Carburetion and petrol port and direct injection

Mixture requirements of SI engines, how a carburettor meters fuel and how to size its jet, and how port and direct petrol injection improve control, with carburettor and injector pulse-width examples.

Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.

Why it matters

A spark-ignition engine only runs well, cleanly and economically if every cylinder gets the right mass of fuel for the air it traps, at every speed and load. The move from carburettors to port fuel injection and then to gasoline direct injection is the story of meeting tighter emission norms and fuel-economy targets, and the carburettor calculation remains a standard university and GATE numerical.

Key ideas

Air-fuel ratio and mixture strength. AFR is the mass of air divided by the mass of fuel. For petrol the stoichiometric (chemically correct) AFR is about 14.7. The excess-air ratio λ = actual AFR / stoichiometric AFR; the equivalence ratio φ = 1/λ. λ > 1 is lean, λ < 1 rich. Petrol–air mixtures burn reliably only between roughly AFR 7 and 20.

What the engine needs.

  • Cold start: very rich (AFR as low as 3–5 at the carburettor), because only the lightest fractions vaporise on cold walls.
  • Idle and very light load: slightly rich (about 12), to overcome dilution by residual exhaust at low manifold pressure.
  • Cruise (part load): lean of stoichiometric for best economy (about 15–17) in a carburetted engine, or exactly λ = 1 in a modern engine with a three-way catalyst.
  • Full load: rich (about 12.5–13.5) for maximum power and to cool valves and pistons.
  • Acceleration: a momentary extra squirt, because when the throttle snaps open the manifold pressure rises, fuel condenses on the walls and the mixture would go lean.

Carburettor. Air passes through a venturi, where its velocity rises and pressure falls. The fuel nozzle, fed from a float chamber at atmospheric pressure, discharges at the throat; the pressure difference pushes fuel out, where it atomises in the fast air stream and partly evaporates in the manifold. A simple carburettor gives a mixture that grows richer as air flow increases (air density at the throat falls while fuel is incompressible), so practical carburettors add: a compensating jet or air-bleed (main metering), an idle system, a choke for cold start, an accelerating pump, and an economiser or power valve for full-load enrichment. Weaknesses: uneven distribution between cylinders, wall wetting in the manifold, throttling loss from the venturi, poor response, icing, and no closed-loop control, which makes meeting Bharat Stage norms impossible.

Port fuel injection (PFI or MPFI). One solenoid injector per cylinder sprays fuel at about 3–5 bar onto the back of the inlet valve. An ECU calculates the fuel mass from air flow (a mass-air-flow sensor, or manifold pressure and speed, the speed-density method), corrects it for coolant temperature, throttle movement and battery voltage, and sets the injector opening time (pulse width). An oxygen (lambda) sensor in the exhaust closes the loop and holds λ ≈ 1 so that the three-way catalyst works. Advantages over the carburettor: equal distribution, no venturi restriction (higher volumetric efficiency), precise cold-start and transient control, and fuel cut-off on overrun. Some wall wetting at the port remains and must be compensated during transients.

Gasoline direct injection (GDI). Fuel is injected straight into the cylinder at about 50–200 bar (newer systems up to about 350 bar).

  • Homogeneous mode: injection during the intake stroke. Evaporation inside the cylinder cools the charge, raising volumetric efficiency and reducing knock tendency, so a higher compression ratio or more boost can be used.
  • Stratified mode (lean-burn designs): late injection during compression creates a rich cloud near the spark plug in an overall lean charge, so the engine can run with little throttling, cutting pumping loss at part load. Lean operation needs a NOx storage catalyst, which is why most current GDI engines run homogeneous and λ = 1.
  • Drawbacks: higher particulate emissions (fuel films on the piston), carbon deposits on inlet valves (no fuel washing them), expensive high-pressure pump and injectors. Gasoline particulate filters are now used.

Formulas

AFR = m_a / m_f and λ = AFR / AFR_st m_a, m_f masses (kg) or mass flows (kg/s); AFR_st ≈ 14.7 for petrol (take the fuel's value from your data book).

ṁ_a = C_da · A_t · √(2·ρ_a·Δp_a) Air flow through a venturi (kg/s); C_da air discharge coefficient (–); A_t throat area (m²); ρ_a air density (kg/m³); Δp_a throat depression (Pa). Incompressible approximation, adequate when Δp_a is a few percent of atmospheric.

ṁ_f = C_df · A_j · √(2·ρ_f·(Δp_a − ρ_f·g·z)) Fuel flow through the jet (kg/s); C_df fuel discharge coefficient; A_j jet area (m²); ρ_f fuel density (kg/m³); z height of nozzle tip above float-chamber level (m); g = 9.81 m/s².

m_f,cyc = ηv · ρ_a · V_s,cyl / AFR Fuel per cylinder per cycle (kg); V_s,cyl swept volume of one cylinder (m³).

t_inj = m_f,cyc / ṁ_inj + t_dead Injector pulse width (s); ṁ_inj injector static flow rate (kg/s); t_dead injector opening delay (s).

