--- title: "Carburettor vs injection; TBI, MPFI, GDI" subject: "Automotive Technology" chapter: 2 key: "semester_5/automotive_technology:carburettor-vs-fuel-injection-systems" source: https://www.bctengineer.com/q/semester_5/automotive_technology/carburettor-vs-fuel-injection-systems site: BCT Engineering --- # Carburettor vs injection; TBI, MPFI, GDI Automotive Technology · Chapter 2: Fuel Supply Systems - Asked in: year not given, 4+6 marks - Video: [FUEL INJECTION: TGDi TSI VS GDi MPFI VS TBI VS CARB #car #gdi #tgdi](https://www.youtube.com/watch?v=34FmQugH6xE) — Mashofreaks Auto - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/automotive_technology/carburettor-vs-fuel-injection-systems - Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-2 ## Question (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. ## 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. ## Answer #### (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. ```text 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). ```text 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is fuel metered in a carburettor compared with EFI? A. Carburettor: ECU pulse width; EFI: venturi vacuum B. Carburettor: venturi vacuum through jets; EFI: ECU controls injector pulse width C. Both use venturi vacuum D. Both use injector pulse width Answer: B. A carburettor is mechanical, with the venturi drawing fuel through jets; EFI is electronic through injector pulse width. (Key point: Fuel metering) 2. What characterises throttle body injection (TBI)? A. One injector per cylinder at the port B. One or two injectors above the throttle valve at about 1 bar C. Injector in the cylinder at 50 to 200 bar D. No injectors, only jets Answer: B. TBI replaces the carburettor with one or two low-pressure injectors above the throttle, so it is simple but distributes unevenly. (Key point: TBI) 3. What is the main advantage of MPFI over TBI? A. Lower cost B. Equal distribution and quicker response C. Direct in-cylinder injection D. Lower pressure Answer: B. One injector per cylinder gives equal distribution, quick response and better economy and emission. (Key point: MPFI advantage) 4. Which is a recognised limitation of GDI? A. Cannot run lean B. Carbon deposits on intake valves and soot emission C. Poorer fuel economy than carburettor D. Only 1 bar pressure Answer: B. No fuel washes the intake valves, so deposits form, and particulate emission may need a gasoline particulate filter. (Key point: GDI limitation) ## Related in Automotive Technology - [Knock and pre-ignition in SI engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aknock-and-pre-ignition-si-engine) — Automotive Technology, chapter 3 - [Air-fuel mixture needs at different operating conditions](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Aair-fuel-mixture-requirements) — Automotive Technology, chapter 2 - [Air flow and fuel flow of an SI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Afuel-consumption-air-flow-numerical) — Automotive Technology, chapter 2 - [Two-stroke versus four-stroke engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atwo-stroke-vs-four-stroke) — Automotive Technology, chapter 1 - [Requirements of SI and CI engine fuels](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Asi-engine-fuel-requirements) — Automotive Technology, chapter 2 ## Related in other subjects - [EFI, MPFI and diesel injection pump](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aefi-mpfi-fuel-pump-injection-systems) — Internal Combustion Engines, chapter 4 - [Differentiate SI and CI engines](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Asi-versus-ci-engine-differences) — Internal Combustion Engines, chapter 2 - [Jet diameter of a simple carburettor](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Asimple-carburettor-jet-size-numerical) — Internal Combustion Engines, chapter 4 - [Simple carburettor and its limitations](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Asimple-carburettor-working-compensation) — Internal Combustion Engines, chapter 4