Engineering of Water Systems

Published On: March 18, 2024By Categories: Pumps and Water Systems, The Water Works

Standby and Backup Systems: Engines

By Ed Butts, PE, CPI

The Water Works’ January installment introduced standby and backup units for emergency and supplemental service. This month, we begin a three-part overview on the specifics of internal combustion engines and their application.

Internal Combustion Engines

Figure 1. Otto (gasoline) cycle.

Engines that are used to generate power necessary to drive a pump, compressor, or generator follow the same basic principles of horsepower and transfer that an electrically powered motor uses by converting one form of energy to another form of energy.

Although the definition of an engine can be applied to fit many types of machines, for the purposes of this discussion, an engine will be limited to that found in an internal combustion type, using fossil fuel as energy sources such as gasoline, natural gas, biogas blends, propane, diesel, or biodiesel blends.

However, in this case, the output rotation and resulting power is derived from the chemical value of the fuel used by converting to an output of kinetic energy through a series of controlled implosions from an assembly of alternating pistons contained within an assembly of closed cylinders rather than the electromotive magnetic force developed from electricity.

The generated horsepower rating and output of an engine also does not follow the same relationship of that within an electric motor due to a massive difference in the energy conversion from the fuel to the engine along with a much lower inherent efficiency.

Figure 2. Diesel cycle.

The principle behind the energy obtained from an engine uses what is known as a combustion power cycle. A combustion power cycle is similar to a vapor power cycle where energy is extracted from the vapor produced from a fluid, such as the vapor from water used for steam generation or boiler feed. However, unlike the vapor cycle, the fluid that is used to generate energy in a combustion cycle is only capable of being used just once in the cycle and cannot be reused or returned to its original state for use in another cycle.

The technical terms applied to a typical combustion power cycle are an air-standard Otto cycle (Figure 1) for gasoline or similar fuels or an air-standard diesel cycle (Figure 2) for diesel fuel.

An Otto cycle is a four-stroke operation that includes an initial air-fuel mixture and compression in a cylinder, followed by ignition in the cylinder through a constant volume combustion explosion. This results in an expansion of the air-gas mixture.

In contrast, a diesel cycle has a compression of the fuel followed by a non-explosive combustion at a relatively constant pressure, resulting in expansion.

Figure 3. Gasoline engine sectional view.

Comparing the two engine types with a given compression ratio, the ideal Otto cycle will be more theoretically efficient. However, a real diesel engine will be more efficient overall since it will have the ability to operate at higher compression ratios. If a gasoline engine were to have the same compression ratio, knocking (self-ignition) would occur and this would severely reduce the efficiency, whereas in a diesel engine, the self-ignition is the desired behavior.

Additionally, both cycles are only theoretical ideas, and the actual behavior does not divide as clearly or sharply. Furthermore, the ideal Otto cycle does not include throttling losses, which do not apply to diesel engines.

Both types of engines convert the predictable value of the potential chemical energy contained in a specific type or blend of fuel by mixing it with an oxidizer to create a combustible mixture. Generally, the source of the oxidizer is oxygen (O2), derived from the open atmosphere from the surrounding ambient air that is a readily available source of oxygen since it comprises about 21% of the volume of atmospheric air at sea level.

In a gasoline engine, between 12 to 18 parts of air (by weight) is used per one part of gasoline (by weight) to create the proper fuel-air mixture. A diesel mixture consists of 15 to 23 parts of air (by weight) per part of fuel (by weight). If there is not enough oxygen for proper combustion, the fuel will not burn completely and will produce less output energy.

An excessively rich air-to-fuel ratio will increase pollutants from the engine. If all the oxygen is consumed because of too much fuel, the engine’s power is also reduced.

For a stationary engine, the fuel is either stored in bulk volume on-site or transmitted and delivered to the engine through a dedicated supply line. With gasoline and similar fuels, it is directed or pumped into a special chamber (a carburetor) and mixed with sufficient air to create a combustible form of thermal energy.

This mixture is then sent or injected to each engine cylinder and combusted to create the mechanical energy. Depending on the type of engine and fuel, the fuel-air mixture or air alone is then routed to the upper region of a cylinder where the mixture is immediately compressed to a typical value between 150-175 psi and then spark-ignited using a high-voltage DC spark plug with gasoline, natural gas, or liquid petroleum gas (LPG).

Figure 4. Diesel engine sectional view.

Conversely, the fuel-air mixture can also be compression-ignited with diesel fuel, using a combined pressure at roughly three to four times the value (450-700 psi) for gasoline, which also adds heat onto the air and results in the explosion of the fuel-air mixture when the fuel is injected into the cylinder and contacts the very hot and highly compressed air.

Thermodynamics and the Relationship to Engines

Gasoline and diesel engines operate using an adiabatic process, that is, a thermodynamic process that occurs without transferring heat or mass between the system and its environment. Unlike an isothermal process, an adiabatic process transfers energy to the surroundings only as work.

The efficiency of a combustion engine is measured as the sum of thermal efficiency. Thermal efficiency is a consequence of thermodynamics. There is both a definition and formula for thermal efficiency. To wit: “Thermal efficiency is a measure of the performance of a power cycle or heat engine.” The strict definition of thermal efficiency is “the ratio of the heat utilized by a heat engine to the total heat units in the fuel consumed.”