Worked examples

Example 1 (standard, GATE style): sizing a carburettor jet. A simple carburettor has a venturi throat of 30 mm diameter and C_da = 0.85. Air density is 1.2 kg/m³ and the throat depression is 0.05 bar. The fuel (density 750 kg/m³, C_df = 0.75) nozzle tip is 5 mm above the float-chamber level. For an AFR of 15, find the air flow and the fuel jet diameter.

  1. A_t = π/4 × 0.030² = 7.069 × 10⁻⁴ m²; Δp_a = 0.05 bar = 5000 Pa.
  2. ṁ_a = C_da·A_t·√(2ρ_aΔp_a) = 0.85 × 7.069 × 10⁻⁴ × √(2 × 1.2 × 5000) = 0.85 × 7.069 × 10⁻⁴ × 109.54 = 0.0658 kg/s (about 237 kg/h).
  3. ṁ_f = ṁ_a/AFR = 0.0658/15 = 4.388 × 10⁻³ kg/s.
  4. Effective fuel pressure difference = Δp_a − ρ_f·g·z = 5000 − 750 × 9.81 × 0.005 = 5000 − 36.8 = 4963.2 Pa.
  5. ṁ_f = C_df·A_j·√(2ρ_f(Δp_a − ρ_f g z)) → A_j = 4.388 × 10⁻³ / (0.75 × √(2 × 750 × 4963.2)) = 4.388 × 10⁻³ / (0.75 × 2728.5) = 2.144 × 10⁻⁶ m².
  6. d_j = √(4A_j/π) = √(2.730 × 10⁻⁶) = 1.65 mm.

Example 2 (GATE level): PFI injector pulse width. A four-cylinder, four-stroke PFI engine of 1.5 L runs at 3000 rpm with ηv = 0.85; intake air density is 1.18 kg/m³. The ECU targets λ = 1 (AFR 14.7). Each injector delivers 3.0 g/s when open and has a dead time of 0.8 ms. Find the fuel per injection, the pulse width and the injector duty cycle (one injection per cycle).

  1. V_s,cyl = 1.5/4 = 0.375 L = 3.75 × 10⁻⁴ m³.
  2. Air per cylinder per cycle = ηv·ρ_a·V_s,cyl = 0.85 × 1.18 × 3.75 × 10⁻⁴ = 3.761 × 10⁻⁴ kg.
  3. m_f,cyc = 3.761 × 10⁻⁴ / 14.7 = 2.559 × 10⁻⁵ kg = 25.6 mg.
  4. Open time for flow = 2.559 × 10⁻⁵ / 3.0 × 10⁻³ = 8.53 × 10⁻³ s; t_inj = 8.53 + 0.8 = 9.33 ms.
  5. One cycle takes two revolutions: 2/(3000/60) = 0.040 s = 40 ms. Duty cycle = 9.33/40 = 23 %, leaving ample margin for full-load enrichment and higher speed.

Common mistakes

  • Using the throat depression alone for the fuel flow and forgetting the nozzle lip head ρ_f·g·z.
  • Mixing up AFR and fuel-air ratio, or λ and φ. λ > 1 is lean; φ > 1 is rich.
  • Assuming a simple carburettor holds AFR constant. It enriches as flow rises; that is why compensating devices exist.
  • Treating "15 % better efficiency" as "15 % less fuel". Fuel use scales as 1/η: 10 L/h becomes 10/1.15 = 8.70 L/h, not 8.5 L/h.
  • Forgetting that a four-stroke cylinder is fuelled once every two revolutions.
  • Believing direct injection raises knock. Its charge cooling lowers knock tendency; its real penalty is particulates.

For GATE ME

Expect the classic simple-carburettor numerical: air flow from venturi size and depression, then jet diameter or AFR, sometimes with the nozzle lip correction. Other questions ask for mixture requirements at idle, cruise and full load, the function of carburettor auxiliary systems, or fuel mass per cycle from air flow and AFR. Practise the Bernoulli-based derivation and keep all pressures in pascals.

Quick check

  1. What is λ for a mixture with AFR 13.2 when the stoichiometric AFR is 14.7?
  2. Why does a simple carburettor give a richer mixture at higher air flow?
  3. Why must a PFI engine with a three-way catalyst run close to λ = 1?
  4. Name one advantage and one drawback of GDI compared with PFI.
  5. Which carburettor device supplies extra fuel when the throttle is suddenly opened?

Answers: 1. 13.2/14.7 = 0.90 (rich). 2. Air density at the throat falls with higher depression while fuel density stays constant, so fuel flow rises faster than air mass flow. 3. The catalyst converts CO, HC and NOx together only in a narrow window around stoichiometric. 4. Charge cooling allows a higher compression ratio (advantage); higher particulate emissions (drawback). 5. The accelerating pump.

