Internal combustion engine

An internal combustion engine comprising a cylinder head having formed therein first and second intake valves for admitting combustion charge into a combustion chamber and first and second exhaust valves for expelling combustion products from the combustion chamber, the intake valves being disposed on one side of a roof of the cylinder head and the exhaust valves being disposed on an opposite side of the roof of the cylinder head, each of which valves comprises a port and an associated valve head permitting opening and closing of the port, wherein the valves are arranged in a circumferential sequence as follows:

• the first intake valve, the first exhaust valve, the second exhaust valve and the second intake valve, characterized by the combination of:
• (i) the first intake valve has a larger port bore and valve head diameter than the second intake valve,
• (ii) the provision of differential valve timing means which act to open the first intake valve before the second intake valve, whereby a greater flow of combustion charge is admitted through the first intake valve as compared to the second intake valve, the differential flow between the two intake valves imparting a swirl to the combustion charge as it enters the combustion chamber, the swirl direction generally corresponding to generally circumferential charge flow in the combustion chamber from the first intake valve, and in sequence past the first exhaust valve, the second exhaust valve and thence past the second intake valve and further characterized in that,
• (iii) the first exhaust valve has a smaller port bore and valve head diameter than the second exhaust port, whereby cross flow between the large first intake valve and the adjacent first exhaust valve is minimized during swirling of the charge.