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The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug. Mister Rudolph Diesel was aware of the gasoline engine (Otto cycle) problems and wanted to improve it. The gasoline engine inherently has problems with efficiency and/or fuel. In order to improve the efficiency one must increase the compression ratio of an internal-combustion engine. However, in the gasoline engine there is a limit – the gasoline-air mixture will self ignite once the compression gets too high. So, either you can have a low-efficient, low-compression engine that uses a cheap fuel, or you can have a high-efficient, high-compression engine that uses expensive, high-refined fuel that wont self-ignite even at high compression levels. In diesel engine this problem is solved. The diesel engine can use much higher compression levels than the gasoline engine reaching higher efficiency. In addition, the diesel engine can use fuel that is not nearly as refined as the high-octane gasoline fuel. More here: http://charming.awardspace.com/otto_diesel/otto_diesel.html
Simply put, the diesel cycle ignites the fuel-air mixture with heat generated by compression (perhaps with help from a low-temperature glow plug to get the cycle started when the engine is cold), whereas the Otto cycle uses a spark plug for ignition on every cycle.
Pretty much Diesel cycles use higher compression ratios to ignite the fuel rather than a spark plug. That's why your diesel truck doesn't have spark plugs.