Two engines. Two methods of functioning. Two philosophies. In karting, the “dominion” of the 2-stroke, every so often the debate is rekindled: what are advantages of this type of engine? What are the reasons for this choice? Why haven’t 4-stroke engines had better luck in this sector? Let’s try to answer these questions, starting with an analysis of the mechanical and performative characteristics, and then by discussing possible future scenariosread more
The combustion engine was born roughly 160 years ago. The first patent belonged to two Italian physicists, Eugenio Barsanti and Felice Matteucci, who, while using a cast-iron cylinder equipped with pistons and valves, realized that the gases produced by combustion generated a vacuum that moved the piston back into position. It was 1853.
But it was the German Nikolaus Otto who, roughly a decade later, was the first to make a truly reliable, functioning engine, laying the foundations for its industrial development. It’s no coincidence that, even today, the “Otto cycle” defines the basic functioning of internal combustion spark-ignition engines. Its success was immediate and, in the wake of this invention, it didn’t take long for the two-stroke engine to make its appearance as well. This time, the inventor was an Englishman, the chemical engineer Dugald Clerk.
The year was 1879 (though the patent only arrived in 1881).
Today as yesterday, the principal difference between the two traditional piston engines (or alternating cycle with crankshaft system) is that that the 2-stroke engine performs all the phases of the cycle in one rotation of the crankshaft, while the 4-stroke engine uses two rotations. Each “stroke” is the movement of the piston, or a half turn of the crankshaft.
The only useful phase of an engine is the expansion of the combustion gases, or what generates power. Considering the cyclical frequency, which in the 2-stroke is double that of the 4-stroke, theoretically you might think that, with equal displacement, the former always generates double the power of the latter. But we will see that this isn’t always the case, because of some limitations in individual efficiency.
An example of the enormous engines used by shipsread more
The complete 2-stroke engine cycle occurs in a single rotation of the crankshaft. In the “1st stroke,” the piston rises toward the TDC (top dead centre), the blend enters from the carter toward the cylinder, and then compression occurs. In the meantime, a mix of air and fuel blend coming from the outside is aspirated into the carterread more
“2nd stroke”: at the TDC ignition takes place. The piston moves down, pushed by the expansion of the combusted gases, and the exhaust phase occurs. Via the side socket, pressure in the carter pushes new blend into the cylinderread more
The four phases of the 4-stroke engine cycle (aspiration, compression, combustion and exhaust) occur in two rotations of the crankshaft, which correspond to 4 strokes of the piston. A stroke means the movement of the piston from one “dead centre” (the point the piston doesn’t pass, and at which it reverses its movement) to the otherread more
The CONNECTING ROD of a 4-stroke engine is more robust, because the presence of an empty cycle in the engine’s functioning subjects it to greater inertia. Generally, the 4-stroke connecting rod comes in two pieces and moves on oiled bronze bushings. The 2-stroke connecting rod is thinner, in a single piece, and moves on a roller cageread more
Generally, the PISTON of a 2-stroke engine is simpler and lighter. The main difference is the skirt, which in the 2-stroke is higher because it has to regulate the opening and closing of the syphons. The 4-stroke piston has more rings (usually two, plus the scraper). The piston crown in the 2-stroke is slightly domed and almost touching the squish band; in the 4-stroke it’s “dirtier,” to allow for the opening and closing of the valvesread more
The HEAD of the 2-stroke is very simple and has the most efficient combustion chamber of all.
The complexity of the 4-stroke, in this case, is far greater, with a head that requires multiple processing phases and a “dirty” shape, to deal with the seats of the valves
Even the CRANKSHAFT has different degrees of complexity: in the 2-stroke it’s simple and round, while in the 4-stroke it has cheeks that act as counterweights and its design must allow for the balancing of alternating massesread more
W = work; t = time
-Machine torque equals the force the connecting rod applies to the crankshaft, multiplied by the radius of the crank.
-Power equals the Force multiplied by the Velocity.
-Work equals the Force multiplied by the distance of movement. Or, the work it does per unit of time.
T = torque in kg-m
P = power (in horsepower)
N = number of rotations per minute (rpm)
Relationship between torque, power and rpms
With this formula the unit of measure for power is the watt; torque is expressed in Nm.
1 Kg=circa 10 N.
1 horsepower = 0.735 Kw.
1 Kw = 1.36 horsepower
A very important parameter, which describes up to what preparation level an engine is pushed, is the “mean effective pressure”: MEP.
You have the maximum MEP at the maximum torque speed, but “MEP” may also be evaluated at the maximum torque speed.
If we know the torque we can calculate the work and, dividing it by the displacement, we get the value of “MEP.”
In a 2-stroke engine aspiration can be handled in different ways: by a collector and syphon placed on the cylinder (solution with less power); directly on the carter, regulated by the reed valve pack; or by the rotating disc commanded by the rotation of the crankshaft. This last solution can give greater power and greater push at high RPMs, but, in reality, the significant development of reed-valve engines has led this type of engine to be the best compromise (the reason for which, in karting, traditional valve engines have been eclipsed).
In 4-stroke engines, on the other hand, aspiration and exhaust are regulated by a distribution valve system.
Without getting too much into the details of engine design, we can say here that in the design phase a theoretical cycle is calculated and designed, and then, in the development phase, the real cycle is measured.
The comparison of the two cycles demonstrates clear differences, most importantly some significant losses of power not envisioned in the theoretical cycle. In substance, we can conclude that the maximum performance of a traditional gas engine turns out to be roughly 30% of its “theoretical” performance, mainly because of heat energy
dispersed by the cooling system and the expulsion of gas.
In practice, an engine’s energy balance is equal to the sum of the single efficiencies of the engine itself. Principally: thermal efficiency, volumetric efficiency, and mechanical efficiency.
Thermal efficiency is the amount of heat that is transformed into work compared to the total heat generated by combustion.
Volumetric efficiency is the engine’s capacity to “breathe well,” or the ratio of the air that the engine is actually able to aspirate compared to what the cylinder could contain.
Mechanical efficiency is the ratio of useful work supplied by the engine to theoretical work that could be achieved without friction.
Some examples of 2-stroke engines for kartsread more