A more practical definition of thermal efficiency is the amount of energy produced when a combustion engine burns fuel in relation to the amount of that same energy that converts to mechanical energy. However, the formula for thermal efficiency may provide the simplest explanation. Thermal energy is the ratio of the amount of heat lost divided by the amount of heat put into a system, with heat being synonymous with energy.

Figure 5. Gasoline engine operational sequence.

The result of dividing loss by input is the thermal efficiency of that system. Thermal efficiency is the amount of energy that goes into powering the crankshaft of a combustion engine, at least those with pistons. There are two laws of thermodynamics that determine the thermal efficiency of a combustion engine.

First Law of Thermodynamics

Thermal efficiency, the efficiency of a combustion engine, is determined by the laws of thermodynamics. According to the first law of thermodynamics, the energy output cannot exceed the energy input.

In other words, the energy an engine produces, whether it is energy lost or energy used for driving a pump, will never be greater than the energy potential of the fuel fed into the combustion chamber.

The first law of thermodynamics is easy to understand and is the same as the law of conservation of energy: “Energy can neither be created nor destroyed.” The first law of thermodynamics is simply another way to prove that energy cannot be created or destroyed, but converted to another form.

Second Law of Thermodynamics

Figure 6. Diesel engine operational sequence.

According to the second law of thermodynamics: 100% thermal efficiency is impossible to achieve. There is a theoretical and practical limit to the potential efficiency of a combustion engine.

The second law, called “Carnot’s Theorem,” states: “Even an ideal, frictionless engine cannot convert anywhere near 100% of its input heat into work.” The limiting factors are the temperature at which the heat enters the engine and the temperature of the environment into which the engine exhausts its waste heat.

Therefore, an extremely large percentage of the energy produced during fuel combustion is lost, and along with friction, is the reason an engine becomes hot. Engine heating is a result of the process of conductive heat transfer. This lost energy in the form of heat is also the reason the air around an engine produces heat and why internal combustion engines are so inefficient.

Rather than producing mechanical energy, the lost energy in the form of this radiated heat raises the temperature of the engine and the immediate atmosphere around the engine. As a result of heat convection and conduction, energy is lost to the air around the engine and to the engine itself because both the engine and the air around the engine produce a lower temperature than the associated temperature of fuel combustion.

Additionally, a huge portion of the energy produced by a combustion engine simply blows out the exhaust, never converting to mechanical energy. The greater the difference in temperature between a fuel’s combustion temperature and that of its surroundings, the lower the thermal efficiency of an engine.

In other words, the greater the difference between the temperature of burning fuel and the metal and air around it, the greater the energy loss. The greater the difference in temperature, the greater the inefficiency of an engine is proven by Carnot’s Theorem. The Carnot limit is the amount of energy produced during combustion that becomes mechanical energy. That limit is determined by the difference in the heat
of combustion and temperature of the atmosphere around the combustion process.

Internal Combustion Engine Components and Function

An internal combustion engine typically consists of two major structural components: the block and the head. Each engine type possesses different components. The typical components of a gasoline engine are illustrated in Figure 3 while the typical components of a diesel engine are shown in Figure 4.

The block is the single heaviest component in an engine and is generally constructed from cast iron and contains the major power-producing components—the pistons, connecting rods, cylinders, crankshaft, and camshaft—as well as open passages to permit the movement of oil lubrication and cooling throughout the engine.

The pistons, connecting rods, and crankshaft function in a common assembly known as the sliding crank assembly. The head, usually constructed from either cast iron or aluminum, attaches to the top of the block, with the gap between the two sealed with a heat-resistant thick gasket, referred to as the head gasket. The head generally includes the combustion-producing components such as spark plugs and the intake and exhaust valves.

Construction of an engine is typically performed in this manner to provide access to the pistons, rings, and cylinders during future repair. Each cylinder internally contains a piston. On an upstroke (stroke #2) either the fuel-air mixture compresses from the sealed action of the piston against the mixture and cylinder wall, or exhaust gases are vented through an open exhaust valve (stroke #4).

An intake valve (to admit the fuel-air mixture [gasoline] or air [diesel] into the cylinder) and exhaust valve (to release the expelled gases from the cylinder following ignition) are each contained at the top of each cylinder in the head.

Both valves are tightly closed during the compression and ignition stages in addition to the sealing occurring between the piston and cylinder (by using rings at the top of each piston). The resulting explosion of the mixture causes an expansion of gases to occur within the top of the cylinder, forcing the piston downward and away from the combustion region.

Each travel of the piston within the cylinder, up or down, is considered a stroke and each complete sequence of the piston from the beginning of its initial travel to the final travel is called a cycle. A four-stroke gasoline engine, consisting of the following four strokes per cycle, is the most common type in use today. Refer to Figure 5 for a visual explanation of each stroke, described in detail below.

