186 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. power as older designs and is more likely to result in deto- nation (pinging). To reduce these problems, some cylinder heads are designed so the spark plug is in the center of the combustion chamber. This shortens the distance that the flame must travel. It also ensures that most of the air-fuel mixture in the combustion chamber burns at the same rate. When the spark plug is installed in the center of the cham- ber, the firing process is nearly perfect. Combustion chambers now have a larger volume when compared to older designs. This increased volume helps to lower the compression ratio. Compression ratio is discussed in the next section. Compression Ratio Since the early 1970s, compression ratios have been lowered from as much as 11:1 to around 8:1. See Figure 10-3. However, improved controls have allowed higher compres- sion ratios in recent years, up to 14:1 on some engines. A lower compression ratio allows the engine to operate on lower-octane gasoline without detonating or dieseling. A lower compression ratio also lowers the temperature of the combustion process. This means oxygen and nitrogen are less likely to combine into NO X . However, a lower compression ratio results in lower engine efficiency. This is because a higher compression ratio develops more heat, which increases the combustion pressure that can be exerted on the piston during the power stroke. The loss from a lower compression ratio must be overcome by other engine systems. Number of Valves All older engine designs had two valves per cylinder. Some current engine designs have two valves per cylin- der. However, most manufacturers now use cylinder-head designs that have four valves per cylinder to produce more power from smaller engines, Figure 10-4. These heads allow more of the air-fuel mixture to flow into the cylin- der when compared to two-valve designs. Also, the design provides a low-restriction escape for the exhaust gases. The use of four valves also allows the combustion chamber to be almost completely hemispherical, improving combus- tion efficiency. Some manufacturers compromise by using two intake valves and one exhaust valve per cylinder, Figure 10-5. Camshafts Valve overlap is the number of degrees of revolution over which both the intake and exhaust valves are open. This occurs during the end of the exhaust stroke and the beginning of the intake stroke. See Figure 10-6. Camshafts now have less overlap than older cam designs. Older cams were engineered with large overlap for maximum airflow on the intake stroke. This led to good high-speed performance. However, this overlap caused the incoming air-fuel mixture to be diluted by exhaust gases at idle and low speeds. On older engines, this dilution was overcome by a rich idle mixture. Figure 10-3. Higher compression ratios produce more power. Ratios have been lowered over the years to reduce oxides of nitrogen. Volume 50 cubic inches Volume 50 cubic inches 5:1 Compression Vol. 10 in3 Vol. 5 in3 10:1 Compression Piston A B Piston Piston Piston