Diesel Engine Combustion Efficiency

Index Key: ENG048
Author: gerald j ballmann
Subject: Diesel engine combustion efficiency.
1) What limits the compression ratio of a diesel engine?
2) What is the best fuel to use in a diesel engine for maximum efficiency?

Response #: 1 of 2
Author: daniel n koury jr
The compression ratio for a particular engine is limited by how strong the
engine block is. Too much pressure (compression) and something will break.
The best fuel will also depend on the design of the engine. But as a
general rule, the greater the energy content of the fuel, the greater the
efficiency (at least in terms of miles per gallon or km per liter). Water
has a low energy content and would not make a good fuel for example.

Response #: 2 of 2
Author: david r munoz
The limits to compression ratio are based on the knock limits of the fuel.
Knock is the term used to describe the auto ignition that occurs when a fuel
ignites because the pressure in the cylinder is such that combustion occurs.
In a spark ignited engine, it is best if the spark control the ignition,
since timing of the piston motion with the motion of the flame front is
critical to the operation of an efficient engine.
In a diesel engine, one relies on auto ignition of the fuel because the
engine is basically compression ignited (there is no spark plug). There-


fore, we want the fuel to be autoignited. However, some types of combustion
waves move faster than others. There are waves called detonation waves that
contribute to sonic velocities of the hot gases in the cylinder. It is
generally, the nonuniformity of the pressure within the cylinder that are
responsible for knock. Some believe that these nonuniformities are due to
detonation and others believe they are due simply to nonuniform combustion
or auto ignition in the cylinder.
The knock limits for a fuel are determined on a special single cylinder
engine called a CFR engine. On this engine, the compression ration can be
gradually changed. In general, a fuel is burned in the engine and the
compression ratio is changed high and low until the engine begins to shake.
This is known as the knock limit for the fuel. The higher the knock limit,
the higher the compression ratio.