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prob 2.9 - In the CI engine only air is compressed during...

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Question 2.9 In the SI engine air and fuel are premixed and then compressed. Ignition happens (via a precisely timed electrical spark) just prior to TDC on the compression stroke. The mixture burns rapidly releasing most of its heat just after TDC (power stroke). This raises the gas pressure to a very high value and thus pushes down on the piston with great force. The fuel must resist autoignition during the compression stroke. (Octane rating). There is a limit to the upper temperature the air/fuel can resist prior to auto ignition so the compression ratio cannot be too high. (~ 10:1)
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Unformatted text preview: In the CI engine only air is compressed during the compression stroke. Fuel is injected at high pressure just before TDC. After a short delay period, the fuel burns to release most of its heat just after TDC on the power stroke. The fuel must exhibit the characteristic of igniting spontaneously upon mixing with the hot compressed air. Building up sufficient level of heat in the compressed air requires a high compression ratio (~ 18:1) and a fuel that ignites with minimal delay. (Cetane rating)...
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