Compression Ignition Engines

While electric cars are starting to gain a following, the vast majority of the cars on the road today are still powered by internal combustion engines.  The two main types of engines used for automobiles are gasoline engines (spark ignition) and diesel engines (compression ignition).  One of the main differences is that in a spark ignition engine a mixture of air and gasoline fumes is compressed, and lit off by a spark at the appropriate time.  In a compression ignition engine, on the other hand, only air is compressed and a fine mist of diesel fuel is injected into the hot air at the appropriate time.

In an earlier post we talked about how compression alone will heat air to pretty high temperatures.  In a compression ignition engine, the air is compressed to a high enough temperature that when the diesel fuel is sprayed into the cylinder, it spontaneously ignites and burns (hence, the name: compression ignition).  This table shows the ignition temperature of various substances.  

Compression ratios for diesel engines are typically in the high teens or low twenties.  This figure shows the temperature that air would reach for a range of compression ratios if it started at 27 deg C (81 deg F), 1 atm.  You can see that the air temperatures can easily exceed the ignition temperatures of many fuels.