Gasoline engines
(car engines) work by igniting a flammable fuel air (gasoline air) mixture when
the piston is at top dead center. The heat of the burning fuel creates a high
pressure in the cylinder and that pressure forces the piston down.
Heat energy (from the fuel burn) is converted
into mechanical energy to turn the wheels of the car.
As long as the piston can travel down, heat
energy gets converted into mechanical energy.
Unfortunately, in real practical engines, the piston reaches bottom dead
center, after a travel in the order of four inches.
At bottom dead center the exhaust valve opens
and the burning mixture, still very hot, is wasted to the atmosphere.
If we could build engines where the piston
could keep going down, for maybe 10 feet, we could get a lot more work out of
the fuel we burn.
Unfortunately such an
engine would never fit under the hood, or even inside the length of a car.
It was found that
the measure of engine efficiency is the ratio of the volume of the combustion
chamber and top dead center, to the volume of the entire cylinder at bottom
dead center. You can raise the
compression ratio by making the combustion chamber really really small, and
living with the four something inch stroke of the typical engine.
In gasoline engines
something else limits the possible compression ratios. The piston compresses a flammable mixture of
fuel and air. Compressing a gas (a fuel air mixture) heats it. Sooner or later the mixture will
spontaneously burst into flame. The
mixture burning on the compression stroke is called knocking in automotive
circles. Most of us have heard the
noise, a kind of banging, when lugging up a hill, rpms too low, load too high. Knocking is more easily heard on standard
shift cars. Slush box cars are
programmed to down shift, get the rpms up, and avoid knocking. The onset of knocking depends upon the grade
of the gasoline. 80 octane regular
gasoline is about as low as we go in the United
States and will not allow a compression
ratio higher than 8:1. 100 octane high
test is quite knock resistant and can support a compression ratio as high as
13:1, beloved of car racers.
In recent years the
advent of microprocessors in cars has allowed compression ratios as high as
10:1 to run on 80 octane regular gas. The
microprocessor detects the onset of knock and adjusts the ignition timing to
control the knocking.
The greenies have
been working hard to reduce car fuel efficiency. They believe that car exhaust emissions of
various nitrogen oxides are the cause of LA smog, and they have written
regulations to reduce nitrogen oxide emissions.
Nitrogen oxides are formed whenever air is heated to extreme
temperatures. High compression ratios
yield extreme temperatures as well as high fuel efficiencies. To pass the greenies nitrogen oxide limits
most car makers are reducing compression ratios and taking the hit to fuel
economy.
In actual fact,
nitrogen oxides all wash down in the rain, all nitrates are soluble. And nitrogen oxides are good for plants; the
botanists call the stuff fixed nitrogen.
When you buy fertilizer for your lawn you are buying nitrogen
oxides.
LA smog happens
when nitrogen oxides react with oily vapors from spilled fuel, loose gas caps,
and worn engines that burn oil. We could
reduce smog by clamping down on oily vapor emissions and permit car engines to
run high compression ratios to get good gas mileage.