Mean effective pressure (MEP). A way of describing the cylinder pressure that can actually be converted into torque. Simply it is combustion expansion pressure (pressure developed in the engine cylinder during the
power stroke) minus the compression pressure. In a four-stroke cycle CI engine it is assumed that cylinder
pressure through the intake and exhaust strokes is zero for purposes of calculating MEP, because through
both there is no signifi cant amount of pressure. MEP describes the relationship between the work performed
by the piston (in compressing the air charge) and the work received by the piston (through its downstroke on
the power stroke). If the engine is going to continue to rotate, there has to be a net gain in terms of work. MEP is an important defi nition. For instance, it is important to understand MEP to explain how an internal engine compression brake functions. The principle of the internal engine compression brake is to apply braking force to the drivetrain by making the piston do all of the work it normally does through the compression stroke but then eliminate the force it would normally be subject to receive during the powerstroke.
Constant pressure cycle. The theoretical diesel cycle engine presumes that the fuel supplied to the cylinder
during the expansion stroke will be at a rate permitting combustion pressure to remain constant through a large portion of the stroke. The theoretical diesel cycle is neither practical nor desirable (see the following section).
Cylinder pressure and throw leverage. The objective of any engine is to transfer the power developed in
its cylinders as smoothly and evenly as possible to the power take-off mechanism, which is usually a flywheel. The relationship between the crankshaft throw (to which the piston assembly is connected) and the crankshaft centerline is that of a lever. A lever is a device that provides a mechanical advantage. The amount of leverage (mechanical advantage) depends on the rotational position of the throw, which ranges from zero or no leverage when the throw is positioned at TDC to maximum leverage when the throw is positioned at a
90-degree angle with the connecting rod after TDC. This makes the relationship between cylinder pressure
(gas pressure acting on the piston) and throw leverage (the position of the piston) critical in meeting the objective of a smooth/even transfer of power from the engine cylinders to the drivetrain.
Let us take a look at this principle in operation.When the piston is at TDC beginning a power stroke, it
is desirable to have minimum cylinder pressure because in this position throw leverage is zero; therefore,
no power transfer is possible. A properly setup fuel system attempts to manage cylinder pressure so that in any given performance mode it peaks somewhere between 10 degrees and 20 degrees ATDC when there is some but nevertheless a small amount of throw leverage. As the piston is forced down through the power stroke, the gas pressure acting on the piston diminishes, but as it does throw leverage increases. Ideally, this relationship between cylinder pressure and crank throw leverage should be managed in a way that results in consistent torque delivery from an engine cylinder through the power stroke until the throw forms a 90-degree angle with the connecting rod: This occurs a little before true 90 degrees ATDC.
Boyle’s law (Robert Boyle, U.K., 1627–1691). States that the absolute pressure that a given quantity of gas at constant temperature exerts against the walls of a container is universally proportional to the volume occupied. In other words, assuming a constant temperature, the pressure of a specifi c quantity of gas depends on the volume of the vessel it is contained in. So to use an example of this law as it applies to a diesel engine, it means that a constant temperature mass of gas (air) in a cylinder as its volume is reduced by moving the piston will exert pressure on the cylinder walls because the number of molecules will remain the same, but they will have less room to move in, thereby causing the pressure rise.
Charles’s law (Jacques Charles, France, 1746–1823). States that the increase in temperature in gases
produces the same increase in volume if the pressure remains constant. In other words, heating a gas must
result in an increase in volume if the pressure is to remain unchanged. Using the Celsius scale, it can be proved that the volume of a gas increases by 0.003663 of its volume at zero Celsius for every one degree of
temperature rise. On the Fahrenheit scale, the volume increases by 0.002174 for every one degree of temperature rise above 32°. By graphing this equation negatively, a point would be reached at which the gas would have no volume; the vibration of the molecules would cease and the gas would contain no heat energy and would cease to exist as a substance. This would occur at absolute zero or – 273°C (– 460°F). To conclude, if the volume of a gas is changed by increasing its temperature while keeping its pressure constant,
then its volume will increase proportionally with temperature rise.
The first law of thermodynamics. States that heat energy and mechanical energy are naturally convertible.
This is predicated by the law of conservation of energy, which states that energy can neither be created
nor destroyed. This means that the total energy available remains constant. However, energy can change its form. Heat energy can be changed into mechanical energy (the operating principle of any internal combustion engine) or vice versa. Similarly, heat energy can be changed into electrical energy and vice versa.
The second law of thermodynamics. States that heat will not fl ow from a cool body to a warmer body
without some kind of assistance, but that it will flow from a warm body to a cooler body. If the objective is to force heat from a cool body to a hot body, such as in an air-conditioning system, some form of external assistance must be applied.
Friction. Force is required to move an object over the surface of another. Friction is the resistance to motion
between two objects in contact with each other. Friction is factored by both load and surface condition.
