- Practice · 4+6 marks
Carburettor vs injection; TBI, MPFI, GDI
Short answer
Carburettor meters fuel mechanically via venturi vacuum; fuel injection uses electronic injectors and ECU for precise control, giving better economy and lower emissions. TBI has a single injector at the throttle (cheap but uneven); MPFI uses one injector per port (even distribution); GDI injects directly into the cylinder for lean burn and higher efficiency, but is more costly.
(a) Differentiate between a carburettor and a fuel injection system for SI engines. (b) Describe the TBI, MPFI and GDI systems with simple block diagrams, stating one advantage and one limitation of each.
(a) Carburettor vs fuel injection
| Point | Carburettor | Fuel injection (EFI) |
|---|---|---|
| Fuel metering | Venturi vacuum draws fuel through jets (mechanical) | ECU controls injector pulse width (electronic) |
| Accuracy of AFR | Poor, varies with altitude and temperature | Accurate, closed loop with lambda sensor |
| Cylinder-to-cylinder distribution | Uneven (long manifold) | Even in MPFI/GDI |
| Cold start | Choke needed | Automatic enrichment |
| Fuel economy and emission | Higher consumption, high CO and HC | Lower, can meet BS-VI |
| Power | Venturi restricts airflow | Higher volumetric efficiency |
| Cost and maintenance | Cheap, simple, but needs tuning | Costly, needs sensors and ECU |
(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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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