Having gained an understanding of the four-stroke cycle and stages of engine operation, it is important to consider how engine manufacturers evaluate and categorize their products. With a wide variety of engines being produced by numerous manufacturers, it becomes necessary to identify the distinguishing factors between them. Typically, engines are classified in one of two ways.
By the amount of power it produces
By the size of the engine
How Engines Are Rated: Work and Torque
Work is the result of applying a force over a certain distance, calculated as force multiplied by distance. This can be expressed as foot-pounds or Newton-meters. Torque, on the other hand, is a force that causes rotation or twisting, calculated as force multiplied by the perpendicular distance from the axis of rotation. In essence, torque is a type of work. In the context of an engine, the torque of the crankshaft rod journal can be determined by multiplying its throw by the force exerted on it.
The most crucial components of the crankshaft are the crankshaft journals. There are two distinct types of journals present on a crankshaft – the rod journals, to which the connecting rods are attached, and the main journals, which serve to secure the crankshaft to the engine block. The length of the rod journal determines the stroke of the piston, acting as a lever. This lever effect represents the engine’s ability to exert a twisting force, known as torque, which is quantified as the engine’s capacity to perform work. Horsepower, on the other hand, is defined as the rate at which the engine can accomplish work. Torque is typically measured in foot-pounds or Newton-meters.
1 foot-pound = 1.355 Newton-meters
1 Newton-meter = 0.737 foot-pounds
How Engines Are Rated: Horsepower
Many years ago, during the early days of engine invention, there was a need to quantify the work output of these new machines. In a time when horses were still the primary source of power for transportation, the inventors of the first engines decided to compare their machines to horses in order to establish a standard unit of measurement. By calculating the amount of work a horse could do in one minute, they were able to determine that 550 foot-pounds per second would become the standard unit of horsepower.
1 horsepower = 550 foot-pounds per second
1 horsepower = 33,000 foot-pounds per minute
Horsepower
Horsepower is defined as the amount of work completed in a given amount of time, representing the power output of an internal combustion engine. It is measured as the rate of work or torque, with one horsepower equivalent to 33,000 foot-pounds of work done in one minute. In the metric system, horsepower is commonly expressed in kilowatts, with 0.746 kilowatts equaling 1 horsepower and 1.341 horsepower equaling 1 kilowatt.
Despite the diminished use of horses for labor, the traditional measurement of horsepower is still utilized to quantify the strength of a gasoline engine. Therefore, when a salesperson informs you that a car has 150 horsepower, you can understand that it possesses the equivalent power of 150 horses working together.
In contemporary advertising and product literature, manufacturers commonly include the horsepower rating of their engines. While this information may not be directly relevant for technicians conducting repairs, it serves as a valuable metric for consumers comparing the power output of different automobile engines. The horsepower of an engine directly correlates to its strength and capacity for work, with various factors influencing this output. To comprehensively understand these factors and their impact on horsepower, a thorough understanding of engine mechanics is essential. As such, the exploration of these factors will be delved into further in the program following a detailed examination of engine operation.
How Engines Are Rated: Displacement
When individuals discuss the size of an engine, they are not referring to its physical dimensions, but rather to the volume of the combustion chamber where the air-fuel mixture is ignited. This parameter is known as the engine displacement.
Engine displacement refers to the amount of space that a piston travels from the lowest to the highest point in its path. This measurement is typically expressed in units of volume, with cubic inches used in the English system and liters used in the metric system. For example, an engine with a displacement of 350 cubic inches means that there is a total volume of 350 cubic inches within its cylinders, while an engine with a displacement of 3.1 liters means there is a total volume of 3.1 liters within its cylinders.
The displacement of an engine plays a significant role in determining the amount of power it can generate. Typically, engines with larger displacements tend to produce more power. However, it is important to note that the size of the engine is not the sole factor influencing horsepower output. Various other factors also impact an engine’s power output. Overall, engines with larger displacements are generally capable of producing more horsepower compared to those with smaller displacements.
The engine displacement of a vehicle is typically listed in the vehicle information as well as in the online service documentation for the engine. In certain instances, the engine size may also be visibly indicated on the exterior of the vehicle. Consulting the service information is generally sufficient to determine the displacement of a specific engine. The process of calculating engine displacement will be covered in a different section of your educational curriculum.
How Engines Are Rated: Compression Ratio
Upon discovering that an engine’s displacement refers to the volume of space within its cylinders, it becomes apparent that the largest cylinder volume occurs when the piston is at its lowest point (BDC) and the smallest volume when the piston is at its highest point (TDC). The comparison between these two volumes is known as the compression ratio, despite it being more accurately described as a volume ratio. In the realm of internal combustion engines, the compression ratio represents the relationship between the total cylinder volume and the clearance volume, calculated as the volume at the start of the compression stroke divided by the volume at the end of said stroke.
A ratio serves as a comparison between two values, such as in a compression ratio where the cylinder volumes at different piston positions, BDC and TDC, are compared. Determining the volumes at BDC and TDC involves complex mathematical calculations and specialized test instruments, a process not typically required of automotive repair technicians. The compression ratio of an engine is typically provided in the vehicle’s service manual, but it is important to understand that this ratio directly impacts the engine’s power output and overall performance.
The compression ratio plays a crucial role in an engine as it dictates the combustion process of the fuel within the cylinder. It is responsible for determining the level of compression the mixture undergoes as the piston ascends. A higher compression ratio results in a greater level of compression for the mixture.
Consider an engine with a compression ratio of 5:1. This indicates that when 5 cubic inches of air-and-fuel mixture enter the cylinder, they will be compressed into a space of 1 cubic inch when the piston reaches top dead center.
As the mixture is compressed, the pressure within it will experience a significant rise, leading to enhanced combustion and increased power output upon ignition. Typically, engines with higher compression ratios exhibit more efficient fuel combustion and faster operation. Nevertheless, it is crucial to consider that excessive compression can potentially harm the engine, while insufficient compression results in reduced power generation. Each engine is unique in its compression ratio requirements.