Stroke 1 (A-Intake): A cycle typically begins with the piston in the uppermost position inside the cylinder following an exhaust stroke (also known as top dead center or TDC). As the piston is drawn down within the cylinder by the connecting rod, an intake valve connected in a common configuration to the other intake valves on the intake manifold—the intake manifold is generally connected to the air inlet through an air filter—also opens due to an eccentric action from a camshaft to admit the new fuel-air mixture into the cylinder’s combustion (upper) chamber via a partial vacuum formed in the cylinder. The rest of the calibrated mixture enters the upper portion of the cylinder from either fuel injection or from the carburetor.

Stroke 2 (B-Compression): At the bottom of the first stroke with the connecting rod linkage collapsed (also known as bottom dead center or BDC), the top region of the cylinder has now been filled with a measured volume of the fuel-air mixture. Also, the inlet and exhaust valves close from the combined forces from the camshaft along with heavy springs designed to help force both valves to close. The piston now begins the reverse travel upward as the mixture compresses to a value between 150-175 psi.

Stroke 3 (C-Power): Near the top of the piston’s upward travel with both valves in the cylinder remaining closed and the connecting rod fully extended, a single spark plug (one per cylinder) fires from a high voltage DC spark. Formerly, a condenser was used to generate the several thousand volts of DC power from the 6- or 12-volt DC engine power source.

However, many engines now use solid-state ignition to perform this action. The mixture ignites and explodes from the high voltage spark transmitted across a calibrated gap on the lower exposed portion of the spark plug within the cylinder. The initial and maintained distance of this gap is critical as the gap must be just wide enough to permit the spark to efficiently cross the gap, but not excessively wide; otherwise, the electrical spark voltage cannot cross the gap to create the needed spark.

Immediately following the explosion of the fuel-air mixture at the top of the piston, an expansive force from the gases developed from the explosion develops that drives the piston down and away from the top of the cylinder. This force creates a rotational moment against and onto a connecting rod that results in a direct circular rotation onto a crankshaft.

The crankshaft crosses underneath and connects to each piston in each cylinder and extends outward from the end of the engine itself. It is the source of the rotation and torque output to the driven element (pump, generator, clutch, etc.). This action is repeated many times per second over the series of several cylinders, each timed to be in sequence to the other cylinders so that ignition occurs to only one cylinder at a time. This was performed on many engines historically through the use of a distributor. The distributor is designed to route the spark signal to each spark plug by using lead or spark plug wires to each cylinder from the camshaft, but to only one cylinder at a time.

Stroke 4 (D-Exhaust): Once the explosion has occurred and the piston is driven to the bottom of its stroke travel (connecting rod collapsed), an exhaust valve at the top of the cylinder now opens to exhaust the spent fuel-air mixture. As the connecting rod extends, the piston is directed upwards to the top of the cylinder from the action produced from the crankshaft. The now inert fuel-air mixture exhaust gases are forced out of the cylinder through the exhaust system (exhaust valve to exhaust manifold to muffler to tailpipe) until the piston reaches the end of its upward travel and the process starts over with another quantity of the combustible mixture entering the cylinder during the following downstroke.

The two valves per cylinder, intake and exhaust, are sequenced and timed through use of a camshaft. The camshaft keeps the engine in the proper sequence of opening and closing valves in each cylinder through a timing belt or chain attached to the crankshaft. Proper timing of the engine is critical to make sure each cylinder fires in the proper sequence and in the proper order.

Diesel Engine Components and Function

The diesel engine works under much the same principle; only with this type of engine, the air is first admitted into the cylinder through an open intake valve. The accompanying strokes of a diesel engine are shown in Figure 6 to mirror the following descriptions:

Intake: Air is admitted into the cylinder at the top of the stroke as the piston begins a downward travel.

Compression: High-pressure compression and the resulting applied heat of this air is performed to a great value of up to 700 psi as the piston travels upward; both intake and exhaust valves are now closed.

Power: Diesel fuel is then injected into the cylinder at the top of the piston’s stroke—where the fuel’s specific thermodynamic properties, combined with the high pressure and heat, of the now fuel-air mixture from the closed intake and exhaust valves creates combustion within the top region of the cylinder. A device called a glow plug is often used to preheat the fuel before injection, providing faster and more complete combustion of the fuel. The resulting expansive force from the combustion and fuel pushes the piston downward from the
top of the cylinder. This creates a rotational moment onto the connecting rod and then crankshaft.

Exhaust: At the bottom of the stroke, action of the connecting rod reverses the direction of the piston, which then begins an upward travel. Exhaust gases are released through a now open exhaust valve.

The common rail-direct fuel-injection process is a fuel injection system now used on many diesel engines. It features a high-pressure (more than 100 bar or 1500 psi) common fuel line (common rail) feeding individual solenoid valves, as opposed to a low-pressure fuel pump feeding individual injectors. Newer common rail diesels now feature piezoelectric injectors for increased precision with fuel injection pressures as high as 2500 bar or 36,000 psi.

______________________________________________

This concludes this installment of The Water Works on the basics of internal combustion engines. In our July and October columns, we will continue this discussion on engines with a review of bore and stroke definitions, cylinder pressure, the lubricating and cooling systems, and determining fuel consumption.

Until next time, keep them pumping!

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

Read the Current Issue

you might also like