130 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Air-fuel mixture Hump Masked valve-seat area S-shaped high-swirl port Intake port Exhaust port Intake valve Exhaust valve Figure 6-8. A swirl combustion chamber uses the shape of the intake port and the entry of the air-fuel mixture into combustion chamber to mix the charge for more-efficient combustion. Exhaust valve Intake valve Spark plug Ignition coil Figure 6-9. A cylinder head with crossflow combustion cham- bers has intake ports on one side of the head and exhaust ports on the other. (Honda) A “squish area” is commonly formed inside a wedge combustion chamber. When the piston reaches top dead center (TDC), it comes very close to the bottom of the cylinder head. This squeezes the air-fuel mixture in that area and forces it (“squishes” it) out into the main part of the chamber. Squish can be used to improve air-fuel mix- ing and burning at low engine speeds. A wedge combustion chamber normally uses a flat top piston. A hemispherical combustion chamber, nicknamed a hemi head, is shaped like a dome when viewed from the side, Figure 6-7C. The valves are canted (tilted) on each side of the chamber. The spark plug is located near the center. This design is extremely efficient. There are no hidden pockets for incomplete combustion. The surface area is very small, reducing heat loss from the chamber. The centrally located spark plug produces a very short flame path for fast and complete combustion. The canted valves help increase the breathing ability of the engine. The hemi head was first used in high-horsepower, rac- ing engines. It is now used in many passenger car and light truck engines. It allows the engine to operate at high rpms and makes it very fuel efficient. It also produces complete burning of the fuel to reduce emissions. One disadvantage of the hemi head is that it often requires a domed piston to obtain the needed compression ratio or compression stroke pressure. The dome adds weight when compared to a flat top piston. The increased piston weight reduces mechanical efficiency during high-engine- speed operation. The pent-roof combustion chamber is similar to the hemispherical combustion chamber, but it has flat, angled surfaces rather than a domed surface. See Figure 6-7D. This design improves volumetric efficiency and reduces emissions. Combustion Chamber Type There are many different types of combustion chamber. A combustion chamber may be classified by one or more of the characteristics described in this section. A swirl combustion chamber uses the shape of the intake and exhaust ports and the shape of the combustion chamber roof to help mix the air-fuel mixture. As shown in Figure 6-8, a curve is provided in the intake port right before the intake valve and seat. Sometimes, a mask area is used to also control the movement of the mixture through the port and into the combustion chamber. Swirling the air-fuel charge improves combustion efficiency. A crossflow combustion chamber has the intake ports on one side of the head and the exhaust ports on the other side. This is pictured in Figure 6-9. During engine opera- tion, the air-fuel charge enters the combustion chamber on the intake stroke from one side of the head. Then, on the exhaust stroke, the burned gases leave on the opposite side. The exiting exhaust gases help pull more air-fuel charge into the combustion chamber to increase power. A noncrossflow (backflow) combustion chamber is illustrated in Figure 6-10 as compared to a crossflow design. A noncrossflow combustion chamber is an older design that has been phased out for the more efficient crossflow com- bustion chamber.