Average piston speed for all engines will normally be in the range of 5 to 15 m/sec (15 to 50 ft/sec), with large diesel engines on the low end and high-performance automobile engines on the high end. There are two reasons why engines operate in this range. First, this is about the
safe limit which can be tolerated by material strength of the engine components. For each revolution of the engine, each piston is twice accelerated from stop to a maximum speed and back to stop. At a typical engine speed of 3000 RPM, each revolution lasts 0.02 sec (0.005 sec at 12,000 RPM). If engines operated at higher speeds, there would be a danger of material failure in the pistons and connecting rods as the piston is accelerated and decelerated during each stroke. This range of acceptable piston speeds places a range on acceptable engine speeds also, depending on engine size. There is a strong inverse correlation between engine size and operating speed. Very large engines with bore sizes on the order of 0.5 m (1.6 ft) typically operate in the 200- to 4oo-RPM range, while the very smallest engines (model airplane) with bores on the order of 1 cm (0.4 in.) operate at speeds of 12,000 RPM and higher. Automobile engines usually operate in a speed range of 500 to 5000 RPM, with cruising at about 2000 RPM. Under certain conditions using special materials and design, high-performance experimental engines have been operated with average piston speeds up to 25 m/sec. The second reason why maximum average piston speed is limited because of the gas flow into and out of the cylinders. Piston speed determines the instantaneous flow rate of air-fuel into the cylinder during intake and exhaust flow out of the cylinder during the exhaust stroke.
For a given displacement volume, a longer stroke allows for a smaller bore (under square), resulting in less surface area in the combustion chamber and correspondingly less heat loss. This increases thermal efficiency within the combustion chamber. However, the longer stroke results in higher piston speed and higher friction losses that reduce the output power which can be obtained off the crankshaft. If the stroke is shortened, the bore must be increased and the engine will be over square. This decreases friction losses but increases heat transfer losses. Most modern automobile engines are near square, with some slightly over square and some slightly under square. This is dictated by design compromises and the technical philosophy of the manufacturer. Very large engines have long strokes with stroke- to-bore ratios as high as 4:1. Minimum cylinder volume occurs when the piston is at TDC and is called the clearance volume Vc. Modern spark ignition (Sl) engines have compression ratios of 8 to 11, while compression ignition (Cl) engines have compression ratios in the range 12 to 24. Engines with superchargers or turbochargers usually have lower compression ratios than naturally aspirated engines. Because of limitations in engine materials, technology, and fuel quality, very early engines had low compression ratios, on the order of 2 to 3. This limit of 8 to 11 is imposed mainly by gasoline fuel properties and force limitations allowable in smaller high-speed engines. Various attempts have been made to develop engines with a variable compression ratio. One such system uses a split piston that expands due to changing hydraulic pressure caused by engine speed and load. Some two-stroke cycle engines have been built which have a sleeve-type valve that changes the slot opening on the exhaust port. The position where the exhaust port is fully closed can be adjusted by several degrees of engine rotation. This changes the effective compression ratio of the engine.
