Fuel Supply Systems

Requirements of a good engine fuel

A good fuel should burn smoothly, give high energy per kg, be easy to store and supply, and cause little pollution.

Spark ignition (petrol) fuel

• High octane number (anti-knock quality), so a high compression ratio can be used.
• Good volatility: easy cold starting, but low enough to avoid vapour lock.
• High calorific value (about 44 MJ/kg) and low sulphur and gum content.
• Non-corrosive, stable in storage, low emission.

Compression ignition (diesel) fuel

• High cetane number for a short ignition delay (smooth, quiet running).
• Proper viscosity so that the injector atomises it well and lubricates the pump.
• Low sulphur, low ash and carbon residue (less deposits and smoke).
• Suitable pour point and cloud point for cold weather.

Octane and cetane number

• Octane number: percentage by volume of iso-octane in a blend of iso-octane (rating 100) and n-heptane (rating 0) that matches the knock behaviour of the fuel in a standard CFR engine. Higher value means better knock resistance. Petrol is typically 87 to 95 RON.
• Cetane number: percentage by volume of cetane (n-hexadecane, rating 100) in a blend with alpha-methyl naphthalene (rating 0) that has the same ignition delay as the diesel fuel. Automotive diesel is typically 45 to 55.

The stoichiometric air-fuel ratio (AFR) of petrol is about 14.7:1 by mass ($\lambda = 1$). Engine demand varies, so the mixture must be changed with operating condition.

Requirements

Maximum economy occurs near 16:1 to 17:1 (lean), maximum power near 12.5:1 (rich), and the three-way catalyst needs 14.7:1 at cruise.

How the mixture is provided

Carburettor

• Choke enriches mixture for starting.
• Idle jet supplies rich mixture below the throttle plate.
• Main jet with compensating jet gives a nearly constant lean mixture for cruising.
• Power (economiser) valve enriches at full load.
• Accelerator pump gives an extra squirt on sudden throttle opening.

Electronic fuel injection

• The ECU reads coolant temperature, throttle position, mass airflow, and lambda sensor.
• It lengthens injector pulse width for cold start, idle, and acceleration, and trims to 14.7:1 in closed loop at cruise.
• At full load it goes open loop to a rich mixture.

(a) Carburettor vs fuel injection

(b) EFI systems

1. Throttle body injection (TBI) - one or two injectors above the throttle valve spray fuel into the intake manifold, replacing the carburettor.

 Fuel tank -> pump -> filter -> [Injector] -> throttle
                                      ^ ECU
 Manifold distributes mixture to cylinders

• Advantage: simple, cheap, low pressure (about 1 bar).
• Limitation: unequal distribution, wall wetting, slow response.

2. Multi-point fuel injection (MPFI) - one injector per cylinder near the intake port, injecting into the port (about 3 to 4 bar).

 Tank -> pump -> filter -> rail -> Inj1..Inj4 -> intake ports
 Sensors (MAF, TPS, CTS, O2) -> ECU -> injector pulse

• Advantage: equal distribution, quick response, better economy and emission.
• Limitation: some fuel still wets the port; costlier than TBI.

3. Gasoline direct injection (GDI) - injector inside the combustion chamber injects high-pressure fuel (50 to 200 bar) directly, with stratified charge at part load.

 Tank -> low-pressure pump -> high-pressure pump -> rail
                                   -> injector in cylinder head

• Advantage: lean burn, higher compression ratio, 10 to 15 % better fuel economy, more power.
• Limitation: carbon deposits on intake valves, high cost, particulate (soot) emission needing a gasoline particulate filter.

Given: $V_d = 1.6\times10^{-3}$ m³, $N = 3000$ rpm, $\eta_v = 0.85$, $\rho_a = 1.18$ kg/m³, $BP = 35$ kW.

(i) Air flow

A four-stroke engine inhales once per two revolutions.

$$\dot m_a = \eta_v \rho_a V_d \frac{N}{2\times 60} = 0.85 \times 1.18 \times 1.6\times10^{-3} \times \frac{3000}{120} = 0.0401\ \text{kg/s}$$

That is $0.0401 \times 3600 = 144.4$ kg/h.

(ii) Fuel flow at $AFR = 14.7$

$$\dot m_f = \frac{\dot m_a}{14.7} = \frac{144.4}{14.7} = 9.83\ \text{kg/h}$$

For petrol density 740 kg/m³ this is $9.83/0.74 = 13.3$ L/h.

(iii) Brake specific fuel consumption

$$bsfc = \frac{\dot m_f}{BP} = \frac{9.83 \times 1000}{35} = 281\ \text{g/kWh}$$

(iv) Full-load mixture

$$\dot m_f = \frac{144.4}{12.5} = 11.55\ \text{kg/h}, \qquad \lambda = \frac{AFR_{actual}}{AFR_{stoich}} = \frac{12.5}{14.7} = 0.85$$

The mixture is 15 % rich ($\phi = 1/\lambda = 1.18$).

Answer: $\dot m_a = 0.0401$ kg/s; $\dot m_f = 9.83$ kg/h at $\lambda = 1$; bsfc $= 281$ g/kWh; full-load fuel $= 11.55$ kg/h with $\lambda = 0.85$.