifa), it makes sense to optimize for highway. Even if you buy such a
car and use it mostly inside the city it is still not bad since you are
not in an efficient regime anyway and much less can be done under
50km/h. (Well, getting a small, light car in this case will make sense,
and screw aerodynamics).

-- 
Oleg Goldshmidt | pub at goldshmidt.org