The piston-stroke engine compresses air or an air-fuel mixture by means of a piston operating inside the cylinder and transmits the pressure developing from the combustion as torque to the drive.
The piston is connected through the connecting rod with the crankshaft. In combination with the piston pin they form the crank mechanism. The piston moves during one rotation of the crankshaft from the bottom-dead-centre (BDC) to the top-dead centre (TDC) and back. In BDC the distance between piston and crankshaft is shortest, in TDC they are the furthest apart. This definition is also clear in case of a horizontal engine (e.g., opposed cylinder engines). A stroke evolves with half a rotation of the crankshaft. Half a stroke corresponds to the centre distance between main bearing and big-end bearing. The pressure on the piston is transmitted as a force to the connecting rod and as torque to the crankshaft.
The piston velocity is very different during the engine run. It begins at zero and increases to more than 22 m/s (=80 km/h), featuring an extra large stroke and/orvery high revs, then falls back to zero, however, again. This happens during every stroke and with every piston. Two-stroke engines and racing engines may reach even clearly higher piston velocities. In the vehicle technology the medium piston velocity is usually calculated as a characteristic for the load of the piston at maximum speed.
How is it possible to overcome empty strokes for engines with less than 4 cylinders? How to overcome the dead centres? No piston-stroke engine (except maybe the radial engine) can start from a standing position. The engine always needs an idling speed. As is the rule, the idling speed is the higher, leaving out other factors, the fewer cylinders the engine has. Some older engines with few cylinders clearly exhibit their untrue idle running. The larger piston weight and perhaps the larger weight of the connecting rod at that time contribute negatively, too. In this case the two-stroke engine should be clearly favoured over the four-stroke engine. Two-stroke engine engineers claim that their three-cylinders are comparable in terms of operational smoothness to four stroke-six-cylinder engines. All connected flywheel masses, like the crank mechanism (today more rotary than up and down moving masses), balance shafts, the flywheel, parts of the motor control unit, auxiliary aggregates and - the secondary flywheel masses in the gearbox if the clutch is non operational - help to reduce this central problem of the piston-stroke engine.
Until today the prestige of a car grows with the number of cylinders under its hood, in spite that the cubic capacity remains the same. Although it is not at all easy to distinguish the noise of a four cylinder engine with offset shafts from that of a six-cylinder car. Let alone to tell apart the eight from the twelve cylinder engine purely by listening. There is no multi-cylinder engine in which there is exactly the same stroke for two cylinders during a whole stroke. By suitable kinks in the crankshaft they help each other in turns over the respective dead centres. This means, e.g., for the eight-cylinder engine that four cylinders are at dead centres and the other four right in the middle of the stroke.
The cylinders or the cylinder block in which the single cylinders are poured with one another, form the basis of the piston-stroke engine. In addition, some engines feature a crankcase which is screwed together with the cylinder block. The crankcase is absolutely necessary in case of individually arranged cylinders. The crankshaft rotates in the crankcase which is connected via the connecting rod and piston pin to the piston as a crank mechanism.
While the bottom is mostly formed by the oil pan, the cylinder head with gasket and cylinder head cover is arranged upwards. We should in fact not use the concepts "top" and "bottom", because there are also engines featuring horizontal cylinders, in line as well as opposed cylinder engines and V engines. The cylinder head accommodates (at least with the passenger car) also quite a few of the motor control unit parts amongst whom are the camshafts and their drives, and the transmission elements with the valves and their springs.
This covers the piston-stroke engine. We did not mention the engine control units, and the cooling and lubrication system. And where do you assign the parts of the exhaust system, which have to do anything with engine management? The starter and the generator should be mentioned, too. 01/08