Okay, a couple of years ago I was watching a program on the History Channel or some such, and they were showing a late 1950s Cadillac with something like a 500 cubic inch engine, three carbs, and the thing got (according to the show) 28 MPG (highway, I’m sure)! Is this accurate? I mean, the car weighs probably close to five thousand pounds and has all the aerodynamic qualities of a brick, and it gets almost 30 MPG? I know that all three carbs would only be working under periods of high acceleration and that the rest of the time, the car would be using only one. Still, the MPG figures claimed for the car seem high. And if they are accurate, then why, some 50 years later have fuel economies improved only slightly when cars have become lighter and more aerodynamic (SUVs excluded, of course)?
Also, I’ve been wondering about possible automotive applications of something I saw in an article about racing boats. The article profiled a new boat design that someone had come up with. It used inlets on the top of the boat to suck air through the hull and exhuast it underneath the boat. This turned the water under the hull into “an aerated foam” which the boat was able to pass through easily, thus cutting the friction it had to overcome to move and allowing the boat to go faster on less fuel and power. If someone were to modify a car (and this would probably be impractical for use in anything other than an experimental vehicle) to exhaust air directly behind the car in the low pressure zone which forms behind the car, would there be a significant improvement in performance? (At normal highway speeds, of course. Doesn’t do you any good if it only works at 100 MPH+.)
The figures for the Caddy seem unlikely. However, they might perhaps have a grain of truth. If, as you say, the tests were completed on a very unrepresentative highway cycle (little or no acceleration, just cruising at, say, 50 mph) on over inflated tyres and with a very gentle touch on acceleration, then the weight of the car won’t have much effect, and the only considerations are rolling and air resistance. Both of those are reasonably marginal (as regards the differences between modern and 50’s cars), so maybe…
On to your second idea, it seems to me that you are unlikely to be able to gain an efficiency increase. The basic problem is to shift air from the high pressure area in front of the car and into the low pressure area behind the car. You have two ways of doing that. One is the conventional method of allowing the air to flow around the car (which isn’t perfect, but works reasonably well). The other is to move air through the car. Assuming the car is going to be useable (ie it’s not going to be simply a tube with no interior space) you are going to have to suck air into the car and then pump it through a comparatively small pipe (compared to the frontal area of the car) and out the back. The energy lost in compressing the air to do this is (I would guesstimate) going to be more than you gain from reduced drag.
The boat idea has been used in underwater high speed missles. Exhaust it emmited throught he nose cone so the missle is travelig through exhaust gas instead of water. The drag is greatly reduced and the speed is greatly increased. Idealy the goal is to get the missle moving so fast that the nosecone will part the water so the missle could travel in a low pressure bubble of just water vapor.
As for something like this to work in a car - I really don’t see the advantages.