INTRODUCTION

We usually think of performance as the maximum torque and power available at any engine speed. However, except on an interstate on-ramp, we seldom use the maximum torque and power. Most of the time we are much more concerned with the behavior of the engine at partial throttle, transient response, and specific fuel consumption. Engine performance is strongly affected by the way in which the fuel is introduced.

CARBURETION VS. INJECTION

Despite the use of Gasoline Direct Injection in about 1932 by Hesselman, until the early 1970s most cars were carburetted. The carburetor has a narrowed throat, or venturi, in which the speed of the air entering the engine increases, causing the pressure to drop. The pressure drop between this point and the atmosphere causes the fuel to flow from the float chamber of the carburetor into the air stream. The fuel entering the airstream enters as liquid droplets. The mass of fuel entering must be controlled so that it is proportional to the mass of air flowing through the venturi. In addition, the droplet size must be controlled, the droplets must be encouraged to evaporate, the resulting vapor must be mixed uniformly with the air, and some source of heat must be provided to prevent the cooling resulting from the droplet evaporation from causing freezing of the water vapor in the air. In addition, provision must be made for modification of the mixture in response to engine temperature and operation (warm-up, idle, cruise, full power or acceleration).