How a CV (Constant Velocity) Carburetor works

A well tuned CV carb can operate nearly as well as computer controlled fuel injection, but without the MAF, O2 sensors and electronics the latter requires. Well tuned CV carbs sometimes actually outperform computer controlled fuel injection in terms of the smoothness of power and torque delivery. It is an elegant design based on Bernoulli's Law, which is a way of expressing conservation of energy in a gas or fluid.

BERNOULLI'S LAW

Bernoulli's law says that, unless power is applied to or drawn from the system, the energy of a gas or fluid remains constant. It expresses that energy in three components, so the sum of the three must be a constant. That is:
Kinetic energy + Potential energy + pressure energy = constant

Now for some simplifying assumptions:

  • Assume potential energy doesn't change, which means the flow is horizontal - not up or down.
  • Ignore constants because all we care about is proportionality

    • This leaves us with:
    (velocity squared) + pressure = constant

    Put differently:
    P := -V^2

    ENERGY

    If we are going to use Bernoulli's law in a carb we have to find a constant for reference: pressure, energy or velocity. Since pressure and velocity are varying, we will use energy. So next we have to determine the energy of the air/fuel passing through the carb throat. The energy can't be constant, because some of the engine's power is used to draw from the intake. At full throttle at high RPM the engine is producing a lot more power than when it's idling.

    The concept I use for energy is as follows:

  • The engine RPM determines the total energy available for the intake.
  • The throttle (downstream butterfly valve) determines how much of the engine's energy is imparted to the intake.

    • The energy in the intake is always determined by the lower of these two constraints: If the throttle is closed, intake energy is low no matter how high the RPM is. If the throttle is open, intake energy is only as high as the RPM is.

    So throttle setting and engine RPM together determine how much energy the intake mixture has.

  • At closed throttle idle, the energy is very low and constant: the engine has little power and the throttle lets little of it through.
  • At open throttle high RPM, the energy is high and constant: the engine has a lot of power and the throttle lets the maximum amount through.
  • At open throttle low RPM, the energy is medium and increasing: the engine has little power but the throttle lets the maximum amount through. As more mixture comes through, the RPM increase, the throttle remains open, so the intake gets more energy.
  • At closed throttle high RPM, the energy is high and suddenly decreasing: the engine has a lot of power but the throttle just closed so only a trickle gets through. But the air already in the system before the throttle was closed, still has all the residual kinetic energy it had a moment ago before the throttle was closed. As that energy is spent, the total energy in the system decreases.

    • One very important concept is that whatever this energy is, it is the same throughout the entire intake system from the carb throttle to the final airbox opening. That is, the energy just upstream from the throttle is the same as at the slides, in the airbox, etc. The key to this analysis is to look at what happens in two separate places in the CV carburetor:

  • At the throttle, where the bore is fixed
  • At the slide (upstream from the throttle), where the bore changes with pressure. Lower pressure causes the bore to increase, and vice versa.

    • In both of these places, the energy has to be the same, but the pressure and velocity can be different. This is the key to understanding how a CV carburetor works.

    VELOCITY VS. AREA

    Another implication of Bernoulli's law is that, if energy is constant, velocity of flow is inversely proportional to cross sectional area or bore. When a fluid or gas moves through a pipe, it slows down in fat sections and speeds up in narrow sections.

    REVVING UP

    Start with the engine at closed throttle idle, open the throttle wide and hold it.

  • The energy is initially low, but suddenly the throttle lets more energy into the intake. This is a sudden increase in energy.
  • According to Bernoulli, this energy has to be manifested throughout the intake as higher pressure or higher velocity (or some combination of both).

    • AT THE THROTTLE

  • The bore at the throttle got bigger, but the bore immediately upstream from the throttle is fixed and unchanged. This is what we will consider.
  • The intake charge has inertia, so the velocity can't increase instantaneously.
  • Thus, the instantaneous change is that the pressure increases, but this is a transient condition. There is nowhere to statically "hold" that pressure, so it can't build up for long.
  • The initial transient pressure increase means less fuel is drawn up the jet - this causes a transient lean condition.
  • Then pressure drops and velocity increases. The intake charge starts rushing through the carb throat faster.
  • The throttle bore is constant, so higher velocity means more air is being drawn through, and more air needs more fuel.
  • As velocity increases, pressure drops and lower pressure means more fuel drawn up the jet. Thus the throttle sees more air and more fuel - mixture remains relatively constant.

    • AT THE SLIDE

  • The slide was at rest (closed), so the bore here is the same as it is downstream near the throttle.
  • The initial transient pressure increase tends to keep the slide down.
  • As with near the throttle, there is nothing to hold a static pressure increase, so the velocity begins increasing.
  • As the velocity increases, the pressure drops.
  • If the energy continues increasing (the engine revs up as the throttle is open), the velocity will continue to increase.
  • At some point, the pressure drops to the point where the slide begins to lift.
  • As the slide lifts, the bore increases, which slows down the intake charge.
  • As the intake charge slows down, the pressure increases, which stops the slide from lifting further.
  • If the total energy continues increasing (engine revving every higher), then the velocity continues increasing.
  • Eventually, as the engine reaches a certain power level, the slide is fully lifted. As long as the energy remains at this level or higher, the slide remains open.
  • The needle is connected to the slide. So even if pressure remains relatively constant as the slide lifts, the needle lifts too which enables the same pressure to draw more fuel up from the jet. Thus the mixture remains relatively constant.

    • The concept here is that once the slide begins to lift, the velocity at the slide will be more or less constant. If the velocity increases, this drops the pressure, which lifts the slide, which slows the velocity back down. If the velocity drops, this raises the pressure, which drops the slide, which speeds the velocity back up.

    The slide is a classic negative feedback system.

    Note that while the velocity remains constant, the energy does not. At a higher RPM, the slide is further open, which means a larger amount of mixture is flowing at that velocity. More mixture (mass) at the same velocity means more total kinetic energy. This is the additional energy from the engine that the throttle is passing through to the intake.

    REVVING DOWN

    Start with the engine at high RPM, wide open throttle, with the slide fully raised. Then suddenly shut the throttle and keep it closed.

  • The energy is initially high, but suddenly drops.
  • The throttle is no longer allowing energy from the engine to be pumped into the intake charge. But the intake charge has inertia - it keeps flowing with the kinetic energy (velocity) it had before the throttle was shut.
  • The closed throttle is a physical barrier to the intake charge, so it has to lose velocity. Thus the pressure must rise - a lot. The kinetic energy is converted into pressure energy.

    • AT THE THROTTLE

  • Pressure increases, so less fuel is drawn up the slow jet.
  • As pressure gradually drops, velocity doesn't speed up because total energy is dropping too.

    AT THE SLIDE

  • The rising pressure causes the slide to lower.
  • As the slide lowers, the bore is reduced which would normally speed up the intake charge. But this can't happen because the throttle is blocking the flow and the engine is not drawing very much. That is, new energy is not being pumped into the intake.
    • Thus the slide drops and stays there as long as the throttle is closed.
  • The engine is still spinning down from high RPM, but the slide is closed.