The water jackets are cavities, cast into the cylinder head and block. These cavities surround the areas of the engine which are exposed to combustion heat. The size and shape of these cavities are designed depending upon the location so that sufficient coolant is brought to that particular area to carry off all of the heat absorbed during each power stroke. The volume of water jackets and the speed of the coolant through the water jackets cause an increase in coolant
temperature of about 6 to 11K between the water pump inlet and the upper outlet. Coolant flow is normally towards the rear through the water jackets in the block, upward into the head, and then forward through the head to the coolant outlet at the front of the head. In many transversely mounted engines, the coolant enters the block at one end and leaves the head at the other end. A V-8 or V-6 engine usually has separate coolant flow along the water jackets in each bank.
Coolant flows through the engine in one of two ways that is either parallel or series. In the parallel flow arrangement, coolant flows into the block under pressure, and then crosses the gasket to the head through holes adjacent to each cylinder. Gasket openings are shown in Fig. 12.4. In the series arrangement, the coolant flows around all of the cylinders on each bank to the rear of the block where large passages allow the coolant to flow across the gasket to the rear of the heads. The series flow gasket openings are shown in Fig. 12.5. The coolant flows forward through the heads to an outlet at the highest point in the engine cooling passage located at the front of the engine. Some engines use a combination of these two systems which is called a series-parallel coolant flow.
Fig. 12.5. Series-type coolant flow.
The cooling passages inside the engine are made in such a way that the whole system can be drained. Also it is designed to avoid pockets in which steam can form. In series flow systems, this is achieved by short circulating a very small amount of coolant through the bleed holes or steam slits in the gasket, block, and head. Often, this short-circulated coolant is directed to flow on hot areas in the heat, such as exhaust valves, spark plugs, and the exhaust cross over.