Interview questions

All Automotive Engines, Emissions and Alternate Fuels interview questions

Try answering each one aloud before you open it.

  1. 1.What is carburetion in an internal combustion engine?Concept

    Carburetion is the preparation of a combustible air-petrol mixture outside the cylinder by a carburettor. Air flowing through a venturi speeds up and its pressure falls, so fuel from a float chamber at atmospheric pressure is pushed out of a nozzle at the throat, atomised by the fast air stream and partly vaporised in the inlet manifold. Auxiliary circuits (idle system, compensating jet, choke, accelerating pump and power enrichment) adjust the air-fuel ratio for starting, idle, cruise, acceleration and full load.

  2. 2.Explain the difference between port fuel injection and direct fuel injection.Concept

    Port fuel injection sprays fuel at about 3–5 bar into the inlet port onto the back of the inlet valve, so the mixture forms outside the cylinder during the intake stroke. Gasoline direct injection sprays fuel at about 50–200 bar or more straight into the combustion chamber, either during intake (homogeneous) or late in compression (stratified). Direct injection gives charge cooling (higher volumetric efficiency and knock resistance, so a higher compression ratio), more precise transient control and possible lean stratified operation, but costs more and produces more particulates and inlet-valve deposits.

  3. 3.Why has fuel injection replaced carburetion in modern petrol engines?Application

    Modern emission norms need the mixture held very close to λ = 1 so a three-way catalyst can convert CO, HC and NOx together; electronic injection with an oxygen-sensor feedback loop can do this, a carburettor cannot. Injection also gives equal fuelling of each cylinder, no venturi restriction, accurate cold-start and transient fuelling, and fuel cut-off on overrun. Direct injection goes further with charge cooling and, in some designs, lean stratified part-load operation.

  4. 4.What happens if the air-fuel mixture is too rich in a carburetted engine?Application

    With too little air for the fuel, combustion is incomplete, so CO and HC emissions rise sharply and fuel consumption increases; black smoke and soot may appear. Spark plugs foul with carbon, deposits build up in the combustion chamber and excess fuel can wash oil off the cylinder walls and dilute the sump oil. A slightly rich mixture actually lowers knock tendency, which is why full-load mixtures are enriched, but a grossly rich one causes misfire and power loss.

  5. 5.How does a carburettor adjust the air-fuel ratio for different engine conditions?Concept

    The main metering system (with a compensating jet or air-bleed) gives a near-constant lean mixture for cruising, correcting the natural enrichment of a simple carburettor as air flow rises. An idle system supplies fuel downstream of the nearly closed throttle, a choke restricts air to give a very rich mixture for cold starting, an accelerating pump injects an extra squirt when the throttle is opened suddenly, and an economiser or power valve enriches the mixture at full load for maximum power.

  6. 6.What are the advantages of using port fuel injection over carburetion?Application

    Each cylinder has its own injector, so fuel distribution is uniform, unlike a carburettor feeding a long manifold. There is no venturi, so volumetric efficiency and power are higher. The ECU meters fuel from measured air flow and corrects it for temperature and transients, and with an oxygen sensor holds λ ≈ 1 for the three-way catalyst, giving lower emissions, better cold starting, fuel cut-off on overrun and better economy. Drawbacks are higher cost and dependence on electronics and sensors.

  7. 7.Calculate the air-fuel ratio if 14.7 kg of air is mixed with 1 kg of petrol, and state whether the mixture is rich or lean.Numerical

    AFR = mass of air / mass of fuel = 14.7/1 = 14.7. This is approximately the stoichiometric ratio for petrol, so λ = 14.7/14.7 = 1: neither rich nor lean. This is the mixture a PFI or GDI engine with a three-way catalyst targets in normal running; a ratio below it is rich (λ < 1) and above it is lean (λ > 1).

  8. 8.How does direct injection affect knock tendency in a petrol engine?Application

    Direct injection reduces knock tendency. Because fuel evaporates inside the cylinder, it absorbs heat from the trapped air, lowering the charge temperature at the end of compression by roughly 10–20 K compared with port injection, so the end gas is less likely to autoignite. This is why GDI engines run about one compression ratio higher, or more boost, than equivalent PFI engines. Knock can still occur with poor mixing or low-octane fuel, so a knock sensor and spark retard are still used.

  9. 9.A carburetted engine uses 10 litres of fuel per hour at a given power. If switching to direct injection raises its brake thermal efficiency by 15 % (same power), estimate the new fuel consumption.Numerical

    For the same power, fuel consumption is inversely proportional to efficiency. New consumption = 10/1.15 = 8.70 L/h, a saving of about 13 %, not 15 %. Writing 10 × (1 − 0.15) = 8.5 L/h is a common error that confuses a 15 % rise in efficiency with a 15 % fall in fuel use.

Finished this topic? Mark it so your progress, study plan and readiness keep up.

Stuck on something here?

PreviousValve timing, port timing and volumetric efficiencyNext Diesel fuel injection: pumps, injectors and common rail