Engine Classification

Piston engines can be simple, single-piston engines, such as those on lawn mowers or string trimmers, or they can be much more complicated, multi-piston engines, such as those in automobiles, trucks, and heavy equipment. Multi-cylinder engines come in various cylinder arrangements.

The following are general engine classifications:

FIGURE 5-2 An in-line engine block has all the pistons in a line and increases torque because of the design of the engine.

FIGURE 5-3 A V block engine has the cylinders angled off of the centerline of the crankshaft, which helps reduce the length of the engine so that it will fit into more applications.

FIGURE 5-4 A slant-designed engine is used to decrease the hood height and still maintain a larger engine size.

FIGURE 5-5 An opposed flat engine design enables a low hood line and is compact.

TECHNICIAN TIP

Engine torque is a result of how much air the engine can ingest every intake stroke and typically varies with rpm. Torque is a force that moves the vehicle at lower speeds, quickly achieving the speed desired while horsepower (wattage) keeps it at that speed. The best way to increase torque is to increase engine displacement (cubic inches or cubic centimeters) in a normally aspirated engine. Forced induction also increases torque by increasing the volumetric efficiency (VE) of the engine. By adding more air, the engine can also mix in more fuel, so a charged engine produces more power overall. Turbochargers, however, experience problems with providing torque. Turbochargers capable of producing peak torque at low revs usually gradually run out of steam at higher rpm, as the vast majority of turbodiesels do. Horsepower is a direct result of the total air that the engine can ingest per unit of time—for example, every second. Increasing the restriction to airflow with a larger intake valve area allows more air to enter the engine. Increased breathing ability also offers better performance at higher rpm before the engine can no longer ingest any more air.

Principles of Thermodynamic Internal Combustion Engines

The general definition of thermodynamics is the branch of physical science that covers heat and its relation to other forms of energy, such as mechanical energy. Energy is required to produce power. This chapter will discuss the usage of heat energy in the internal combustion engine to generate power and do work.

Converting the heat energy from fuel released during combustion into mechanical energy describes the operation that an engine performs. This mechanical energy propels a vehicle and provides the power required for the automobile’s onboard systems. An engine can be classified as either an external or an internal combustion engine. The main difference between an internal and an external combustion engine is that combustion takes place outside the cylinder in an external combustion engine. Combustion occurring outside the cylinder transfers the heat to the working fluid, whereas in an internal combustion engine, the working fluid burns inside the cylinder (FIGURE 5-6).

FIGURE 5-6 A. A Stirling engine uses an external heat source to create motion.

FIGURE 5-6 B. An internal combustion engine uses the heat source contained within the cylinder.

External Combustion

Steam engines and Stirling engines are examples of external combustion engines. External heat engines are typically steam engines. Steam engines consisted of many of the same parts used in the internal combustion engine. Steam engines were used for powering trains, boats, ships, automobiles, and more during the Industrial Revolution (FIGURE 5-7). The steam engine power was from any combustible fuel. Typical fuel sources included burning coal or wood to turn water into steam.

FIGURE 5-7 External combustion engines. A. Steam engine.

FIGURE 5-7 External combustion engines. B. Stirling engine.

In an external combustion engine, ignition takes place outside the cylinder and then transfers the heat created into the working fluid and cylinder performing the work. Combustion gases do not enter the cylinders of an external combustion engine. They must be in thermal contact with the engine, however.

Steam engine use included either moving a piston up and down or turning a turbine. Steam engines use heat created in a heat exchanger or external boiler to raise steam, transferring the movement to the piston. As the piston moves, it creates kinetic energy, used to turn a mechanical device. If steam is applied to only one side of a piston, known as a single-acting steam engine, it is not very efficient, because it applies power to the piston only half the time. A double-acting (or counterflow) steam engine was also used where a valve allowed high-pressure steam to act alternately on both faces of a piston, increasing the engine’s efficiency and power output.

Steam engines are very inefficient due to burning the combustible material from the cylinder that turns the heat into mechanical energy. Due to this inefficiency in the conversion process, a significant amount of time must pass before enough heat can build up before it is available to use; this amount of time is significantly greater compared to that required for an internal combustion engine.

The Stirling engine is also an external combustion engine. It holds promise as an alternative source of power, but it has not become popular for transportation, since its output is not easily variable. Solar-powered Stirling engines are gaining popularity as home energy sources because they are environmentally friendly. Since solar energy is the source of energy providing heat for the engine, it produces no byproducts of combustion. Stirling engines can run almost silently and are therefore beneficial in applications where an engine would operate near people without disturbing them.

Internal Combustion

Regardless of its design, gasoline (SI) or diesel (CI), the internal combustion engine (ICE) has reigned supreme as the power source for automotive and other transportation applications. According to Charles’s law, when a gas is heated, it expands. An ICE converts the chemical energy contained in the fuel into a mechanical force. When the fuel is ignited, the heat released from burning fuel in the combustion chamber increases the temperature of the gases in the cylinder. When the gas temperature rises, the pressure of the gases in the cylinder also rises. The pressure created by the expansion of gases acts on the top of the piston or turbine wheel, creating a mechanical force and converting that energy into a reciprocating movement (force) that is used to power the vehicle. This chapter focuses on SI ICEs since they are the most widely used engines in modern automobiles.

ICEs burn the fuel internally, in the cylinder, making it much more efficient than an external combustion engine. The fundamental operation of both SI and CI is almost identical. The significant difference between the two is the method of combustion that occurs in both engines. The spark plug is located in the cylinder head of an SI engine, creating an electrical spark inside the cylinder (FIGURE 5-8). The timing of the spark event controls when combustion occurs. The piston creates a compression of the air-fuel mixture, and the combustion event takes place at a constant volume in the cylinder. In CI engines, the fuel source does not require a spark; instead, high compression that develops extremely high cylinder pressures ignites the fuel. The timing of the combustion event depends on the fuel injection timing into the cylinder. The fuel burns at a constant pressure. Both spark and compression engines can operate in either two-stroke or four-stroke cycles.

FIGURE 5-8 An SI engine has a spark plug that ignites and starts the combustion process.

The ICE is classified as either a reciprocating piston or a rotary engine (FIGURE 5-9). The gasoline piston engine uses the reciprocating movement of the pistons in their cylinder bores and converts it into a rotary motion of the crankshaft. A rotary engine (Wankel engine) uses a rotating combustion chamber with a triangular rotor that spins inside a housing, as opposed to the reciprocating movement of a piston engine. Both the piston and the rotary engine descriptions will continue in detail later in this chapter.

FIGURE 5-9 The rotating assembly includes the piston/connecting rod assembly and the crankshaft.