Smooth surfaces produce less friction than rough surfaces, and if a lubricant such as water or oil is added,
friction diminishes. Lubricants coat and separate two surfaces from each other and reduce friction, but the
lubricant itself provides some resistance to movement, which is known as viscous (fl uid) friction. A friction
bearing such as a crankshaft main bearing provides a sliding friction dynamic, whereas ball bearings provide
a rolling friction dynamic that usually offers less resistance to motion.
Static friction. Describes the characteristic of an object at rest to attempt to stay that way. For example,
the engine piston at its travel limit stops momentarily before the crankshaft and connecting rod reverse its
movement. When the piston is momentarily stopped, the crankshaft must overcome the static friction of the
stationary piston, which places both the connecting rod and a portion of the crankshaft under tension. This
tensile loading of the connecting rod and crankshaft is amplified as rotational speed increases.
Inertia. Describes the tendency of an object in motion to stay in motion or, conversely, an object at rest to
remain that way. Kinetic inertia describes the characteristic of an object in motion to stay in motion. For example, an engine piston moving in one direction must be stopped at its travel limit, and its kinetic inertia must be absorbed by the crankshaft and connecting rod.
The inertia principle is used by the engine vibration dampener and the flywheel—the inertial mass represented by the fl ywheel would have to be greatest in a single-cylinder, four-stroke cycle engine. As the number of cylinders increases, the inertial mass represented by the flywheel can be reduced due to the
greater mass of rotating components and the higher frequency of power strokes.
Joule’s heat effi ciency (James Prescott Joule, U.K., 1818–1889). Joule established the relationship between the units of heat and work that could be done. This relationship is used to describe the potential energy
of a fuel and is known as Joule’s mechanical heat equivalent.
1 Btu of potential heat energy = 778 ft.-lb of
mechanical energy
1 J (joule) of potential heat energy = 1 J of
mechanical energy
1 J = 1 Newton/meter (N·m) = 0.7374 ft.-lb
Calorifi c value. The potential heat energy of a fuel. A heat engine attempts to convert the potential heat energy of a fuel into kinetic energy: The thermal efficiency of the engine is a measure of how successful
this is. The calorifi c value of fuels is measured in Btu (English system) or joules and calories (metric
system).
British thermal unit (Btu). A Btu is a measure of heat energy. One Btu is the amount of heat required to
raise the temperature of 1 pound of water 1 degree Fahrenheit.
Thermal efficiency. A measure of the combustion efficiency of an engine calculated by comparing the
heat energy potential of the fuel (calorifi c value) with the amount of usable mechanical work produced. Electronically controlled CI engines can have thermal efficiency values exceeding 40%.
Rejected heat. That percentage of the heat potential of the fuel that is not converted into useful work
by an engine. If a CI engine operating at optimum efficiency can be said to have a thermal effi ciency of
40%, then 60% of the calorifi c value of the fuel has to be discharged as rejected heat. Half of the rejected
heat is typically transferred to the engine hardware to be dissipated to the atmosphere by the engine
cooling system, and the other half exits in the exhaustgas. A turbocharger makes use of rejected heat by
compressing the intake air forced into the engine cylinders, thereby increasing the thermal effi ciency of
the engine. Figure 4–9 shows how the potential energy of the fuel (100%) is released in a typical diesel
engine: 33% is converted into useful mechanical energy delivered to the fl ywheel. The remainder of
that potential heat energy totaling 67% is released as rejected heat. The rejected heat is dispersed as
radiated heat (7%) to engine coolant (30%), and into the exhaust (30%).
Important Points:
• Most diesel engines are rated by their ability to produce power and torque. The tendency is to rate gasoline-fueled auto engines by their total displacement.• Diesel engines have high-compression ratios so they tend to be undersquare.
• Almost every medium and large bore highway diesel engine is manifold boosted; that is, it is turbocharged.
• The most common diesel cycle is a four-stroke cycle consisting of four separate strokes of the piston occurring over two revolutions; a complete engine cycle is therefore extended over 720 degrees.
• The two-stroke diesel cycle enables every downstroke of a piston to be a power stroke, so in theory,
it has the potential to produce more power per engine pound than the four-stroke cycle. In practice, this is unobtainable.
• MEP is the average pressure acting on the piston through the four strokes of the cycle. Usually the intake and exhaust strokes are discounted, so MEP is equal to the average pressure acting on the piston through the compression stroke subtracted from the average pressure acting on the piston through the power stroke.
• Ideally, engine fueling should be managed to produce peak cylinder pressures at somewhere around 10 to 20 degrees ATDC when the relative mechanical advantage provided by the crank throw position is low. This means that as cylinder pressure drops through the power stroke, throw mechanical advantage increases, peaking a little before 90 degrees ATDC and providing a smooth unloading of force to the engine fl ywheel.
• An engine attempts to convert the potential heat energy of a fuel into useful mechanical energy: the degree to which it succeeds is rated as thermal efficiency.
• That portion of the heat energy of a fuel not converted to kinetic energy is known as rejected heat. Rejected heat must be dissipated to the atmosphere by means of the engine cooling and exhaust systems.
