Compression Ratio. What is it? How high is enough?

   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.