What causes a diesel engine to be more efficient than a 4 stroke engine?

Answer 1

A diesel is a 4 stroke engine. The diesel ignites the fuel through the process of compression rather than spark ignition. Diesels are more efficient because the last longer, get better fuel economy, and produce more torque at a lower rpm. This makes the diesel ideal for pulling heavy loads.

Diesels and gasoline engines come in both 2- and 4- stroke designs, but all current auto and truck engines use 4-stroke designs, primarily for cleaner emissions and fuel efficiency. The diesel's greater efficiency comes from its higher compression ratio that is required to cause the fuel to spontaneously ignite without a spark. As the piston rises on a cylinder that has just received a charge of fuel and air, the mixture is compressed to some multiple of atmospheric pressure. If the final cylinder pressure achieved at the end of the compression stroke was 147 psi the compression would be expressed as a ratio of 10 to 1 - ten times atmospheric pressure. In a gasoline/spark engine the compression ratio generally falls around 9 or 10 to 1. In order to heat the fuel/air mixture enough to enable the diesel fuel to self-ignite it must be much more highly compressed - compression ratios of 20 to 1 and higher are typical. This produces more power during the combustion and expansion of the fuel/air mixture for a given amount of fuel, or what is called thermal efficiency.