How Engines Are Rated

Now that you understand the four stages of engine operation, let's take a look at how engine manufacturers rate and classify the engines they make. Since so many different engines are produced by so many different manufacturers, what factors can we use to distinguish one from another? Well, an engine is normally described in one of two ways:

• By the amount of power it produces
• By the size of the engine

Many years ago, when the first engines were invented, no one knew how to express the amount of work they could do. It was decided that a standard was needed. At that time, horses still provided most of the power for transportation. As a result, the inventors of the first engines compared their engines to horses in order to describe the amount of work the engines could do. It was known that an average horse could do approximately 550 foot-pounds of work in one second (Figure 17). This fact was used to develop the standard unit of horsepower.




FIGURE 17-An average horse can complete 550 foot-pounds of work in one second. This fact was used to develop the standard unit of horsepower. The more horsepower an engine produces, the more work it can perform.

Even though horses are seldom used to perform work anymore, we still use the standard unit of horsepower to describe the power output of a gasoline engine. So, the next time you're looking at a car in a showroom and the salesperson tells you it has 150 horsepower, you'll know that the car has as much power as if it was being pulled by 150 horses. Pretty impressive, isn't it?

Today, manufacturers will often list the horsepower output of their engines in their advertising and product information brochures. For a technician, knowing the horsepower of an engine isn't very useful for making repairs. However, this information can be used by consumers to compare the power of different automobile engines. The more horsepower an engine produces, the stronger the engine is, and the more work it can perform. The amount of horsepower that an engine will produce will depend on many factors. To understand how these factors affect the horsepower, you must first have a complete understanding of how an engine operates. Therefore, we'll look at the various factors that affect horsepower later in the program, after we examine the engine in more detail.

When you hear people refer to the size of an engine, they don't mean the physical size of the engine. Rather, they're referring to the size of the space inside the engine where the air-and-fuel mixture is burned. The size of this space is called the engine displacement.

By definition, engine displacement is the volume of space that the piston must travel from the very bottom of its path to the very top of its path. The distance that the piston travels up and down in a cylinder is called the stroke of the engine. Displacement is measured in units of volume. In the English system of measurement, displacement volume is measured in cubic inches. In the metric system, displacement volume is measured in liters. (Even if you're not too familiar with the metric system, you're probably aware of liters-two-liter plastic bottles are commonly used to hold soft drinks.) So, if an engine has a displacement of 350 cubic inches, this means that it has a total volume of 350 cubic inches inside its cylinders. If an engine has a displacement of 3.1 liters, it has a total volume of 3.1 liters inside its cylinders. An engine's displacement value has an effect on the amount of power that the engine will develop. In most cases, the larger the displacement, the more power the engine will develop. However, this doesn't mean that a smaller engine can never develop more horsepower than a larger one. As mentioned earlier, there are many factors besides displacement that affect an engine's power. In general, however, an engine that has a larger displacement will develop more horsepower than one with a smaller displacement.

An engine's displacement will usually be stated in the service manual for the engine, or it may be printed on the engine. In some cases, the engine size is even labeled on the outside of the vehicle body itself. A check of the vehicle or manual is all that's usually needed to find the displacement of a particular engine.

You've just learned that an engine's displacement is the volume of space inside its cylinders. When a piston is at its lowest point in a cylinder (BDC), the cylinder volume is at its largest. When the piston is at its highest point in the cylinder (TDC), the cylinder volume is at its smallest. The ratio of the largest cylinder volume to the smallest cylinder volume is called the compression ratio. A ratio is simply a comparison between two values. In a compression ratio, the two values are the cylinder volumes at the two different piston positions, BDC and TDC.

The volumes at BDC and TDC can be determined by using a combination of mathematical calculations and special test instruments. Automotive repair technicians will never be required to measure these volumes, so we won't get into the details of how they're determined. (Measuring the volume is a complicated procedure that requires special equipment.) Besides, an engine's compression ratio is usually listed in the vehicle's service manual. However, you should be aware that an engine's compression ratio affects the amount of power that the engine develops, which in turn affects the engine's performance.

The compression ratio is important in an engine because it determines how fuel will be burned in the cylinder. An engine's compression ratio will determine how much the mixture will be compressed when the piston rises. The higher the compression ratio, the more the mixture will be compressed.

So, imagine that a particular engine has a compression ratio of 5 to 1. What exactly does this mean? Well, suppose 5 cubic inches of air-andfuel mixture enter the cylinder. Because this engine has a compression ratio of 5 to 1, when its piston rises to TDC, the 5 cubic inches of mixture will be compressed into a 1-cubic-inch space.

When the mixture is compressed, the pressure of the mixture will increase dramatically. This large increase in pressure makes the mixture burn more completely and produce more power when it's ignited. In general, the higher an engine's compression ratio, the stronger the fuel mixture burns and the faster the engine will run. However, every engine has its limitations. If an engine's compression ratio is too high, the excessive pressure can damage the engine. If the compression ratio is too low, the engine doesn't develop much power. Different engines have their own particular compression ratios. However, most automobile engines have compression ratios that are between 5 : 1 and 9 : 1.