|View single post by pc|
|Posted: 12-17-2008 07:02 am||
|Fuel delivery in a carburetor is a function of air volume, i.e. Velocity x Area. In the case of a constant area carb (Weber, Delorto, Holley, Mikuni,…) fuel flow varies with velocity and the venturi cross section is fixed. In the case of a constant velocity carb (Zenith-Stromberg, SU, Hitachi,…) fuel flow varies with throat cross section and velocity is fixed.
Because carburetors deliver based on volume, changes in air density (mostly due to altitude and ambient temperature) cause changes in A/F ratio. Factory tuned street engines are optimized for a “nominal” conditions so that they’ll operate reasonably at extremes. Highly tuned race engine carbs are re-jetted for running on tracks at different altitude from where they were originally dyno’ed/tested/tuned.
EFI fuel delivery is a function of whatever the system designers builds it to be.
basic electronic control schemes commonly used include:
Alpha-N – fuel flow is based on rpm’s and throttle angle. This scheme suffers from the same sensitivity to changes in air density as carburetors but is simple and not sensitive to low manifold vacuum. It’s often used on motorcycles and racecars, which operate at wide open throttle (WOT) much of the time.
Speed-Density - fuel flow is based on calculated mass air flow derived from rpm’s, measured MAP (manifold absolute pressure) and air temperature combined with a lookup table of estimates of combustion chamber volumetric efficiency.
Mass Air Flow - fuel flow is based on measured mass air flow
Those are the basic schemes but real systems can include additional sensor inputs to “tweak” the controls to more precisely approximate actual fuel demand.
Real systems often combine modes. They may for example operate as speed-density at part throttle and/or midrange rpm’s then transition to alpha-N at WOT and high rpm’s.
Typical throttle body injection systems are called “electronic carburetors” because they operate like carburetors, introducing fuel at the throttle plates, mixing the air and fuel in the intake manifold, fully wetting the intake tract and are not synchronized to valve operation/piston position.
The intent is to have a uniformly mixed air/fuel charge in the intake tract that is available any time the intake valve opens, making fuel intake to the combustion chambers independent of valve timing. “Electronic carburetor” TBI systems can use any of the three control schemes.
Typical EC/TBI systems do fire injectors on a timed cycle but not synchronized with valve opening. They’re timed to regulate fuel deliver. Since injector solenoids are simply either on or off they can’t regulate fuel flow in a variable manner. They are either not flowing anything or they’re flowing all they can.
The control computer cycles them on and off so the total fuel flow is the average of the fuel delivered between the on and off periods. This is called “pulse width modulation.” The farther up the intake tract the fuel spray occurs the more time it has to vaporize and mix to a uniform air/fuel charge by the time it reaches the cylinder. If it is well mixed in the intake plenum it will be uniform in the intake runners to each cylinder.
A timed, sequential port injection system can base flow volume on any of the schemes but synchronizes the “on” signal for the injectors with intake valve operation. These systems will have the injectors positioned very close to the intake port and include crank position sensing, not just speed sensing, inputs to the control computer.
Since the Patton Machine installation notes mention neither MAF nor crank position sensors but does mention MAP and throttle position sensors it appears to be a speed-density/electronic carburetor system.
Last edited on 12-17-2008 07:04 am by pc