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  BACKGROUND   

Piston Engine

The reciprocating piston internal combustion engine that is now widely used, as its four operating processes: intake, compression, power and exhaust are carried out in the same cylinder, the stroke length is the same for all four phases and the compression ratio for the air-fuel mixture is equal to the expansion ratio for the burned gas. Therefore, the high temperature and pressure burned gas cannot expand sufficiently in its power process. Consequently, there is about 35% of the gross thermal energy taken away by the hot exhaust gases approximate 1000 Cº doing with the pressure of 500kPa. As a result,  the thermal efficiency deteriorates; and a very large exhaust noise is created, and  the exhaust valve which is difficult to cool down is damaged.

Cooling water will take away another 1/3 of the gross thermal energy. Most losses concentrate in the periphery of the combustion chamber; in addition, it is a concern of the surface-to-volume ratio of the combustion chamber. The structure of the combustion chamber of the reciprocating piston engine is flat cylinder-shaped which has a high surface-to-volume ratio, so the heat loss is great. Moreover, with its large surface-to-volume ratio, the quenching effect which produces HC and CO emissions would be higher.

There are also about 2% of fuels wasted in the gaps between piston and cylinder, piston rings and groves, and spark plug (these occupied a total of approximate 10% of final compressed volume) because the air-fuel mixtures in these gaps can't burn at all. Those unburned mixtures together with the quenching effect of the combustion chamber become the main source of HC and CO emissions. Modern engines equipped with direct injection or lean burning system would produce less HC and CO emissions, as these gaps and the quenching zone are charged with much leaner mixture.

Furthermore, while the power acting on the piston is to be transmitted through the connecting rod and crankshaft, there are some component forces that are not only influencing the power transmission efficiency but also causing great friction between piston and cylinder wall to reduce the mechanical efficiency.  Meanwhile, there are the inertia losses due to reciprocating of the valves and pistons, and these losses would be increasing with the raise of the engine speed. Also, these inertia effects would severely affect the acceleration characteristic and top velocity of the engine, and make the engine unbalanced. Moreover, the driving of valve train would consume part of engine power ( about 3-5% of gross power ), and create a lot of noise.

Finally, the piston engine is also restricted by its own structure and operation principle; the mixture of air-fuel is insufficient; burning time is too short; combustion is incomplete, it results in much more CO and HC emissions and lower fuel use rate. 

Consequently, as there is a great deal of energy lost during engine operation, the efficiency of the conventional piston engines could be as low as 20 percent and are seldom above 25 percent. Practical limitations prevent higher efficiencies. Therefore fuel is high, with serious pollution. What is more, with its large bulk, heavy weight, complicated structure, consisting of thousands of parts, heavy use of manufacturing technology, high cost, and high fault rate, it is unable to meet modern requirements. Although some new technologies, such as Direct Fuel Injection, Lean Burning, Sequential Variable Valve Timing, Variable Intake Manifold, Valvetronic, Camless Valvetrain, Multi-valves per cylinder, Tumble Swirl Multiplex and Air Assisting Injection, and even Miller-cycle etc, have been applied, these limited improvements only have slightly promoted the performance of the piston engines. Its development is in fact stagnant now.