Reverse-flow cylinder head

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In engine technology, a reverse-flow or non-crossflow cylinder head is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction to exit the head. This is in contrast to the crossflow cylinder head design.

The main advantage of the reverse-flow cylinder head is that both the entering inlet charge and the exiting exhaust gas cause a tendency to swirl in the same direction in the combustion chamber.^[1] In a crossflow head the inlet and exhaust gases promote swirl in opposite directions so that during overlap the swirl changes directions. The constant swirl during overlap which results in a reverse-flow cylinder head promotes better mixing, hence better scavenging of the end gas. The fact that the inlet charge must change direction before exiting the exhaust makes it less likely that fresh mixture will exit the exhaust before mixing during overlap. Overall this improves volumetric efficiency and reduces emissions.

In carbureted engines, poorly atomized fuel reduces efficiency and power at lower rpm (at higher rpm the large air speed keeps the mixture in suspension). The inlet manifold of a reverse-flow cylinder head may be connected to the exhaust by a heat riser to transfer further heat, improving low rpm response and emissions as a result.

Costs can be reduced in production engines by casting the inlet and exhaust manifolds as one unit. This also transfers further heat to the inlet eliminating the need for manifold heating and other related devices. Such an engine is overall simpler, and has improved cold starting.

The reverse flow design is generally considered^{[according to whom?]} to be inferior to a crossflow design in terms of ultimate engineering potential for two reasons. Firstly, there is limited space when inlet and exhaust ports are arranged in a line on one side of the head meaning a reduction in port area compared to a crossflow head. This mainly affects power delivery at high rpm by limiting airflow. Secondly, since inlet and exhaust manifolds are both on the same side of the engine and in close proximity, the inlet manifold and carburetor (if applicable) are heated by the exhaust. This heating reduces the density of the inlet charge and hence the volumetric efficiency of the engine. In a spark-ignition petrol engine the heat also increases the likelihood of pre-ignition or detonation which limits the allowable compression ratio reducing both power and efficiency.^{[citation needed]}

Engineering has found a number of solutions to the first problem such as staggering the ports by placing the inlet ports on a higher level than the exhausts. This way larger ports can be used while still leaving enough room for