So the problem with low engine design speed is increased weight. A heavier engine costs more to propell and it causes other parts of the car to be heavier too and its also bigger. In all then it's more expensive both to buy and to run.
Nevertheless what you are suggesting has been done at least to an extent. Many cars were (are?) equipped with overdrive. This was based on there being some power reserve at cruising speed in top gear (for acceleration) and providing an extra higher gear to drop the cruising revs which reduced the power output and took away the acceleration margin but provided more economical and quieter cruising. In these days of 5 and 6 gear transmissions this may be obsolete or incorporated anyway but no longer highlighted by a special name. Anyhow it was widely in use a few years ago and presumeably still is. It doesn't go as far as you're suggesting.
Check out this link for a lot more detail:
http://en.wikipedia.org/wiki/Overdrive_(...
As was said above, horsepower to weight ratio is important. An engine that makes a lot of power at low RPM's needs to by physically larger than one running at higher RPM, all other things being equal.
And then the efficiency of an internal combustion engine is only partially dependent on it's operating speed. It's also dependent on how much thermal energy created by the explosion is converted into mechanical energy. The more the gas expands and keeps pusing on the piston, the more energy is derived from a single explosion, increasing efficiency. This usually means a longer stroke and a slower RPM because it takes more time to move the piston up and down. But it's not a direct relationshp of slower=better.
An engine gives its best performance at a certain RPM. Designers aim to make the engine operate in a range around that certain RPM for the various conditions of drive. Normally, that certain RPM is a lot higher than your suggested 1000 - 1500 RPM for highway drive.
You get the best efficiency by running the engine close to that certain RPM. Your concept that the lower the RPM the higher the efficiency is wrong. If you design your car so that the engine runs at just 800 RPM at highway speeds, then you would have a very inefficient set up, as the engine operates in an unfavourable condition.
Some large bore gas engines can operate at 200 or 300 rpm, like the original Otto stationary gas engines produced in the late 1800's. They could operate on many different fuels, but were extremely heavy (tons of weight for 25 horsepower). Today, manufacturers have to consider weight of the engine, as a heavy engine will use more fuel due to the large inertia of all that weight. Diesel engines are better in that they produce more torque at lower speed than gasoline engines, and are becoming more popular since ways have been found to lighten the diesel engine and still have the reliability of a gasoline engine.
Your transmission is the device that changes the ratio between engine speed and car speed. In order for an idle engine speed (my Honda is about 750 rpm) to produce enough energy to keep a car going at highway speed, one would need an extremely high gear ratio. Perhaps you are familiar with the term "overdrive" which is a higher than unity gear ratio. You are looking for a gear ratio that is maybe double what you are used to seeing for a normal overdrive. While theoretically feasible, with a normal car type gas engine you would have very little torque with that kind of ratio. It wouldn't work well because any little variation in load (a bump, change in wind, a very slight hill) would cause it to shift down if it were an automatic to get more torque.
Power is torque time speed. If you decrease the speed of the engine, you have to increase the torque to achieve the same power. I would guess that engines perform best over the speed range you gave.
The higher the RPM's an engine runs at, when it doesn't need to, the more fuel it wastes. Yet every vehicle I know of has to operate at least 1000 - 1500 RPM's at highway speeds. Why aren't engineers able to just design a vehicle that runs at, say, 800 RPM at highway speeds when the vehicle is not accelerating?
There has to be a reason this has never been done, I'm just wondering what that reason is.
