- Practice · 6 marks
Air-fuel mixture needs at different operating conditions
Short answer
Cold start and idling require very rich mixtures (2:1 to 12:1) to overcome weak vaporisation and residual gas. Cruising uses a lean mixture (15:1-17:1) for economy; full load and acceleration need a rich mixture (~12.5:1) for maximum power. A carburettor uses choke, idle and main circuits plus a power valve and accelerator pump; EFI varies injector pulse width.
Explain why an SI engine needs different air-fuel mixture strengths for cold starting, idling, cruising and full-load (acceleration). State the approximate air-fuel ratio in each case and how a carburettor or injection system provides it.
The stoichiometric air-fuel ratio (AFR) of petrol is about 14.7:1 by mass (). The engine's demand varies, so the mixture strength must change with the operating condition.
Requirements
| Condition | AFR (approx.) | Mixture | Reason |
|---|---|---|---|
| Cold start | 2:1 to 5:1 (supplied) | Very rich | Little fuel vaporises on cold walls; only the light ends burn |
| Idling | 10:1 to 12:1 | Rich | Throttle nearly closed, high residual gas fraction, poor mixing |
| Part-load cruising | 15:1 to 17:1 | Lean | Best fuel economy, throttle partly open |
| Full load / maximum power | 12:1 to 13.5:1 (about 12.5:1) | Rich | Gives maximum power; excess fuel cools the charge |
| Acceleration | Momentarily rich | Rich | Throttle opens suddenly and the fuel lags behind the air |
Maximum economy occurs near 16:1 to 17:1 (lean) and maximum power near 12.5:1 (rich). Engines with a three-way catalyst are held at 14.7:1 during cruise.
How the mixture is provided
Carburettor
- The choke enriches the mixture for starting.
- The idle jet supplies a rich mixture below the throttle plate.
- The main jet with compensating jet gives a nearly constant lean mixture for cruising.
- The power (economiser) valve enriches the mixture at full load.
- The accelerator pump gives an extra squirt of fuel when the throttle is opened suddenly.
Electronic fuel injection
- The ECU reads the coolant temperature, throttle position, mass airflow and lambda sensor signals.
- It lengthens the injector pulse width for cold start, idle and acceleration, and trims the mixture to 14.7:1 in closed loop at cruise.
- At full load it switches to open loop and gives a rich mixture.
✓ Answer checked by Claude Opus 5.5 (high effort)
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