Chapter 10 Emission Control and Exhaust System Fundamentals 185 Copyright by Goodheart-Willcox Co., Inc. also lowers emissions. However, the main advantage of com- puter control is that it makes electronic fuel injection possi- ble. Electronic fuel injection allows for more precise control of the air-fuel mixture and, therefore, emissions. On newer vehicles, the emission controls must be working properly for maximum driveability and minimum emissions. Integrated design and computer control give the technician a smaller range of options for dealing with problems in the emission control system when compared to older systems. Emission System Tampering Although the emission control and other engine control systems are critical for good vehicle operation, tampering remains a major cause of many driveability problems. There are two types of emission-system tampering. Accidental tampering occurs when a change is made without the intent of altering the system. For example, a technician inadver- tently misrouting a vacuum hose while looking for the cause of a driveability problem is considered accidental tamper- ing. Intentional tampering occurs when a change is made that is known to alter the system. For example, plugging vacuum hose to correct a driveability problem is considered intentional tampering. In the process of doing this, an emis- sion or other driveability problem is created. Inspection of the emission control system is covered in more detail in Chapter 18. Caution: Never intentionally remove or disable any emission-control device under any circumstance. This is a violation of federal and, in most cases, state law. Do not modify any emission control system component or device or the system itself unless the modification is approved by the original equipment manufacturer. Internal Engine Components Internal engine components are designed to balance the needs of performance, fuel economy, emission control, and driveability. Some of these designs were developed many years ago and are still used on the latest vehicles. It is important to understand that internal engine design affects engine operation and, therefore, plays a critical role in driveability. While it is not practical and sometimes illegal to correct a driveability problem by substituting internal engine components, knowing about these modifications can make the job of diagnosing driveability problems easier. This is especially true if tampering is suspected. The most common of these internal modifications are explained in the following sections. Combustion Chamber Shapes The combustion chambers in the cylinder heads are designed to reduce the amount of HC in the exhaust. Older combustion-chamber designs had complex shapes, Figure 10-2A. The goal was to create a high amount of tur- bulence in the combustion chamber, thoroughly mixing the air-fuel mixture. Most pistons had areas on their heads that almost touched the combustion chamber when the piston was at the top of the cylinder. This added turbulence to the mixture as the piston came up on the compression stroke. The shapes of these combustion chambers produced high power with little detonation. However, they caused unburned fuel to condense and cling to the sides of the combustion chamber. This fuel would eventually find its way into the exhaust without burning, resulting in HC emissions. Now, combustion chambers have simple shapes and as little surface area as possible. The most efficient shape for the combustion chamber is the hemispherical design, Figure 10-2B. Limiting the surface area reduces the oppor- tunity for fuel to become trapped and not burn. However, this type of combustion chamber does not produce as much Figure 10-2. A—Older head designs produced more power, but also produced high hydrocarbon levels. B—The hemispherical head is the ideal design for lowest emissions and maximum power. Old-style combustion chamber Off-center placement of spark plug Tight clearance between combustion chamber and piston head Valves Valve Valve Central placement of spark plug Hemispherical combustion chamber Piston Piston A B