Compression ignition engines: which statement is true?

• A. Mix oil into the fuel for lubrication
• B. Will compress air only on the compression stroke
• C. Will compress air only on the power stroke
• D. Require a spark timing device

Answer: (B)

Compression ignition engines rely on heating the air by compressing it, then injecting fuel so it auto-ignites from that high temperature. The air is created and compressed during the compression stroke, and once the fuel is injected near the top of that stroke, ignition happens without any spark. That’s why no spark timing device is needed. The lubrication system uses separate engine oil, not oil mixed into the fuel, so that option isn’t correct. The power stroke is the expansion phase after combustion, not the phase where air is compressed. So the statement that air is compressed only on the compression stroke best fits how these engines operate.