Carburetor & Fuel Injection systems

Function

The carburettor atomises petrol and mixes it with air in the correct ratio (about 15:1 for cruising) for the SI engine under all running conditions.

Construction

        air in
          |
     [ choke ]
          |
     \         /
      \ venturi/<-- fuel nozzle <-- jet <-- float chamber
       \  __  /                         (float + needle)
        |    |
     [throttle]
          |
     to inlet manifold

Main parts:

• Float chamber with float and needle valve: keeps fuel at a constant level slightly below the nozzle tip.
• Metering jet and discharge nozzle projecting into the venturi throat.
• Venturi (choke tube): narrowing that raises the air velocity and creates a depression at the throat.
• Throttle valve: butterfly valve after the venturi controlling the mixture flow.
• Choke valve before the venturi for cold starting.

Working

During suction the piston draws air through the venturi. At the throat the velocity is high and the pressure falls below atmospheric pressure. The float chamber is at atmospheric pressure, so the pressure difference forces petrol through the jet and nozzle. It is broken into fine droplets and mixed with the air. The throttle controls the quantity of mixture and therefore the power.

Flow of air and fuel (neglecting the nozzle lip):

$$\dot{m}_a = C_{da} A_t \sqrt{2\rho_a \Delta p}, \qquad \dot{m}_f = C_{df} A_f \sqrt{2\rho_f \Delta p}$$

Both flows depend on $\sqrt{\Delta p}$, but the air density changes with the throat pressure while the fuel is incompressible, and the discharge coefficients change with flow. As a result, the simple carburettor supplies a mixture which gets richer as the throttle opens.

Why it fails and the remedies

Given: $d_t = 0.032$ m, $p = 101.3$ kPa, $T = 300$ K, $\Delta p = 5000$ Pa, $C_{da} = 0.85$, $C_{df} = 0.65$, $\rho_f = 740$ kg/m$^3$.

Air density and throat area

$$\begin{aligned} \rho_a &= \frac{p}{RT} = \frac{101\,325}{287 \times 300} = 1.177\ \text{kg/m}^3 \\ A_t &= \frac{\pi}{4}(0.032)^2 = 8.042 \times 10^{-4}\ \text{m}^2 \end{aligned}$$

Air flow

$$\begin{aligned} \dot{m}_a &= C_{da} A_t \sqrt{2 \rho_a \Delta p} \\ &= 0.85 \times 8.042 \times 10^{-4} \times \sqrt{2 \times 1.177 \times 5000} \\ &= 0.85 \times 8.042 \times 10^{-4} \times 108.48 = 0.0742\ \text{kg/s} \end{aligned}$$

(Equivalent to 267 kg/h.) The throat velocity is $\sqrt{2\Delta p/\rho_a} = 92.2$ m/s, which is well below sonic, so the incompressible assumption is acceptable.

Fuel flow for A/F = 15

$$\dot{m}_f = \frac{0.0742}{15} = 4.944 \times 10^{-3}\ \text{kg/s}$$

Jet area and diameter

$$\begin{aligned} A_f &= \frac{\dot{m}_f}{C_{df}\sqrt{2\rho_f \Delta p}} = \frac{4.944 \times 10^{-3}}{0.65 \times \sqrt{2 \times 740 \times 5000}} \\ &= \frac{4.944 \times 10^{-3}}{0.65 \times 2720.3} = 2.796 \times 10^{-6}\ \text{m}^2 \\ d_f &= \sqrt{\frac{4 A_f}{\pi}} = 1.887 \times 10^{-3}\ \text{m} \end{aligned}$$

Answer: air flow $= 0.0742$ kg/s (267 kg/h); fuel flow $= 0.00494$ kg/s (17.8 kg/h); jet diameter $\approx 1.89$ mm.

Valve timing is the setting of the crank angles at which the inlet and exhaust valves open and close with respect to the dead centres. In theory the valves would open and close exactly at TDC and BDC, but this does not work in a real engine.

Typical diagram (petrol engine, crank angles)

           TDC (end of exhaust, start of suction)
              EVC 15 | IVO 10
         . - ' ' -  -|-  - ' ' - .
      /     overlap 25 deg         \
    |       exhaust  |  suction     |
    |                |              |
      \   EVO 40     |      IVC 35 /
         ' - . _ _ _|_ _ _ . - '
           BDC (EVO before, IVC after)

Angles are in crank degrees: EVC after TDC, IVO before TDC, EVO before BDC, IVC after BDC.

(For diesel engines the figures are similar, with injection at about 15-25 degrees before TDC.)

Reasons

• EVO before BDC: the gas pressure is still high, so the valve is opened early to let most gas escape (blowdown) and reduce the back pressure on the piston in the exhaust stroke. The small loss in expansion work is less than the gain.
• EVC after TDC: the moving gas column has inertia, so the cylinder is scavenged more fully.
• IVO before TDC: so that the valve is fully open when the suction stroke starts. This gives less throttling.
• IVC after BDC: the fresh charge keeps moving in due to its inertia (ram effect), so cylinder filling is better and the volumetric efficiency is higher.

Valve overlap

The period during which both valves are open together near TDC at the end of exhaust and start of suction (here 25 degrees). It helps scavenging of exhaust gas and cools the valves, but too much overlap lets fresh mixture escape at low speed.

MPFI system

In MPFI one injector is fitted in the inlet manifold near the inlet valve of each cylinder. An electronic control unit (ECU) decides how much fuel is injected.

 Sensors --> [ ECU ] --> Injectors (one per cyl.)
 MAP/MAF,       |             ^
 throttle,   ignition      fuel rail <- pump <- tank
 coolant T,   timing         + pressure regulator
 O2, rpm

Components

• Electric fuel pump, filter, fuel rail with pressure regulator (about 2.5-3 bar).
• Solenoid injectors, opened for a short pulse width set by the ECU.
• Sensors: air-flow or manifold pressure, throttle position, engine speed and crank position, coolant temperature, intake air temperature and exhaust oxygen (lambda) sensor.
• ECU with stored maps.

Working: the ECU uses the sensor data to calculate the air mass, and opens each injector for the time needed to give the required air-fuel ratio (about 14.7:1 with a three-way catalyst, using the lambda sensor as feedback). Injection may be sequential, timed with the inlet valve.

Advantages over a carburettor

• Accurate mixture to every cylinder, better distribution.
• Higher volumetric efficiency, since there is no venturi restriction; more power.
• Better fuel economy and lower emissions; works with catalytic converters.
• Better cold starting and throttle response, no icing.

Fuel feed pump

A low-pressure diaphragm or plunger pump driven by a cam lifts diesel from the tank, passes it through filters, and supplies it to the injection pump at about 1-2 bar.

Jerk-type injection pump

A plunger moves in a barrel with an inlet port and a spill port. The cam lifts the plunger. After the plunger covers the ports it raises the pressure of the trapped fuel to 150-300 bar or more, which opens the delivery valve and injector (the "jerk"). A helical groove on the plunger uncovers the spill port, the pressure collapses and injection stops. Rotating the plunger changes the effective stroke and therefore the quantity of fuel injected for each load.