- 2071 Magh · 6 marks
Spark ignition vs compression ignition engine
Short answer
Spark ignition (SI) engines use the Otto cycle with petrol, low compression ratio (6–10), spark plug, constant-volume heat addition. Compression ignition (CI) engines use the Diesel cycle with diesel, high compression ratio (14–22), self-ignition, constant-pressure heat addition. P-V and T-S diagrams for each show isentropic compression/expansion; heat addition differs.
Compare spark ignition and compression ignition engine. Sketch P-V and T-S diagrams for them.
A spark ignition (SI) engine follows the Otto cycle and a compression ignition (CI) engine follows the Diesel cycle.
| Point | SI engine (Otto) | CI engine (Diesel) |
|---|---|---|
| Fuel | Petrol, light and volatile | Diesel oil, heavier |
| Air-fuel intake | Air + fuel mixed (carburettor or injection) during suction | Only air drawn in; fuel injected at end of compression |
| Ignition | By spark plug | By heat of compressed air (self-ignition) |
| Compression ratio | Low, 6-10 (knock limit) | High, 14-22 |
| Heat addition | At constant volume | At constant pressure |
| Efficiency | Lower in practice, ~25-30 % | Higher in practice, ~35-40 % |
| Weight and cost | Light, cheaper | Heavier, costlier |
| Speed | High-speed | Lower speed |
| Governing | Quantity governing (throttle) | Quality governing (fuel quantity) |
| Pollution | More CO and HC | More NOx and smoke |
Diagrams
Otto cycle (SI engine)
P-V diagram T-s diagram
P T
| 3 | 3
| |\ | /|
| | \ | / |
| | '. | / |
| | '. | .' |
| 2 '-. | 2 .' |
| \ '-. | | 4
| '. '-. | | /
| '-. '-. 4 | | .'
| '-._ | | | .'
| '-.._ | | 1.-'
| '-- 1 |
+-----------------------> V +--------------------> s
V2 V1
Diesel cycle (CI engine)
P-V diagram T-s diagram
P T
| 2 _______ 3 | 3
| | \ | .'|
| | '. | .' |
| | '. | .' |
| \ '-. | 2 _.' |
| \ '-. 4 | | 4
| '. | | | .'
| '-. | | | .'
| '-._ | | | .'
| '--.._ | | 1.-'
| '--- 1 |
+-------------------------> V +--------------------> s
V2 V1
Otto: 1-2 isentropic compression, 2-3 constant-volume heat addition, 3-4 isentropic expansion, 4-1 constant-volume heat rejection. η = 1 − 1/r^(γ−1).
Diesel: the same, except that 2-3 is at constant pressure. η = 1 − (1/r^(γ−1)) (r_c^γ − 1)/(γ(r_c − 1)).
✓ Answer checked by Claude Opus 5.5 (high effort)
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