Spark Ignition Vs. Compression Ignition: How Do These Two Engines Differ?
A fundamental difference between gasoline and diesel engines is that a gasoline engine uses spark ignition while a diesel engine uses compression ignition. Before we delve deeper, let's understand how a four-stroke engine works, as modern gasoline and diesel engines are four-stroke engines. The engine consists of one or more pistons that move up and down a cylinder. The piston is connected to a crankshaft via a connecting rod and the reciprocating piston movement is converted into rotary movement of the crankshaft.
The four strokes are intake, compression, power and exhaust. A piston moves down a cylinder, sucking air (or an air-fuel mixture, in a gasoline engine) into the cylinder. Then the piston moves back up, squeezing the contents of the cylinder. The contents are ignited and a mini explosion occurs, pushing the piston down. Then the piston moves back up, pushing the burnt contents out of the cylinder.
In a gasoline engine, air and fuel are mixed before entering the cylinder. Once the mixture is compressed, it's lit up with the help of a spark plug, similar to lighting a match. In a diesel engine, there is no spark plug and only air is introduced into the cylinder. There the air is compressed and gets hot. Compressing a gas makes it hot, and this happens to the air inside the cylinder. It gets hot enough to ignite diesel when it's sprayed into the combustion chamber. This is a more efficient method of combustion, and a primary reason why diesel engines offer better economy than gas engines.
The reason gasoline needs a spark and diesel needs a squeeze for combustion has a lot to do with both fuels' properties, specifically their flash points and autoignition temperatures. A flash point is the minimum temperature at which vapors of a liquid ignite via an external ignition source, while autoignition temperature is the minimum temperature at which a liquid self-ignites in the presence of air, but without an ignition source.
The flash point temperature for gasoline is minus 45 degrees while it autoignites at 536 degrees. For diesel, they are 126 and 410 degrees, respectively. This means that gasoline emits vapors at temperatures as low as minus 45 degrees and will easily light up with a small spark at that temperature. It will ignite without a spark if you heat up air around gasoline to 536 degrees. Diesel is difficult to light up. It emits vapors at 126 degrees, so you'll have to hold a match to diesel fuel for a long time for it to catch fire. However, it's easier to ignite diesel if you compress the air around it to 410 degrees. Nothing as dramatic as a diesel runaway, though.
The more you compress air in a diesel engine, the more efficiently it will burn. This is a primary reason why diesel engines use higher compression ratios than gasoline engines. Gasoline engines have lower compression ratios, as their fuel ignites easily when compressed. If the air-gasoline mixture inside the engine ignites too early, it can damage the engine.
