In The Beginning
In 1673, Christian Huygens invented the first engine. It was designed to pump water from the Seine River to King Louis XIV's Palace of Versailles. Built with an open combustion chamber, it consumed gun powder as a fuel. Though it was the first documented engine to be built, it had a striking resemblance to a 1509 Leonardo da Vinci drawing, which never came to fruition.
German Nicholaus Otto is generally credited with the creation of the first successful spark-ignition, four-stroke powerplant that bears any resemblance to today's modern engine. Frenchman Alphonse Beau de Rochas fathered the original concept 14 years earlier in 1862, but he was not completely successful in getting it to work. Otto's original goal was to improve on the efficiency of the French Lenoir two-stroke, non-compression engine, and his inspiration came from watching steam rise from a smokestack. As a result, his surname is often used synonymously for the four-stroke cycle.
Rudolf Diesel invented the compression ignition engine in 1897, though his first working model was produced in 1893. Shortly thereafter, refining technology produced gasoline, albeit not in today's current form. Gasoline was basically a derivative of kerosene, a waste byproduct of early oil refining. It was considered a nuisance and was disposed of by being dumped on the ground or into rivers.
Diesel vs. Gas
The major difference between the gasoline and diesel engine is that the former relies on spark ignition and the latter on compression ignition. More specifically, the combustion process in the diesel engine is initiated by spontaneous ignition of the fuel when it is injected into a highly compressed charge of air, which has reached approximately 1,400 degrees Fahrenheit. Diesel engine combustion also tends to occur at constant pressure rather than at constant volume, as in a gasoline engine. This means that in the diesel engine the combustion pressure continues to rise steadily as the piston retreats and the cylinder volume increases, whereas in the gas engine, the combustion process is so rapid that there is very little movement of the piston while it occurs and, thus, very little increase in cylinder volume. The expansion of the flame at a constant pressure is responsible for the diesel's reputation for extreme torque along with a flat torque curve.
When the displacement is the same, the diesel engine produces only about two-thirds the horsepower of the gasoline engine. This poses a question: Why is the power output lower even though the combustion pressure is higher?
An IDI engine employs a prechamber that houses the glow plug. The combustion starts there and then travels in to the main combustion chamber. This style of cylinder head has more in common with a gas engine than a DI engine. Our subject is from a GM 6.5 diesel. | 
The piston on the GM 6.5 diesel resembles a gasoline engine, only it's thicker and heavier. Notice the valve relief in the crown. | |
Since combustion is an oxidation reaction, there is a specific weight of air that will completely oxidize one gram of fuel without leaving excess oxygen. This weight of air is called the stoichiometric air/fuel ratio. The gasoline engine operates with an air/fuel mixture very near stoichiometric. This is due to the fact that a mixture much leaner than stoichiometric is difficult to ignite in a gasoline engine with a spark plug, and an extremely rich ratio is very inefficient. The mixture is supplied to the gasoline engine by a carburetor or fuel injectors in the manifold and is well mixed and nearly homogeneous.
In the diesel engine, the fuel is injected into the combustion chamber near the end of the compression stroke and ignites spontaneously. This is responsible for the combustion sound that a diesel engine generates that is music to the ears of everyone who reads this magazine. As mixing between the fuel and air occurs, burning continues. This process is very heterogeneous (since the fuel and air are mixed in a combustion chamber it is not as uniform as in a gas engine that has the mixture created prior to entering the cylinder head). Soot is formed during combustion because some of the fuel burns with insufficient oxygen, and the combustion of the fuel is not completed. As additional fuel is injected, more and more soot is produced. Therefore, the air/fuel ratio of the diesel engine must always be leaner than stoichiometric to prevent excessive amounts of smoke. For this reason, a modified, high-output diesel will blow black smoke because it is fueled for power alone with no concern for soot generation. The smoke-free diesel has less fuel present in the cylinder than in the cylinder of the gasoline engine, and diesel power is therefore reduced in comparison.
Older, naturally aspirated diesel engines can use only about 70-80 percent of the fuel employed by a gasoline engine with the same swept volume and still avoid black smoke.