Engine power can range from a few watts in small model airplane engines to thousands of kW per cylinder in large multiple-cylinder stationary and ship engines. There is a large commercial market for engines in the 1.5- to 5-kW (2-7 hp) range for lawn mowers, chain saws,
snow blowers, etc. Power for outboard motors (engines) for small boats typically ranges from 2 to 40 kW (3-50 hp), with much larger ones available. Modern automobile engines range mostly from 40 to 220 kW (50-300 hp). It is interesting to note that a modern midsize aerodynamic automobile only requires about 5 to 6 kW (7-8 hp) to cruise at 55 mph on level roadway.
snow blowers, etc. Power for outboard motors (engines) for small boats typically ranges from 2 to 40 kW (3-50 hp), with much larger ones available. Modern automobile engines range mostly from 40 to 220 kW (50-300 hp). It is interesting to note that a modern midsize aerodynamic automobile only requires about 5 to 6 kW (7-8 hp) to cruise at 55 mph on level roadway.
Both torque and power are functions of engine speed. At low speed, torque increases as engine speed increases. As engine speed increases further, torque reaches a maximum and then decreases as shown in Figs. 2-8 and 2-11. Torque decreases because the engine is unable to ingest a full charge of air at higher speeds. Indicated power increases with speed, while brake power increases to a maximum and then decreases at higher speeds. This is because friction losses increase with speed and become the dominant factor at very high speeds. For many automobile engines, maximum brake power occurs at about 6000 to 7000 RPM, about one and a half times the speed of maximum torque.
Greater power can be generated by increasing displacement, mep, and/or speed. Increased displacement increases engine mass and takes up space, both of which are contrary to automobile design trends. For this reason, most modern engines are smaller but run at higher speeds, and are often turbocharged or super- charged to increase mep.
These parameters are important for engines used in transportation vehicles such as boats, automobiles, and especially airplanes, where keeping weight to a minimum is necessary. For large stationary engines, weight is not as important. Modern automobile engines usually have brake power output per displacement in the range of 40 to 80 kW/L. The Honda eight-valve-per-cylinder V4 motorcycle engine generates about 130 kW/L, an extreme example of a high-performance racing engine [22].One main reason for continued development to return to two-stroke cycle automobile engines is that they have up to 40% greater power out- put per unit weight.
In the early 1990s, Honda produced a racing motorcycle with a V4 engine, of which each cylinder had four intake valves and four exhaust valves. The engine was developed by modifying a V8 engine so that the motorcycle could be raced under rules restricting engines to four cylinders. A four-valve-per-cylinder V8 engine block was modified by removing the metal between each set of two cylinders. Special pistons were built to fit into the resulting non-round, oblong cylinders. This resulted in each cylinder having eight valves and a piston with two connecting rods using a common piston pin.
