Question

In a SI engine, combustion stage I takes 1 ms and combustion stage II takes 1.5 ms when the engine runs at 1000 rpm. If stage I time duration is independent of engine speed what will be the additional spark advance necessary when the engine speed is doubled?
1. 0° 
2. 6° 
3. 12° 
4. 24° 

Answer 1

Correct Answer - Option 2 : 6° 

Given 

Time taken by stage I (T₁) = 1 ms

Time taken by stage II (T₂) = 1.5 ms

Speed of engine = 1000 rpm

Spark advance time = T₂ – T₁ = 1.5 – 1 = 0.5 ms

Now, If the speed of the engine is doubled, N = 2000 rpm

then, additional spark advance necessary will be

Speed - 2000 rpm → 4000π degrees per minute → 4000π/60 degrees per sec 

Spark advance time is 0.5 ms → 0.5 × 10^-3 × (4000 × 180/60) degrees = 6°