(c) Four-Stroke versus Two-Stroke Cycle. Even though the two-stroke cycle is theoret ically superior to the four-stroke cycle,yet the latter predominates. We can very readily see by comparison why this is true. The main ad vantages of the four-stroke cycle are its simple construction, high speed and low cost for small engines; its disadvantages being the varying torsional moment, heavy flywheel, small specific output, large dimensions and contamination of the new charge by the burned gases. As to the main advantages of the two-stroke cycle, they are a specific output which is about 75 per cent to 90 per cent larger than in the four-stroke cycle; the size of the engine is about half of that of a four-stroke cycle engine for the same power output, hence it has lower weight ; good scavenging with a correspondingly purer mix ture and larger unit charge i elimination of valves by using slots or ports in cylinder walls, thus assuring a more reliable construction ; more uniform torque; ease of reversibility; large overload capacity; ratio of cooling surface to displacement volume larger, hence better cool ing effect and higher compression than in the four-stroke cycle. On the other hand, we have disadvantages which may be stated as follows: the charge must be moved through two cylin ders, hence greater frictional resistance and lower mechanical efficiency; low speed; heat losses during the power stroke larger than in the four-stroke cycle. Quite recently, however, the two-stroke cycle has come into wider use, especially for high power units operating on crude oil as fuel. Among experts it is a doubt ful question as to the superiority of the one over the other, the more so as the two-stroke cycle engine is not yet fully developed. Only the future can tell which of the two will be the final victor in the race for supremacy in design, construction, operation and economy.
(d) Constant Volume and Constant Pres sure Cycle. In both the four-stroke and two stroke cycle engines, ignition with consequent combustion of the charge, or the introduction of heat into the cycle, may take place either at ap proximately constant volume with pressure in creasing rapidly (Fig. 11), or at approximately constant pressure with volume increasing grad ually (Fig. 12). In the first case the engine is usually spoken of as an or tOtto) engine, while the constant pressure en gine is usually called a 'Combustion* or engine. The late August N. Otto of Cologne, and the late Rudolph Diesel of Munich were the first successful builders of the con stant volume and constant pressure engines, re spectively.
There have been many other cycles invented and more or less successfully applied (see His torical Development), yet the fact remains that almost all internal combustion engines in prac tical operation to-day are running either on the Otto cycle or on the Diesel cycle, the former being the older and considerably in the lead.
In comparing these two cycles as to theo retical thermal efficiency, it is assumed that: (1) the gas used follows the law of a perfect gas; (2) the ratio of the specific heats a is con stant; (3) the cylinder in which the cycle takes place is absolutely heat tight, i.e., adiabatic heat
conditions exist; (4) the inlet, and outlet valves open and close exactly at dead centre; (5) the work consumed during the suction and the exhaust strokes is neglected.
Otto Cycle. Referring to. Fig. 13, we can follow through the different operations of the cycle. Suction occurs along the line 0-1; adia batic compression along 1-2; ignition and com bustion at constant volume and addition of heat, Qa, along the line 2-3, with a resulting in crease of the absolute temperature Ts to T, and pressure p, to p,; adiabatic expansion along 3-4; and exhaust along 4-1-0, whereby heat, Qr, is rejected along 4-1. This completes the cycle and the heat. Q has been transformed into work represented by the area 1-2-3-4-1. As the efficiency is the ratio of the output over the input we can write the expression: 1) es 2 = Qa . Qr According to the laws of thermodynamics, 2) ts Wc s(Ts-Ts) =heat added; 3) Wcs(Ts-T,) =heat rejected; where is the weight of the charge and the ;pecific heat at constant volume. Then, since T2 T, 4) Ti and for adiabatic curvesT.
Ti I VA = equation 1,assumes the form la) Pr, = 1 1 theoretical thermal efficiency rie of the Otto cycle, being the ratio of cora l', pression, Thus the thermal efficiency of the Otto cycle increases with the compression ratio (4-", as well as with the ratio of specific heats s Since the latter is a function of the mixture ratio of gas to air, the efficiency de pends also on the mixture ratio. Lean mix tures give higher values of K and better effi ciencies than rich ones. For instance, for = r = 2 and r = 1.2, rtt a 13 per cent while V, for ==r=.- 10. in Gls 60 per cent.
Diesel Cycle.As in the Otto cycle, we can determine the thermal efficiency from the the oretical card. Referring to Fig. 14, suction takes place along 0--1; adiabatic high compres sion of the air along 1-2; spraying of fuel into the cylinder by means of highly compressed air, self-ignition and combustion along 2-3, thus adding the heat, Qa, to the cycle; adiabatic expansion along 3-4; exhaust along 4-1-0 whereby heat Qr is rejected. The cycle is thus completed, the heat being trans formed into work 'represented the area 1-2-3-4-1. Using the same notation as above, we have, Q (2, -- Qr Qa Qs Qa 7) Qa = We p (T,-r.) =heat added; 8) Qr = Wcs( Ts-Ts) = heat rejected: T3 Vi 9) = Tr, being the ratio of the loads or ratio of cut off volume to compression volume.
= 10) Equation 6 takes the form 6a) ne 1 X s1 giving the theoretical thermal efficiency of the Diesel cycle. The thermal efficiency of the Diesel cycle thus depends not only on the ratio of compression r, and the ratio of specific heats x, but also on the ratio of the loads L.